Symposium EM2 : Rare-Earths in Advanced Photonics and Spintronics

Integration of cubic (bixbyite-type) sesquioxides on a Si platform has high potential to introduce the new performance of Si circuits by incorporating functionalities as not only alternative gate oxides but also light emitting materials. Among the oxides, Sc₂O₃ is one of the most promising as a new host material of rare-earth ions-doped light emitters on Si substrates. The thermal conductivity of Sc₂O₃ is the largest among the bixbyite-type oxides, including Y₂O₃, Gd₂O₃ and rare earth oxides, and larger than that of yttrium aluminium garnet (YAG) crystal. The highest thermal conductivity could enable us to obtain high-power light emission on Si with effective heat dissipations for both electrical and optical pumping. So far, epitaxial growth of Sc₂O₃ films have been realized on Si(111) by molecular beam epitaxy (MBE), even though the misfit between lattice constant of Sc₂O₃ (=0.983 nm) and two times that of Si (=1.0862 nm) is about 9% [1]. However, growth of Er-doped Sc₂O₃ has not been achieved on Si as a light emitting material on Si. In this work, we grow Er_xSc_2-xO₃ films on Si(111) by MBE as a function of x and growth temperature, and systematically characterize the films using synchrotron grazing incidence X-ray diffraction (GIXD), cross-sectional transmission microscopy, extended X-ray absorption fine structure (EXAFS), and hard X-ray photoemission spectroscopy (HAXPES). We also characterized optical properties of the films after subsequent thermal annealing and oxidation of the as-grown films by photoemission spectroscopy (PL) and Raman spectroscopy. The GIXD measurements clearly show that the films are strained in-plane and out-of-plane directions depending on x and the growth temperature, and thermal expiation coefficients of the epitaxial Er_xSc_2-xO₃ (x=2) and Er_xSc_2-xO₃ (x=0) films are obtained. EXAFS measurements and theoretical fittings of the EXAFS profiles show that the Er³⁺ ions in the films are preferentially occupied in the C_3i site of the bixbyite films when x is less than 0.02 and in the C₂ and C_3i sites of the films when x is equal to 0.1. HAXPES measurements show that the first energy level of 4f in Er³⁺ ions (⁴I_15/2) is located at 2.0 eV below the valence band maximum of the Er_xSc_2-xO₃ film when x=0.06. PL measurements show that the PL intensity becomes almost double with increasing the temperature of subsequent thermal annealing and oxidation of the as-grown films. Note that the subsequent oxidation improves the PL intensity of the films effectively compared with the thermal annealing above 500 degree C. The improvement of the PL intensity is considered to be due to the strain relaxation of the as-grown films and to the formation of interfacial SiO₂ layers by oxidation which effectively hinders energy transfers from Er³⁺ ions in the film to Si substrates when pumped.
[1] D. O. Klenov, L. F. Edge, D. G. Schlom, and S. Stemmer, Appl. Phys. Lett. 86, 051901 (2005).

Imaging complex materials at the single-molecule level reveals spatial and temporal heterogeneities that are lost in ensemble imaging experiments. An ongoing challenge is the development of probes with the photostability, brightness, and continuous emission necessary for single-molecule imaging, all while limiting laser intensities to levels that do not cause photodamage. We have found that lanthanide-doped upconverting nanoparticles (UCNPs), which absorb multiple photons in the NIR and emit at higher energies in the NIR or visible, have unusual properties that make them valuable luminescent reporters, including: no measurable photobleaching, even under prolonged single-particle excitation; no overlap with autofluorescence in biological systems; an absence of on-off blinking; and efficiencies 5-6 orders of magnitude higher than the most efficient 2-photon fluorescence processes (1-3). The brightness and sensitivity of UCNPs has been limited by a poor understanding energy transfer and relaxation within individual nanocrystals and unavoidable trade-offs between brightness and size. Using a combination of advanced characterization and theoretical modeling, we have recently engineered UCNPs that are 2 orders of magnitude brighter under single-particle imaging conditions than the brightest ensemble compositions(4-5), allowing us to visualize single upconverting nanoparticles at fluences (<1000 W/cm²) typically found in 1-photon confocal imaging. We find that factors known to increase brightness in bulk experiments are unimportant at higher excitation powers, and that, paradoxically, the brightest probes under single-molecule excitation are barely luminescent at the ensemble level.

1. Wu, S., Han, G., Milliron, D.J., Aloni, S., Altoe, V., Talapin, D.V., Cohen, B.E., & Schuck, P.J. (2009) Non-blinking and photostable upconverted luminescence from single lanthanide-doped nanocrystals. Proc.Natl. Acad.Sci. U.S.A. 106, 10917-10921.
2. Cohen, B. E. (2010) Biological Imaging: Beyond fluorescence. Nature 467, 407-408.
3. Ostrowski, A.D., Chan, E.M., Gargas, D.J., Katz, E.M., Han, G., Schuck, P.J., Milliron, D.J., & Cohen, B.E. (2012) Controlled synthesis and single-particle imaging of bright, sub-10 nm lanthanide-doped upconverting nanocrystals. ACS Nano 6, 2686-2692.
4. Gargas^, D.J., Chan, E.M., Ostrowski, A.D., Aloni, S., Altoe, V., Barnard, E.S., Sanii, B., Urban, J.J., Milliron, D.J., Cohen, B.E., & Schuck, P.J. (2014) Engineering bright sub-10-nm upconverting nanocrystals for single-molecule imaging. Nature Nanotech. 9, 300-305.
5. EM Chan, ES Levy & BE Cohen. Rationally designed energy transfer in upconverting nanoparticles. Adv. Mater. 27, 5753-5761 (2015).

Er_xY_2-xSiO₅ crystalline compounds have been investigated as the C-band light sources and optical gain media for integrated photonics. Förster energy transfer between Er ions causes the excitation energy dissipation, such as the cooperative upconversion and the diffusion limited relaxation in high Er content materials. Increasing Er density causes not only the optical gain per unit length but also the excitation energy dissipation owing to the energy transfer. Considering the trade-off relation, the Er content was optimized in order to obtain high optical gain.
On the other hand, the energy transfer from Yb to Er ions in the co-doped materials can enhance the Er 1.53µm luminescence (⁴I_13/2 -> ⁴I_15/2) due to the 980nm excitation (⁴I_15/2 -> ⁴I_11/2), because absorption cross-section of Yb³⁺ ion is about one order larger than that of Er³⁺ at the 980nm excitation band and the absorbed energy is preferentially transferred to the second exited state of 4f-electrons in Er ions (⁴I_11/2). Then the existence of Yb replacing Y also can block the energy transfer between Er ions because of no transition in 4f-electronic states of Yb ion corresponding to the Er intra-transition between ground and first excited states (⁴I_13/2 -> ⁴I_15/2). We have assumed that Yb replaced with a part of Y in the optimized Er_xY_2-xSiO₅ system take a site enough close to Er to transfer the excitation energy. This study also shows the extremely effective sensitization of the Er-related emission in Er/ Yb system.
From the experimental results, the sensitization effect above 10 times in Er_xYb_yY_2-x-ySiO₅ crystals with the optimized ratio Yb/Er=1 has been obtained. The optical net gain of 45dB/cm with low threshold is demonstrated, and then the effective excitation cross section is estimated by the threshold to be 3.3x10^-19 cm² which corresponds to about 10 times of the absorption cross section of Er ions. These results suggest great efficient sensitization owing to the energy transfer from Yb ions.

Er-Doped Fibre Amplifiers (EDFA) have been used in terrestrial systems for many years, because of their reduced size, lightweight and mechanical flexibility. Their application fields range from optical communications to several on-board navigation instruments. However, their use in harsh environments, such as space application, is strongly limited by their reduced resistance and stability at temperatures above 850 °C and under hard radiation exposure. Here, our objective is to identify the key structural and phase changes that can improve the thermal stability of Er/Si ion-implanted fused silica. This understanding will be used to determine potential routes for the development of EDFA with superior resistance to thermal and radiation stimuli.
We study the nucleation and growth of Er/Si nanoclusters obtained in co-implanted fused silica upon annealing by transmission electron microscopy (TEM). Bright-field and high angle annular dark-field TEM image show that the diameter of the formed nanoclusters increases with the annealing temperature (1000-1200 °C). The microstructure of nanoclusters is investigated by high-resolution TEM imaging, showing the presence of structural defects, whereas their chemical composition is measured by energy-dispersive X-ray spectroscopy. Besides Si and Er₂O₃ nanoclusters, we find Er₅Si₃ nanoclusters which formed for thermal annealing at 1200 °C. We also carry out Raman measurements to study the optical performance of each sample and correlate their macroscopic properties with the microstructure of Er/Si nanoclusters.

Laser induced crystallization of rare earth doped glasses is a spatially selective process which has the potential to produce active photonic integrated circuits in a glass matrix. Previous work has already shown that laser induced crystals in undoped glass can function as waveguides [1]. In this work, we present our studies on the properties of rare earth ions (Er, Nd, Pr) within laser-induced crystals-in-glass and compare them with data obtained from LaBGeO₅ glass-ceramics. We find that the rare earth ions incorporate at predominantly one majority side and are able to quantify the energy levels of this site. With the goal to assess if these incorporated rare earth ions are suitable for laser or amplifier applications, we will discuss our efforts to quantify radiations lifetimes, up-conversion probabilities, and absorption cross-sections using time resolved emission spectroscopy and waveguide absorption and gain measurements.
[1] Stone, A. et al., “Direct laser-writing of ferroelectric single-crystal waveguide architectures in glass for 3D integrated optics”, Scientific Reports 5, 10391 (2015).

Research on white-LED (oxy)nitride phosphors doped with Eu²⁺/Ce³⁺ ion has been extensively conducted this century [1]. It is widely believed that (oxy)nitride phosphors, with high covalence apparently leading to longer emission peak than oxide phosphors (nephelauxetic effect), may exhibit small thermal quenching of photoluminescence (PL), due to rigid Si-N network (stiffness) that may induce small Stokes shift. Nonetheless, researchers have gradually noticed that such naïve expectation is somewhat optimistic, by observing not a few exceptions. Non-empirical approach for analysis/design for phosphors is thus wanted.
In this talk, we will review our theoretical approach from bulk properties to optical properties of Eu²⁺/Ce³⁺-doped phosphors. It is stressed that emission color should be analyzed from the viewpoint of polarizability (dielectric constant) rather than covalence/ionicity, as an indicator of nephelauxetic effect [2]. Still, it is difficult to non-empirically identify luminescent centers in (oxy)nitrides where multiple cation sites are available for Eu²⁺/Ce³⁺ dopant.
We have thus developed method for luminescent center identification by considering excited-state geometry relaxation for Eu²⁺/Ce³⁺-dopant. Two cases are exemplified: 1) Ba₃Si₆O₁₂N₂:Eu²⁺ (green phosphor) vs. Ba₃Si₆O₉N₄:Eu²⁺ (bluish green phosphor): despite similar physical properties of the bulk systems [3], Ba₃Si₆O₁₂N₂:Eu²⁺ has weak thermal quenching at working temperature (around 100 °C), while Ba₃Si₆O₉N₄:Eu²⁺ exhibits so strong thermal quenching (little luminescence at room temperature). The both phosphors have rather narrow PL spectra despite multiple Ba sites available for Eu²⁺-dopant. Our analysis indicates that the narrow PL spectra may originate from ONLY ONE luminescent center; non-luminescent centers are also identified. The temperature quenching behaviors can be explained by the energy gap between Eu²⁺ 5d level at its emitting state and the conduction band bottom (auto-ionization) [4]. 2) LaSi₃N₅:Ce³⁺(blue phosphor) vs. La₃Si₆N₁₁:Ce³⁺ (yellow phosphor) : LaSi₃N₅ has one non-equivalent La³⁺ crystallographic site whereas La₃Si₆N₁₁ has two non-equivalent La³⁺ sites (symmetric and asymmetric). Our recent calculation not only quantitatively explains the emission color difference but also identifies luminescent and non-luminescent centers [5].
Hence, our theoretical method may be expected as ab initio crystal site engineering (CSE) for design as well as analysis of Eu²⁺/Ce³⁺-doped phosphors. We will refer to some benchmarks as assessment of our ab initio CSE.

[1] Nitride Phosphors and Solid-State Lighting, R.-J. Xie et al., eds. CRC Press, Boca Raton (2011).
[2] M. Mikami, ECS J. Solid State Sci. Technol. 2 (2013) R3048.
[3] B. Bertrand and S. Poncé et al., Phys. Rev. B88 (2013) 075136.
[4] S. Poncé, Y. Jia, M. Giantomassi, M. Mikami, and X. Gonze, J. Phys. Chem. C 120 (2016) 4040.
[5] Y. Jia, A. Miglio, S.Poncé, X. Gonze and M. Mikami, Phys. Rev. B93 (2016) 155111.

A tri-doped YBO3 phosphor, which is capable of producing white light by combining blue, green, yellow, orange and red emissions when excited at 365 nm ultraviolet (UV) light, was developed. A general hydrothermal method was used to synthesize tri-doped YBO3:Tb3+, Eu3+ and Dy3+. The samples showed strong photoluminescence spectra at 485, 541, 578, 591, 611, and 627 nm under the excitation wavelength of 365 nm (Ultraviolet Light) indicating the presence of blue, green, yellow, orange and red light. Tri-doping is seen to be more successful in the creation of white light. The effect of the variations in doping percentages were also evident.

Erbium doped optical fibers based on silica are key elements in the construction of infrared lasers for telecommunications, a platform which is mature and a commercial success. Miniaturization of photonic components for integration with silicon photonics requires further material development in order to enhance optical gain characteristics of rare-earth materials on short length scale, while maintaining low temperature deposition and annealing processing to minimize the thermal budget on a photonics/CMOS process flow.
Concurrently, recent progress in thin film transistors based on metal oxides deposited at room temperature have shown attractive electronic devices operating in a variety of amorphous substrates, which show great promise for integration as smart, active optical claddings in silicon photonics.
In this work, we study the crystal morphology, electron transport and photo-electroluminescence of erbium-doped nanolaminates of ZnO, TiO₂ and SiO₂, deposited by RF-sputtering, with and without an annealing step. The effect of the nanolaminate on the interface roughness, erbium distribution on the laminate and its correlation to the photo- and electroluminescence (visible and infrared domains) is presented

Zinc stannate (Zn2SnO4) is a transparent n-type semiconducting oxide with a diverse array of applications, such as in lead-free ferroelectrics, gas sensors, transparent conductors, lithium-ion batteries, dye-sensitized solar cells and photocatalysis. As a wide band gap material, it also has good potential for full color phosphors, however there are only few reports on the possibility of Zn2SnO4 hosting an activator ion for this kind of application. Among different possible dopants, the rare-earth elements are the most attractive because of their characteristic electronic transitions that could lead to sharp luminescence features from the ultraviolet (UV) to infrared (IR) range. Additionally, scalable methods of production are desired, as long as the material properties can be controlled. Thus the main objective of this work is to utilize a simple method of synthesis – mechanochemistry – in order to solve the comparatively complex and challenging task of nanocrystal doping. The mechanically initiated chemical reactions of oxides are typically accompanied by cation redistribution, formation of defect centers with unsaturated oxygen coordination, phase transformations and etc.



This work investigates the structural and luminescence properties of mechanochemically synthesized Zn2SnO4 nanoparticles and the changes induced by europium doping. Nanocrystalline Zn2SnO4 owders doped with Eu3+ ions were synthesized via a mechanochemical solid-state reaction method followed by post annealing in air at 1200 oC. X-ray diffraction (XRD), energy-dispersive X-ray (EDX), Raman and photoluminescence (PL) spectroscopies provide convincing evidence for the incorporation of Eu3+ ions into the host matrix on non-centrosymmetric sites of the cubic inverse spinel lattice. Microstructural analysis shows that the crystalline grain size decreases with the addition of Eu3+ Formation of a nanocrystalline Eu2Sn2O7 secondary phase is also observed. Luminescence spectra of Eu3+ doped samples show several emissions, including narrow-band magnetic dipole emission at 595 nm and electric dipole emission at 615 nm of the Eu3+ ions. Excitation spectra and lifetime measurements suggest that Eu3+ ions are incorporated at only one symmetry site. According to the crystal field theory, it is assumed that Eu3+ ions participate at octahedral sites of Zn2+ or Sn4+ under a weak crystal field, rather than at the tetrahedral sites of Zn2+, due to the high octahedral stabilization energy for Eu3+ Activation of symmetry forbidden (IR-active and silent) modes is observed in the Raman scattering spectra of both pure and doped samples, indicating a disorder of the cation sublattice of Zn2SnO4 nanocrystallites. These results were further supported by the first principle lattice dynamics calculations. The spinel-type Zn2SnO4 shows effectiveness in hosting Eu3+ ions, which could be used as a prospective green/red emitter. This work also illustrates how sustainable and simple preparation methods could be used for effective engineering of material properties.

Besides excellent thermal and chemical stability, the oxide phosphors are environmental friendly what are required for their growing applications such as in display systems, lasers energy-conversions, photocatalysis, optical imaging, medicals, and several others. Synthesis of oxide phosphors of self-confined size in a nanostructure is important to tailor selective optical and other properties for the applications. In this investigation, we report synthesis of Eu³⁺ doped CaIn₂O₄–carbon hybrid nanostructure with selective Eu³⁺ dosages of 0.1-1 mol % in optimizing the optical properties for using in as light-emitters and other devices. A green synthesis route using fresh nectar extracted from aloe vera leaves is used to obtain the samples with tailored microstructure and optical properties. This is known as pollution controlled biological method. The nectar, which is a natural hydrogel, was mixed to an aqueous solution in calcium, indium and europium salts, wherein the gel traps the metal ions forming a mixed hydrogel as ‘a precursor’ to obtain the Eu³⁺:CaIn₂O₄–carbon hybrid nanostructure. The gel on self-dried at room temperature was burnt with camphor (a fuel) in air that yields the final product of small crystallites. At this stage, the sample contains a lot of free carbon which was removed by annealing the powder at 400 and 600 °C in air. Light-emission spectra were studied from the as-prepared and annealed powders, dispersed in distilled water, nearly 0.2 mg per mL. In exciting at 240 nm wavelength from a xenon lamp, two broad emission bands appear at 431 nm, as an intense blue emission in the ⁷F₂→ ⁵D₃ transitions, and 671 nm, as a red emission in the ⁵D₀ → ⁷F₃ transitions of Eu³⁺ ions. On the Eu³⁺ dosages above 0.2 mol%, the emission intensity gradually decreases in the concentration quenching process. The results are described in correlation to the microstructure.

The effect of Gd on structural, optical and magnetic properties of ZnO has been investigated in a large concentration range. On the one side ZnO substrates have been ion-implanted with Gd leading to an average volume concentration of 5 x 10¹⁸ -10²⁰ /cm³ [1]. On the other side Gd_xZn_1-xO-films have been sputtered reactively on sapphire substrates yielding Gd concentrations of 1.3 to 16% [2]. Ion implantation of ZnO with RE atoms like Gd usually leads to a distortion of the lattice which has been simulated by theory [2] and evidenced with XRD. In the sputtered films it could be shown with XLD that Gd mainly substitutes for Zn in the lattice. With increasing Gd percentage the lattice distortion becomes massive and secondary phases occur. Eventhough there are many defects in the RE doped ZnO, there is no long range magnetic order, even for high concentrations of 16% Gd in ZnO. This was confirmed on a global scale with SQUID and element selective with XMCD. The occurrence and shift of defect bands in ZnO due to doping will be shown with PL. Finally the results for Gd:ZnO will briefly be compared with those for Sm, Ho and Eu implanted ZnO substrates.
[1] V. Ney et al, J. Appl. Phys. 104, 083904 (2008)
[2] V. Ney et al, Phys. Rev. B 85, 235203 (2012)

For the longest time, hematite and rare earth oxides have been reported to have very low electrical conductivity and carrier mobility. Here we report an exciting class of new amorphous ternary oxides made of iron, terbium, and dysprosium (a-Fe-Dy-Tb-O), the thin films of which demonstrate a unique combination of room temperature ferromagnetism, semiconducting behavior, and optical transparency. Thin films with varying metal composition were prepared using electron-beam co-evaporation and were subsequently oxidized by air exposure under ambient conditions. The films were characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy (XPS), optical spectroscopy, glancing angle x-ray diffraction (GIXRD), 4-probe electrical properties and magnetization. The amorphous microstructure was found for compositions between R = 0.6 to 20, with R = (at % Fe)/(at % Dy + at % Tb). The film with R = 0.60, displayed visible light transparency of ~50% with semiconductor behavior confirmed by valence band analysis in XPS. The semiconducting film showed exceptionally low resistivity and sheet resistance (7.15 × 10⁻⁴ Ω-cm and ~200 Ω/sq respectively) as well as soft room temperature ferromagnetism (coercivity ~40 Oe and saturation field ~ 10.5 x 10³ Oe). This novel a-Fe-Dy-Tb-O system opens a new class of materials for potential applications as a transparent conducting layer in light harvesting applications such as in solar cells, and could be further useful for spintronic devices.
Reference: Taz et al. Scientific reports (2016) www.nature.com/articles/srep27869

When the complexity of a metallic compound reaches a certain level, a specific location in the structure may be critically responsible for a given fundamental property of a material while other locations may not play as much of a role in determining such a property. The first-principles theory has pinpointed a critical location in the framework of a complex intermetallic compound - Gd₅Ge₄- that resulted in a controlled alteration of the magnetism of this compound using precise chemical tools.

This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. The research was performed at Ames Laboratory, which is operated for the U.S. DOE by Iowa State University under contract # DE-AC02-07CH11358.

Observation of the phenomenon such as negative magnetism and associated magnetic exchange bias (HE) have made many chromate systems important for spin-value, magnetic recording, sprintronic devices. These phenomena originate due to competing instabilities and, for example, the resulting unidirectional anisotropy simulated in a ferromagnetic component due to another antiferromagnetic phase, which appears as the sample is cooled across the Neel temperature. In general, the above phenomenon can be observed in a multiphased magnetic material, which may comprise of phases such as a ferromagnet (FM), ferrimagnet (FIM), antiferromagnetic (AFM), and/or a spin glass. In this paper, we show that, in Y_1-xPr_xCrO₃ (0.0≤ x≤ 1.0), the magnitude of negative magnetism is significantly tuned as the structural transformation takes place, with changing concentration of the dopant. The different possible arrangement of the magnetic order in Y_1-xPr_xCrO₃ is presented in the paper. It is also shown that, beyond a critical concentration, positive magnetism reappears in the system.
Results of M-T and M-H experiments are presented at length. The experimental observations could be explained by the changing strength of the Dzyaloshinskii-Moriya interaction (DMI) and uniaxial anisotropy. The results actually indicate that the exchange bias can be observed without a magnetic reversal. Results of temperature dependent X-ray diffraction and particle morphological studies are also presented in the paper.
Another aspect that remains interesting in these materials is the structural evolution as a function of pressure. The paper also presents the structural phase transition studies in Y_1-xPr_xCrO₃ in the pressure range 0.0≤ pressure ≤ 30 GPa. The nature of phase transition changes as we investigate the samples having varying concentrations of dopant. But, in most samples, atleast 2 pressure dependent phase transitions were observed in the synchrotron XRD studies. The paper discusses these in detail.

A new family of Faraday rotation (θ_F) garnets, namely terbium iron garnet (TIG), is introduced that can be grown directly onto Si and quartz without seedlayers. Doping these garnets with Ce and Bi provides a palette of chirality and rotation for photonics designers, including θ_F = +500, -500, -2600°/cm (λ =1550nm) for TIG, Bi:TIG, and Ce:TIG, respectively. Unlike the best Faraday rotators (-3700 to -4500°/cm) which are Ce-doped yttrium iron garnets (Ce:YIG), none of the TIG garnets require a seedlayer to crystallize successfully on Si-compatible substrates or waveguides. To demonstrate the importance of this newly discovered material, simulations were used to compare Ce:TIG to the best Ce_xY_3-xFe₅O₁₂ (Ce:YIG) claddings silicon-on-Insulator (SOI) waveguides. The results show that Ce:TIG-claddings are three times more efficient than Ce:YIG claddings which require seedlayers to get the highest Faraday rotations. Next, sputter deposition and rapid thermal annealing (200-800^oC) were used to grow TIG garnets directly onto Si and quartz substrates with high quality crystallization and the Faraday rotations given above. Ce:TIG claddings were then used in quasi-phase matched SOI waveguide isolators. These Faraday rotators work for all polarizations (TE through TM), unlike interferometers and ring resonators that only isolate TM light because they use non-reciprocal phase change (NRPS) rather than Faraday rotation. In addition to excellent performance, these new garnets with involve fewer processing steps, and thus are promising new materials for integrated isolator devices that will enable polarization diversity in photonic integrated circuits.

In this presentation I describe the electronic structure of the rare-earth elements, as obtained by the so-called Hubbard I approximation. I show that theory reproduces all measured features of both occupied as well as unoccupied states, without significant deviations between observations and theory. I will also present cohesive

properties like the equilibrium volume and bulk modulus, where in general there is a good agreement between theory and measurements. In addition, spin and orbital moments of these elements, as they are reflected from measurements of the saturation moment, are reproduced by

theory. I also present calculations of the interatomic exchange parameters of an effective spin Hamiltonian for the heavy rare earths.
It is found that the Hubbard I approximation gives results which are consistent with calculations where 4f electrons are treated as core states for Gd. The latter approach was also used to address the series of the heavy/late rare-earths. Via Monte Carlo simulations, ordering temperatures have been estimated, which reproduce measurements within about 20%. The Hubbard I approximation is compared to other theories of the electronic structure, and I argue that it is superior.

We investigate magnetic and optical properties on Eu-doped GaN using multi-scale modeling combining an ab initio calculation and two kinds of Monte Carlo simulations. So far, various experiments have been performed on this system. Firstly, Eu-doped GaN is commonly known as a luminescent material emitting red light. Secondly, ferromagnetic or superparamagnetic behavior has been observed as the magnetic property. Lastly, self-organized spinodal decomposition has been observed as the chemical property. [1,2]
Nevertheless, little has been reported for relations among the nanostructures, the magnetic, and optical properties in rare earth doped semiconductors. In this study, the local electronic structures and the chemical and magnetic pair interactions are estimated using the Akai-KKR code. [3] These chemical and magnetic pair interactions between Eu ions are short-range attractive and ferromagnetic interactions, respectively, and suggest that the double exchange interaction is dominant more than the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. These chemical pair interactions generate nanostructures through the first Monte Carlo simulation. [4] This simulation provides us with various nanostructures which are quantum dots called Dairiseki-phase, nano rods called Konbu-phase, and so on. The other simulations demonstrate that these nanostructures denote paramagnetic and superparamagnetic behaviors when the external magnetic field is applied. [5]

References:
[1] M. Hashimoto, et al., Jpn. J. Appl. Phys. 42 (2003) L1112.
[2] B. Mitchell, et al., Sci. Rep. 6 (2015) 18808.
[3] H. Akai, J. Phys. Soc. Jpn. 51 (1982) 468.
[4] T. Fukushima, et al., J. Phys. Soc. Jpn. 76 (2007) 094713.
[5] K. Sato, et al., Jpn. J. Apple. Phys. 46 (2007) L682.

Rare-earths nitrides form an interesting class of materials with intriguing magnetic and electronic properties. We will review the methodologies used to deal with the challenge of f-electrons in describing their electronic structure. We will discuss the ambiguities in the LSDA+U method and the need to ensure Hund's rules by an appropriate choice of starting density matrix. Recent results using the quasiparticle self-consistent GW method will be presented. For example, it gives rise to a completely spin -polarized valence band and conduction band edge in DyN. [Phys. Rev. B 92, 035134 (2015)] related to a strong hybridisation of an f-level of minority spin with the valence band nitrogen p-states. The need for theories including multiplet splittings will be illustrated with the case of EuN. This can be done by means of dynamical mean field theory in the Hubbard-I approximation but there is a need to combine this with GW theory. The status of our understanding of magnetism in GdN will be reviewed. While it is found to be ferromagnetic in LSDA+U calculations for the pure material, the magnitude of the exchange interactions depends sensitively on the opening of a gap, lattice spacing and so on. The best calculations suggest a Tc much smaller than the experimental value, which is most likely influenced by carrier mediated ferromagnetism due to extrinsic doping or defects. Turning to GaN doped with rare-earth elements, we will review the case of Gd-doped GaN. The exchange interactions related to nitrogen and oxygen interstitials calculated by the linear response approach in a KKR formalism show that these are too short-range to explain the development of ferromagnetism. Ga-vacancies can have 3 μ_B magnetic moments when in the neutral charge state. The latter is unlikely for isolated vacancies but may occur in vacancy clusters or other extended defects which pin the Fermi level locally. These moments are located on the neighboring N and hence lead to a lower percolation threshold for forming a connected network of magnetic sites. The occurence of vacancy clusters can result from a sort of phase seggregation mechanism during growth as shown by kinematic Monte Carlo simulations by Thiess et al. [Phys. Rev. B 86, 180401(R) (2012)]. Simulations of the magnetic properties of such clusters suggest a two temperature regime and superparamagnetism rather than ferromagnetism. Finally, we will discuss open problems and remaining challenges to understand the optical properties of rare-earth doped nitrides. In particular the energy transfer mechanism between band gap excitations and the f-electron multiplet excited states remains an unsolved problem. Various aspects of this work were done in collaboration with Alexander Thiess, Stefan Blügel, Peter Dederichs, Rudolf Zeller, Tawinan Cheiwchanchamnangij, Paul Larson, Chandrima Mitra, Axel Svane, Mark van Schilfgaarde, Ben Ruck and Joe Trodahl and have been funded by NSF and DOE.

The rare-earth nitrides (LN, L a lanthanide element) have been characterised variously as metallic, semi-metallic, semiconducting and even insulating. There is little doubt that these extremes were driven by poor stoichiometry; the LNs are plagued by a propensity for both N vacancies and reaction with oxygen. The earliest investigations, based on ceramic polycrystalline pellets, identified them as metallic. Later studies on single crystals of GdN, and less intensively YbN, returned a small carrier concentration of order 10²¹ cm^-3, in which a semi-metallic state was supported by a stoichiometry measured to be within 1% of 1:1. In the past ten years there have been both polycrystalline and epitaxial thin films in which the doping control of carrier concentration and both the magnitude of an optical band gap and its Moss-Burstein shift invite a semiconductor interpretation. Furthermore the theoretical treatment of these materials are complicated by the need to accommodate strong correlation, where especially the 4f shell is not easily handled. The predictions cover the same range of behaviours as experiments, with in addition a proposed topological insulator state. This presentation will review both historical and more recent literature in an attempt to resolve the disagreeing data.

The spin-Seebeck effect (SSE) is expected as a core technology of spin-driven thermoelectric conversion devices. The device has a simple bilayer structure with a magnetic film (e.g. YIG) and a metal film (e.g. Pt). In this device, a spin current is generated from a heat current by the SSE in the magnetic film. The spin current is injected into the metal film, then is converted to an electric current by the inverse spin Hall effect (ISHE). Therefore, we can detect a spin-driven thermoelectric voltage V_SSE on the metal film. Here, we firstly show that V_SSE is improved by partially substituting a rare-earth element (Yb) for the c-site in YIG. Secondly, we show the further enhancement of V_SSE in a Pt/Bi:YIG/Yb:YIG trilayer structure.
Device preparation and characterization were done as follows: magnetic films (Yb:YIG or Bi:YIG) were formed by using a metal organic decomposition (MOD) method. The Pt films were deposited on the magnetic films by sputtering. The Yb:YIG film were prepare with different Yb substitution rate. The V_SSE was detected on the Pt film under the condition where a magnetic field and a temperature gradient were applied in the in-plane and out-of-plane directions, respectively. We also performed a ferromagnetic resonance (FMR) measurement to evaluate a mixing conductance g_r, which represents an efficiency of spin current injection from the magnetic film into the metal film.
As a result of V_SSE and g_r measurements on the Pt/Yb:YIG system, it was found that V_SSE increases with increasing the Yb substitution rate, whereas g_r decreases. These results suggest that the efficiency of the spin current generation from the heat current is enhanced by the Yb substitution.
We also measured V_SSE for trilayer structures consisting of Pt/Bi:YIG/Bi:YIG, Pt/Bi:YIG/Yb:YIG, Pt/Yb:YIG/Bi:YIG and Pt/Yb:YIG/Yb:YIG. It was found that Pt/Bi:YIG/Yb:YIG exhibits the largest V_SSE, and that the FMR measurements indicate that g_r is enhanced by inserting the Bi:YIG film in between the Pt and Yb:YIG. This result indicates that the spin current generated in the Yb:YIG film is more efficiently injected into the Pt film with the Bi:YIG at the interface.

Laser-diode pumped solid-state micro lasers are compact (~cm), highly stable and efficient. They have been used in various applications, e.g., micromachining, ignition plugs for cars, laser radars. Giant pulse power (several megawatts) has been demonstrated via passive Q-switching with saturable absorbers (e.g. Cr⁴⁺:YAG and InGaAs). Especially Cr⁴⁺:YAG could be grown on the Nd:YAG crystals using liquid phase epitaxy method since they have the similar crystalline structure and lattice constant, which allows the micro lasers being miniaturized. We suggest using rare-earth substituted iron garnet (RIG) film as a novel active Q-switch. RIGs have large magneto-optical (MO) effects and can be used in thin-film form, allowing us to develop an actively Q-switched micro laser. Active Q-switched lasers are favorable to obtain large peak power and higher repetition rate. Since the MO Q-switched laser has not been reported a lot so far, we prepared the Nd:YAG crystal and RIG film and demonstrated a 14 mm length cavity MO Q-switched laser using pulsed magnetic field.

A 190 μm thick single crystalline RIG [(Bi, Tb)₃(Fe, Gd)₅O₁₂] was prepared by liquid phase epitaxy on single crystalline Gd₃Ga₅O₁₂ substrate. This RIG was sandwiched by a three-turn 5.3 mm diameter Helmholtz coil, and used as a MO Q-switch. The RIG used had a Faraday rotation angle of 47 degree and a transmissivity of 78% at a wavelength of 1064 nm. The film also has approximately 50 μm width maze-like magnetic domain structure with no externally applied field, and the difference of the domain the light go through is appeared as the spatial incoherency. While field is applied perpendicularly to the film, the domains disappear and the coherency of light increases, which modulate the Q-factor of the cavity. This component was installed into the laser cavity which was 14 mm in length. The lasing material Nd:YAG was end-pumped by a fiber coupled quasi-CW diode laser at a wavelength of 808 nm. The resonator mirror has a curvature radius of 300 mm and partial reflectance of 95%. The demonstration of Q-switching was operated at repetition rate of 1 kHz.

The electrical pulse used to drive the applied magnetic field had a pulse width of 2.3 μs and peak current of 56 A, which led to enough intensity for saturating the RIG. The obtained pulsed width was 27 ns and the pulse energy was 53 μJ/pulse, which corresponds to a peak power of 2.0 kW, at the wavelength of 1064 nm. This peak value and repetition rate generated by MO-Q switched laser was the highest in the reported experiments. These results indicate that actively Q-switched micro lasers can be achieved with MO RIGs. Further optimization of the cavity structure and switching speed of the pulse circuit would improve the output characteristics.

Coordination polymers, also known as metal–organic frameworks (MOFs) or rare earth organic frameworks, have been reported due of their promising applications in gas storage, separation, catalysis, luminescence, magnetism, drug delivery, and so on. As a type of organic–inorganic hybrid materials, the properties of coordination polymers could be chose by deliberately selecting the organic or inorganic components. Rare earth organic frameworks have received considerable attention due to their properties such as porosity, luminescence and magnetism.

In order to explore the possibility of preparing rare earth frameworks for upconversion applications hydrothermal synthesis was employed. We used terephthalic acid (T), 1,4-diazabicyclo[2.2.2]octane (D), Ytrium, Ytterbium and Erbium in water for several days under 160°C. The semi-rigid 1,4 –benzodicarboxylic acid (H₂tereph) coordinates to RE³⁺ ions and is able to form the polyfunctional bridge. Methods such as hydrothermal synthesis are important as a strategy to control the structural and morphological properties as well as the composition of the target compounds.

The crystalline products were characterized by power X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), dispersive spectroscopy (EDS), thermogravimetric analysis (TGA) and photoluminescence. X-ray diffraction (XRD) is an effective method to investigate crystalline properties of a synthesized material. The solid crystal obtained in the synthesis show peaks at 2θ< 10θ/°, that indicated the MOF formation. In the FTIR, the peaks at 1589 and 1500 cm^-1 correspond to the asymmetric stretching vibration of –COO. The peak at 1383 cm^-1 assigned to the symmetric stretching vibration of –COO. The TGA results were used to calculate molecular weights of the products. The chemical composition of the compounds were studied by means of energy dispersive spectroscopy (EDS) and the pattern confirms the presence of Y, Yb, Er, oxygen and carbon.

Feng Wang and Xiaogang Liu; Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals; Chemical Society Review; 976–989; 2008.

Liangjun Li, Sifu Tang, Xiaoxia Lv, Jinjun Cai, Chao Wang and Xuebo Zhao; A series exceptionally robust luminescent coordination polymers based on a bipyridyldicarboxylate ligand and rare earth metal ions; European Journal of inorganic chemistry; 6111-6118; 2013.

Ping Wang, Rui-Qing Fan, Xin-Rong Liu, Li-Yuan Wang, Yu-Lin Yang, Wen-Wu Cao, Bin Yang, WuLiJi Hasi, Qing Sub and Ying Mub; Two-/three-dimensional open lanthanide–organic frameworks containing rigid/flexible dicarboxylate ligands: synthesis, crystal structure and photoluminescent properties; CrystEngComm; 1931-1949; 2013

Recently, terahertz (THz) waves with frequencies in a range from 0.1 to 10 THz are attracting much attention, since they can be used as a new analyzing tool for spectroscopic study of substances. In this study, we observe absorption spectra of orthorhombic perovskite YAlO₃ (YAP), which has wide applications in many fields such as laser materials.
The samples we examined are YAP (100) single crystal plates of a square shape (10 × 10 × 0.5 mm³) made by the Czochralski method. First, the directions of the [001] axis and the (010) plane of the plate were confirmed by measuring in-plane X-ray diffraction patterns. Then, the sample was set on a substrate of a THz time-domain spectrometer (TAS7500TS, Advantest) so that its [001] axis is aligned parallel to the direction of electric field of THz waves. We call this angle θ = 0°. After that, THz absorption spectra were measured at room temperature in a range from 0° to 180° at intervals of 5°.
A strong absorption peak appears in a range from 3.0 to 4.0 THz when θ is in between 50° and 60°. This peak becomes strongest at 3.4 THz when θ = 55°. This angle, 55°, is the angle at which the [0-11] axis is parallel to the electric field of the THz waves.
It has been known that absorption due to lattice vibrations appears at THz frequencies in several inorganic crystals. For example, LaMnO₃, TbScO₃, or DyScO₃ exhibits an absorption peak in a range from 3.6 to 3.9 THz. The frequencies of these peaks, including the peak appearing in YAP (this research), increase almost linearly with an increase in the reduced mass of the constituent rare-earth element and oxygen. As for YMnO₃ with a crystal structure similar to YAP, it has been reported that the phonon density of states (PDOS) occupies about 60 and 40% around the elements of Y and Mn, respectively, at about 3.4 THz. If PDOS is also higher in the vicinity of Y in YAP, the present absorption peak at 3.4 THz that we observed in YAP should originate from a certain lattice vibration associated with Y. Since infrared-active vibrational modes of rare-earth metals in the above-mentioned orthorhombic perovskite rare-earth oxides are modes B_1u and B_3u normal to the [010] axis in addition to the mode B_2u parallel to the [010] axis. On the other hand, as for Al, there are no symmetry restrictions on the direction of its vibration for all IR-active modes. Therefore, the present 3.4-THz absorption seems to be related to a certain vibrational mode of Al or Y.
The important findings we obtained in this research are as follows:
(1) A sharp absorption peak is observed at 3.4 THz, when the [0-11] axis of YAP becomes parallel to the electric field of THz waves.
(2) The 3.4-THz absorption is attributable to one of the three vibrational modes B_1u, B_2u, and B_3u of Y or a certain vibration of Al.

The study of Dy3+ ion embedded in different matrices has aroused much interest. This interest stems from the relevance of the spectroscopic properties of the Dy3+ ions on the phosphate matrices. The emission and absorption cross sections, together with the fluorescence lifetime and thermoluminescent are the properties relevant to characteristics of this material. However, these properties depend on the host material. The emission and excitation spectra were reported for the phosphate glasses. As preliminary results were observed emission bands at 574 nm yellow. Besides the increase of the emission cross section, the fluorescence decay time requires further optimization. Phosphate glass has been identified as one of the most promising precursors of laser materials. As for the characteristics of the material, we should mention that it is soft and transparent from the ultraviolet (UV) to near infrared (IR), and relatively strong from the mechanical point of view. Phosphate glass also has a relatively high density, high refractive index, and thermal conductivity, besides its high phonon energy. Moreover, phosphate glass is easy to prepare, in addition, it is versatile and can provide wide spectra of emission and absorption as compared with other crystalline materials.

The phosphate glasses characterization co-doped with Dy3+ and silver nanoparticles is apresented.
A particular interes is in the phenomenological analysis of the silver nanoparticles influences on
the properties of the glass, i.e., the spectroscopic effects and a quantitative measure of emission
and absorption cross section, decay time, and thermolumi- nescent properties.

Seven glass samples were prepared (hereafter we refer to them as samples A, B, C, D and E),
whose compositions are NaH2PO4H2O, Dy2(SO4)3, and AgNO3 respectively. In preparation process,
the reagents were melted at 1000�C for 4 hours in a porcelain crucible exposed to air. Then,
the melt was poured in an aluminum-steel brass mold in open air and cooled down to room temperature.
Afterwards, the samples were annealed at 400�C for 2 hours in order to produce Ag metal
nanoparticles. The glasses were then cut into a size of 3 mm and 3 mm in thickness, and polished.

The emission and excitation spectra were reported for the phosphate glasses. As preliminary results
were observed emission bands at 574 nm yellow. High resolution images of the samples are
obtained from scanning electron microscopy (SEM: JOEL JSM-7401F), demonstrates the presence
of the silver nanoparticles.

In this work, we report a large magnetocaloric effect in TbMn_1-xFe_xO₃ (x=0, 0.75) single crystals by rotating them in a variable external field in the temperature range from 2 to 50 K (near the Tb³⁺ antiferromagnetic ordering temperature). We demonstrate the large magnetic entropy change exists between a and c axes with strong magnetic anisotropy in TbMnO₃ as well as TbMn_0.25Fe_0.75O₃. The maximum of magnetic entropy change and refrigeration capacity of TbMnO₃ (TbMn_0.25Fe_0.75O₃) reach 19.20 J/Kg K(14.84 J/Kg K), 411.97 J/Kg(260.80 J/Kg) under an external magnetic field of 7 T, respectively. Based on nonextensive thermodynamics, we further extend the q-statistics method to demonstrate the good consistency between computed and experimental results of magnetization versus rotating angles. Our results clearly indicate the magnetocrystalline anisotropy energy plays a decisive role in the large differences of the magnetic, structure and magnetocaloric properties between a and c axes in TbMn_1-xFe_xO₃. Furthermore, it’s shown by Lorentz Transmission Electron Microscopy technique the TbMn_1-xFe_xO₃ samples display good crystalline purity and phase uniformity. Utilizing electron holograms, the single domain structure and tendency of magnetic induction lines were revealed, which is consistent with the distribution of magnetocrystalline anisotropy energy and neutron diffraction results from our findings.

Rare earth (RE) element doped semiconductors have attracted to a lot of researchers owing to their unique magnetic properties. For example, the appearance of superparamagnetism or carrier-induced ferromagnetism (FM) in the nanoparticles (NPs) are expected for the combination of RE elements and semiconductor NPs. Dy³⁺ ion is one of the element with the largest magnetic moment among RE ones. However, FM is sometimes reported to have concerns about originating in the clusters of magnetic dopants without doping them into host oxide in the case of using physical manufacturing processes such as sputtering. In this study, we tried to use chemical thermolysis in air to produce Dy-doped ITO (Dy-ITO) NPs . Theby chemical thermolysis as facile synthetic method has recently been developed, which can be performed without requiring an expensive vacuum system. We have already reported magneto-transport properties up to 6 T in the chemically synthesized ITO NP thin films [1].
Dy-ITO NPs were synthesized from the precursor complexes; indium carboxylate, tin octylate and dysprosium nitrate. Indium carboxylate and tin octylate were stirred and dissolved in a flask at approximately 160 degrees °C for 1 hour. After that the mixture was heated up to around 340 degrees °C for 1 hour, and then dysprosium nitrate was put into the mixture and kept at 340 degrees °C for 1 hour. in order to synthesize NPs. The resultant mixture was cooled down to room temperature and Dy-ITO NPs were collected by decantation and put in toluene. By means of performing filtration of the solution, we finally fabricated some kinds of Dy-ITO NPs with the various Dy and Sn concentrations confirmed by energy dispersive x-ray fluorescence analysis.
Dy-ITO NPs were characterized by transmission electron microscope images. The variation of the size was 10-50 nm and the shape was spherical or polygonal. X-ray diffraction measurements showed a clear cubic indium oxide (222) diffraction peak; we found that Dy-ITO NPs had good crystallinity without a Dy₂O₃ phase and the clusters of Dy. Thermogravimetryic and differential thermal analysis measurement resulted the decreases mass of organic compound.
We examined magnetic field dependence of magnetization (M-H) for the pellets (thickness 1 mm, radius 3 mm) of Dy-ITO NPs with the various concentrations at room temperature. The NPs for the magnetization measurement were annealed at 500 degrees °C for 30 minutes. Hysteresis loops were clearly observed the samples. Furthermore, the magnetization significantly decreases without Sn doping. This enhancement of the magnetization reminds us carrier-induced FM, and this suggests that the conduction electrons of Dy-ITO NPs can strongly couple with f-electrons of Dy ions.

[1] A. Fujimoto et al., J. Phys. Soc. Jpn. 82, 024710 (2013).

Recently, photoluminescence properties of Ca-based perovskite phosphors such as rare earth-activated CaTiO₃ and CaSnO₃ have been extensively investigated. The crystal structure of Ca-based perovskites is highly distorted from the ideal cubic perovskites as SrTiO₃. Thus, such emission should be derived from low site-symmetry of Ca site. Photoluminescence properties of the oxide phosphors are strongly dependent on not only crystal structure of the host materials, but also preparation condition of the samples. Conventional solid-state reaction (SSR) method is not fit for the synthesis of phosphor materials with complicated compositions because the degree of cation mixing is relatively low due to slow interdiffusion of various cations. Therefore, in order to improve the photoluminescence properties of the Ca-based perovskite phosphors, we investigated the preparation of Pr³⁺-activated CaTiO₃ and CaSnO₃ via an aqueous-solution process using aqueous metal complex. Pr³⁺-activated CaTiO₃ and CaSnO₃ phosphors were prepared by amorphous metal complex (AMC) method using stable water-soluble Ti⁴⁺ or Sn⁴⁺ complex with lactic acid. Especially, we developed a new stable water-soluble Sn⁴⁺ complex with lactic acid synthesized from SnCl₄ for the preparation of Pr³⁺-activated CaSnO₃. Photoluminescence properties of Pr³⁺-activated CaTiO₃ and CaSnO₃ under UV irradiation were significantly sensitive for the small deviation of the cation ratios of Ti/Ca and Sn/Ca. When both the ratios were less than 1.00, both the perovskite phosphors showed low emission, compared with the phosphors prepared by SSR method at the optimum condition. In contrast, in the case of the cation ratios of Ti/Ca and Sn/Ca from 1.00 to 1.05, the emission intensities of both the phosphors prepared by AMC method were higher than those prepared by SSR methods. Especially, Pr³⁺-activated CaSnO₃ prepared by AMC method at the optimum condition showed high emission intensity under UV irradiation that was 1.5 times higher than those by SSR method. This work was partially supported by Grant-in-Aid for Scientific Research (C) (no. 15K06445) from MEXT, Japan

Amorphous ternary oxides (Me)₂O₃ with primary metal (Me) being Iron, along with two Lanthanides, Terbium and Dysprosium, were recently reported to show a combination of very high optical transparency, electrical conductivity, and Hall mobility [1]. The Hall mobility values observed in this material are comparable to the best indium-based transparent conductive oxides and an order of magnitude better than in amorphous silicon. That makes the material a potential candidate for a wide range of electronic applications.

Thin films of Fe-Tb-Dy-O were prepared by electron beam co-evaporation technique with different Iron content measured by Iron to Lanthanides ratios (in at %) derived from energy dispersive x-ray analysis (EDS). The film structure was amorphous which was confirmed by the glancing incidence x-ray spectroscopy. As deposited, films with lowest Iron concentration exhibited semiconductor-like optical and electronic properties, while films with high iron content were metallic. The evolution of the transport properties of the Iron-rich films was studied in-situ during repeated heating-cooling cycles in the temperature range from 290 to 700 K under low vacuum of 5 mTorr. With continued oxidation of the material, temperature dependence of electrical conductivity demonstrated metal to semiconductor transformation. Room temperature conductivity decreased by over an order of magnitude (from 2x10⁴ to 3x10³ S/cm) and stabilized within 10% of the value following thirteen cycles. X-ray photoemission spectroscopy was used to investigate the changes in metal cation state due to the thermal cycling, which we believe are responsible for the evolution of the transport properties.

[1] Malasi A. et al. Novel Iron-based ternary amorphous oxide semiconductor with very high transparency, electronic conductivity, and mobility. Sci. Rep. 5, 18157; doi:10.1038/srep18157(2015).

Strongly correlated electronic systems such as Rare Earth Transition Metal Oxides often possess various mid-gap states originating from intrinsic defects in these materials. In this work, we have investigated extremely sharp Photoluminescence (PL) transitions originating from such defect states in various widely used perovskites, LaAlO₃, SrLaAlO₄, LSAT [(LaAlO₃)_0.3-(Sr₂AlTaO₆)_0.7)] and SrTiO₃. A detailed study of the PL as a function of temperature (10K-300K) and magnetic field (up to 8T) has been conducted to understand the behavior and origin of the transitions involved. We observe opposite temperature dependence of the PL peak positions for some of these materials which helps us clearly distinguish the PL origin. Such sharp PL lines from these widely used materials make them promising candidates for laser applications. Our results reveal the presence of a spin/orbital character in these materials which is evident from the splitting of these defect energy levels under a high magnetic field. We have also conducted PIXE measurements to comprehend the possible role of impurities in this sharp luminescence. These observations indicate the potential of these materials, which are widely used as substrates in oxide electronics for non-contact thermal and magnetic field sensors.

Recently, rare earth (RE)-doped gadolinium orthovanadate (GdVO₄:RE) nanoparticles (NPs) providing photobleaching resistant and narrow emissions are successfully prepared and have attracted considerable attention for biomedical applications such as multifunctional bio-probes capable of luminescent probing as well as magnetic resonance imaging (MRI). However, most biomedical applications of nanoparticles need to understand the influence of metal ions in human blood (Mg²⁺, Ca²⁺, Fe²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺ and Al³⁺) because adsorbed metal ions can affect the function of nanoparticles to limit their sensitivity, performance, stability, and/or resolution in applications. In this study, the adsorption of various metal ions at the surface of GdVO₄:Eu NPs was studied to assess their spectral filter effect on the fluorescence of GdVO₄:Eu NPs. The intensive adsorption reaction of GdVO4:Eu NPs with metal cations was caused by the electrostatic attraction due to the negative surface potential. Compared to other metal cations, a distinct fluorescence quenching of GdVO4:Eu NPs was induced in the presence of Cu²⁺ ions. The fluorescence intensity was reduced by more than 25% even in 0.05 μM Cu²⁺ solution. It is proposed that the complementary overlap of the emission band of GdVO₄:Eu NPs with the absorption band of Cu²⁺ results in the effective filter effect to quench the red emission. When we examined the selectivity and sensitivity of filter effect by Cu²⁺ ion in aqueous M²⁺ (M = Mg, Ca, Fe, Cu, Zn, Cd, Hg, and Pb) and M’³⁺ (M’ = Al and Cr) solutions, the GdVO₄:Eu NP suspension showed small or negligible change in the relative intensity before and after the adsorption for most of these metal ions regardless of their concentration. In contrast, the filter effect higher than 90% by Cu²⁺ ions was clearly distinguished from other metal ion adsorptions even at a concentration of 1.0 μM. Significantly reduced emission from GdVO₄:Eu NPs confirmed that other metal ions of substantial concentrations in the biological solution prepared with C57BL/6 mouse blood do not strongly interfere with the fluorescence quenching by Cu²⁺ adsorption.

We present an investigation of the optical constants of the near stoichiometric GdN thin fillms. Transmission and reflection spectra are collected for the paramagnetic and the ferromagnetic GdN in the photon energy range of 0.5-5.5 eV. In the ferromagnentic phase, behaviors of minority and majority spin states are specifically focussed, which indicate spin-split joint density of states. The results confi rm the LSDA+U estimates of energy gap associated with the majority-spins and also the magnitude of spin splitting.

We have determined the magnetic properties of epitaxially grown Dy islands on graphene/SiC(0001) that are passivated by a gold film for ex-situ X-ray magnetic circular dichroism measurements. Our sum-rule analysis of the Dy M_4,5 XMCD spectra at low temperatures (T = 15 K) as a function of magnetic field indicates that the islands can be adequately described in terms of paramagnetic Brillouin functions of Dy³⁺ (spin configuration ⁶H_15/2). This and no detectable magnetic hysteresis indicates a major difference from bulk hcp Dy which is ferromagnetic below 88 K. Temperature dependence of the magnetic moment (extracted from the M₅ XMCD spectra) shows anomalies at temperatures greater than 130 K below the first transition of bulk Dy to helical magnetic structure at 179 K. Below ≈ 130 K the inverse magnetic moment (extracted from the XMCD) is linear in temperature as commonly expected from a paramagnetic system suggesting different behavior of Dy nano-particles or possible inadvertent oxidation to Dy₂O₃ through the Au film. We also examine specific sum rules for Dy³⁺ that enable us to extract the average spin Z> and angular-momentum Z> moments, the dipolar Z> term, and the total moment total>.

Superconducting spin valves are trilayer devices in which changing the magnetic alignment of two magnetic layers alters the critical temperature (Tc) of a superconducting layer. Although it has been predicted that a complete suppression of superconductivity is possible for parallel magnetic alignment, experiments using transition metal ferromagnets have generated only small (mK) variations in Tc. Much larger changes have been reported recently using rare-earth ferromagnet and rare-earth ferromagnetic insulator devices. The reasons for this and the potential for applications will be discussed in this presentation.

Developing viable semiconductor spintronics has led to intense research into the dilute magnetic semiconductors, while more recently the intrinsic ferromagnetic semiconductors within the rare-earth nitride series have shown promise for low temperature semiconductor spintronics applications. The wide range of magnetic properties across the REN series suggests applications involving hard/soft magnetic structures, such as tunneling magnetoresistance devices, but recent work which we present here suggests more exotic possibilities. As we will discuss, an intriguing phenomenon occurs in the light rare-earth nitrides where the magnetism is determined by the orbital moment, as established for NdN [1] and SmN [2]. In these cases the net moment is parallel to the orbital moment, with the smaller spin moment lying antiparallel. This was recently shown to drive a twisted magnetization phase in semiconducting SmN/GdN heterostructures [3], driven by competition between interface exchange and the orbital-dominant Zeeman coupling of SmN. Recently, this unconventional twisted phase (or exchange spring phase) has also been observed in a NdN/GdN superlattice, where ferromagnetic interface exchange competes with the large orbital dominant moment of NdN. By giving an overview of magnetometry and element specific x-ray magnetic circular dichroism, we make the case that these rare-earth nitride based heterostructures establish a new class of semiconducting exchange-spring systems, and could host novel magnetotransport effects due to their unique electronic and magnetic structure.

[1] E.-M. Anton, J.F. McNulty, et al., Phys. Rev. B 93, 064431 (2016)
[2] E.-M Anton, B.J. Ruck et al., Phys. Rev. B 87, 134414 (2013), J.F McNulty, B.J. Ruck, H.J. Trodahl. Phys. Rev. B. 93, 054413 (2016).
[3] J.F. McNulty, E.-M, Anton, et al. Phys. Rev. B 91, 174426 (2015)

After reviewing the state of the art about ferromagnetism in GaN doped with gadolinium, we present an experimental study on layers grown by plasma-assisted molecular beam epitaxy on various substrates. Based on a comprehensive analysis of structural, electronic and magnetic properties, the origin of the ferromagnetic signatures observed in part of the samples is discussed and the prospect of GaN based dilute magnetic semiconductors is considered.
The report of room temperature ferromagnetism and the observation of giant magnetic moments in highly diluted GaGdN layers have triggered a strong interest in this material system. Several questions concerning the authenticity and the underlying physics of the giant magnetic moments remain unanswered, making the discussion about ferromagnetism in GaN:Gd still controversial. Possible spurious sources or artifacts for the magnetic behavior experimentally observed have been investigated, both theoretical and experimental works point out the possible role of defects in the magnetic coupling, and other experimental studies strongly support an intrinsic ferromagnetic ordering at room temperature.
Unwittingly, sets of magnetic and non-magnetic samples were obtained during the growth campaign with nominally identical layer composition. Electrical transport characterization of the GaN:Gd epitaxial layers reveal different types of electronic transport, correlated with the non-reproducibility of the magnetic properties. The coexistence of variable-range hopping transport and ferromagnetism at very low Gd-concentrations – below the residual doping level - suggests that Gd induces deep states which account for electron localization, contrary to the expected effect of substitutional Gd³⁺ on the electronic structure of GaN. Therefore the presence of certain defects formed during growth, which compensate and/or suppress the formation of shallow oxygen donors, and account for the formation of an impurity band deep in the GaN bandgap is inferred.
Furthermore, it can be ruled out by positron annihilation spectroscopy that gallium vacancies are responsible for the formation of a ferromagnetic order in these layers.

We have intensively investigated rare-earth (RE)-doped III-V semiconductors grown by atomically controlled organometallic vapor phase epitaxy (OMVPE). Eu-doped GaN (GaN:Eu) has been identified as a promising red emitter because it has excellent luminescence properties in the red spectral region. A main emission line with a half width of less than 1 nm was observed at 621 nm, which can be assigned to the ⁵D₀–⁷F₂ transition of Eu³⁺ ions. The Eu²⁺ state is very attractive for future application of GaN:Eu because it has a potential to emit blue/green light due to the 5d-4f transition and to provide a magnetic moment. In this contribution, we demonstrate that the valence state of Eu ions in GaN:Eu can be controlled by adjusting conditions of OMVPE growth and impurity codoping.
GaN:Eu layers were grown on (0001)-oriented sapphire substrates by OMVPE. TMGa, NH₃, EuCp^pm₂, Ar diluted O₂ and MMSi were used as Ga, N, Eu, O and Si sources. The growth was initiated by a low-temperature GaN buffer, which was followed by the growth of an undoped GaN layer, a 300 nm GaN:Eu layer, and an undoped GaN capping layer. The GaN:Eu layer were grown at 650°C, 700°C, 750°C and 800°C, respectively. The GaN:Eu layer was also prepared at 700°C with Si and O codopants as follows; O₂: 5 nmol/min, MMSi: 8 nmol/min, and MMSi: 15 nmol/min.
X-ray fluorescence analysis (XRF) and x-ray absorption near edge structure (XANES) were performed to evaluate Eu concentration and the valence state of Eu ions in these samples, respectively. Electrical properties were characterized by Hall measurements. Magnetic properties were measured by a SQUID. XANES spectra from a sample grown at 1030°C as a reference only exhibited a peak due to Eu³⁺ ions. On the other hand, samples grown at low temperatures exhibited a shoulder due to Eu²⁺ ions at the lower energy side. The amplitude ratio of the Eu²⁺ peak to the Eu³⁺peak (A_Eu2+/A_Eu3+) depended on the growth temperature was calculated, and the maximum ratio of 0.39 was obtained at 700°C. In addition, according to XANES spectra from samples with Si or O as a co-dopant, the Eu²⁺ signal was increased with electron concentration.
M-H curve for the 700°C sample was reconstructed by subtracting a M-H curve of the 1030°C sample without the Eu²⁺ signal from that of the 700°C sample. Considering the total Eu concentration [Eu] of 4.12×10¹⁹ cm^-3, the ratio of the Eu²⁺ concentration [Eu²⁺] to the [Eu], [Eu²⁺]/[Eu], was 55%, which was calculated from saturated magnetization and the magneton number of a Eu²⁺ ion (7μ_B). This sample exhibited no ferromagnetism but paramagnetism because the Eu²⁺ ions were doped lightly compared with Ref. [1]. However, the analysis of the shape using the Langevin function revealed an effective magneton number (μ_eff = 24μ_B) larger than that of a single Eu²⁺ ion. This result indicates the formation of a superparamagnetic cluster consisting of 3.4 (= 24/7) Eu²⁺ ions on average.
[1] M. Hashimoto et al., Jpn. J. Appl. Phys. 42, L1112 (2003).

Many experiments have established that rare earth (RE) doping of III-N semiconductors leads to ferromagnetic behavior in these materials [1]. However, there has been considerable variation in the observed magnetic properties reported by different research groups. In this talk, general issues relating to methods for introducing RE ions into the III-N host will be reviewed as well as theoretical models predicting room temperature ferromagnetic properties. Two specific RE doped material systems, Er-doped GaN and Nd-doped GaN, will be discussed in detail. A comparison will be made of the doping methods, the magnetic characterization, and the correspondence to theoretical models. Finally, prospects for achieving dilute magnetic semiconductor devices based on these materials will be put forward.

[1] Rare Earth and Transiton Metal Doping of Semiconductor Materials: Synthesis, Magnetic Properties and Room Temperature Spintronics, Ed. V. Dierolf, I.T. Ferguson and J.M. Zavada (Elsevier Woodhead Publishing, 2016).

Members of the rare earth nitride series exhibit a unique combination of tunable electrical conduction coupled with ferromagnetic order comprising a varying mixture of spin and orbital moments [1]. This immediately suggests they may have use in novel spintronics devices. Here we present a magnetic tunnel junction that exploits two members of the series, gadolinium nitride and samarium nitride, as electrodes with strongly contrasting magnetic states. Both are ferromagnetic at low temperatures, with the strong magnetic moment in GdN residing purely in the spin of the Gd 4f shell. The moment in SmN contains also an orbital contribution from the 4f electrons, with a magnitude very similar to that of the spin but with opposite alignment, resulting in a near-zero moment ferromagnetic state. Crucially, the orbital moment is slightly larger than the spin moment, so the spin aligns opposite to an applied magnetic field [2].

We have made polycrystalline magnetic tunnel junctions fabricated with a GdN and a SmN electrode separated by a thin AlN barrier. The devices show a magnetoresistance as high as 200%, implying strong spin polarisation in both electrodes. In contrast to conventional tunnel junctions the resistance is largest at high fields, a direct result of the orbital-dominant magnetisation in samarium nitride that requires the spin in this electrode aligns opposite to that in the gadolinium nitride when the magnetisation is saturated. The magnetoresistance at intermediate fields is controlled by the formation of a twisted magnetisation phase in the samarium nitride, a direct result of the orbital dominant ferromagnetism [3]. Thus, new functionality can be brought to magnetic tunnel junctions by use of novel electrode materials, in contrast to the usual focus on tuning the barrier properties.

[1] F. Natali, B.J. Ruck, N.O.V. Plank, H.J. Trodahl, S. Granville, C. Meyer, and W.R.L. Lambrecht, Rare-earth mononitrides, Prog. Mats. Sci. 58, 1316 (2013).

[2] J.F. McNulty, B.J. Ruck, and H.J. Trodahl, On the ferromagnetic ground state of SmN, Phys. Rev. B 93, 054413 (2016).

[3] J.F. McNulty, E.-M. Anton, B.J. Ruck, F. Natali, H. Warring, F. Wilhelm, A. Rogalev, M.M. Soares, N.B. Brookes, and H.J. Trodahl, Twisted phase of the orbital-dominant ferromagnet SmN in a GdN/SmN heterostructure, Phys. Rev. B 91, 174426 (2015).

The anomalous Hall effect (AHE), noted already by E.H. Hall in 1881,¹ is a contribution to the conventional Hall voltage that follows the spin imbalance in the conduction channel of a ferromagnetic conductor. Its root cause is firmly recognised as lying in the spin-orbit effect on electrons in the conduction channel.² It has been investigated frequently in both metals and diluted magnetic semiconductors (DMS), but the detailed interpretation as regards an intrinsic and various impurity-focussed scenarios is still uncertain. In contrast to those materials, the rare earth nitrides comprise the only series of ferromagnetic materials in which both scattering and the carrier concentration can be controlled by doping, offering potential new insights about the mechanism responsible for the effect.

We will report AHE investigations on the two most studied materials among the rare-earth nitrides: gadolinium nitride (GdN) and samarium nitride (SmN). In addition to the fundamental importance, these two are of particular interest in spintronic device manufacturing, based on the contrast of their magnetic response: GdN has a large magnetic moment (7 μ_B/Gd ion) and a small coercive field, while SmN features a very small magnetic moment (0.035 μ_B/Sm ion) and an enormous coercive field.³ The AHE in GdN carries the same sign as the ordinary Hall effect (OHE) and follows the inverse of the carrier concentration for concentrations below 10²¹ cm^-3 ; we will demonstrate that these correlate perfectly with the LSDA+U band structure of GdN. The strong hysteresis in SmN demands careful measurement to separate the Hall from a parasitic resistance voltage, but with care we show that its AHE is opposite to the OHE, revealing an orbital dominance to the Sm³⁺ 4f magnetic moment in SmN, again in agreement with the LSDA+U band structure. In both cases the signal clearly supports the intrinsic AHE mechanism.

1 E.H. Hall, Phil. Mag. 12, 157 (1881).
2 Naoto Nagaosa, Jairo Sinova, Shigeki Onoda, A. H. MacDonald, and N. P. Ong, Rev. Mod. Phys. 82, 1539 (2010).
3 F. Natali, B.J. Ruck, N.O.V. Planck, H.J. Trodahl, S. Granville, C. Meyer and W.R.L. Lambrecht, Progress in Material Science 58, 1316 (2013).

Elements in the rare earth nitride (REN) community attract considerable attention owing to the changing occupation numbers in their 4f shell, which in turn leads to interesting magnetic and electronic properties that makes them interesting candidates for spintronic devices [1]. NdN is one interesting member in the REN family, which is relatively less studied, due to the difficulty in preparing bulk samples with high density and purity [2]. We investigate the magnetic and electronic properties of NdN thin films grown along the NaCl (001) direction using the generalized gradient approximation including the Hubbard parameters (GGA+U). Non collinear magnetic calculations are performed with spin-orbit coupling to estimate the spin and orbital contribution to magnetism as well as to understand the preference of the axis of magnetization. Magnetic anisotropy energy (MAE) is calculated as the differences in total energy between parallel and perpendicular orientation of spin moments to the direction of film growth. We see that the magnitude and direction of MAE can be tuned from parallel to perpendicular with film thickness as well as with the concentration of N vacancies. The density of states analysis shows that the films possess half metallic character with only one spin channel available for conduction around the Fermi level. The perpendicular magnetic anisotropy and half metallic character exhibited by NdN thin films make them suitable candidates for spintronic applications.
References

F. Natali et al., Prog. Mater. Sci. 58, 1316 (2013).
J. Adachi et al. J. Nuc.Mater. 376(1): 83-87 (2008)

The rare-earth nitrides have recently come into focus as a promising material system for the development of spintronics applications, due to most of the series combining intrinsic ferromagnetism and semiconducting behavior. The magnetic properties of each rare-earth nitride vary considerably across the series, holding promise for hard/soft magnetic device applications. For example, GdN has a large 7 μ_B spin-only moment and a very small coercive field, while SmN is a hard ferromagnet with a vanishing magnetic moment due to nearly equal spin and orbital moments with antiparallel alignment. Until now, NdN has not yet been systematically studied in thin-film form. It has been known for some time that bulk NdN is ferromagnetic, and theoretical considerations point towards a dominant role of the orbital magnetism, similar to the case of SmN. Recently we developed the ability to grow NdN thin films by molecular beam epitaxy, and have established the optical and transport behavior of high quality NdN samples, showing it is semiconducting with an optical gap of 0.9 eV [1]. Magnetic SQUID and X-ray magnetic circular dichroism studies revealed ferromagnetic order with a magnetic moment of 1 μ_B , strongly reduced from previously reported bulk moments, pointing towards thermally induced lattice strain. An overview over the recent progress in understanding the properties of this promising spintronics material will be provided.


[1] E.-M. Anton, et al. Phys. Rev. B. 93, 064431 (2016)

Realization of practical solid-state quantum memories critically depends on identifying materials that offer a demanding combination of physical properties along with massively parallel channel scalability to provide required data rates, for example in secure communication networks. Already, experimental demonstrations and theoretical analysis by groups across the world have shown that rare-earth ions doped into dielectric crystals are one of the most promising candidates to meet all of the requirements for multimode optical quantum information storage protocols.

The phenomenon of spectral hole burning (SHB) of inhomogeneously broadened optical absorption lines in these materials at cryogenic temperatures enables spectrally distinct ensembles of ions within the high density environment to be individually manipulated and interrogated, leading to the potential for 100,000 or more multiplexed frequency channels within a single absorption line. To realize these advances, we must continue to improve fundamental understanding and practical control of the physical processes that govern ion-ion, ion-spin, and ion-lattice interactions. At the same time, new knowledge is crucially needed regarding the detailed role of material chemistry and fabrication processes in determining, and sometimes limiting, the properties and consistency of existing materials.

With these motivations, we review the unique properties of rare-earth-doped crystals designed and characterized for solid-state quantum memories and outline active areas of materials research aimed at understanding and optimizing these properties. In particular, we highlight the engineering of lattice defects to manipulate both static and dynamic disorder and the resulting effects on optical coherence, spectral multiplexing capacity, spin-state lifetimes, and other key parameters. Ongoing efforts also focus on a better understanding of the nature and consequences of defects introduced during the growth and sample fabrication processes. All of these concepts are applied to specific rare-earth-doped material systems and illustrated by our recent results on quantum memory materials with enhanced properties. Finally, we discuss how the new perspectives and insights into structural and chemical defects of the high quality crystalline materials required for quantum information applications may also be applied more broadly to improve materials for traditional luminescence applications such as laser materials, phosphors, and scintillators.

We show the reversible magnetization switching of the perpendicular magnetized ferrimagnetic insulator Tm₃Fe₅O₁₂ (TmIG) by inducing a spin-orbit torque via a spin current by an adjacent layer of platinum. Spin-orbit torque has been characterized extensively in all-metallic systems, but in the case of magnetic insulators, although charge currents can not flow, spin currents can still propagate. However, the current-induced control of the magnetic state in a magnetic insulator has so far remained elusive.

We first demonstrate robust perpendicular magnetic anisotropy in strained ultra thin TmIG films of high structural quality down to a thickness of which retain a magnetization close to bulk, ~100 emu/cm^-3. Xray circular magnetic dichroism confirms the ferrimagnetic arrangement of Tm and octahedral and tetrahedral Fe trivalent cations in the TmIG, and magnetometry and magnetic force microscopy demonstrate a high remanence state with out-of-plane magnetization. Platinum deposited on TmIG possesses large spin Hall magnetoresistance (SMR), which indicates efficient spin transmission across the TmIG/Pt interface. By measuring the SMR on a Pt/TmIG Hall bar device, we quantify the real and imaginary values of the interfacial spin mixing conductance as G_r=1.3x10¹⁴ Ω^-1m^-2 and G_i=4.8x10¹² Ω^-1m^-2, respectively, which are comparable to the values of already known Pt/Y₃Fe₅O₁₂ interfaces. We demonstrate that spin currents injected across this interface lead to deterministic magnetization reversal at low current densities in the presence of a small in-plane magnetic bias field. This electrical reversal and detection of the magnetization state paves the road towards ultralow dissipation spintronic devices based on magnetic insulators.

Oxyfluoride glasses have received much attention as photoluminescent materials because of its low phonon energy and chemical stability. New oxyfluoride glasses of Eu₂O₃-doped 50BaF₂-xAl₂O₃-(50-x)B₂O₃ (x=0-25) and yMgF₂-(100-2y)BaO-yB₂O₃ (y=0-60) with high fluorine contents were prepared by using a conventional melt-quenching method. After heat-treatment on the glasses, optical non-linear fluoroborate nanocrystals (BaAlBO₃F₂, BaMgBO₃F) were formed. The glass structure, optical and photoluminescence properties of the glasses and nanocrystallized glasses were investigated. The glasses and nanocrystallized glasses have deep UV cutoff wavelength and showed excellent red luminescence with high quantum yield and small concentration quenching, i.e. h=97 % for 1Eu₂O₃-doped and h=72 % for 10Eu₂O₃-doped 50BaF₂-25Al₂O₃-25B₂O₃ glass at the excitation of l=396 nm. The new fluoroborate glasses and nanocrystallized glasses have a high potential for new phosphors with rare-earth ions.

When metallic nanoparticles are arranged in a period comparable to the wavelength of light, light diffraction via the periodic structure can mediate the radiative coupling between the surface plasmon polaritons (SPPs) in neighboring nanoparticles. Consequently the SPPs in each nanoparticle oscillate in phase to provide a collective plasmonic mode. The coherent oscillation of SPPs yields a collective response that is stronger than the simple sum of each SPP.
The distinctive difference of the collective modes from an isolated SPP is the field distribution. While an SPP is a localized mode bound on the metallic surface, these modes accompany the electric field that extends to the neighboring nanoparticles. From the viewpoint of efficient use of light, such plasmonic array is a good platform for light management: the light energy is efficiently trapped in the plane of the array and/or the dielectric layer and can be utilized as energy for further reactions. These collective modes have proven useful for surface enhanced Raman scattering (SERS), intensified fluorescence, and to increase the efficiency of solar cell applications owing to their characteristic field distributions.
In the present study, we examined directionally enhanced photoluminescence (PL) from a phosphor layer coupled to a plasmonic array. Highly luminous Eu (III) complex with a high quantum yield, a large Stokes shift, and a long fluorescence lifetime was selected as an emission center. PL enhancement as large as 5 times occurs at the emission angles and wavelengths corresponding to the excitation conditions of collective plasmonic mode. We also demonstrated the tuning of emission direction by the pitch of the array.

References
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S. Murai, M. A. Verschuuren, G. Lozano, G. Pirruccio, S. R. K. Rodriguez, and J. Gómez Rivas, Opt. Express 21 4250–4262 (2013).
G. Lozano, D. J. Louwers, S.R.K. Rodríguez, S. Murai, O.T.A. Jansen, M. A Verschuuren, and J. Gómez Rivas, Light: Sci. Appl. 2, e66 (2013).

Powder X-ray diffraction (XRD) pattern of Ce(OH)₂Cl was successfully refined with the monoclinic space group P2₁/m (No. 11) by Rietveld method; a = 6.2898(2) Å, b = 3.9456(1) Å, c = 6.8680(2) Å, β = 113.535(2)°, and R_exp = 5.93, R_wp = 7.74, and R_p = 6.02. This structure is characterized by an alternate arrangement of Ce³⁺-O^2- and Cl^- layer. Four binding energies obtained by fitting X-ray photoelectron spectroscopic curves were very close to those observed for CeCl₃, supporting essentially 3+ valence state of Ce in Ce(OH)₂Cl. Under approximately 273 and 312-nm excitations, Ce(OH)₂Cl showed a characteristic blue emission band centered at ~ 422 nm, which is attributed to the allowed transitions of Ce³⁺ from the lowest energy level of the 5d¹ excited state to the ²F_5/2 and ²F_7/2 levels of the 4f¹ ground state. In contrast, the blue emission was significantly reduced and instead the strong green emission was observed in photoluminescence spectra of Ce(OH)₂Cl:Tb, indicating an efficient energy transfer from Ce³⁺ to Tb³⁺. The green emission from Ce(OH)₂Cl:Tb was quenched after adsorption of manganate ions. Such a “turn-off” fluorescence behavior is explained by the inner filter effect (IFE); absorption of both excitation and emission light for fluorescent Ce(OH)₂Cl:Tb by manganates. Films of Ce³⁺ hydroxychlorides, Ce(OH)₂Cl and Tb-doped Ce(OH)₂Cl, were simply fabricated on slide glass and FTO glass substrate using hexamethylenediamine (HMT) in aqueous cerium chloride solutions. Interestingly, completely oriented film was obtained on FTO glass. The disappearance of (h0l) reflections and strong increase of (0k0) intensities in XRD pattern of Ce(OH)₂Cl film on FTO glass suggested the preferred crystal growth with the c-axis parallel to the substrate. The turn-off fluorescence efficiency of this oriented film by the manganate adsorption was highly enhanced compared to that of Ce(OH)₂Cl film on the slide glass.

In this study, we dispersed YAG:Ce³⁺ phosphor with Sn-P rich glass frit as an inorganic encapsulant for light-emitting diodes (LEDs). We obtained the phosphor-in-glass (PiG)-based down-converted white LED (DC-WLED) at a low sintering temperature of 200 °C, which can decrease thermal defects in commercial phosphors. The commercial-phosphor-converted LED (pc-LED) consists of an InGaN blue LED and YAG:Ce³⁺ phosphor dispersed in silicone or epoxy resin. However, the organic encapsulant can easily become yellow, affecting the optical properties of the WLEDs with, for instance, a decline of the luminous efficacy (LE). The phosphor-in-cup WLED consisting of an InGaN blue LED (λ_max = 450 nm) and YAG:Ce³⁺ dispersed in glass frit realized a high LE of 134 lm/W at 4500 K, an EQE value of 0.38 and a CRI level of 71 under an applied current of 350 mA. Furthermore, it shows high temperature resistance in comparison with commercial silicone resin in a temperature-dependence test. The photoluminescence properties of phosphor were measured using a Xe lamp and a spectrophotometer. Images of PiG-based WLED were obtained by scanning electron microscopy (SEM). The PiG-based WLEDs were measured in terms of the electroluminescence (EL) in an integrated sphere with an applied current ranging from 50 mA to 700 mA.

Compact semiconductor laser sources with temperature-insensitive oscillation wavelengths are under investigation for widespread application in optical communications. For this application, the Er-2O luminescent center formed in GaAs thin films by co-doping Er and O atoms,¹ is a promising candidate. This center exhibits a sharp and significantly temperature-stable (0.001 nm/K) emission spectrum at the wavelength of 1.5 um^2,3. Electrically-driven light-emitting diodes based on Er,O co-doped GaAs have been proposed, however, lasing from the Er-2O luminescent centers has not yet been reported.
In order to achieve lasing with Er-2O, a cavity with high Q-factor is required. To address this, we have focused on the use of a 2-dimentional photonic-crystal⁴ nanocavitiy (PCNC). Si-based PCNCs with ultra-high Q-factor of 8 million have been reported⁵. Furthermore, PCNCs are also suitable for application in optical communication, as they are very small and can be easily integrated in optical circuits using photonic-crystal waveguides. The aim of this work is to achieve a solid-state laser by combining Er-2O luminescent centers and PCNCs, where the PCNCs are fabricated on a thin film of Er,O co-doped GaAs.
A GaAs-based PCNC structure with a high Q-factor was designed by using finite difference time domain (FDTD) calculations. The sample was prepared as a double-heterostructure, which has been reported to have an ultra-high-Q factor⁶. To adjust the Q-factors, the position of the holes in the PCNC structure was modified. The position of each hole changes the properties of the internal electric field of the cavity. When an internal electric-field distribution in a cavity changes abruptly at the cavity edges, the Q-factor significantly decreases due to leakage. In our simulations, the highest Q-factor obtained was 5.8×10⁶, which is high enough for lasing with the optical gain of Er-2O co-doped GaAs (8 cm^-1)⁷.
Also, inaccuracies that are introduced in the processing of these structures can significantly lower the Q-factor. Therefore, it is helpful to investigate the minimum Q-factor required for lasing. Simulations were performed on several types of structures with Q-factors ranging from 5000 ~ 5.8 million. The minimum Q-factor required for lasing was determined to be 100,000, which is feasible for fabrication purposes⁸. We believe that this study will pave the way for the use of light sources in optical communication in the future.

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In this work, calcium fluoride (CaF₂) and Tb³⁺-doped calcium fluoride (CaF₂:Tb³⁺) thin films were prepared by electrodeposition technique. According to XRD results, the CaF₂ thin films go through a structural deformation from cubic to tetragonal unit cell as a function of the doping concentration. The main peaks show a very clear shift towards lower angles. On the other hand, optical absorption results showed the typical range of transparency from IR to UV, and from photoluminescence (PL) spectra results it is possible to identify the f-f transition of Tb³⁺. The PL at low temperature allows finding the quenching concentration when the PL intensity decreases at the highest doping concentration value. However, the results also showed that the transition of Tb³⁺ does not seem to be affected by the mentioned deformation. In the case of these kind RE-doped CaF₂ thin films materials, they can be used with a double purpose for solar cell applications, not only as antireflection coating, but also as a host for lanthanide doping for down conversion of light. These properties could be very useful for photovoltaic applications, so that the spectral range of light conversion could be increased to achieve higher conversion efficiency values.
Acknowledgements: This work was partially supported by Red Temática PROMEP (SEP-Mexico) and DGPI-BUAP (Mexico) and CONACYT grant No. 167993.

Due to a wide middle-infrared (mid-IR) transparency range and low phonon cut-off frequency, II-VI wide-bandgap semiconductors are promising host materials for rare earth (RE)-doped mid-IR lasers. Many RE ions such as Er, Nd, Pr, and Ho have strong mid-IR transitions. When doped into II-VI crystals, these ions may allow mid-IR lasing under electrical excitation. In addition, co-doping of II-VI media with RE ions and transition metal (TM) ions offers flexibility in the excitation of optical centers via TM«RE energy transfer processes. RE ions could be used as donor or acceptor centers in this energy transfer. The advantages of the RE as an acceptor (laser) center is their long upper level lifetimes at room temperature, which for some multiplets can be in excess of 10 ms. Such long upper level lifetimes enable high pump power storage and is critical for development of high energy lasers. One could use the near-IR absorption bands of TM ions in association with conventional pump sources. Excitation of TM ions followed by resonant energy transfer to RE ions could then result in effective lasing over the 4-6 µm range. RE ions could also be utilized as the donor in the RE®TM energy transfer process. Since numerous well-developed high power laser sources exist that can pump Er-doped II-VI crystals, energy transfer to a TM ion such as Fe²⁺ (with strong emission in the 3-3.5 µm range) could lead to effective lasers in this spectral range. In this work we explore the fabrication of Pr- and Er-doped ZnSe thin films by pulsed laser deposition for photoluminescence (PL), electroluminescence and stimulated emission studies. A KrF excimer laser (2.0–4.0 J/cm²) is used to ablate a pressed, sintered target produced by mixing powders of ZnSe and Pr (or Er) precursor compounds at various concentrations. Targets are sintered in sealed quartz ampules at 950°C for 7 days and then polished for a smooth ablation surface. Targets are ablated at pressures below 1′10^-6 Torr with substrate temperature kept at 450°C. Deposited thin films are produced on sapphire and (100) GaAs wafers. Film thicknesses from 500 nm to 3.5 µm are obtained as determined by deposition rate calibrations and scanning electron microscopy. Films are analyzed by atomic force microscopy for surface morphology, and X-ray diffraction and Raman spectroscopy to characterize the crystalline quality of the films. The optical absorption and emission characteristics of the films are used to verify the incorporation of Pr and Er ions into the II-VI host and evaluate the effect of film characteristics (crystallinity, grain size, and dopant concentration) in the potential of the thin films for stimulated emission. Results are compared with RE-doped ZnSe crystals prepared by thermal diffusion, which in the case of Pr exhibit room temperature PL over the 2–2.8 μm range under 1.56 μm excitation with a peak emission around 2.3 μm and lifetimes of approximately 10 μs. RE-TE co-doping will also be discussed.

Holographic displays reconstruct realistic three-dimensional (3D) images without any special glasses. We have developed a 3D-magneto-optic spatial light modulator (3D-MOSLM) which reconstructs holographic 3D images with magneto-optic effect. Magnetic pixels of the MOSLM are driven by thermomagnetic writing method. Optical absorption of magnetic media in MOSLM is a key characteristic for writing and reconstructing processes. High optical absorption material is good for thermomagnetic writing because of its low light energy consumption. In contrast, low optical absorption is good in reconstruction process because of its high diffraction efficiency, showing bright reconstruction 3D images.
To solve such a trade-off, in this study, we fabricated a multilayer media comprising two different magnetic materials as magnetic hologram media showing high diffraction efficiency and low writing energy. A floating magnetic field from the writing layer transcribes magnetic hologram to the reconstruction layer.
Amorphous TbFe (a-TbFe) was chosen as the writing layer because of having the remanent magnetization value of 1617 G and the writing energy value of 2.5 mJ/cm². The reconstruction layer was bismuth, dysprosium, aluminum substituted yttrium iron garnet (BiDyAl:YIG) with high diffraction efficiency (η = 8.2×10^-3%) and small coercivity (H_C = 200 Oe ). However, these materials rotated light polarization in the opposite direction. Therefore, both magnetic layers should be separated optically and be coupled magnetically via an Al separator layer. A multilayer film was fabricated by ion beam sputtering and had a structure of substituted gadolinium gallium garnet substrate / BiDyAl:YIG (576 nm) / Al (20 nm) / a-TbFe (100 nm) / SiN (50 nm). To transcribe magnetic information from a-TbFe to BiDyAl:YIG, Al layer was chosen the thinnest thickness that both layers were optically separated. The film was evaluated magneto-optical properties in reflection. The Kerr rotation angle θ_K and the reflectivity of multilayer was 1.5 deg. and 23.9% severally. As a result, Al separator was functioning from the fact that Kerr rotation angle of multilayer was up about 1.5 times from BiDyAl:YIG monolayer. The theoretical diffraction efficiency was obtained from the following formula, η = (4/π²) Tsin²(θ_F), T is the transmissivity, θ_F is Faraday rotation angle, respectively^[2]. At reflection, the formula substitutes T and θ_F into R and θ_K: R is the reflectivity of the magnetic film. The theoretical diffraction efficiency of the multilayer was 7.0×10^-3%, it was 1000 times larger than that of a-TbFe film. The multilayer was had a low writing energy value of 4.8 mJ /cm², it was approximately 25 % of BiDyAl:YIG monolayer writing. In addition, the magnetic pixels of writing layer were magnetically transferred to reconstruction layer. The 3D image could be visualized the cubic frame from the multilayer.

A nanoscale texturing technique of glass substrate with the simple printing process of yellow (Ba,Sr,Ca)₂SiO₄:Eu⁺² silicate phosphor paste is introduced here to achieve enhanced white luminescence performance. The luminous efficacy of the resulting printed phosphor layer was enhanced by ~16% as a result of controlling surface roughness of the substrate up to 151 nm. The substantial improvement obtained by texturing both sides of the substrate is attributed to the reduction of total internal reflection of rays at the glass–air interface, combined with reduction of specular reflection at the phosphor–glass interface. According to 3D ray tracing simulations, more rays were assumed to be extracted with a widely scattered radiation pattern on the surface. Far-field luminance uniformity is also found to be significantly improved as a result of the texturing technique.

In this reasearch, compact optical detection systems for photometry and fluorometry were constructed from an organic light emitting diode (OLED) based on rare earth metal complexes as a light source, and a hetero-junction organic photodiode (OPD) based on p-type and n-type layers as a photo-detector on a microchip, which was prepared from poly(dimethylsiloxan)(PDMS). For the photometric detector system, an OLED based on a europium complex, which emits a light of 612 nm with 8 nm FWHM, was used for the determination of phosphate in a flow injection system on a PDMS microchip. An ion association reaction of malachite green with molybdophosphate complex in sulfuric acid was used. The detection limit (S/N=3) of 20 ppb was obtained. Good recoveries of 97 - 99% were obtained for river water samples, which were spiked by the standard phosphate solution.
For the fluorometric detector system, an OLED based on a terbium complex, which emits a light of 545 nm with 9 nm FWHM, was used for the immunoassay for nonionic surfactant, alkylphenol polyethoxylates (APnEOs). A competitive enzyme-linked immunosorbent assay for APnEOs, where an anti-APnEOs antibody was immobilized on the surface of the channel of the PDMS microchip, was adopted. After incubation a sample solution of APnEOs containing a horseradish oxidase-labeled APnEOs with the anti-APnEOs immobilized on the surface of the microchannel and the fluorescence of resorufin generated just after the introduction of the mixed solution containing Amplex Red and H₂O₂ into the microchannel was measured by the fluorometric detector system. As a result, a calibration curve between the photocurrent and the APnEOs concentration with the detection limit of ca. 2 ppb was obtained.

Photothermal therapy (PTT) is widely explored as a promising technology for cancer treatment [1]. Gold nanomaterials exhibit great photothermal properties due to their strong optical absorption provided by surface plasmon resonance (SPR) effect [2]. Gold nanorods (AuNRs) are especially attractive because their high efficient of absorbing near infrared (NIR) light and converting photon energy to heat. The upconversion luminescence (UCL) of Lanthanide (Ln³⁺)-doped fluoride upconversion nanoparticles (UCNPs) have many advantages to be served as imaging or therapy agents because they can absorb low energy photons in the near-infrared and emit high energy visible photons, through a process known as upconversion [3]. By combining rapid light-induced heat transfer from AuNRs to environment, it may be possible to engineer up-conversion fluorescence processes at the nanoscale. In this study, a NIR laser is used as an excitation light source because bio-tissues have a maximum transmittance at NIR region. Herein, we design a novel multifunctional nanocomposites based on the conjugating of AuNRs with NaYF₄:Yb³⁺,Er³⁺ UCNPs to combine SPR and UCL effects for fulfilling both photothermal and fluorescent labeling functions. The AuNR-NaYF₄:Yb³⁺,Er³⁺ nanocomposites developed in this work exhibit effective resonance absorption at 980nm which produces significant photothermal and UCL effects. Noteworthy, we found the AuNR-NaYF₄:Yb³⁺,Er³⁺ nanocomposites exhibit better photothermal property compared with pure AuNRs and the blending of AuNRs and NaYF₄:Yb³⁺,Er³⁺ UCNPs. Mesoporous silica shellis coated between AuNRs and NaYF₄:Yb³⁺,Er³⁺UCNPs to serve as a separation layer for those two nanomaterials. We investigate the thickness effect of silica shell on photothermal property of the AuNR-NaYF₄:Yb³⁺,Er³⁺ nanocomposites.

In order to examine details of the energy levels of rare earth ions in crystalline materials, we developed a new experimental method that Is based on combining combined excitation and emission spectroscopy (CEES)[1] with the ability to perform angle-dependent magneto-optical Zeeman measurement as well as light polarization of both the excitation and emission transitions. The automated system is able to collect, within a relatively short time, multi-dimensional data sets of emission spectra as a function of excitation wavelength, sample orientation, magnetic field and light polarization. The power of this method will be demonstrated using erbium doped stoichiometric lithium tantalate as an example. Emission and excitation spectra for this host material exhibit very distinct sharp lines originating from several specific incorporation sites [2]. The different sites are distinct due to the charge compensation that is required to accommodate the trivalent erbium ion on a lithium site. Datasets produced for this study are used to determine a detailed emission energy map, the local symmetry and the magnetic g-factors for each observed incorporation site. These results are then used for a crystal field analysis that delivers robust parameters for the crystal field splitting of the levels. Using excitation at around 980nm, we are further able to characterize those incorporation sites which are most efficient for emission at 1.5µm and compare them to those that preferentially undergo excitation up-conversion followed by an emission in the red and green spectral region.

[1] V Dierolf, M Koerdt, Physical Review B 61 (12), 8043 (2000).
[2] K Miyahara, A Toulouse, N Woodward, P Capek, V Dierolf Journal of Physics: Conference Series 249 (1), 012011 (2010).

ABSTRACT
Layered rare-earth hydroxides (LRHs), as a newly intriguing family of layered materials, have attracted increasing attention, due to the excellent properties arising from lanthanide elements. The delamination of LRHs to obtain nanossheets can propel the assembly of film luminesecent materials. We demonstrate the facile delamination of the HPTS/OS-LTbH composite (LTbH is layered terbium hydroxide; HPTS is the fluorescent molecule of 8-hydroxy-pyrene-1,3,6-trisulphonate; OS is 1-octane sulfonic acid sodium) and tunable luminescence behavior. Varied luminescence color was present for delaminated colloidal suspensions of HPTS_0.02OS_0.98-LTbH composite in changed FM/water volume ratios. At higher FM/water ratios such as 9:1 (with more FM), one much strong blue emission at ∼440 nm (companying a weak green emission at 516 nm) was observed. At lower FM/water ratios such as 3:7, 2:8 and 1:9 (with more water), an almost pure green emission (516 nm) was achieved. This suggests the puried function of the LTbH layer for the emission color of the composite materieal.

ACKNOWLEDGMENTS
This work is supported by the National Science Foundation of China 21271028.

Layered rare-earth hydroxides (LRHs) have attracted increasing interest because of their rich interlayer chemistry and luminescence property. Herein, we demonstrate the intercalation of two organic compounds, the coumarin-3-carboxylic acid deprotonated by NaOH aforehand and the surfactant 1-octane sulfonic acid sodium into the gallery of the layered europium hydroxide (LEuH) via an ion exchange method. In formamide (FM), the composites were facilely delaminated, and different molar ratios of the organic guests gave rise to different emission intensity. The luminescence spectra indicate that the organic sensitizer of coumarin-3-carboxylic acid can markedly enhance the red luminescence of the layer Eu³⁺ ions due to efficient energy transfer from the interlayer coumarin-3-carboxylic acid guest to the layer Eu³⁺. These composites maybe promising materials to achieve efficient application on display devices with red emission.

ACKNOWLEDGMENTS
This work was supported by the National Science Foundation of China 21271028.

We use spectroscopic methods to probe and map distributions and enhancements of optical electric and magnetic fields at nanoscale. Our methods are based on use of Eu ions which have both electric and magnetic dipole transitions. We show that both electric and magnetic dipoles can strongly affected by surface plasmons, resulting in modification of the specta and angular radiation pattern. Enhancement of the relative intensity of the magnetic transition is observed in the systems demonstrating magnetic resonance at optical frequencies.

Rare earth upconversion nanocrystals enable large anti-Stokes shifts with relatively sharp emission and long excited-state lifetimes, leading to a variety of applications. Fundamental aspects however had hindered a more general applicability, i.e. in photovoltaics, where lower irradiances precluded this materials from being practical. In this work, unique core-shelled nanocrystals, where the active upconverting material is encapsulated by a protective shell, are systematically investigated as optical gain and lasing media as a function of dopant type and concentration. Polystyrene microbeads of an average size of 5 µm are loaded with nanocrystals and used as resonators to achieve upconversion lasing by excitation of whispering gallery modes (WGM). These may greatly extend the broad functionality of UCNPs for background free imaging, theranostics, volumetric displays, active waveguiding and photonics structure assembly. We observe that the concentration of lanthanides in the crystal lattice reveal a correlation with lasing thresholds and optical gain, and measure lasing thresholds as low as 1 µW cm^-2 in pulsed excitation.

The realization of red GaN-based light-emitting diodes (LEDs) enables the fabrication of a nitride-based monolithic optical device, which is composed of red, green, and blue LEDs for full color LED displays. Red emission from GaN-based LEDs using Eu-doped GaN (GaN:Eu) as an active layer has been achieved,¹ however, the light output is not yet sufficient for practical use. In this contribution, GaN:Eu was inserted into a microcavity structure, which resulted in a modulation of the directionality, photoluminescence (PL) intensity, and lifetime of the Eu emission.

The samples in this study were grown by organometallic vapor phase epitaxy (OMVPE) on (0001) sapphire substrates. A half λ microcavity was constructed with a GaN:Eu layer inserted between an AlInN/GaN distributed Bragg reflector (DBR), with a design wavelength of 622 nm, and a ZrO₂/SiO₂ DBR deposited on the surface. The Q-factor of the microcavity was determined to be 546 based upon the reflectivity of the DBRs. Another sample was prepared without the cavity structure for reference.

The emission spectra from each sample are markedly different depending on the emission angle θ, defined with respect to the surface. The resonance wavelength (λ_res) shifts towards shorter wavelength for the higher q, in agreement with theoretical calculations. As for the θ dependence on the PL intensity, it was observed that the PL intensity of the sample with a microcavity was enhanced by 12.9 times at θ = 0°, but decreased drastically compared to the sample without the microcavity for θ > 0°.

Time-resolved PL (TR-PL) profiles were measured for the samples with and without a microcavity and the decay curves were analyzed by fitting a stretched exponential. The PL lifetime of the sample with the microcavity was found to be 1.2 times larger than the sample without the microcavity. Thus, it is inferred that other contribution to the overall enhancement of the PL intensity was due to a 10.8 times improvement of the light extraction efficiency. These results indicate that the Eu emission is drastically affected by the modulation of the optical mode in the microcavity, and suggest that the microcavity structure is effective for improving the light output from GaN:Eu-based red LEDs.

References
[1] A. Nishikawa, Y. Fujiwara et al., Appl. Phys. Exp. 2, 071004 (2009).

Epitaxial layers of GaN doped with Europium have demonstrated great promise for red light emitting diodes that would provide the missing link to monolithically integrate an active three color light source that is solely based on the hexagonal nitride platform. Extensive spectroscopic work has provided ample evidence that the two main emitting centers, Eu1 and Eu2, are complexes consisting of a Eu ion on gallium site accompanied either nitrogen (V_N) or (V_Ga) gallium vacancy [1]. These vacancies act as donors and acceptors respectively. Despite these electrical signatures, the question of the charge state and the ability to change it with temperature has not been addressed. In addition, the nature of several incorporation sites remains unclear. Most notable, a center labeled OMVPE 8 [2] stands out by its ability to emit efficiently under UV excitation and by a magnetic g-factor that is too high to be explained by a regular Eu3+ ion. Using temperature dependent spectroscopy under excitation (1) of the host using above band gap light (2) of deep traps using below the band gap light, (3) of the distinct Eu ion defect configurations using light that is resonant with the ⁵F₀ to ⁵D₀ transition, and (4) of combinations of the above cases using simultaneously multiple wavelengths, we are able to demonstrate that the Eu2 and the OMVPE 8 complex are the same basic defect configuration distinct only by the presence or absence of charges. Consequently, we can observe, above a certain temperature (T>120K), that these centers can convert into each other, even while they are excited such that the emission of one type of center is detected while the other is excited and vice versa. For these temperatures, the conversion happens quite freely given rise to the observation that both centers emit with similar strength. Even at very low temperatures (T~10K), the relative number of the two centers can be manipulated by the presence of charge carriers that are created using light. We will discuss how the ability of the Eu-V_Ga to capture and release carriers competes with other deep and shallow that traps are present in the material.

[1] B Mitchell, J Poplawsky, D Lee, A Koizumi, Y Fujiwara, V Dierolf
Journal of Applied Physics 115 (20), 204501 (2014).
[2] N. Woodward, A. Nishikawa, Y. Fujiwara and V. Dierolf, MRS Proceedings, 1342, mrss11-1342-v05-06 (2010).

Eu-doped GaN (GaN:Eu) is a promising material for the active layer in nitride-based red light-emitting diodes (LEDs). The output power of these LEDs has reached the sub-mW level, but further improvement is strongly desired for practical application. Recently, we reported that the growth of GaN:Eu at a lower temperature (~960°C) nearly doubled the photoluminescence (PL) emission intensity. Multilayer structures (MLS) consisting of 40 pairs of alternating GaN and GaN:Eu were grown at this temperature, where the thickness of the GaN layers was held fixed at 10nm, while that of the GaN:Eu layers was changed from 1, 2, 3, and 4 nm.
PL measurements revealed that, compared to a bulk GaN:Eu sample (~300nm thick), all of the MLS samples had narrower PL spectra. The PL efficiency per Eu ion also increased for all MLS samples as compared to the bulk. The largest enhancement was ~163 times for the MLS sample with a 1nm thick GaN:Eu layer. In this contribution, we will discuss the growth of MLS, the beneficial effect on the optical properties, and propose a model explaining the origin of this enhancement.

We have been investigating a luminous Eu centers in GaN by Mg co-doping method using NH3-MBE. Although Mg co-doping enables improvement of the luminescence properties from Eu inner-shell transitions, the enhanced optical sites reported from some groups were different and depending on the fabrication method. In case of Eu and Mg co-doped GaN (GaN:(Eu, Mg)) grown by NH3- MBE, the optical properties of Eu-Mg related site showed high stability against both thermal annealing process and laser irradiation.
From the combined excitation emission spectroscopy (CEES) technique, new four optical site groups were found in addition to the previously reported two major luminescence sites A (620.5 and 633.8 nm) and B (621.9 and 622.8 nm), which were dominant under indirect excitation of the Eu ions through GaN. These optical sites are inconsistent with Eu-Mg related sites reported for GaN:(Eu, Mg) grown by OMVPE, indicating that the optical site constitution strongly depends on the growth method.
The site A had a high cross section and contributed to as high as 22% of the total PL integrated intensity for GaN:(Eu, Mg) grown by NH3-MBE, which resulted in a high PL intensity. The relative existence of site A compared with other sites was highest in the low total Eu concentration region, but saturated around at 1e19cm3. Other sites increased after the saturation of site A.

In this work, an InGaN/GaN superlattices (SLs) and an Eu³⁺ ion implanted InGaN/GaN SLs (SLs:Eu³⁺) were was investigated. The SLs grown by metal-organic chemical-vapor deposition (MOCVD) with 25 periods of 5 nm InGaN well/5 nm GaN barrier. The designated quantum well (QW) indium percentage is ~7% and the SLs was grown on a buffer of 1.0 μm undoped GaN/2.5 μm n-type GaN grown on (0001) sapphire substrate. SLs:Eu³⁺ sample was implanted with Eu ions dose of 5 x10¹⁵ cm^-2. The as-grown SLs and SLs:Eu³⁺ were subjected to thermal annealing at different temperatures ranging from 200 °C to 950 °C in nitrogen ambient for 3 min.
Samples were characterized by X-ray diffraction (XRD) for investigating the interface quality of the annealed SLs and their structural recovery from Eu ion implantation induced damage. Photoluminescence (PL) and cathodoluminescence (CL) of Eu ion implanted InGaN/GaN SLs reveals QW excitonic emission band at 395 nm and characteristic Eu³⁺ ion emission lines that can be assigned to different transitions between ⁵D₀-⁷F_J levels with the strongest emission peaks at 620 nm (CL) and 622 nm (PL). A broad defect emission band induced by photon excitation was observed in PL between 500 and 700 nm; however it is absent in CL spectra. The spectral position of the excitonic peak in SLs:Eu³⁺ was blue-shifted with respect to undoped InGaN/GaN SLs in all studied samples. Theoretical simulations indicate that the experimentally observed emission blue-shift is due to the europium-induced change of MQW’s internal stress and strain-induced electric field in InGaN/GaN:Eu³⁺ SLs. It was found that compressive stress developed in Eu ion implanted InGaN/GaN SLs can be controlled by changing the annealing conditions affecting active QW thickness. PL kinetics study was performed for excitonic emission decay in MQWs as a function of annealing temperature to further understand strain-related effects. Furthermore, Laplace deep level transient spectroscopy (LDLTS) and optical DLTS techniques are applied to investigate electrically the defect levels in the materials. Finally, we attempt to correlate the nature of Europium induced defects tested by optical and electrical means for better understanding of the excitation mechanism of Eu³⁺ ions in studied InGaN/GaN SLs.

Wide bandgap semiconductor materials like III-Nitrides doped with rare earth (RE) ions (Er, Yb, Eu, etc.) have attracted lot of attention in the recent days due to their potential applications in photonic, optoelectronic, spintronic and display device applications from ultraviolet to infrared range. In this study, we present a comparative study of the pure and RE (Er³⁺ and Yb³⁺) doped high indium content In_xGa_1-xN (x=0.24 to 0.30) thin films grown directly on sapphire substrate without any buffer layer by plasma assisted molecular beam epitaxy (rf-MBE). Highly crystalline growth has been revealed by X-ray diffraction (XRD) and reflection high energy electron diffraction (RHEED). The presence of rare earth ions in the films has been confirmed by the X-ray Photoelectron spectroscopy (XPS) and Photoluminescence (PL). Undoped InGaN samples showed presence of InN secondary phase, whereas the secondary phase has been completely suppressed by RE doping. Defect free sharper visible near band edge (NBE) emission (FWHM=39.5 nm) has been observed in Yb doped InGaN thin films compared to the InGaN (FWHM=61.1 nm). However, the films with Er doped has showed quit broader NBE emission (FWHM=145.8 nm) compared to other samples. A weak PL quenching has been observed in all the samples and an inverse S-shaped behavior of emission position has been observed from 11 K to 300 K.
The unusual behavior of broadening and shifting in the NBE emission could be due to the fluctuation of the indium content, which can be explained by using the crystal field calculations. The IR emission related to RE³⁺ ions can be analyzed by introducing surface defects and defect complexes using the crystal field theory. A comparative study of the luminescent properties of InGaN:Yb³⁺ and InGaN:Er³⁺ thin films with InGaN showed the presence of rare earth induced structural isovalent (RESI) traps. Based on these rare earth optical emission lines, the crystal field theory will be explained in details in terms of the splitting of the 4f Stark energy levels and the details of the optical properties will be discussed.

The doping of Europium into gallium nitride (GaN) has a promising future for optoelectronic applications.¹ It has been shown that the Eu ions incorporate themselves into the GaN host in multiple defect environments or centers.² At the nano-scale, the local defect environments around the Eu ions play a large role in their optical and magnetic properties. Some of the defect environments facilitate efficient energy transfer from the host the Eu ion, while others do not.¹ It now appears that one Eu center is actually a charged derivative of another, with a temperature dependent meta-stability. The effective g-factor of the ⁷F₂ multiplet for this center is far above the theoretical expectation for an isolated Eu ion,³ and over three times larger than that of the other Eu centers, which may be related to its charged nature.
On the macroscale, the overall concentration of oxygen, which can be detrimental to device performance, was shown to influence the incorporation of the Eu ions. However, this can be overcome by using delta-doping growth, with alternating layers of GaN and GaN:Eu.⁴ Recently, it has been also found that the incorporation of Eu into the GaN matrix can drastically influence the material properties, even at dilute levels (< 1%). By using the delta-doping structure and lowering the growth temperature, the surface morphology, as well as the size and concentration of threading dislocations, can be controlled. This control led to the fabrication of an LED with the highest output power per Eu layer thickness, and the lowest threshold voltage reported for this system.

[1] Y. Fujiwara and V. Dierolf, Jpn. J. Appl. Phys. 53, 05FA13 (2014).
[2] N. Woodward et al., Opt. Mater. 33, 1050 (2011).
[3] N. Woodward et al., MRS Proceedings, 1342, mrss11-1342-v05-06 (2010).
[4] B. Mitchell et al., Scientific Reports 6,18808 (2016).

The advent of wide band gap semiconductors doped with rare-earth (RE) elements has become increasingly important in the field of optoelectronics. In particular, Europium (Eu)-doped gallium nitride (GaN) has been extensively investigated in recent years for red light emission. Because of the screening of the outer closed 5s and 5p shells of Eu⁺³ ions, the crystal field of the GaN host weakly interacts with the 4f shells of Eu⁺³ ions. More specifically, electron-hole pairs present in the host can be captured from traps in the vicinity of Eu⁺³ ions to form complexes. Following the recombination of those pairs (de-excitation of complex), part of the released energy can be transferred to a nearby Eu⁺³ ion and excite the intra 4f electrons. Despite the realization of electrically driven GaN:Eu based red light emitters, the internal quantum efficiency (IQE) is limited to ~1%. In addition, the observed higher IQE in the optically pumped as compared to the electrically driven GaN:Eu devices, has resulted in unexplained limiting factors in the electrically driven devices. In this work we develop a physically intuitive current injection efficiency (CIE) model for a AlGaN/GaN:Eu/AlGaN based quantum well (QW) LED. The model is constructed based on the solution of coupled rate equation of the carriers, complexes and excited Eu⁺³ ions under steady state conditions. Through manipulation of different parameters associated with the excitation path of Eu⁺³ ions such as capture rate from defects, energy transfer rate to Eu⁺³ ions, radiative lifetime of Eu⁺³ ions as well as energy loss mechanisms and transport phenomena that take place, high IQE can be achieved both for optically pumped and electrically driven devices. The simulation results indicate that much higher than the current state of the art IQE can be achieved by carefully engineering this excitation path. More specifically, from our numerical analysis we report an IQE of ~6% for the electrically driven and ~20% for the optically excited GaN:Eu QW. In addition, we report special cases in which the excitation path could be engineering in such way to provide an IQE 16.5% - 35% for the electrical case and 47.5% for the optical case. The CIE model serves an important purpose for understanding the discrepancies observed between the electrically- and optically-driven devices. In addition, the CIE model provides the pathway towards achieving high IQE for these GaN:Eu devices. Our analysis of the carrier mechanisms in the GaN:Eu active region provides a strong indicator of the device efficiency, and provides the deep understanding of device physics that will lead to the realization of high efficiency GaN:Eu QW based red light emitters.

Gallium nitride (GaN)-based light-emitting diodes (LEDs) are widely used as blue and green light sources. Ternary-compound semiconductors of In_xGa_1-xN with a well-controlled bandgap energy can be formed through In incorporation, which has been applied for the emission color tuning of LEDs. However, with In incorporation, it is difficult to realize the emission of red light because the higher values of x for In_xGa_1-xN having a narrow bandgap induce a strong local internal field because of piezoelectric polarization, reducing the internal quantum efficiency. A possible solution to the problem is inserting an active layer for red light emission—GaN doped with europium (GaN:Eu)—at the p–n junction of the LED. Because Eu dopants have intra-4f transitions (typically, ⁵D₀ → ⁷F₂) at a wavelength of ~620 nm, the red LED is expected to be realized by transferring energy from the GaN host to the Eu dopants. Despite the recent progress in red LED made of GaN:Eu¹ having sharp emission lines due to the intra-4f transition of Eu³⁺, unexpected subsidiary Eu emission centers radiate several satellite lines. We investigated these subsidiary emission centers by analyzing the harmonic contents using forward current modulated by an electronic micro square wave (duty cycle: 50 %), and observed the originally forbidden even harmonics in a specific frequency region of 23–45 MHz. The even-harmonic generation was formulized with a binary response caused by the electronic coupling of emission centers in valence states, i.e., dimerization. The coupling was consistent with the results of the optical analyses of GaN:Eu thin films in former studies. The binary response was experimentally quantified by using a parameter such as the phase difference between the responses of coupled centers, and a significant phase difference of 63° (> π/4) was observed at 36 MHz. The injection charges were cooperatively captured by the coupled emission centers and were branched into the constituent centers for recombination, resulting in undesired satellite emission lines.
This work was partly supported by KAKENHI with a Grand-in-Aid for Scientific Research (Nos. 26420287 and 24226009).
¹Y. Fujiwara and V. Dierolf, Jpn. J. Appl. Phys. 53, 05FA13 (2014).

Erbium doped GaN have attracted much attention due to their capability to provide highly thermal stable optical emission in technologically important wavelengths. There is a continued need to exploring effective mechanisms to further improve the quantum efficiency of the 1.54 micron emission in this material. We report photoluminescence and direct evidence of two mechanisms responsible for the excitation of Er3+ ions in GaN/AlN multiple quantum wells (MQWs:Er) grown by metal organic chemical vapor deposition. The emission intensity from our MQWs:Er increases significantly, compared with those from a single layer. We will discuss the influence of the quantum well and barrier width on the photoluminescence emission at 1.54 micron. These results demonstrate that MQWs:Er provide a basis for efficient photonic devices active at 1.54 micron.

This presentation will focus on the growth of highly anisotropic Er-group-V (Sb and As) nanostructures embedded in a III-V semiconductor matrix by self-assembly during molecular beam epitaxial growth of Er_xIII_1-x-V by codeposition. In-situ scanning tunneling microscopy in combination with molecular beam epitaxy allows for atomic scale characterization during different stages of growth. For growth of GaSb(001) with increasing Er concentration, ErSb embedded nanostructures change from nanoparticles to vertical nanorods, nanotrees, horizontal nanorods and nanosheets[1]. The resulting Er_xGa_1-xSb nanocomposites are single crystalline with a continuous Sb-sublattice. The vertical nanorods are continuous throughout the Er_xGa_1-xSb layer, their axes are parallel to the [001] growth direction, and they self-assemble into ordered arrays aligned along the [-110] direction. The horizontal nanorods grow in the [-110] direction.
In the case of GaAs, ErAs nanorods can also form by self-assembly during molecular beam epitaxial growth of Er_xGa_1-xAs by codeposition. In this case the nanorod formation with the rods growing in the <211> direction was found for growth on GaAs (h11)A surfaces. In contrast, ErAs nanorods do not form on GaAs (h11)B or GaAs (001) surfaces.
Scanning tunneling spectroscopy and angle resolved photoemission spectroscopy were used to measure the electronic bandstructure of embedded RE-V nanostructures of varying dimensions, namely 0D nanoparticles, 1D nanorods, and 2D thin films.
The growth mechanisms for Er_xGa_1-xSb and Er_xGa_1-xAs that result in embedded vertical and horizontal nanorod formation will be discussed. The atomic scale growth mechanisms are a result of surface diffusion and wetting characteristics which are used to explain the differences for the self-assembly of nanostructures for Er_xGa_1-xSb and Er_xGa_1-xAs.


[1] J. K. Kawasaki, B. D. Schultz, H. Lu, A. C. Gossard, and C. J. Palmstrøm, Nano Letters 13, 2895 (2013)

Semiconductor lasers with a temperature-independent oscillation wavelength are highly desired for applications in optical communications. Er,O co-doped GaAs¹ (GaAs:Er,O) is employed as a gain material because its emission wavelength (1538 nm) is suitable for optical communications, and it has temperature-stable spectrum (0.001 nm/K). For lasing to occur, the Er emission centers of GaAs:Er,O should be put in an optical cavity. In this contribution, PC nanocavities were used due to their small modal volume, high Q-factor, and compatibility with integrated optical circuits. However, the emission properties of Er emission centers in high-Q PC nanocavities have not been well studied. Therefore, numerical simulations of the emission from PC nanocavities including Er emission centers were performed using the finite-difference time-domain (FDTD) method. The results were compared with experimental observations.
L3 PC nanocavities² were used in this study. L3 PC nanocavities are fabricated in a triangular lattice of circular holes, where the nanocavities themselves are formed by intentionally omitting three consecutive holes. At first, numerical simulations using FDTD were performed under two situations. One in which the oscillation wavelength of the PC nanocavity was tuned to the emission wavelength of Er emission center (on-resonant), and another where it was not (off-resonant). Then, GaAs:Er,O was grown by organometallic vapor phase epitaxy and L3 PC nanocavities with various lattice constants, that is, various oscillation wavelengths, were fabricated by electron-beam lithography and plasma etching technique. Photoluminescence (PL) measurements were performed on the sample at room temperature with an Ar laser (488 nm) as the excitation source. The PL spectrum under the on-resonant condition and off-resonant condition were compared with the FDTD calculation. FDTD calculations showed that Er emission centers in an L3 PC nanocavity emit 3.4 times brighter under the on-resonant condition as compared to the off-resonant condition, which is due to the Purcell effect³. Experimentally, the PL emission under the on-resonant condition was 20 times brighter than under the off-resonant condition, which was not in agreement with the FDTD calculation. This extreme enhancement is due to the interaction between Er ions, which was not considered in the FDTD calculation.

(1) K. Takahei and T. Taguchi, J. Appl. Phys. 76, 4332(1994).
(2) Y. Akahane et al., Nature 425, 944 (2003).
(3) E. M. Purcell, Phys. Rev. 69, 681 (1946).

The rare earth nitrides (REN) as a class of materials are especially interesting from a spintronics application view. Many of the REN series have been shown to be intrinsic ferromagnetic semiconductors¹. They therefore offer an alternative to dilute magnetic semiconductors for filling a gap in appropriate materials for semiconductor-based spintronics. Additionally, members of the REN series have been shown to be epitaxially compatible with the wide bandgap III-nitrides GaN and AlN^2,3,4. The III-nitrides are the basis of high efficiency blue light-emitting diodes (LEDs) allowing the cheap and efficient modern LED lighting we have today, along with other applications in other high-power, high-efficiency electronics. Here we present work on combining the properties of these two classes of materials for novel devices; understanding of the epitaxial growth process is an essential step in the fabrication of device structures.

SmN is notable among the REN series due to its near-zero moment from the nearly equal and opposing spin and orbital moments⁵. Heavily-doped SmN has also recently been shown to have a superconducting transition, marking the first known material to show coexisting ferromagnetic, semiconducting and superconducting behaviour⁶. We report here on another recent discovery that epitaxial SmN thin films on AlN(0001) surfaces can be selectively grown either (1 1 1)- or (0 0 1)-oriented by varying the growth temperature, resulting in a highly unusual cube-on-hexagon epitaxial relationship.

The growth of SmN is performed via ammonia MBE under nitrogen rich conditions on 100 nm thick AlN(0 0 0 1) buffer layers grown on Si(1 1 1) substrates. For substrate temperatures in the range 400-800°C, epitaxial growth of SmN is confirmed via in-situ RHEED and ex-situ XRD measurements. A striking transition from the expected (1 1 1) orientation to (0 0 1)-oriented SmN is observed for growth temperatures above 700°C. The 4-fold on 6-fold symmetry results in the formation of three rotational domains rotated at 30° with respect to each other. Accompanying this transition in growth orientation is a decrease in surface roughness and improved crystalline quality.

This ability to control the epitaxial growth orientation on a fixed substrate is unique among the REN series members which have been successfully grown epitaxially. It provides further opportunities in exploring novel thin film REN heterostructures whose properties may depend on the film's orientation as well as in the integration of REN-III-V devices.

1. Natali, F, et al. "Rare-earth mononitrides." Progress in Materials Science 58.8 (2013): 1316-1360.

2. Scarpulla, M. A., et al. Journal of Crystal Growth 311.5 (2009): 1239-1244.

3. Natali, F., et al. Journal of Crystal Growth 312.24 (2010): 3583-3587.

4. Natali, F., et al. physica status solidi (c) 9.3-4 (2012): 605-608.

5. Meyer, C., et al. Physical Review B 78.17 (2008): 174406.

6. Anton, E-M., et al. arXiv preprint arXiv:1505.03621 (2015).

The ultimate frontier in nanomaterials engineering is to realize their composition control with atomic scale precision to enable fabrication of nanoparticles with desirable size, shape and surface properties. Such control becomes even more useful when growing hybrid nanocrystals designed to integrate multiple functionalities. Here we report achieving such degree of control in a family of lanthanide-doped upconversion nanomaterials. We experimentally verify the co-existence and different roles of oleate anions (OA^-) and molecules (OAH) in the crystal formation. We identify that the control over the ratio of OA^- to OAH can be used to directionally inhibit, promote or etch the crystallographic facets of the nanoparticles. This control enables selective grafting of shells with complex morphologies grown over nanocrystal cores, thus allowing the fabrication of a diverse library of monodisperse sub-50 nm nanoparticles. With such programmable additive and subtractive engineering a variety of three-dimensional shapes can be implemented using a bottom–up scalable approach.

The luminescent properties of the lanthanide-doped upconversion nanocrystals strongly depend on their composition and morphology. The relationships among these parameters have been interpreted in this report. The sensitizer doping concentration affects the luminescent emission more than the size of nanocrystals. Furthermore, the luminescence of upconversion nanocrystals is determined by both the surface deactivations and internal crystal defects. The engineering of upconversion nanocrystals is capable to manipulate the luminescent properties through the controlled crystal growth process.

SiAlON phosphors with rare earth doped have attracted significant attention on the application of LEDs with good color rendering ability, high efficiency, a long lifetime and so forth. The Ca-α-SiAlON:Eu²⁺ phosphors, featuring high phase purity, uniform particle size of 3-5μm and good luminescent properties with a yellow emission spectrum under blue light excitation, were prepared by a simple yet highly efficient combustion synthesis (CS) method. A certain amount of NaCl was applied as an innovative additive to regulate and control the properties of synthesized phosphors. It was demonstrated that the appropriate addition of NaCl could remarkably influence the CS process and promote the photoluminescence properties of the synthesized phosphors. The intensity of the emission spectrum reached a maximum for the sample doped with 6 wt% of NaCl, nearly 40% more than the sample which was not NaCl-doped. Further, the effects of NaCl on the properties of products were systematically investigated, and the immanent influencing mechanisms were rationally proposed herein. It was found that the effect on accelerating nitridation and crystallization played a dominant role in the reaction when the content of NaCl was low, while the effect of absorbing reaction heat through vaporization was dominant with the further-increased content of NaCl. Moreover, a continuous blue shift phenomenon in emission spectra was observed with the increased content of NaCl. This work would be of great reference for the efficient synthesis of Ca-α-SiAlON:Eu²⁺ phosphors with outstanding PL behavior by CS.

Bismuth-substituted yttrium iron garnet film was one of the promising magneto-optical (MO) materials utilized in the nonreciprocal optical devices but facing the difficulties of integration on the traditional semiconductor materials like silicon. In this talk, a novel method to realize MO garnet films growing on the silicon substrate will be introduced. In our recent work, a single-step PLD method with subsequent rapid thermal annealing has been proposed to develop Bi:YIG/YIG bilayers. Existence of YIG as an in-situ seed layer promoted the growth and crystallization of Bi:YIG portion. The multilayer films with different layer-thickness ratio were deposited under different sequence. The relationship between microstructure and properties of the film will be discussed. The surface morphology of the films was characterized by AFM. The interface structure between layers and substrate were observed by HRTEM. As our previous results, this single-step PLD process will enable the Bi:YIG layer to contact the photonics platform directly attempting to improve the device performance.

Transmission electron microscopy (TEM) with in situ laser annealing was used to identify a novel two-step anneal to produce fully-crystallized large-grained yttrium iron garne (YIG) films on silicon dioxide (SiO₂) substrates. Thin films of yttrium YIG are necessary for proximity layers in spin-Hall and topological insulator devices and also as seed layers in non-reciprocal photonics. However, integration onto Si-compatible substrates has yielded low quality, multi-phase films, even when standard diffraction patterns exhibit only garnet peaks. When examined closely, we found standard anneals produce YIG crystallites in a nanocrystalline matrix. Here, a low-temperature pre-anneal was discovered to allow YIG nucleation after which crystallites grow with front velocities of 280nm/sec during a subsequent laser anneal at standard annealing temperatures thus preventing the formation of the nanocrystalline matrix. Also found was that large (10um) grains of YIG could be formed by in situ annealing with a TEM heating holder due to nucleation from the electron beam. From these in situ TEM results, a commercially feasible two-step rapid thermal anneal (RTA) was identified (400 °C, 3min; 800 °C, 3min) and found to successfully produce phase-pure garnet films on SiO₂ substrates.