Symposium EP03 : Materials and Processes for Nonlinear Optics and Nonlinear Photonics

Cellular imaging has become very relevant in such important fields as biophysics, diagnostics and therapeutics. The advances of nonlinear optics for this imaging modality gradually become obvious. Two-photon fluorescence induced by near-infrared light has brought a number of advantages, such as higher resolution and intrinsically confocal imaging, and less scattering and absorption, leading to reduced phototoxicity and longer photostability.

More recently, it has become obvious that an additional imaging modality is possible with the same pulsed laser and microscope equipment. Second-harmonic imaging only requires detection at a different wavelength, but provides additional information, different from one- or two-photon fluorescence. Because these are odd-order processes (either first of third order) they are not sensitive to symmetry. Second-harmonic imaging, being an even-order nonlinear process, is symmetry-sensitive and will provide structural information based on the symmetry requirements for the superstructural features of the probe, while fluorescence will only provide information about the where-about of the probe.

We will report on the design and engineering of new molecular probes for this combined two-photon fluorescence and second-harmonic imaging of cellular structure. These can be molecular probes with combined advanced structural (amphiphilic properties) and optical functions (second- and third- order optical nonlinearities), based on a number of different strategies. We will also report on our approach to screen fluorescent proteins for second-harmonic imaging and our effort to rationally design a new fluorescent protein for this new function of second harmonic imaging.

Multi-photon excitation microscopy involves simultaneous multi-photon absorption by a luminescent probe. Because the absorption rate depends on the square or cube of the intensity of the excitation source, a tightly focused laser beam enables three-dimensional spatial selectivity for excitation. For this reason, multi-photon excitation microscopy is a powerful tool for the three-dimensional imaging of cells, tissues and organs. In order to realize effective multi-photon excitation processes, a pulsed laser with a large peak intensity is required. On the one hand, the averaged power of excitation laser should be minimized to avoid phototoxicity. Therefore, a femtosecond (fs) pulsed Ti:sapphire laser has conventionally been employed as the excitation light source of multi-photon excitation microscopy.
Recently, several fs Yb-doped fiber lasers operating at 1030–1070 nm have been commercialized. The fs Yb-doped fiber laser is an attractive excitation light source for multi-photon excitation imaging. This is because the fiber laser oscillator is much smaller than that of a Ti:sapphire laser. Furthermore, the fs fiber laser is stable over wide temperature and humidity ranges, and requires less maintenance than a Ti:sapphire laser. However, there have been only a few reports on in vivo probes excitable by using a fs fiber lasers.
Our group has developed a salient two-photon absorption probe suitable for excitation using a fs fiber lasers [1], PY [(4,4’-((1E,1’E)-(3,8-dibutylpyrene-1,6-diyl)bis(ethene-2,1-diyl))bis(1-methylpyridin-1-ium) iodide)]. Maximum wavelength of fluorescence of PY in DMSO was 650 nm and the quantum yield was 0.8. Two-photon absorption maximum of PY was 950 nm with the cross-section of 1100 GM (1 GM = 10^-50 cm⁴ per photon per molecule). Even at 1050 nm, the two-photon absorption cross-section was larger than 200 GM. PY exhibited sufficient water solubility (higher than 10^-6) for staining a living cell and localized at mitochondria. Owing to the large fluorescent quantum yield and two-photon absorption cross-section of PY, HEK293 cell stained with PY showed a bright two-photon excitation microscope image even when a few mW of laser beam from a fs fiber laser was employed for excitation.
We have synthesized several fs fiber laser excitable two-photon absorption probes other than PY. In addition, three-photon absorption probes suitable to fs fiber laser excitation have been developed. The details of such probes will be discussed.

References:
[1] Y. Niko, H. Moritomo, H. Sugihara, Y. Suzuki, J. Kawamata, and G. Konishi, J. Mater. Chem. B 3, 184 (2015).

Studies of the interactions between dye molecules and nucleic acids and proteins are crucial for any applications of the dyes for imaging or manipulating biomolecules. The intercalation or groove binding lead to optical changes which can be used for monitoring the binding processes. Host-drugs complexation can be thus studied by numerous spectrophotometrical methods: UV-Vis absorption, fluorescence, circular dichroism and in some cases by nonlinear optical fluorescence response. Analysis of the induced spectral effects reveals considerable detail about the host-drug binding site size as well as thermodynamics of complex formation which is needed e.g. for efficient drug design.
A series of new photochromic aminoazobenzenes molecules was synthesized and their spectroscopic properties were determined. The kinetics of photoisomerisation in solvents of various polarity and viscosity was measured prior to the studies involving biomolecules. An important feature of the new aminoazobenzenes is high water solubility which is crucial for any biological applications. Moreover, the molecules showed fluorescent properties, unusual for this class of materials, and very interesting from the point of view of application as potential markers in biology [1]. It has been found that the cis form of aminoazobenzene is thermally stable and the energy of activation (E_a) of the dark cis-trans reaction is 133 kJ/mol which is about 30% higher than that for unsubstituted azobenzene and results in 4 days of the thermal recovery of up to 50% of the trans form at room temperature.
A detailed description of the binding with salmon sperm DNA and Human Serum Albumin at physiological conditions (pH 7.25) was performed for a water soluble chromophore with star-shaped oligomeric arms, named Ant-PHEA and a similar molecule Ant-PIM. The results achieved by applying UV-vis absorption, fluorescence, Fourier transform infrared (FT-IR) and circular dichroism spectroscopy confirm that Ant-PHEA interacts mainly with the bases and the phosphate groups of DNA. The binding constants calculated at 298, 304 and 310K are 2.63x10³, 2.70x10³, and 4.67x10³ L mol^-1, respectively. The melting temperature (T_m) of the DNA and DNA-Ant-PHEA were determined [2]. A significant increase of the two-photon induced fluorescence in the adducts was observed, especially with the detection of the non-canonical structures of DNA such as e.g. quadruplex DNA.

Fig.1 Ant-PHEA chemical structure

Acknowledgments
Funding from NCN HARMONIA grant UMO-2016/22/M/ST4/00275 is highly acknowledged.

[1] M. Deiana, Z. Pokladek, J. Olesiak-Bańska, P. Młynarz, M. Samoć, K. Matczyszyn, Scientific Reports 6 28605 (2016)
[2] M. Deiana, B. Mettra, K. Matczyszyn, D. Pitrat, J. Olesiak-Bańska, C. Monnereau, C. Andraud, M. Samoć, Biomacromolecules 17 (11), 3609-3618 (2016)

Monitoring mitochondrial membrane potential (MMP) by luminescence microscope is important for understanding activity of a living cell. Our research group has created a novel type of MMP sensitive two-photon excitable green fluorescent probe [1]. When MMP is high, the compound accumulates on mitochondria and transfers to nucleus as the mitochondrial membrane potential decreases. This translocation is reversible. A key parameter of this ability has been found to be polarizability of the compound. In this study, we have undertaken to extend the color variation of this type of MMP sensitive probe. Namely, we have designed red or blue fluorescent probes that exhibit MMP sensitivity.
In order to obtain a red fluorescent probe, a compound possessing a low polarizability but a rather longer π-electron system must be designed. To satisfy this dilemma, we designed pyrene derivatives possessing substituents at 1 and 3 or 1 and 8 positions, 4,4’-[1,3-pyrenediyldi-(1E)-2,1-ethenediyl]bis(1-methylpyridinium) diiodide (1,3-PY), and 4,4’-[1,8-pyrenediyldi-(1E)-2,1-ethenediyl]bis(1-methylpyridinium) diiodide (1,8-PY). As for a blue fluorescent MMP sensitive probe, 1,1’-dimethyl-4,4’-(2,6-naphthalenediyl)dipyridinium diiodide (NPJ) has been designed. This compound possessing rather smaller π-electron system compared to that of the previous biphenyl derivative. The optical properties and MMP sensitivities of these compounds were compared with those of the previously reported MMP sensitive fluorescent probe, 4,4’-[(1,1’-biphenyl)-4,4’-diyldi-(1E)-2,1-ethenediyl]bis(1-methylpyridinium) diiodide (BP).
The wavelengths of absorption maxima of 1,3-PY, 1,8-PY, BP and NPJ were 451, 470, 384 and 338 nm, while the fluorescence maxima were 630, 618, 527 and 455 nm, respectively. As we expected, red, green and blue fluorescent probes were obtained. The maximum value of the two-photon absorption cross-section of these compounds was 952, 1664, 250 and 42 GM (1 GM=10^-50 cm⁴ s photon^-1 molecule^-1), respectively. These values were sufficient for applying to a two-photon microscope. MMP sensitivity of these compounds were confirmed by a multi-photon fluorescence microscope. Before decreasing MMP, the probes were localized at mitochondria of HEK293 cells. After decreasing MMP by addition of carbonyl cyanide m-chlorophenylhydrazone (CCCP), these compounds transferred from mitochondria to the nucleus. When recovering the MMP by washing out the CCCP, the probes were transferred from nuclei to mitochondria.

Reference
[1] H. Moritomo, et al. Cell. Struct. Funct., 2014, 39, 125-133

Intensive researches are currently conducted on the miniaturization of photonic devices and on the combination of photonics and electronics to decrease the power consumption and to create novel functionalities for a myriad of applications including datacom, telecom, sensing and quantum optics, to name few. In this context, functional oxides have emerged as promising materials to expand the functionalities of current photonic circuit thanks to their wide range of properties including multiferroicity, piezoelectricity and optical nonlinearities. Among these materials, Yttria-Stabilized Zirconia (YSZ) is well known as a buffer layer for oxide-based thin films heterostructures, extensively used for LiNbO₃, PbTiO₃, Pb(Zr,Ti)O₃ and YBa₂Cu₃O₇ integration on silicon. YSZ is also well known for its extraordinary thermal and chemical stability, as well as its hardness and mechanical durability. An increasing quantity of applications uses this material for the combination of its excellent mechanical and optical properties, such as its high refractive index, large optical band gap and transparency from the ultraviolet (UV) to the mid-infrared (mIR). However, no work has been performed for its use in integrated optics, yet.
This work has been mainly focus on the integration of YSZ functional oxide on sapphire substrates. Due to the refractive index contrast between sapphire (1.75) and YSZ (2.12) at a wavelength of 1300 nm, optical YSZ waveguides can be designed with good mode confinement. The study and optimization of growth parameters using Pulsed-Laser Deposition (PLD) technique allowed achieving high quality YSZ films on sapphire (0001) with remarkably sharp X-Ray diffraction rocking curve peaks in 10^-3 degrees range for two different out-of-plane orientations. We have demonstrated that a thermal annealing of the sapphire substrate before the growth, played an important role in thin film orientation. Passive photonic structures including grating couplers, single-mode waveguides and resonators have been then designed and fabricated using electronic lithography and Ion-Beam Etching (IBE) techniques. Preliminary optical characterization results of integrated structures will be presented such as propagation losses in YSZ waveguides as low as 2 dB/cm, ring and disk resonators with Q factors up to 12000, narrow-band Bragg filters with 16.2 dB extinction ratio for 1.5 nm bandwidth. Finally, large third-order nonlinear susceptibility has been estimated to be three times larger than in silicon with the absence of two photon absorption at telecom wavelengths. Our results demonstrated the fabrication of passive devices and basic building blocks, which combined with the outstanding nonlinear properties, provide a promising robust platform for functional oxides-based nonlinear on-chip applications. Furthermore, the first integration of YSZ on silicon photonics platform will also be reported.

As a class of semiconductors, transition metal dichalcogenides (TMDCs) have the formula MX2, where M stands for a transition metal (i.e., Mo, W, Ti, Nb, etc.) and X stands for a chalcogen (i.e., S, Se or Te). TMDCs show graphene-like layered structure. Strong covalent bonds in layers and weak van der Waals interaction between layers allow TMDCs to form a robust 2D nanostructure. In a TMDC monolayer, the single transition metal layer is sandwiched between the two chalcogen layers. Owing to the specific 2D confinement of electron motion and the absence of interlayer coupling perturbation, 2D layered TMDCs show unique photonics-related physical properties, e.g.,
1) Sizable and layer-dependent bandgap, typically in the 1-2 eV range;
2) Indirect-to-direct bandgap transition as the decreasing of the number of monolayer;
3) Fairly good photoluminescence and electroluminescence properties;
4) Remarkable excitonic effects, i.e., high binding energy, large oscillator strength and long lifetime.
In combination of the ultrafast carrier dynamics and molecular-scale thickness, the prominent properties manifest the 2D TMDCs a huge potential in the development of photonic devices and components with high performance and unique functions.

In this work, the saturation of two-photon absorption (TPA) in four types of layered TMDCs (MoS2, WS2, MoSe2, WSe2) was systemically studied both experimentally and theoretically. It was demonstrated that the TPA coefficient is decreased when either the incident pulse intensity or the thickness of the TMDCs nanofilm is increased, while TPA saturation intensity has the opposite performance, under the excitation of 1.2 eV photons with pulse width of 350 fs. A three-level excitonic dynamics simulation indicates that the fast relaxation of the excitonic dark states, the exciton-exciton annihilation and the depletion of electrons in the ground state contribute significantly to TPA saturation in TMDC nanofilms. Large third order nonlinear optical response makes these layered 2D semiconductors strong candidate materials for optical modulation and other photonic applications.

Z-scan is one of the common measurement method to characterize the third-order optical nonlinearity of materials and has ability to determine both real and imaginary components of the nonlinearity with a single scan. With the optical intensity and its distribution in time and space properly measured, each component of the nonlinearity can be determined without measuring the reference material whose nonlinearity was already known (i.e. absolute method). However, precise characterizations of the incident optical intensity and its distributions are laborious tasks, especially for femtosecond pulses for a wide range of the wavelengths. Thus, to save measurement time for reproducible results, the relative method by comparing unknown sample with a known reference material is still effective for the Z-scan method. For two-photon absorption (TPA), i.e. the imaginary component, a number of dyes have been reported as reference materials.[1,2] However, their TPA spectra have been reported from 550 nm or longer. Virtually no reliable reference material was reported for the shorter wavelength region than that although the TPA property of the blue-green region is interesting because of the potential for high resolution in applications such as 3D photolithography.
As standard material, strong and broad spectrum of TPA is desireble around the wavelength of interest, in addition to stability and availability of the materials. Thus, here we propose to use GaN as standard for the TPA measurement for the blue-green excitation. Semiconductor is known to have structureless, broad TPA spectrum well descried by the band theory at the excitation photon energies higher than the half of the band gap energy (E_g).[3] As E_g=3.4 eV for GaN, its TPA spectrum covers all wavelengths of the blue-green region. Reported TPA coefficients (β) of GaN were found to be scattered and different from a report to another. We measured TPA spectrum of single crystal of GaN substrate (thickness = 270 μm) by the femtosecond Z-scan method from 400 nm to 700 nm. The obtained magnitude of the TPA coefficient was found to be close to that reported by van Stryland et al.[4] although it is smaller than other reports. The TPA spectral measurement of a short wavelength TPA dye, bis(o-methylstyrylbenzene), based on the GaN as standard gave consistent result of the previous report. In conclusion, GaN can be use as convenient standard material for the Z-scan measurement in the blue-green region.
[1] C. Xu and W. W. Webb, J. Opt. Soc. Am. B 1996, 13, 481.
[2] N. S. Makarov, et al., Opt. Expr. 2008, 16, 4029.
[3] M. Sheik-Bahae, et al., IEEE J. Quantum Electron. 1991, 27, 1296.
[4] E. W. van Stryland et al., Opt. Lett. 1985, 10, 490.

Silicon photonics is a large volume and large scale integration platform for applications from long-haul optical telecommunications to intra-chip interconnects. Extension to the mid-IR wavelength range is now largely investigated as it could potentially lead to key advances in several disciplines including molecular sensing, early medical diagnosis or secure communications, among others. Among the materials used in silicon photonics, germanium (Ge) is particularly compelling as it has a broad transparency window up to 15 µm and a much higher third-order nonlinear coefficient than silicon which is promising for the demonstration of efficient non-linear optics based active devices. In that regard, we have demonstrated that graded-index Ge-rich SiGe alloys, with Ge concentrations larger than 70%, can be an ideal material choice to develop such mid IR photonic platform since they permit an accurate control of the optical properties by fine tuning the Ge concentration along the growth direction. Nevertheless, the material nonlinearities are very sensitive to any modification of the energy bands, so Si_1-xGe_x alloys are particularly interesting for nonlinear device engineering. We report on the first third order nonlinear experimental characterization of Ge-rich Si_1-xGe_x waveguides, with Ge concentrations x ranging from 0.7 to 0.9, where previous numerical modeling disagree as there is a change between direct-like and indirect-like bandgap approaches. To this end, we have developed a novel non-destructive single beam technique that includes reliable measurement of the injected power in the waveguide, called bidirectional top-hat D-Scan. The D-Scan method, a temporal analogue of the Z-Scan technique, consists in measuring the output spectral broadening of transmitted pulses by varying the dispersion coefficient φ⁽²⁾ introduced to the input pulses. For Z-Scan, the spatial deformation of a laser beam is analyzed while the nonlinear bulk-material is displaced through the focused spot and for increasing power. The optical (or thermal) Kerr effect induces an intensity dependent spatial variation of the refractive index of the material, playing the role of a spatial lens that modifies the beam propagation. Similarly, the D-Scan consists in recording the spectral broadening behaviors experienced by linearly chirped pulses with increasing incident power in order to characterize the temporal Kerr lens introduced by the nonlinear material. There is an analogy between Z-Scan and D-Scan as they both rely on the interplay between linear terms, respectively diffraction and dispersion, and nonlinear Kerr effect. The characterization performed at 1580 nm is compared with theoretical models and a discussion about the prediction of the nonlinear properties in the mid-IR is introduced. These results will provide helpful insights to assist the design of nonlinear integrated optical based devices in both the near- and mid-IR wavelength ranges.

Chalcogenide materials exhibit a unique portfolio of properties which has led to their wide use for non-volatile memory applications such as optical data storage or more recently Phase-Change Random Access Memory [1]. The huge electrical conductivity nonlinearities observed in some chalcogenide glasses (CGs) under electrical field application led the latter as being considered as promising materials to be used as innovative selector element in 3D back-end-of-line memory arrays [2]. Besides, as CGs exhibit a high transparency window in the infrared range and large optical nonlinearities, they offer also opportunities for elaboration of innovative mid-infrared (MIR) components such as MIR supercontinuum (SC) laser sources, optical sensors, IR micro-lens arrays and all-optical integrated circuits [3]. Up to now, the state-of-the-art MIR SC sources have been mainly demonstrated with CGs containing Arsenic such as As₂S₃, As₂Se₃ fibers or GeAsSe rib waveguide [4, 5]. However, the REACH European recommendation – which calls for the progressive substitution of the most dangerous chemicals – and the World Health Organization have both identified Arsenic as one of the ten most harmful chemicals for human health. In that context, the linear and nonlinear optical properties of As-free amorphous chalcogenide thin films are investigated here with a particular care on their compatibility with CMOS technologies for future realization of on-chip MIR components. By means of magnetron co-sputtering of chalcogenide compounds targets in an industrial 200 mm deposition tool, we show here how to tailor GeSb_wS_xSe_yTe_z chalcogenide amorphous thin films aiming to find the best compromise between good glass stability of S-based chalcogenide and the huge nonlinear refractive index of Te-based compositions. Modeling of spectroscopic ellipsometry measurements allowed to determine complex components of linear refractive index and to approximate the optical band gap energy of the different amorphous materials. Optical transmission losses and the real part of refractive index at 1548 nm were obtained using M-line technique. FTIR and Raman spectroscopy in the 100-500 cm^-1 range allowed to get information on the amorphous structure of the films. Advanced optical characterizations of nonlinearities in rib and ridge waveguides with a tailored group velocity dispersion were performed and compared to the nonlinear refractive Kerr index n₂ of each CGs calculated by means of analytical and empirical models. Finally, the origin of the enhanced optical nonlinearities observed in some of the amorphous GeSb_wS_xSe_yTe_z chalcogenide compositions will be probed by means of ab initio simulations.

References
[1] P. Noé et al., Topical review in Sem. Sc. And Tech. (2017).
[2] A. Verdy et al., IEEE 9th Int. Mem. Work. , IMW (2017).
[3] D. Tsiulyanu et al., Sensor. Actuat. B-Chem. 223, 95-100 (2016).
[4] O. Mouawad et al., Opt. Lett. 39, 2684 (2014).
[5] Y. Tu et al., Laser & Phot. Rev. 8, 792 (2014).

Perovskite oxides have attracted much attention owing to their excellent nonlinear optical properties. Moreover, these properties can be strongly affected and regulated by the doping strategy. In this work, a novel solvothermal method was designed to synthesize Er³⁺ doped single crystal KNbO₃ nanosheets. XRD analysis indicated that, with increasing content of Er, Er³⁺ could firstly substitute into the B site (Nb⁵⁺) and then A site (K⁺) in the ABO₃ perovskite structure. The orthorhombic structure of the pure KNbO₃ and Er³⁺ doped KNbO₃ was confirmed according to the Raman spectra. The as-obtained products were nanosheets with a thickness of about 70 nm and diameter of 3 μm in terms of the SEM observations. The single crystal structure of as-obtained products was confirmed by HRTEM. Furthermore, the growth mechanism of Er³⁺ doped KNbO₃ nanosheets was rationally elaborated and proposed. The as-obtained Er³⁺ doped KNbO₃ nanosheets exhibited excellent up-conversion (UC) photoluminescence (PL) properties with a strong green emission and a weak red emission under the excitation of a laser with a wavelength of 980 nm. The most outstanding UC PL property was endowed to the KNbO₃ nanosheets by the doping of 0.4 mol% Er³⁺. It was confirmed that two photons contributed to this UC PL behaviors based on the spectra exited by different laser power. As a conclusion, the as-synthesized perovskite KNbO₃ nanosheets with excellent UC PL property could be the potential alternative materials for the development of nonlinear optical devices.

Two compounds in the Mg-Si-As system have been synthesized using solid state methods and structurally characterized. MgSiAs₂ has been reported theoretically multiple times as a part of the well-known II-IV-V₂ family, but has never been successfully synthesized until now. Similar to other II-IV-V₂ compounds, it crystallizes with a chalcopyrite structure in the space group I-42d (No. 122). Mg₃Si₆As₈ is a cubic compound with a new structure type that crystallizes in the space group P4₃32 (No. 207). Both compounds crystallize in chiral space groups though only MgSiAs₂ is expected to exhibit non-linear optical activity. Mg₃Si₆As₈ belongs to the single non-centrosymmetric crystal class that falls outside of the group of non-linear optically active compounds. According to band structure calculations, both compounds possess pseudo-direct band gaps of 1.03 eV for MgSiAs₂ and 1.55 eV for Mg₃Si₆As₈. The additional property of high chemical resistance to oxidizers and acids make them contenders for optoelectronic materials.

Frequency combs are spectrally broad light sources where each line of the comb is incrementally spaced from adjacent lines. Generation of a comb in silica whispering gallery mode (WGM) optical resonant cavities relies on nonlinear optical effects, most notably the Kerr nonlinearity, which allows for four-wave mixing (FWM) to generate new sideband photons. Because silica has a low Kerr coefficient, considerable input power is necessary in order to obtain a broad spanning comb. An important consideration for obtaining a wide comb is the material dispersion. The overlap of the whispering gallery mode with the comb modes dictated by FWM is crucial for efficient comb generation. The material dispersion plays a role in changing the spacing of whispering gallery modes, limiting the wavelength range before mismatch of the comb modes and resonator modes causes loss to exceed gain. One material that has shown promise in frequency comb generation due to its low dispersion is calcium fluoride (CaF₂). Single-crystalline CaF₂ WGM resonator-based frequency combs have been demonstrated, but the difficulty in device fabrication limits potential applications.
In this study, we have shown that coating the surface of a silica WGM resonator with a low dispersion material effectively reduces the effective overall dispersion of the material, allowing for higher spanning frequency combs to be generated. We first synthesize CaF₂ nanocrystals using a simple chemical co-precipitation method. We then coat them onto the surface of silica whispering gallery mode resonant cavities and show that frequency comb span is enhanced compared to uncoated resonant cavities. The normalized thresholds of the FWM processes in coated devices are unchanged compared to uncoated devices, and are on the order of hundreds of microwatts. At input powers of ~4 mW, the comb span in a CaF₂-coated microsphere reaches 300 nm, while a comb of just 30 nm is present in an uncoated silica microsphere at the same power levels. The largest spanning comb on a coated microsphere observed is over 400 nm wide at an input power of ~6 mW.

Optoelectronic applications such as solar cells and light emitting diodes would benefit from glass substrates with both high transparency and high haze to improve power conversion and extraction efficiencies, respectively. In this letter, we demonstrate a new nanograss fused silica glass that displays ultrahigh transparency and ultrahigh haze (both over 95% at wavelength 550 nm). The nanograss may be fabricated through a scalable maskless reactive ion etching (RIE) process in fused silica where the height may be controlled through the etch time. We demonstrate that shorter nanograss (< 2.5 um height) improves the antireflection properties of the glass, and that longer grass tends to increase haze monotonically. Ultrahigh haze over 99% may also be achieved with longer nanograss (> 6 um), though the transmission decreases slightly to less than 92% at such ultrahigh haze. Finally, we demonstrate that various fluids with a similar index of refraction as the glass may be utilized to permeate the superhydrophilic hazy nanograss, such that it resembles a uniform flat glass substrate with little haze. Upon removal of this fluid, the nanograss recovers its original hazy state. This haze switchability may have application in various smart glass.

Potassium titanyl phosphate (KTP) is a widely used frequency-doubling crystal for realizing high-efficiency laser sources from near infrared to visible wavelengths through phase matching (PM) and quasi-phase matching (QPM) configurations ^[1]. Optical waveguides are basic photonic components which confine the light propagation in very small volumes with dimensions of micron or sub-micrometer scales. Swift heavy-ion irradiation (with energies not less than 1 MeV/amu) has emerged to be a very powerful technique for waveguide fabrication in various materials ^[2].
In this work, the KTP sample with size of 6.0×3.1×1.7mm³ was cut along the direction optimal for second harmonic generation (SHG) of 532 nm from the fundamental wavelength of 1064 nm (θ = 90°, φ = 23.5°) under the Type II phase matching configuration, i.e., e^ω + o^ω → e^2ω. Firstly, the sample was irradiated by 17 MeV O⁵⁺ ions at fluence of 1.5×10¹⁵ ions/cm² on one surface (6.0×1.7 mm²) through the 3MV tandem accelerator, forming a planar waveguide layer with thickness of ~8 μm beneath the sample surface. Afterwards, a diamond rotating blade on top of the irradiated planar waveguide surface moving in the direction parallel to the blade was used to construct air grooves with depth of 12 μm. The rotating speed and cutting velocity were set to 20.000 rpm and 0.2 mm/s, respectively. As a result, a ridge waveguide with 20-μm width and 8-μm depth was formed by controlling the distance of two adjacent grooves.
An end-face coupling system was arranged to realize SHG through KTP ridge waveguide. We utilized 1064 nm pulsed wave laser as the fundamental light source. The pulsed laser beam (80 µJ pulses with duration of 11.05 ns at a repetition of 5 kHz) was coupled into the KTP waveguide using a convex lens. The generated SH laser beam of 532 nm from the waveguide exit facet was captured by using a 20× microscope objective and characterized by a CCD camera, a spectrometer, and a power meter. We can find that the fundamental wave at 1064 nm was with TE polarization and the generated wave at 532 nm exhibited TM polarization, which was in agreement with the bulk Type II (e^ω+o^ω→e^2ω) 1064 nm→532 nm of KTP crystal. The maximum SH peak power obtained was 17.6 W, when the 1064 nm pump power was 73 W, resulting in a conversion efficiency of η_pulsed=24.1%. Compared with the reported value of Nd:GdCOB ridge waveguide ^[3], the conversion efficiency of KTP ridge waveguide is significantly higher. These results imply potential applications for O⁵⁺ ion irradiated ridge KTP waveguide to be an efficient frequency-doubling device.

Reference:
[1] X. Deng, et al, Opt. Express 21(22), 25907-25911 (2013).
[2] G. C. Righini, et al, Opt. Eng. 53(7), 071819 (2014).
[3] Y. Jia, et al, Opt. Mater. 34(11), 1913-1916 (2012).

Due to the properties of high sensitivity and room-temperature operation, Terahertz detectors based on nano-structured materials, such as semiconductor 1-D nanowires [1] or 2-D nanomaterials [2], have recently been considered as the next generation alternatives to the Terahertz technologies. In order to extend their practical utilizations, our present work reports the fabrication of Terahertz detector arrays based on multiple orderly aligned GaAs nanowires field-effect transistors (FET). The GaAs nanowires were synthesized by chemical vapor deposition growth and then transferred onto the substrate by using a contact printing method [3]. Field-effect transistors were then constructed through photolithographic template-assisted processes. For the terahertz detection, devices were tested under different terahertz sources with various frequencies. Importantly, our GaAs nanowire FET detectors exhibit outstanding photoresponse without any sacrifice of the noise level. Also, as compared to bulk GaAs materials [4] and single nanowire detectors [5], our devices demonstrate the better and more stable properties in terahertz detection.

References

[1] L. Romeo, D. Coquillat, E. Husanu, D. Ercolani, A. Tredicucci, F. Beltram, L. Sorba, W. Knap, and M. S. Vitiello, “Terahertz photodetectors based on tapered semiconductor nanowires Terahertz photodetectors based on tapered semiconductor nanowires,” vol. 231112, pp. 1–5, 2014.
[2] D. Spirito, D. Coquillat, S. L. De Bonis, A. Lombardo, M. Bruna, A. C. Ferrari, V. Pellegrini, A. Tredicucci, W. Knap, and M. S. Vitiello, “High performance bilayer-graphene Terahertz detectors,” Appl. Phys. Lett., vol. 104, no. 6, 2014.
[3] N. Han, Z. Yang, F. Wang, S. Yip, G. Dong, X. Liang, T. Hung, Y. Chen, and J. C. Ho, “Modulating the morphology and electrical properties of GaAs nanowires via catalyst stabilization by oxygen,” ACS Appl. Mater. Interfaces, vol. 7, no. 9, pp. 5591–5597, 2015.
[4] A. Lisauskas, W. von Spiegel, S. Boubanga-Tombet, A. El Fatimy, D. Coquillat, F. Teppe, N. Dyakonova, W. Knap, and H. G. Roskos, “Terahertz imaging with GaAs field-effect transistors,” Electron. Lett., vol. 44, no. 6, pp. 6–7, 2006.
[5] M. S. Vitiello, D. Coquillat, L. Viti, D. Ercolani, F. Teppe, A. Pitanti, F. Beltram, L. Sorba, W. Knap, and A. Tredicucci, “Room-temperature terahertz detectors based on semiconductor nanowire field-effect transistors,” Nano Lett., vol. 12, no. 1, pp. 96–101, 2012.

Hybrid metal/organic subwavelength-structured media can exhibit highly unusual photonic properties. In particular, such media permit a fine tuning of the interactions between constituents, giving birth to new excitation modes, such as plasmon-mediated collective oscillations or strongly-coupled hybrid exciton-polariton waves. This offers original coupling schemes between light and matter of high interest in particular for light emission, light amplification or nonlinear optical processes.
Self-organized molecular systems permit a soft « bottom-up » fabrication of self-replicated structures with hierarchical organizations from the molecular scale up to the macroscopic scale. Liquid crystals embedding various π-conjugated mesogens are outstanding examples combining multi-scale organized structures with fluidity and self-healing. Perylene bisimide dye (PTCDI) is the best archetype as it is amenable to form a large variety of structures and offers a unique combination of (i) one-dimensional nature, (ii) ability for pi-stacking and (iii) intense luminescence even in condensed phases.
We experimentally demonstrated the strong coupling between dense H-aggregate-like PTCDI-based self-organized lamellar phases and surface plasmon polaritons (SPP) propagating at the interface with the gold substrate.[1] Experimental wavevector-resolved reflectance spectra evidence the formation of hybrid states that display a clear anticrossing, attesting the strong coupling regime with a Rabi splitting energy of ~100 meV at room temperature. The strength of the observed strong coupling regime results from the high degree of organization of the dense layers of self-assembled molecules at the nanoscale which results in the concentration of the oscillator strength in a charge-transfer Frenkel exciton, with a dipole moment parallel to the direction of maximum SPP electric field. We compare our results to numerical simulations of a matrix model and reach a good qualitative agreement with the experimental findings.
The effects of exciton-SPP coupling in emission processes was also investigated. Most of the exciton radiative relaxation is funneled into the SPP. At large pumping rates exciton-exciton recombination is the main limiting factor of the optical – or plasmonic – gain in this hybrid architecture.
[1] Strong Coupling between Self-Assembled Molecules and Surface Plasmon Polaritons J. Bigeon, S. Le Liepvre, S. Vassant, N. Belabas, N. Bardou, C. Minot, A. Yacomotti, A. Levenson, F. Charra, S. Barbay, Journal of Physical Chemistry Letters 2017.

All-optical signal processing / switching applications require materials with large third-order nonlinearities and low nonlinear optical losses. At the molecular level, it means maximizing the the real part of the third-order polarizability (γ) and minimizing its imaginary part. Practical applications require the figure of merit, Re(γ)/Im(γ), to be larger than 4p. Cyanine dyes such as the bis(seleno-pyrylium) heptamethine dye can exhibit a real part of γ that is exceptionally large throughout the wavelength range used for telecommunications, and an imaginary part of γ, a measure of nonlinear loss, that can be two orders-of-magnitude smaller [1]; this combination is critical to enable low-power, high-contrast optical switching. In this presentation, we will first provide a quantum-chemical description of the real and imaginary nonlinear optical properties of relevant polymethine-type molecules. We will then discuss the strategies that need to be followed in order to translate the properties of the isolated molecules into the solid state. We will describe some recent examples based on zwitterionic cyanines [2] and Pd(PPh₃)₂Cl-substituted cyanines.

References

[1] J.M. Hales et al., Science 327, 1485 (2010).
[2] S. Shiring et al., J. Phys. Chem. C 121, 14166 (2017).
[3] I. Davydenko et al., submitted (2017).

In order to develop new chromophores for nonlinear optics applications our group has synthesized and characterized numerous platinum acetylides. Recently, we synthesized a model series of nominally centrosymmetric chromophores trans-Pt(PBu₃)₂(CC-Phenyl-X)₂, where X = diphenylamino(DPA), NH₂, OCH₃, t-Bu, CH₃, H, F, benzothiazole(BTH), CF₃, CN and NO₂. We collected linear- and two-photon absorption spectra and performed DFT and TDDFT calculations on the ground- and excited state properties of these compounds. Two-photon absorption spectra obtained from these compounds allowed estimation of the change of permanent electric dipole moment upon vertical excitation from ground state to S₁ state. The corresponding calculated values calculated from nonplanar molecular conformation showed good agreement with the experimental data indicating that the two-photon spectra resulted from nonplanar ground state conformations. We also measured triplet state spectra, including phosphorescence and time-resolved transient absorption spectra. Through DFT and TDDFT calculations we calculated triplet state energy, geometry and descriptors for the location of the triplet exciton on the molecule. The calculated triplet state energies agreed well with measured phosphorescence emission and excitation spectra. The triplet state lifetime was found to be a strong function of the average distance between the triplet exciton and the central platinum atom, showing the importance of spin-orbit coupling effects of the central platinum atom. We found substituent effects in the vibronic envelope of the phosphorescence spectra. The intensity of the Franck-Condon vibronic bands correlated with triplet state bond length changes in the aromatic ring, showing the DFT calculations described the displacement of the triplet state potential energy surface relative to the ground state. The intensity trends suggested the triplet exciton of compounds with electron-donating ligands resided primarily on the ligand's benzene ring. In contrast, the triplet exciton of compounds with electron-withdrawing ligands formed a charge-separated state, with one electron on the benzene ring, while the other electron residing on the variable portion(X) of the ligand. These two spectroscopic studies show success in using DFT methods to calculate the two-photon and triplet state spectra of a series of model platinum acetylides with systematic variation in electronic properties of the ligand. These methods are now being applied to more complex systems.

Iridium(III) complexes are promising opto-electronic materials due to their intriguing electrochemical and photophysical properties. Because of the large spin-orbit coupling constant of the Ir(III) ion, Ir(III) complexes typically possess high triplet quantum yields and thus intense phosphorescence at room temperature. This makes Ir(III) complexes good candidates for applications in organic light-emitting diodes (OLEDs), luminescent biological reagents, molecular sensors, and light-emitting electrochemical cells (LEECs). In addition, many Ir(III) complexes exhibited broad and strong excited-state absorption in the visible to the near-IR region, which makes it possible to utilize these complexes for optical limiting application based on their reverse saturable absorption. For practical application, optical limiting materials are required to exhibit broadband spectral and temporal responses. For this purpose, we designed and synthesized two series of Ir(III) complexes with different π-conjugated cyclometalating C^N ligands. We found that the natures of the lowest singlet and triplet excited states were switched to the C^N ligand based ^1,3CT (charge transfer) /^1,3π,π* transitions when the π-conjugation of the core C^N ligand is larger than that of the core diimine (N^N) ligand. The increased π-conjugation of the C^N ligand red-shifted the spin-forbidden ³CT transitions in the UV-vis absorption spectra of the Ir(III) complexes, but reduced the intensity of the triplet excited-state absorption and the triplet lifetimes. Consequently, the optical limiting performance at 532 nm for ns laser pulses is reduced for the complexes with more extended π-conjugated C^N ligand. However, these complexes potentially could be used as broadband optical limiting materials in the near-IR region.

The family of hydrogen-bonded molecular crystals can be considered as a promising result of crystal engineering of novel materials for nonlinear optics (NLO). These crystalline materials (i.e. salts and co-crystals) are based on properly arranged organic molecules with highly delocalized π-electron systems acting as carriers of NLO properties. The energy of formed hydrogen bonds counteracts the natural tendencies of the organic molecules (ions) to form centrosymmetric pairs. In addition, the formed hydrogen-bonded structures frequently gain advantageous chemical and physical properties.
Several molecular crystals exhibit exceptional NLO properties based on χ⁽²⁾- and χ⁽³⁾- nonlinearities. Examples of these properties that can be employed for technical applications include harmonic generation (e.g. second harmonic generation – SHG), sum- and difference-generation, intensity dependence of the complex refractive index, light-by-light scattering, and stimulated light scattering. A very recent application of hydrogen-bonded salts of organic molecules (e.g. guanylurea hydrogen phosphite [1]) is based on stimulated Raman scattering (SRS). This χ⁽³⁾ NLO phenomenon is used for the development of compact and efficient frequency converters of the one-micron laser emission based on neodymium or ytterbium lasants. [2]
The most of physical properties (including the optical ones) of crystalline materials are intimately related to the symmetry of their crystal structures, and the eventual phase transitions are frequently accompanied by the changes of their physical properties. In addition to the ordinary phase transitions (e.g. first and second order transitions), other rarely observed effects, such as glass transitions and isostructural phase transitions [3], can occur in the family of molecular hydrogen-bonded crystals.
This contribution deals with preparation and characterization of molecular crystals based on inorganic and organic salts of aminopyrimidines. Particular attention will be focused on 2,4,6-triaminopyrimidinium phosphates and dicarboxylates (novel prospective SHG materials) and their low temperature phase transformations. The detailed explanation of the mechanism of the observed transitions associated with low-temperature proton transfer between cations and anions in the crystal structure is based on combination of experimental (i.e. vibrational spectroscopy, XRD and calorimetry) and theoretical (solid state quantum-chemical calculations) methods.
[1] A.A. Kaminskii, P. Becker, H. Ree, O. Lux, A. Kaltenbach, H.J. Eichler, A. Shirakawa, H. Yoneda, I. Nemec, M. Fridrichova, L. Bohaty, Physica Status Solidi B 250(2013) 1837.
[2] A.A. Kaminskii, Laser and Photonics Reviews 1 (2007) 93.
[3] D. Chernyshov, M. Hostettler, K.W. Tornroos, H.B. Burgi, Angewandte Chemie- International Edition 42 (2003) 3825.

Financial support from the CUCAM project (project No. CZ.02.1.01/0.0/0.0/15_003/0000417) is gratefully acknowledged.

Metal nanoclusters are now excellent candidates for applications in optical limiting and other nonlinear optical processes. The use of nanoclusters as building blocks for larger structures has found new interest due to the unique optical and electronic properties. In this presentation it will be shown that one can synthesize chromophore-Au₂₅ nanocluster oligomers and investigate their linear and nonlinear optical properties. The chromophore-Au₂₅ nanocluster oligomers were separated by polyacrylamide gel electrophoresis and characterized by matrix assisted laser desorption ionization mass spectrometry and scanning transmission electron microscopy imaging. The linear optical properties of the systems were investigated by steady state UV-vis absorption and fluorescence spectroscopy. The chromophore-Au₂₅ nanocluster oligomers showed increased oscillator strength and transition dipole moment compared to single Au₂₅ nanoclusters. Energy transfer from the chromophore 4,4’-thiodibenzenethiol (TBT) to the metal cluster was observed in the chromophore-Au₂₅ nanocluster dimer system. The excited state and fluorescence dynamics were investigated by transient absorption spectroscopy, time-resolved fluorescence up-conversion and time-correlated single photon counting. The chromophore-Au₂₅ nanocluster oligomers have a long-lived surface state, due to the contribution of energy transfer by two nanocluster cores. The two-photon absorption cross sections of the chromophore-Au₂₅ nanocluster oligomers showed an increasing enhancement trend with increasing oligomer length. An enhancement factor of up to 68 times was found compared to single Au₂₅ nanoclusters. These Chromophore-Au-Chromophore oligomers are excellent candidates for applications in nonlinear optics.

A synthetic entry to novel dyes based on the dipyrrolonaphthyridinedione core was developed via the Heck reaction. These weakly fluorescent compounds bearing double bond linkages between the core and the peripheral units absorbed strongly in the far-red/NIR region and possessed large values of two-photon absorption cross-sections (up to 5180 GM). Additionally, analogous dyes bearing triple bond linkages were also efficient TPA materials with relatively large two-photon absorption cross-sections (up to 2840 GM) as well as two-photon brightness (up to 1450 GM). The centrosymmetric nature of both of these families of dyes is responsible for the location of the maxima of two-photon absorption at much higher energy than the ones corresponding to the double wavelength of the lowest-energy one-photon absorption. Theoretical calculations clarified that the enhancement of the TPA by the peripheral substitutions arose through different mechanisms depending on either the electron-donating or electron-withdrawing ability of given substituent to the ambipolar core. Change in electron distribution of HOMO and HOMO-1 by the push-pull effect was found to govern the strength of the lowest-energy TPA-allowed transition. Importantly, compounds from both series possessed a beneficial ratio of σ₂/MW (1.6-9.8 GM/g).

Typically, various nonlinear optical effects are investigated in materials in the form of inorganic or polymer glasses, liquid solutions, single crystals or nanoparticles, the notable exception being powder SHG studies. On the other hand, there exist classes of materials such as metal-organic frameworks (MOFs) that are mostly available in the form of crystalline powders. It appears that their NLO properties may also be of interest, for example some MOFs are very efficient multiphoton absorbers showing also quite high values of the luminescence quantum yields. Exploitation of multiphoton induced luminescence of MOFs is an example of the concept of application of NLO materials in the form of "NLO pigments": materials that must be used in the solid state form but the intended application allows for the use of microcrystalline powders.

We have been studying the nonlinear optical (NLO) properties of metal alkynyl complexes, in studies
ranging from small complexes to dendrimers. While our early focus was on molecular quadratic and cubic NLO
coefficients, materials with multi-photon absorption (MPA) properties have become of increasing interest for
applications such as microfabrication, bioimaging, photodynamic therapy, and frequency upconversion lasing. We
have recently been exploring the MPA behaviour of rod-like, star, and dendritic OPVs and oligo(phenylenethynylene)s
(OPEs) bearing bis(diphosphine)ruthenium moieties; the results of these studies will be
presented.

Electro-optic (EO) polymers have attracted attention for applying to ultrafast modulators for long-haul optical communication as well as short range optical interconnection, electromagnetic wave sensors, terahertz (THz) wave generators and detectors, etc. Large EO coefficient and modulation bandwidth are the advantageous features of EO polymers and are used to evaluate the performance of EO polymers. Although absorption coefficient (α) and EO coefficient (r) are known as in a trade-off relation for EO polymers, the absorption has been assumed to be small enough to be neglected. That is because the absorption at NIR wavelength region is less than the measurable limit in the thin film absorption measurement. However, propagation losses of waveguides made with recently developed EO polymers having very large EO activities are more than a few dB/cm and it is suspected that the polymers have significant absorption. We measured the absorption at NIR wavelength region by making thick films of EO polymers and revealed that the magnitude of absorption is small but definitely not negligible. Therefore, we re-evaluate EO polymers by the figure-of-merit for modulators including absorption (FOM_MOD = n³r/α) and determine the molecular structure of EO polymers optimized for each of the specific device applications such as ultrafast modulators for telecommunication, Si hybrid modulators for data communication, phase modulator array for LiDAR.
EO polymers also show the large FOM for THz wave generation (FOM_THz = n_opt⁶r²/16n_THz), long coherence length and relatively small absorption coefficients in a broadband THz frequency region, compared to the other inorganic and organic nonlinear optical materials such as lithium niobate (LiNbO₃), zinc telluride (ZnTe), and DAST. We evaluated refractive index and absorption coefficient of our EO polymers at THz frequency region and confirmed these advantages. In order to make efficient THz generation and detection devices, EO polymer waveguides have to be formed with the claddings of THz-wave low-loss materials. However, the conventional fabrication process of the EO polymer waveguide devices requires the conductive polymer-based claddings as well as the electrodes for poling the EO chromophores, and the conductive cladding materials or the electrodes strongly absorb the THz waves. We developed a novel fabrication process that the poling procedure was made before forming the waveguide. Then we successfully fabricated the EO polymer waveguides on the substrate of a cyclic olefin polymer (COP), whose absorption coefficients are very small in the THz region, with complete elimination of the poling electrodes. Such novel waveguides can be made only with the recently developed thermally stable EO polymers whose EO activity is kept unchanged even after experiencing high temperature around 100 deg C during the waveguide fabrication processes.

Integrating rare-earth iron garnets with silicon structures for non-reciprocal photonic devices is rife with challenges specific to material processing. In this work, we present the integration of a seed-layer free garnet for TE mode waveguide along with enhancement of high-temperature annealing processes. Current research in integration has been limited to Si-garnet wafer bonding or vacuum deposition (sputtering or pulsed laser deposition) of garnet top claddings followed by annealing. Some of these techniques are difficult to scale and all pose limitations to TE mode waveguides which require significant sidewall coating. Here, we introduce the use of cerium-doped terbium iron garnet, which can be sputtered on integrated photonic structures, have acceptable sidewall coating, and do not require seed layers for garnet formation. These terbium-based garnets have Faraday rotations of -2600°/cm at 1550nm. This value is similar or higher than the effective Faraday rotation of Ce-doped yttrium iron garnet grown by many of the vacuum deposition techniques. It is also important to minimize the thermal processing budget, an important industry metric, without sacrificing the properties of garnet. This work focusses on the application of a two-step anneal and optimizing the quenching rate, resulting in better crystallization and by extension superior material properties. Our results show that the two-step anneal can reduce the peak temperature for crystallization and increase the Faraday rotation by almost 10%. This co-optimization of growth techniques and thermal processing has resulted in better performance for garnets on TE-mode structures.

In this paper, we present our recent research effort in developing cladded crystal fibers for high power single frequency fiber lasers. Glass fiber lasers, have been very successful to date, with demonstrations of several kW’s of output power, but they have several intrinsic issues that ultimately limit how much power they can produce. These relate to their low thermal conductivity (~1W/m/K) coupled with low usage temperature (Tg<<1000°C), and low dopant concentrations (<1%), which necessitate the use of long fiber lengths. In addition, for single frequency operation, the high stimulated Brillouin (SB) cross-section of glass leads to SB scattering which limits the achievable power. On the other hand, single crystal fibers based on YAG exhibit reduced nonlinearities such as SB scattering and increased thermal conductivity which allows power scaling to higher average and peak powers than in glass core fiber. If designed correctly, their laser output powers can exceed those of all-glass fiber lasers by more than a factor 10, before nonlinear and thermal issues have a detrimental impact. In this paper, we report on the fabrication and physical/optical characterization of cladded single crystal RE:YAG core fiber with the architectures analogous to those seen in LMA silica fiber for application in high power lasers and amplifiers.

Recent development of highly efficient organic electro-optic (EO) materials based photonic devices has opened promising opportunities to complement and improve current inorganic semiconductor-based technologies. In this talk, examples will be given based on molecular engineering of the shape, size, interactive force and interface of organic and inorganic hybrid materials to significantly improve the performance of devices for efficient low power, ultrafast information processing.

These molecularly engineered and optimized EO materials can be used to provide a full array of optical functions and can be processed at temperatures that are compatible with CMOS integrated circuits. In response to exponentially increasing demands for operational bandwidth, nanostructured organic EO materials are expected to play critical role in organic/silicon hybrid nanophotonics in the near future. Integrated optical circuits based on organic/silicon hybrid EO materials will enable low cost, mass production of novel nanophotonic devices for broadband optical signal processing and communications.

Silicon-based optical integrated circuits and modules will be required for future chip-level optical interconnection and optical data transmission. We engage in fabrication and characterization of integrated optical waveguides and modules using hybrid structure of high performance functional photonic polymers and silicon photonics.
In this paper, recent progress of photonic polymers and waveguide-type device technologies for next generation integrated optical circuits is presented. High performance photonic polymer is a potential candidate for functional optical waveguide material by combining silicon waveguide platform. Athermal silicon optical waveguide has been realized using an organic-inorganic hybrid optical material (a TiO₂ nanoparticle doped polymer) with large negative thermo-optic (TO) coefficient as a top cladding material of silicon waveguide to compensate the positive TO coefficient of silicon core. We can tune the TO coefficient as well as refractive index of hybrid materials by changing the concentration of nanoparticles of hybrid materials to realize the exact athermality even if there is a process size error of silicon waveguide. Using an electro-optic (EO) polymer as the top claddings of silicon waveguide limit of modulation bandwidth and/or driving voltage of a pure silicon waveguide modulator can be overcome. Light-induced self-written (LISW) waveguide technology enables optical interconnection between optical components such as optical waveguide and/or optical fiber using a photocurable resin. In this research two-photon-absorption-chromophore doped monomer was used for short wavelength infrared laser (1550 nm) pumping and curing, and optical interconnection between single-mode fibers was performed. By combining high performance functional photonic polymers and silicon waveguide, integrated optical devices and modules for next-generation optical interconnection application will be realized.
Part of this work was supported by the Strategic Promotion of Innovative Research and Development (S-Innovation) program, “Photonics Polymer,” of the Japan Science and Technology Agency (JST).

Hybrid organic-inorganic metal halide perovskites (PVSKs) have attracted interest as next generation solar cell materials due their potential for high power conversion efficiency and simple fabrication methods. Despite the rapid development of PVSKs in the material science field, the characterization of photo-physical properties and dynamics is far from exhaustive, leaving open questions, such as the role of trap states, influence of defects, and the origin of luminescence. PVSK layers show grain-level spatial variations, and conventional spectroscopic analysis tools (such as transient optical absorption spectroscopy) often cannot resolve the heterogeneous grains. Nano imaging tools, such as scanning electron microscopy, can resolve the spatial structure but provide little information on dynamic optical and electronic properties.
Here we demonstrate the use of time-resolved nonlinear optical microscopy, pump-probe microscopy, to measure optical and electron dynamics in PVSKs on the femto- to pico-second timescale and sub-micron spatial scale. We investigate crystalline CH₃NH₃PbI₃ and CH₃NH₃PbI_3-xCl_x perovskite thin layers deposited on glass substrates and map charge carrier dynamics that result in various nonlinear contrasts: two photon absorption, ground state depletion, and excited state absorption. Based on these contrasts, we can visualize the heterogeneous grains and investigate the influence of different fabrication methods or the presence of electron accepting layers on the electronics properties. In this submission, we will present progress in utilizing pump-probe microscopy for mapping of charge carrier dynamics in order to explore the effects of PVSK composition, manufacturing, and aging.

Two-photon polymerization is a rapidly expanding mask-less lithographic technique with applications in photonics and biotechnology. This technology which can be termed as 3D printing for the microscale can be used to fabricate high resolution microstructures with arbitrary shapes. Majority of work reported using this technique involves the use of polymeric precursors and fabrication of polymeric microstructures. Current presentation summarizes our efforts to incorporate inorganic nanomaterials into microstructures through various strategies.
Incorporating noble metal nanoparticles or semiconductor quantum dots into polymeric microstructures allows the modulation of their optical properties. This presentation will highlight our ongoing efforts at achieving, silver or quantum dot containing microstructures through two-photon lithography. We describe chemical functionalization of semiconductors QDs, post functionalization of fabricated structures, as well as direct writing of 3D structures containing inorganic materials.

Three-D printing is a key technology that is set to change manufacturing in the so called fourth industrial revolution. While 3D printing has been demonstrated for many different materials for millimeter scale upwards, attaining smaller dimensions are still challenging. Two-photon stereolithography is one of the contenders for fabricating structures with micron- or sub-micron resolution. Generally, two-photon stereolithography is based on a photopolymerization reaction directly or indirectly initiated by a nonlinear optical molecule capable of simultaneously absorbing two-photons. When a near-infrared ultrashort-pulsed laser is closely focused into a volume of photoactive chemical medium (photoresist), real 3D microstructures can be fabricated using a layer-by-layer accumulating technique. In this lecture, high resolution patterns of polymers, ceramics, noble metals and semiconductors incorporated microstructures fabricated by two-photon-initiated polymerization will be presented. The 3D microstructures containing noble metals or semiconducting quantum dots are of great interest for applications in optoelectronics, photonics and biophotonics due to their ability to change the dielectric properties and refractive index of polymeric structures. In addition, recent developments of novel 3D cancer cell chips for the in vitro 3D cell growth simulation of tumor cells and the activity detection of anticancer drugs is also reported.

Visual information is first processed in retinal ganglion cell (RGC), transmitted to lateral geniculate nucleus (LGN) of the brain and then to primary visual cortex (V1) in the cerebrum. These visual cells have limited fields of view, or windows, called receptive field. Neurons in RGC and LGN have a coaxial-shaped receptive field structure formed by the excitatory region and the inhibitory region. The on-centre simple cell receptive fields are elongated with an excitatory central oval, and an inhibitory surrounding region. Both receptive fields have antagonism between the excitatory and inhibitory regions. The RGC receptive field structure can be represented by DOG (difference-of-Gaussians) function, which is the difference between two Gaussian functions. The classical receptive field of simple cells can be approximated well with Gabor function.
In the cell membrane of Halobacterium Salinarum, photosensitive protein that precedes photosynthesis exists. This protein is called bacteriorhodopsin (bR), because it resembles the visual pigment rhodopsin of animals. BR shows differential response with a polarization reversal and we interpret this response as an equivalent for excitatory/inhibitory response of receptive filed. Employing this particular feature of bR we propose two types of optical filters, DOG and Gabor filters, and aim to apply for image processing.
We have fabricated receptive fields consisting of two oppositely coated bR films on the front and rear ITO plates. These films relate to the excitatory and inhibitory regions, respectively. We demonstrated temporal and spatial frequency characteristics of vision using such bR-based visual receptive fields. Our DOG filter, which mimics on-centre RGC receptive fields, had the function of a Laplacian filter and acted as an edge detector. The X-type receptive field responses obtained by the filter, for a variety of stimuli, were compared with available electrophysiological recoding. An on-centre Gabor filter strongly responded to a grating with particular spatial-frequency and orientation.
The goal of this research is the development of artificial DOG and Gabor optical filters for improving the analog image processing performance. Notably, the low cost and simplicity of fabricating single element bR-based filters, and no requirement for external connections are two of the major advantages over conventional silicon semiconductor technology.

Ultrahigh quality factor (UHQ) microresonators have very long photon lifetimes, enabling high circulating power inside the cavities. As a result of the amplification of the input optical power, these devices are able to excite various nonlinear optical phenomena, such as optical parametric oscillation (OPO) and stimulated Raman scattering. Previously, OPO has been demonstrated using silica toroidal resonators via the Kerr non-linearity. By combining degenerate and non-degenerate four-wave mixing (FWM), frequency combs have been generated with high input power. Additionally, due to the moderate value of the third order nonlinear susceptibility, >5mW of input power was required to obtain a broad (>100nm span) frequency comb.
In this work, we experimentally demonstrate that by coating a silica microtoroid with a Zirconium (Zr) doped silica solgel film, the threshold and comb span improve. Due to the higher third order susceptibility value of the Zr-doped silica layer as compared to silica, the efficiency of the four wave mixing process, especially OPO, has been improved. The frequency comb behaviors of different concentrations of Zr coated devices are experimentally investigated in the near-IR (1550nm), and the comb spans are compared between the various devices. Comb spans in excess of 300nm are achieved with ~mW input powers. In conclusion, the frequency comb performance is dramatically improved by incorporating a thin film of Zr-doped solgel into the silica toroidal cavity.

Disordered optical media are ubiquitous in nature, and a proper description of light propagation in such materials is crucial for a number of applications ranging from the characterization of these materials to the fabrication of new types of lasers [1].

We have shown earlier that light diffusion is critically dependent on the finite lateral size in disordered meso-macroporous materials with a cylindrical shape [2]. In a second step, we showed the light diffusion behaviour in (HIPE)-based isodense polystyrene foams [3], a study which strongly motivated further investigations into the limits of validity of the observed scalability between the light transport mean free path and the pore sizes as the scattering strength of the system increases.

Recently, we have numerically predicted and experimentally shown the coexistence and competition of random lasing (RL) and stimulated Raman scattering (SRS) in active disordered random media [4]. We developed a simple model which includes both mechanisms coupled through diffusion equations. We found that the prevalence of a nonlinear mechanism over the other is determined by the degree of scattering. The competition was explained in terms of disorder-dependent pump depletion and fluorescence saturation.

In this talk, we will provide a short review of the salient features of these effects. Then, based on our experience in the realization and characterization of 3-dimensional disordered porous materials, we will show that the infiltration of molecules or nano-crystals with second order (Hyper Rayleigh Scattering, Second Harmonic Generation) nonlinear optical properties in such disordered 3D porous materials lead to a nonlinear enhancement factor. The effect of coherence in such processes will be discussed. These preliminary results are well accounted for by numerical FEM simulations, which will also be presented.

[1] D. S. Wiersma, Nat. Phys. 4, 359 (2008); B. Redding, M.A. Choma, H. Cao, Nat. Photonics 6, 355 (2012); D. S Wiersma, Nat. Photonics, 7, 188 (2013).
[2] P. Gaikwad, S. Ungureanu, R. Backov, K. Vynck and R.A.L. Vallée, Opt. Express 22, 7, 7503, (2014).
[3] S. William Reginald, V. Schmitt and R.A.L. Vallée, EPL 107, 64003 (2014).
[4] N. Bachelard, P. Gaikwad, R. Backov, P. Sebbah and R. A. L. Vallée, ACS Photonics, 1(11), 1206–1211 (2014); P. Gaikwad, N. Bachelard, P. Sebbah, R. Backov and RAL Vallée, Advanced Optical Materials, 3(11), 1640–1651 (2015).

Exciton-polaritons in organic microcavities are of growing interest in both fundamental investigations and applications. In particular, the large exciton binding energy and large oscillator strength provide for strongly and ultrastrongly coupled polaritons at room temperature. These facts open the way for new generations of lasers and photonic devices. Multiple coupled microcavities introduce additional degrees of freedom, and have attracted increasing attention. Moreover, given the remarkable nonlinear optical properties of organic materials, organic cavity polaritons present intriguing possibilities for nonlinear optics that arise from the collective nature of the unique hybrid light-matter states. We report here on linear and nonlinear optical properties of ultrastrongly coupled organic cavities containing a well-researched conjugated organic glass.

We have fabricated low-Q single and double cavities using silver as reflectors enclosing a neat organic dye glass (DCDHF-6-V). The organic dye glass has very broad absorption spectra, and the strong inhomogeneous broadening gives rise to a large oscillator strength. The linear dispersion of the polariton states for single and coupled double cavities were studied using angle-resolved reflection spectroscopy. We observed anti-crossing dispersion of cavity polaritons from a single cavity, where the vacuum Rabi splitting energy is 1.12 eV for both TM and TE polarization. This value is 52% of exciton transition energy, thus indicating ultrastrong exciton-photon coupling.

Anti-crossing dispersion of four cavity-polariton branches is observed from the double cavity. The vacuum Rabi splitting between UP and MP2 branch is 1.11 eV, while the splitting between LP and MP1 is 1.08 eV. The observed broken degeneracy of two Rabi splittings is due to higher-order anti-resonant Hamiltonian terms beyond rotating wave approximation in the ultrastrong coupling regime.

Third harmonic generation dispersion was studied in single cavities resonant with the polariton states. Nonlinear transfer matrix formalism predicts that resonant THG follows the dispersion of cavity polariton, and its intensity is larger when the THG wavelength is closer to exciton wavelength. The magnitude of the dispersion is greater in the exciton dominated branch of the dispersion. We experimentally verify these predictions using THG measurements for a single cavity sample. The basic features of the experimentally determined dispersion are in agreement with transfer matrix calculations.

Cavity polaritons exhibiting ultrastrong coupling in low-Q cavities at room temperature along with the robust nonlinear optical response of conjugated organic materials could lead to promising applications in lasers and photonics. Further, the ability to adjust the dispersion of the nonlinear optical response via angle-tuning of the polariton band, suggests wavelength agile nonlinear optical devices.

Funding from National Science Foundation: Grant DMR-1609077

The third-order optical nonlinearity, n₂, of hybrid lead iodide perovskites is selectively enhanced in the two-dimensional (2D) Ruddlesden-Popper series, (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1 (n = 1–4), where the layer number (n) is engineered for bandgap tuning from = 1.60 eV (n = ∞; bulk) to 2.40 eV (n = 1). Despite the unfavorable relation, n₂ ∝ E_g^-4, strong quantum confinement causes these wide-bandgap 2D perovskites to exhibit four times stronger third harmonic generation at mid-infrared when compared with the 3D counterpart, (CH₃NH₃)PbI₃. The corresponding n₂ values of the 2D perovskites indeed outshine conventional semiconductors having bandgaps larger than 1.0 eV. Surprisingly, however, the impact of dimensional reduction on two-photon absorption, which is the Kramers-Kronig conjugate of n₂, is rather insignificant as demonstrated by broadband two-photon spectroscopy. The concomitant increase of bandgap and optical nonlinearity is truly remarkable in these novel perovskites, where the former increases the laser-induced damage threshold for high-power nonlinear optical applications.

Our recently developed ultrafast nonlinear beam deflection technique [1] allows for clear separation of slow and fast nonlinearities and the experimental results enable the prediction of nonlinear properties for much longer pulses. The technique is also highly sensitive, allowing testing of measurement of weak nonlinearities, such as observed in vapors and gases.[2] Essentially, the beam deflection method provides the impulse response function for nonlinear refraction, thus providing information that can be used to predict the nonlinear response as measured by Z-scan or other methods for arbitrary pulse shapes and durations.[3] We have characterized a large number of common solvents in this way. Many solvents have significant non-instantaneous contributions to NLR, and when these solvents are used to dissolve molecules with negative (i.e. self-defocusing) nonlinear refraction, we find that the effective nonlinear refraction becomes highly dependent on pulse width, and for certain combinations of pulse width and concentration, the effective n₂ may be exactly zero. This becomes particularly interesting for solvents with noninstantaneous nonlinearities, where it is possible to have a zero average nonlinear refraction, yielding very low sepf focusing, but where the derivative of the index change is still relatively large, giving significant self-phase modulation. Such materials systems are of interest for generation of white light continuum, without suffering from catastrophic self-focusing. The beam deflection method may also be used to reveal the extremely large nonlinear absorption and refraction in semiconductors that occurs when two very different wavelengths are used. We have also used these methods in combination with the dual-arm Z-scan method to test a simplified 3-level model for organic materials that predicts nonlinear refraction knowing only the linear and 2-photon absorption spectra.[4]
References
M. R. Ferdinandus, et. al., Opt. Lett. 38, 3518-3521 (2013)
M. Reichert, et. al., Optics Express 24(17), pp.19122-19122, (2016).
M. Reichert, et. al., Optica 3(6), pp.657-658, (2016)
T.R. Ensley, et. al., J. Opt. Soc. Am. B 33(4), pp.780-796, (2016).

Triplet-triplet annihilation up-conversion (TTA-UC) has attracted considerable attention recent years because of its high up-conversion quantum yield (UC-QY) and low excitation intensity comparable with the solar radiation. These features allow a variety of potential applications such as spectral management of sunlight for solar cell and biological imaging.
TTA-UC have been extensively studied for the systems converting from the visible (Vis) region of wavelength mostly in solution or polymeric media. We have realized efficient solid-state UC from green to blue wavelength in the binary solid fabricated from the rapid-drying casting method. In this method, we used the saturation concentration of the emitter, allowing faster solidification of emitter (matrix) [1].
To step forward for efficient use of solar energy, it is needed to extend the excitation wavelength to the near-infrared (NIR), which accounts for more than 50% of the entire energy of solar radiation. In this work, we develop solid-state NIR (785 nm)-to-yellow (570 nm) TTA-UC system with relatively lower excitation intensity and higher UC-QY, by using the rapid-drying casting method. For NIR-to-Vis conversion, we applied PdTPTAP (Pd-tetrakis(3,5-di-t-buthylphenyl)tetraanthrophophyrin) as a sensitizer and rubrene as a emitter. These molecules have the close triplet energy levels to each other. The binary solid of the sensitizer (guest) and emitter (host) was fabricated on a slide glass from the mixed solution of them in tetrahydrofuran at saturated concentration of the emitter. Many particles of the binary solid were obtained with the size of 10-100 µm and the thickness of 1-3 µm. Under excitation with a NIR laser diode (785 nm) though a 20x objective lens (excitation intensity 0.5-1 W cm^-2 at the sample position), the particles were found to show yellow UC emission, originating from delayed fluorescence emission of rubrene. In TTA-UC system, the threshold intensity (I_th) defined as the excitation intensity of the crossing point of quadratic dependence of the UC emission to the excitation intensity at lower excitation intensities and linear dependence at higher excitation intensities. I_th of the binary particles was found to be 0.2-0.8 W cm^-2 depending on particles. The I_th were in the similar order or less than that of the reported solution system with similar sensitizer [2]. The UC-QY of 30 individual single particles was examined under an optical microscope. The values were as high as ∼1% under air condition. The UC-QY was unchanged after storing the binary solid for 80 days or more under Ar environment.
We successfully developed the NIR-to-Vis TTA-UC in binary solid by using of the rapid-drying casting method with relatively low threshold intensity and high UC-QY. This result will open the way to solid state TTA-UC by NIR excitation by using simple fabrication method of solution casting.
[1] K. Kamada, et al. Mater. Horiz. 2017, 4, 83.
[2] V. Yakutkin, et al. Chem. Eur. J. 2008, 14, 9846.

Upconversion luminescence is an active field of research since the pioneering work by Auzel [1]. The past two decades the discovery of upconversion nanocrystals has triggered renewed interest, also because of the potential applications in bio-imaging, solar cells, sensor and security [2-4]. The Yb³⁺-Er³⁺ ion couple is one of the most efficient systems of upconversion and is widely used for infrared to green upconversion. However, the upconversion efficiency, especially in nanocrystals, is still quite low, typically below a few percent, even with approaches aimed at enhancing the efficiency, including core-shell architectures, sensitization with dye-antennas, photonic and plasmonic enhancement [2-5]. It is of prime importance to understand the underlying reason why the upconversion process is intrinsically inefficient.
A major loss mechanism is related to the high concentrations of rare earths ions like Yb³⁺ and Er³⁺ in upconversion materials. Upconversion relies on multi-step energy transfer. For this reason high dopant concentrations are crucial to realize efficient transfer between lanthanide neighbors. On the other hand, the high dopant concentrations give rise to energy migration and cross-relaxation quenching. The trade-off between efficiency losses by concentration quenching and efficiency gain by energy transfer upconversion determines the maximum upconversion quantum yield but is not well understood. Here we present a systematic investigation on the concentration dependence of luminescence quenching for Er³⁺ and Yb³⁺ in NaYF₄ nanocrystals, the upconversion model system. Yb³⁺ and Er³⁺ concentrations are varied between 1 and 60% for core and core-shell nanocrystals where an undoped isocrystalline shell is grown around the lanthanide doped core. Luminescence spectra and luminescence lifetime measurements, by both indirect and direct excitations, are reported and analyzed. The results show that the concentration quenching is strongly reduced in core-shell geometries. For Yb³⁺ concentration quenching is limited up to the highest concentrations of 60%. For Er³⁺ concentration quenching varies for different emitting levels. The strongest quenching occurs for the ⁴I_11/2 level which is an important intermediate state in the IR to green upconversion process. Variation of the solvent reveals that a major loss mechanism is multi-phonon relaxation due to coupling with high energy vibrations of the coordinating ligands and solvent. These results give more insight of decay dynamics and concentration quenching for lanthanide ions involved in upconversion process. The results can serve to optimize the upconversion efficiency by careful tuning of concentrations and core-shell design and thus boost the application of upconversion nanomaterials.

References:
[1] Chem. Rev. 2004, 104, 139-173.
[2] Chem. Soc. Rev. 2015, 44, 1379-1415.
[3] Adv. Optical Mater. 2015, 3, 510-535.
[4] Chem. Soc. Rev. 2017, 46, 4150-4167.
[5] Angew. Chem. Int. Ed. 2014, 53, 11702-11715.