Symposium S.EL04 : Materials for Nonlinear and Nonreciprocal Photonics

Our research group has a long history of developing chromophores that exhibit non-linear absorption. Substantial progress was made to this end with the synthesis of two-photon chromophores substituted with transition metals. The inclusion of the metal center facilitates the formation of triplet excited-states in the two-photon chromophore. If this leads to a triplet state with strong excited-state absorption at wavelengths where two-photon absorption is active, the nonlinear optical performance of the chromophore is enhanced.¹ The champion molecule from these initial studies was found to be bis(phenylethynyl)bis(tributylphosphine)platinum (II) bearing two alkynyl-benzothiazolylfluorene ligands. Recently, Au(I) analogs of this complex containing fluorene-benzothiazole and alkynyl-fluorene-benzothiazole ligands have be synthesized with differing ancillary ligands attached to the Au(I) center. Varying the ancillary ligand on Au(I) from a phosphine to a N-heterocyclic carbene alters the photophysical properties of the chromophoric ligand. UV-vis, steady-state and time-resolved luminescence, and transient absorption results will be discussed. This talk will compare the photophysical properties and their impact on the nonlinear performance of the original platinum (II) alkynyl-fluorene-benzothiazole complex and the Au(I) fluorene-diphenylamine and Au(I) alkynyl-fluorene-diphenylamine complexes.

References
1. Cooper, T. M. et al. Platinum Acetylide Two-Photon Chromophores. Inorg. Chem. 46, 6483-6494 (2007).

Probing the nonlinear response in light-matter interactions offers the advantage of lifting some fundamental limitations existing within the scope of linear operations. One of the most striking examples is the ability of breaking the reciprocity of light transmission. Transmission through a medium characterized by a symmetric, time-independent, and linear permittivity tensor stays the same when reversing the excitation direction, or interchanging the source and the detector. While serving as an innate foundation in any optical design, it presents a fundamental challenge for applications requiring a non-reciprocal directionality of light propagation.

Here, we show that the option of non-reciprocity in the directionality and the efficiency of nonlinear generation is inherently incorporated in the response of nanoelements exhibiting magnetic dipolar (or higher-order multipolar) Mie resonance(s) in their linear response. In particular, we show that nonlinear generation from a nanoelement, where it regularly occurs nearly isotropically in all directions (due to the lack of phase matching constraints within nanoscale interaction volumes), can be made both unidirectional and non-reciprocal, such that the generation occurs predominantly in a single direction which, additionally, remains unchanged with respect to a fixed laboratory coordinate system when reversing the direction of the fundamental beam(s). In contrast with the previous studies, the proposed approach does not require asymmetry in either the geometry or the material composition of the nanoelement. Rather, it relies on the multiplicative nature of the nonlinear response which, in turn, allows a simultaneous change in phase of both the electric and the magnetic, nonlinearly induced, dipolar modes, when switching the phase of a single (either electric or magnetic) vector of the fundamental field. Furthermore, the interference can occur between various pathways within the electric and magnetic (nonlinearly produced) dipolar modes. As a result, non-reciprocity in terms of just a change in the efficiency of nonlinear generation when reversing the direction of any subset of the fundamental beams is inherent to and expected in the nonlinear response of most nanoelements, even the symmetric ones, and for most of the nonlinear processes. Targeted engineering of the relative strengths of various pathways within each (nonlinearly produced) multipolar mode may then allow an interferometric cancellation of the generated field for a given nonlinear process, for certain respective directions of the fundamental beams.

Reliance on multipolar interference in the suggested approach inherently assumes the manifestation of the described phenomena on the nanoscale, through the response of subwavelength-scale elements. These nanoelements can thus be used as building blocks to construct a metasurface or a medium with similar unique features in its nonlinear response. As an example, we present a metasurface operating as a one-way nonlinear mirror, where the image is formed by a process of difference frequency generation on one and the same side of the metasurface independently of the object location.

Both non-reciprocal directionality and inhibition of the nonlinear response require, however, a careful engineering of the respective strengths of various pathways within each (electric or magnetic) type of the nonlinearly produced multipolar partial wave. As such, these phenomena are not expected to manifest in natural nonlinear materials, even those possessing natural magnetic dipolar transitions. They, however, can be achieved via a tailored design of the effective magneto-electric nonlinear polarizabilities of a nanoelement.

Achieving room-temperature quantum-mechanical strong coupling, or vacuum Rabi splitting, between a single emitter and a plasmon resonance has been a longstanding goal. Recently, two peaks have been observed in the scattering spectra of plasmonic metal nanostructures coupled to single molecules and single quantum dots, and this was taken as evidence of strong coupling. However, a two-peak scattering structure can also arise at intermediate coupling strengths, below the strong-coupling threshold, due to Fano-like interference between the plasmon and emitter dipole.
We unambiguously distinguish between intermediate and strong coupling by measuring both the scattering spectra and the photoluminescence spectra of coupled plasmon-emitter structures. Specifically, we couple single colloidal quantum dots to a plasmon resonator by placing them in the gap between a gold nanoparticle and a silver film. We observe weak, intermediate, and strong coupling in these hybrid metal-semiconductor structures at room temperature, depending on the detailed nanoscale structure of the metal nanoparticle.
These structures have the potential to provide ultrafast, low-power optical nonlinearities on the nanoscale. Both induced transparency and strong coupling can be canceled by absorbing a photon in the quantum dot, leading to a strong change in extinction at the quantum dot transition frequency. Since only a single photon must be absorbed by the QD for this to happen, the energy needed for modulation has the potential to be extremely low, and the structure has the potential to enable all-optical information processing, possibly including neuromorphic computation and quantum information processing.
One limitation to the practical application of these structures, however, is the random nature of their assembly and the resulting low yield of structures exhibiting the desired transparency or strong coupling. One approach we have taken to this low yield is to replace the metal nanoparticle with a metal scanning-probe tip. In this way, we are able to achieve reversible, controllable strong coupling to several individual quantum dots, one at a time. Our current efforts are focused on developing bottom-up techniques of chemical synthesis and assembly for the high-yield production of strongly-coupled metal-semiconductor hybrid nanostructures.

In recent years hybrid organic-inorganic perovskites have become a very promising materials for photonic applications and optical devices. Special attention is given to the two-dimensional (2D) perovskites. In the two-dimensional crystalline form these materials behave as a self-organized multiple quantum-well heterostructures. Compared to analogous semiconductor heterostructures 2D-layered perovskites displays stronger dielectric confinement in the inorganic layers, whereas excitonic resonances in such materials are characterized by a high binding energy and large oscillator strengths which leads to a great stability even at room temperatures, as well as exhibit strong non-linearities.
We report the realization of a tunable planar dielectric microcavities containing a 2D-layered perovskite-type semiconductor, CH₃NH₃PbI₃, showing the strong-coupling regime at both room and liquid helium temperatures. 2D-layered perovskite structure was synthesized from the organic solution which was deposited by spin-coating on a dielectric mirror.
The tunable cavity was constructed of two dielectric mirrors. The distance between the mirrors was controlled through piezo positioners. It allow, at the same time, for the realization of high fineness open micro-cavities (high Q) with tunable photonic mode and to avoid deterioration of perovskite crystals caused by the growth of the upper mirror. A strong coupling regime between the perovskite exciton and the confined photon mode is evidenced at room and liquid-helium temperatures from angular-resolved reflectivity and photoluminescence experiments. The observed exciton-photon coupling strength is of W~110 meV and photonic mode can be tuned over the range of 100 meV.
The scientific significance of this work is based on the realization of a room-temperature new material platform allowing for the observation of phenomena previously carried out at cryogenic temperatures.

Hybrid materials are commonly used for wide range of optical applications (sensors, filters, imaging, photocatalysis…)..[1] The use of interactions between emitting or absorbing systems and plasmonic nanostructures have also been an intensive field of research due to the possibility to tune and optimize the optical responses using the local electromagnetic field. The synthesis of gold nanostructures combining high yield, purity, large scale and plasmon resonance spreading from the visible to the NIR wavelengths was developped.[2,3] These nanostructures were functionalized in order to allow their homogeneous incorporation in transparent hybrid silica matrices using the sol-gel process.[4,5] Co-dispersion of the metallic structures with nonlinear dyes was successfully achieved.[6] The role of the concentration, shape and size of the metal nanoparticles on the optical response was evaluated. The respective impact on the nonlinear optical response of the dyes will be discussed both in the visible and the near infrared wavelengths.

References
[1] S. Parola, B. Julian-Lopez, L. D. Carlos, C. Sanchez, Adv. Funct. Mater. 2016, 26 (36), 6506-6544
[2] D. Chateau, A. Liotta, F. Vadcard, J. R. G. Navarro, F. Chaput, J. Lermé, F. Lerouge, S. Parola, Nanoscale 2014, 7, 1934.
[3] D. Chateau, A. Desert, F. Lerouge, G. Landaburu, S. Santucci, S. Parola, ACS Appl. Mater. Interfaces 2019, doi.org/10.1021/acsami.9b12973.
[4] D. Chateau, A. Liotta, D. Gregori, F. Lerouge, F. Chaput, A. Desert, S. Parola, J. Sol-Gel Sci. Technol. 2016, DOI 10.1007/s10971-016-4116-y.
[5] H. Lundén, A. Liotta, D. Chateau, F. Lerouge, F. Chaput, S. Parola, C. Brännlund, Z. Ghadyani, M. Kildemo, M. Lindgren, C. Lopes, J. Mater. Chem. C 2015, 3, 1026.
[6] D. Chateau, A. Liotta, H. Lundén, F. Lerouge, F. Chaput, D. Krein, T. Cooper, C. Lopes, M. Lindgren, S. Parola, Adv. Funct. Mater.2016, DOI: 10.1002/adfm.201601646

Computation of response functions, based on linear response theory and first-principles electronic structure methods, has been extensively utilized for analyses and predictions of various funcitonal materials such as ferroelectrics, piezoelectrics, and multiferroics. For the study of strongly correlated materials, however, methods based on standard density functional theory need substantial improvement; because of the need of incorporating the so-called two-particle vertex correction in evaluating response functions, which is computationally demanding, an extensive study of functional and correlated materials has remained a challenging task. To mitigate this difficulty and facilitate investigations of correlated functional materials, we propose a new theoretical method to compute linear response functions of strongly correlated Mott insulators. Our approach starts from the strongly correlated atomic regime, unlike other first-principles-based methods, which circumvents evaluating vertex correction terms and enables computationally cheap evaluation of material properties in Mott systems. As an example, we present our simulation results of nonreciprocal directional dichroism in Ni₃TeO₆, which is a wide-gap Mott insulator, showing a promising agreement with experimental observations.

In this talk, we will describe a materials self-assembly approach to controllably weld gold nanorods end-to-end, forming much higher aspect ratio dimers in macroscale quantities, linking the nanorods using unfocused femtosecond light. Electrostatic-based molecular assembly controls the discrete selection of end-to-end dimer assemblies, preventing larger structures. Large, high-quality yields of welded dimers can be produced by illuminating these dimer suspensions at their capacitively coupled plasmon resonance wavelength using femtosecond light pulses. In-situ absorbance measurements demonstrate an isosbestic point which results from the conversion of single nanorods to capacitively coupled plasmon dimers, indicative of direct population conversion. We will discuss the underlying physical mechanisms for the welding process as well as the optical properties of the newly formed high aspect ratio dimers.

It is well known in the literature that for a two photon nonlinear absorbing dye to be the most effective, high concentrations are needed. The problem is that most photophysical studies are done at low concentration and in a solution. These low concentration studies are important for understanding inherent materials properties but it is also important to understand what happens in a material at high concentration. In addition to this, efforts have been made to study the effects of incorporating a dye into a solid matrix environment to better understand the constraints this environment has to a given material. Here we present the results of a study of a two photon absorbing platinum dye, E1-BTF, incorporated into various hosts including PMMA, epoxy, polyurethane, and sol gel. Similar results were found in all hosts that a triplet excited state derived excimer forms at the highest concentrations. This is due to close packing of the E1-BTF molecule where both static and dynamic quenching may occur. The host material defined the highest achievable concentrations for each polymer host. We will report on the overall photophysical properties of E1-BTF in these polymer hosts.

Nonreciprocal directional dichroism is an unusual light-matter interaction that gives rise to diode-like behavior in low symmetry materials. The chiral varieties are particularly scarce due to the requirements for strong spin-orbit coupling, broken time reversal symmetry, and a chiral axis. We bring together magneto-optical spectroscopy and first principles calculations to reveal high energy, broad band nonreciprocal directional dichroism in Ni₃TeO₆ with special focus on behavior in the metamagnetic phase above 52 T. In addition to demonstrating this effect in the magnetochiral configuration, we explore the transverse magnetochiral orientation in which applied field and light propagation are orthogonal to the chiral axis and by so doing, uncover an additional configuration with a nonreciprocal response in the visible part of the spectrum. In a significant conceptual advance, we use first-principles methods to analyze how the Ni²⁺ d-to-d on-site excitations develop magnetoelectric character and present a microscopic model that unlocks the door to theory-driven discovery of chiral magnets with nonreciprocal properties.

Colloidal semiconductor quantum dots (QDs) are attractive materials for realizing highly flexible, solution-processable optical gain media with readily tunable operational wavelengths [1, 2]. However, QDs are difficult to use in lasing due to extremely short optical gain lifetimes limited by nonradiative multicarrier Auger recombination [3]. This, in particular, is a serious obstacle for realizing cw optically and electrically pumped lasing devices. Recently, we have explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination [4, 5]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [4], which has been a long-standing goal in the field of colloidal nanostructures. Further, we have applied these dots to practically demonstrated the viability of a ‘zero-threshold optical gain’ concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [5, 6]. Such charged QDs are optical-gain-ready without excitation, which allows us to reduce the lasing threshold to record-low values that are well below a fundamental single-exciton-per-dot limit [6]. Most recently, we have developed QD devices that operate as both an electroluminescent (EL) structure and a distributed feedback optically pumped laser [7]. By carefully engineering a refractive-index profile across the device stack, we have been able to demonstrate low-threshold lasing even with a very thin EL-active region, which comprises only three monolayers of the QDs. All of these recent developments demonstrate a considerable promise of colloidal QDs for implementing solution-processable optically and electrically pumped lasers operating across a wide range of wavelengths.

[1] V.I. Klimov, A.A. Mikhailovsky, S. Xu, A. Malko, J.A. Hollingsworth, C.A. Leatherdale, H.J. Eisler, M.G. Bawendi, Science290, 314 (2000).
[2] V.I. Klimov, S.A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J.A. McGuire, A. Piryatinski, Nature 447, 441 (2007).
[3] V.I. Klimov, A.A. Mikhailovsky, D.W. McBranch, C.A. Leatherdale, M.G. Bawendi, Science 287, 1011 (2000).
[4] J. Lim, Y.-S. Park, V.I. Klimov, Nat. Mater. 17, 42 (2018).
[5] K. Wu, Y.-S. Park, J. Lim, V.I. Klimov, Nat. Nanotech. 12, 1140 (2017).
[6] O.V. Kozlov, Y.-S. Park, J. Roh, I. Fedin, T. Nakotte, V.I. Klimov, Science 365, 672 (2019).
[7] J. Roh, Y.-S. Park, J. Lim, V.I. Klimov, Nat. Comm., in press(2019).

Optical modes can couple to sufficiently strongly absorbing vibrational transitions to create new polaritonic states in the mid-infrared. Transient infrared spectroscopy of these new states reveals rich spectra that report on coherences between and population of both the polaritons and so-called reservoirs of uncoupled modes. Our previous work, focusing on one-dimensional, pump-probe spectroscopy has shown that cavity-coupled vibrations have potential for photonic devices and can exhibit modified relaxation dynamics compared to uncoupled systems. Here, we present new results from two dimensional infrared spectroscopy (2D IR) and pump filtering experiments that give deeper insight into the spectroscopic responses of the vibrationally excited polaritonic system. We report on the spectroscopic polaritonic signatures in cavity-coupled systems and the influence of the homogeneous linewidth of the vibrational mode on the population dynamics of coupled systems.

Recently, interest in organic-inorganic perovskites has increased due to their application in photovoltaics, photonics and optoelectronics. They have also been used as a strong light emitters in the microcavities, due to the possibility of obtaining strong light-matter coupling regime, and the occurrence of coherent macroscopic effects, such as Bose-Einstein condensation of exciton-polaritons. Compared with multi quantum wells heterostructures produced by epitaxial growth of inorganic semiconductors, 2D perovskites displays stronger dielectric confinement in the inorganic layers, whereas excitons in such materials are characterized by a higher binding energy and exhibits strong nonlinearities. It indicates that the devices based on polaritons in 2D perovskite layers can work stable at room temperature. In our work we present a novel construction of a tunable planar dielectric cavity containing free standing layers of 2D-perovskite. Perovskites are deposited on thin films obtained from solution P2611:NMP.
The realization of such microcavity is innovative due to specific and unique technique of preparing emitter from thin films of layered perovskite-type semiconductor (C₆H₅(CH₂)₂NH₃)₂PbI₄. The frame of free-standing films from polyimide solution were deposited by spin-coating technique on silica substrate. After heating, obtained structure served as matrix to deposit 2D-layered perovskite. Fabrication of ultrathin polyimide films allows to isolate lots of arbitrary materials from the substrate.
Obtained thin film of perovskite were placed between dielectric mirrors controlled through piezo positioners, which allowed to shift photonic mode over the range of 100 meV. Such 2D perovskites can be considered as spontaneous realizations of a quantum well, in which organic material is sandwiched between organic cations acting as a potential barrier. The advantage of an emitter based 2D perovskites strongly coupled to the cavity field is its enhanced emission rate. Moreover, the regime of exciton-polaritons condensate compared to typical lasers is the ability to obtain laser action without the need to create population inversion. The additional advantage of our tuneable structure is the possibility to adjust the emission wavelength. A strong coupling regime between the perovskite exciton and the confined photon mode is evidenced at room temperature from angular-resolved reflectivity and photoluminescence experiments. The observed exciton-photon coupling strength is of Ω~110 meV.
The scientific meaning of this work is concentrated on the realization of new approach to obtain efficient, thin layer of organic-inorganic materials in a microcavity and developing a room-temperature devices for the observation of strong light-matter coupling and lasing phenomena.

Infrared-to-visible upconversion (UC) bears the potential to surpass the Shockley Queisser limit for single junction photovoltaics. Here, bulk-perovskite sensitized UC has been shown to be a promising approach in which free electrons and holes can be injected into the triplet state of the upconverting semiconductor. In these UC systems, a bulk-perovskite thin film is employed as the absorber layer and rubrene, a p-type tetraphenyl derivative of tetracene, is used as the annihilator layer. To boost the UC quantum yield, the rubrene layer is usually doped with dibenzotetraphenylperiflanthene (DBP) in a commonly used host-guest/annihilator-emitter approach^1,2.
DBP is used to harvest the singlet excited state from rubrene through a Förster resonance energy transfer, which prevents rubrene from undergoing the reverse process of singlet fission. To understand and optimize the role of DBP in the upconversion devices, we fabricate bilayer devices consisting of a lead halide perovskite sensitizer followed by a layer of rubrene/DBP with varying concentrations of DBP. Steady state and time resolved spectroscopy are then used to study the energy transfer processes in the bilayer devices. As expected, an increase in the DBP concentration results in the reduction of the rubrene emission, indicating a higher transfer efficiency. In addition, we observe an increase in the upconverted light emission when the rubrene/DBP ratio is varied. However, the time-resolved dynamics indicate that DBP may not merely be a guest in the system, but rather directly influences the observed upconversion dynamics and the device performance. These insights provide a path to further optimize and improve the efficiencies of bulk-perovskite sensitized UC devices.
References:
(1) Wieghold, S.; Bieber, A. S.; VanOrman, Z. A.; Daley, L.; Leger, M.; Correa-Baena, J.-P.; Nienhaus, L. Triplet Sensitization by Lead Halide Perovskite Thin Films for Efficient Solid-State Photon Upconversion at Subsolar Fluxes. Matter 2019, 1 (3), 705–719.
(2) Wieghold, S.; Bieber, A. S.; VanOrman, Z. A.; Nienhaus, L. Influence of Triplet Diffusion on Lead Halide Perovskite-Sensitized Solid-State Upconversion. J. Phys. Chem. Lett. 2019, 10 (13), 3806–3811.

Chalcogenide glasses (ChGs) have long been of interest to the photonics community for their unique properties like high refractive index, photo-induced phase changes, and high optical nonlinearities. The latter are essential for all-optical systems but can be difficult to tailor due to a lack of understanding of their complex origins. Here we use open-aperture Z-scan to compare the two-photon absorption (2PA) spectra of arsenic (III) sulfide ChG samples with different levels of local bonding disorder. We find that the difference between the 2PA coefficients of the samples depends on the ratio of the photon energy to the band gap energy, giving rise to two distinct regions in the spectra. We then explain these observations using an effective mass model that links the 2PA coefficient of a glass to its level of structural disorder. Our results offer a generalized structure-property relation for semiconducting glasses that can be used to enhance optical nonlinearities for photonic applications.

Recently, Cu₂BaSnS₄ (CBTS) and its mixed-chalcogen analog Cu₂BaSn(S,Se)₄ (CBTSSe) have emerged as promising new semiconductors for photovoltaic (PV) and photoelectrochemical (PEC) applications. This potential is due to CBTS and CBTSSe having high absorption coefficients, abundant and non-toxic elemental makeups, and reduced tendency for antisite disorder between cations.^[1,2] Beyond CBTS and CBTSSe, various other compounds within the I₂-II-IV-VI₄ (I = Li, Ag, Cu; II = Ba, Sr, Eu, Pb; IV = Si, Ge, Sn; VI = S, Se) family have drawn attention for applications in thermoelectrics (e.g. Ag₂BaSnSe₄), nonlinear optics (Li₂BaSnSe₄), and solar energy conversion (Cu₂SrSnS₄).^[3-5] Therefore, it is useful to explore the larger I₂-II-IV-VI₄ space beyond CBTS for promising semiconductors. This talk will explore several new compounds within this space. Cu₂BaGeSe₄ (CBGSe) has been shown theoretically and experimentally to have favorable solar-energy conversion prospects^[6] and the material properties of this compound can be tuned through cationic alloying (namely Sn for Ge) similar to substituting Se for S in CBTSSe.^[7] Close examination of the crystal structures of CBGSe and the fully-Sn substituted Cu₂BaSnSe₄ reveals related structural moieties upon which the lattices are built. This has led to the design and discovery of three new semiconductors Ag₂SrSiS₄, Ag₂SrGeS₄, and Ag₂SrSnS₄. These compounds have band gaps within relevant ranges for single- and multi-junction PV as well as PEC applications. Future work will investigate further the properties of these compounds and hypothetical related semiconductors for solar-energy conversion applications.

[1] D. Shin, et al. Chem. Mater. 2016, 28, 4771.
[2] D. Shin, et al. Adv. Mater. 2016, 1606945.
[3] Y. Li, et al. Materials Today Physics 2019, 9, 100098.
[4] L. Nian, et al. Inorg. Chem. 2018, 57, 3434.
[5] A. Crovetto, et al. ACS Appl. Energy Mater. 2019, 2, 7340.
[6] T. Zhu, et al. Chem. Mater. 2017, 29, 7868.
[7] G. C. Wessler, et al. Chem. Mater. 2018, 30, 6566.

Quantum emitters placed in resonant optical cavities have shown modified spontaneous emission rates and frequency when they are coupled in the weak coupling regime. In the so-called strong coupling regime, however, the coupled oscillators, i.e., emitters and optical modes, exchange energies inextricably, creating new hybrid states called polaritons. Recently, this half-light half-matter quasi-state has been demonstrated in a system where molecular vibrations are coherently coupled to optical cavity modes. In this talk, we will focus on non-linear transient response probed in the ultrafast regime, cavity-modified saturable absorption, and cavity-modified chemical reactivity. Specifically, we find excited-state spectra that reveal coherences between populations of both cavity polaritons and so-called dark states and explore the influence of the homogeneous linewidth of the vibrational modes. Our recently published results[1] show that the saturation fluence (typically, an intrinsic parameter of a material) can be tailored via cavity coupling and exhibits a counterintuitive scaling with system parameters (cavity length and molecular concentration). Lastly, we monitor transmission spectra of the Fabry-Pérot microcavity filled with species that participate in a simple addition reaction. Both reactants and products have strong molecular vibrations that couple to the optical cavity modes, resulting in quantifiable vacuum Rabi splittings. We examine the reaction rates in and out of the cavity to expose the influence of vibrational strong coupling on reaction kinetics. Our results will extend the potential of cavity-modified material properties, which will have important implications for ultrafast optical devices and chemical synthesis and catalysis.

[1] Dunkelberger, A. D.; et. al., Saturable Absorption in Solution-Phase and Cavity-Coupled Tungsten Hexacarbonyl, ACS Photonics, doi.org/10.1021/acsphotonics.9b00703

Reverse Saturable Absorption (RSA) is a nonlinear optical effect in which the excited state of a molecule or material exhibits greater absorptivity than its corresponding ground state. Traditional approaches to optimize this effect in transition metal compounds and materials are to employ extended aromatic rings to enhance absorption at longer wavelengths and to increase the excited state lifetime. In contrast, we have created a new class of chelating ligands comprising phosphorous and sulfur donor atoms. These types of ligands impart beneficial non-emissive properties to transition metal complexes. Complexes of Ir containing these new P,S ligands with derivatives of phenylpyridine will be prepared. In addition to standard characterization techniques (absorption spectroscopy, electrochemistry, NMR spectroscopy, crystallography, etc), these complexes will be interrogated by transient absorption spectroscopy from the femtosecond to the millisecond time scale. These data will determine the excited state spectra and RSA response as a function of wavelength. These data and others will be presented.

Metal-organic frameworks (MOFs) are a diverse class of highly ordered and tunable nanoscale materials that are increasingly employed in several solar energy conversion schemes. Investigation of light-harvesting and energy transfer processes within the three-dimensional framework of such nanoscopic materials results in more efficient design of biomimetic chromophore arrays for artificial photosynthesis. Here we present the synthesis and photophysical investigation of three anthracenic MOFs. The MOFs were synthesized by following a one-pot solvothermal synthetic protocol, and the powders were structurally characterized with the help of X-ray powdered diffraction (PXRD) patterns and scanning electron microscopy (SEM) images. Steady-state and time-resolved spectroscopic techniques aided in exploring the photophysical behavior in MOFs as a function of 3D structure. Upon surface functionalization of a sensitizing palladium porphyrin, excitation of the composite with green light resulted in upconverted emission. The role of the 3D incorporation of the anthracenes and resultant spacing of the anthracene units dictates the efficiency of the upconversion process.

Hybrid materials are commonly used for wide range of optical applications (sensors, filters, imaging, photocatalysis…)..[1] The use of interactions between emitting or absorbing systems and plasmonic nanostructures have also been an intensive field of research due to the possibility to tune and optimize the optical responses using the local electromagnetic field. The synthesis of gold nanostructures combining high yield, purity, large scale and plasmon resonance spreading from the visible to the NIR wavelengths was developped.[2,3] These nanostructures were functionalized in order to allow their homogeneous incorporation in transparent hybrid silica matrices using the sol-gel process.[4,5] Co-dispersion of the metallic structures with nonlinear dyes was successfully achieved.[6] The role of the concentration, shape and size of the metal nanoparticles on the optical response was evaluated. The respective impact on the nonlinear optical response of the dyes will be discussed both in the visible and the near infrared wavelengths.

References
[1] S. Parola, B. Julian-Lopez, L. D. Carlos, C. Sanchez, Adv. Funct. Mater. 2016, 26 (36), 6506-6544
[2] D. Chateau, A. Liotta, F. Vadcard, J. R. G. Navarro, F. Chaput, J. Lermé, F. Lerouge, S. Parola, Nanoscale 2014, 7, 1934.
[3] D. Chateau, A. Desert, F. Lerouge, G. Landaburu, S. Santucci, S. Parola, ACS Appl. Mater. Interfaces 2019, doi.org/10.1021/acsami.9b12973.
[4] D. Chateau, A. Liotta, D. Gregori, F. Lerouge, F. Chaput, A. Desert, S. Parola, J. Sol-Gel Sci. Technol. 2016, DOI 10.1007/s10971-016-4116-y.
[5] H. Lundén, A. Liotta, D. Chateau, F. Lerouge, F. Chaput, S. Parola, C. Brännlund, Z. Ghadyani, M. Kildemo, M. Lindgren, C. Lopes, J. Mater. Chem. C 2015, 3, 1026.
[6] D. Chateau, A. Liotta, H. Lundén, F. Lerouge, F. Chaput, D. Krein, T. Cooper, C. Lopes, M. Lindgren, S. Parola, Adv. Funct. Mater.2016, DOI: 10.1002/adfm.201601646

Reverse Saturable Absorption (RSA) is a nonlinear optical effect in which the excited state of a molecule or material exhibits greater absorptivity than its corresponding ground state. Traditional approaches to optimize this effect in transition metal compounds and materials are to employ extended aromatic rings to enhance absorption at longer wavelengths and to increase the excited state lifetime. In contrast, we have created a new class of chelating ligands comprising phosphorous and sulfur donor atoms. These types of ligands impart beneficial non-emissive properties to transition metal complexes. Complexes of Ir containing these new P,S ligands with derivatives of phenylpyridine will be prepared. In addition to standard characterization techniques (absorption spectroscopy, electrochemistry, NMR spectroscopy, crystallography, etc), these complexes will be interrogated by transient absorption spectroscopy from the femtosecond to the millisecond time scale. These data will determine the excited state spectra and RSA response as a function of wavelength. These data and others will be presented.

Metal-organic frameworks (MOFs) are a diverse class of highly ordered and tunable nanoscale materials that are increasingly employed in several solar energy conversion schemes. Investigation of light-harvesting and energy transfer processes within the three-dimensional framework of such nanoscopic materials results in more efficient design of biomimetic chromophore arrays for artificial photosynthesis. Here we present the synthesis and photophysical investigation of three anthracenic MOFs. The MOFs were synthesized by following a one-pot solvothermal synthetic protocol, and the powders were structurally characterized with the help of X-ray powdered diffraction (PXRD) patterns and scanning electron microscopy (SEM) images. Steady-state and time-resolved spectroscopic techniques aided in exploring the photophysical behavior in MOFs as a function of 3D structure. Upon surface functionalization of a sensitizing palladium porphyrin, excitation of the composite with green light resulted in upconverted emission. The role of the 3D incorporation of the anthracenes and resultant spacing of the anthracene units dictates the efficiency of the upconversion process.

Probing the nonlinear response in light-matter interactions offers the advantage of lifting some fundamental limitations existing within the scope of linear operations. One of the most striking examples is the ability of breaking the reciprocity of light transmission. Transmission through a medium characterized by a symmetric, time-independent, and linear permittivity tensor stays the same when reversing the excitation direction, or interchanging the source and the detector. While serving as an innate foundation in any optical design, it presents a fundamental challenge for applications requiring a non-reciprocal directionality of light propagation.

Here, we show that the option of non-reciprocity in the directionality and the efficiency of nonlinear generation is inherently incorporated in the response of nanoelements exhibiting magnetic dipolar (or higher-order multipolar) Mie resonance(s) in their linear response. In particular, we show that nonlinear generation from a nanoelement, where it regularly occurs nearly isotropically in all directions (due to the lack of phase matching constraints within nanoscale interaction volumes), can be made both unidirectional and non-reciprocal, such that the generation occurs predominantly in a single direction which, additionally, remains unchanged with respect to a fixed laboratory coordinate system when reversing the direction of the fundamental beam(s). In contrast with the previous studies, the proposed approach does not require asymmetry in either the geometry or the material composition of the nanoelement. Rather, it relies on the multiplicative nature of the nonlinear response which, in turn, allows a simultaneous change in phase of both the electric and the magnetic, nonlinearly induced, dipolar modes, when switching the phase of a single (either electric or magnetic) vector of the fundamental field. Furthermore, the interference can occur between various pathways within the electric and magnetic (nonlinearly produced) dipolar modes. As a result, non-reciprocity in terms of just a change in the efficiency of nonlinear generation when reversing the direction of any subset of the fundamental beams is inherent to and expected in the nonlinear response of most nanoelements, even the symmetric ones, and for most of the nonlinear processes. Targeted engineering of the relative strengths of various pathways within each (nonlinearly produced) multipolar mode may then allow an interferometric cancellation of the generated field for a given nonlinear process, for certain respective directions of the fundamental beams.

Reliance on multipolar interference in the suggested approach inherently assumes the manifestation of the described phenomena on the nanoscale, through the response of subwavelength-scale elements. These nanoelements can thus be used as building blocks to construct a metasurface or a medium with similar unique features in its nonlinear response. As an example, we present a metasurface operating as a one-way nonlinear mirror, where the image is formed by a process of difference frequency generation on one and the same side of the metasurface independently of the object location.

Both non-reciprocal directionality and inhibition of the nonlinear response require, however, a careful engineering of the respective strengths of various pathways within each (electric or magnetic) type of the nonlinearly produced multipolar partial wave. As such, these phenomena are not expected to manifest in natural nonlinear materials, even those possessing natural magnetic dipolar transitions. They, however, can be achieved via a tailored design of the effective magneto-electric nonlinear polarizabilities of a nanoelement.

Nonreciprocal directional dichroism is an unusual light-matter interaction that gives rise to diode-like behavior in low symmetry materials. The chiral varieties are particularly scarce due to the requirements for strong spin-orbit coupling, broken time reversal symmetry, and a chiral axis. We bring together magneto-optical spectroscopy and first principles calculations to reveal high energy, broad band nonreciprocal directional dichroism in Ni₃TeO₆ with special focus on behavior in the metamagnetic phase above 52 T. In addition to demonstrating this effect in the magnetochiral configuration, we explore the transverse magnetochiral orientation in which applied field and light propagation are orthogonal to the chiral axis and by so doing, uncover an additional configuration with a nonreciprocal response in the visible part of the spectrum. In a significant conceptual advance, we use first-principles methods to analyze how the Ni²⁺ d-to-d on-site excitations develop magnetoelectric character and present a microscopic model that unlocks the door to theory-driven discovery of chiral magnets with nonreciprocal properties.

Colloidal semiconductor quantum dots (QDs) are attractive materials for realizing highly flexible, solution-processable optical gain media with readily tunable operational wavelengths [1, 2]. However, QDs are difficult to use in lasing due to extremely short optical gain lifetimes limited by nonradiative multicarrier Auger recombination [3]. This, in particular, is a serious obstacle for realizing cw optically and electrically pumped lasing devices. Recently, we have explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination [4, 5]. Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping [4], which has been a long-standing goal in the field of colloidal nanostructures. Further, we have applied these dots to practically demonstrated the viability of a ‘zero-threshold optical gain’ concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption [5, 6]. Such charged QDs are optical-gain-ready without excitation, which allows us to reduce the lasing threshold to record-low values that are well below a fundamental single-exciton-per-dot limit [6]. Most recently, we have developed QD devices that operate as both an electroluminescent (EL) structure and a distributed feedback optically pumped laser [7]. By carefully engineering a refractive-index profile across the device stack, we have been able to demonstrate low-threshold lasing even with a very thin EL-active region, which comprises only three monolayers of the QDs. All of these recent developments demonstrate a considerable promise of colloidal QDs for implementing solution-processable optically and electrically pumped lasers operating across a wide range of wavelengths.

[1] V.I. Klimov, A.A. Mikhailovsky, S. Xu, A. Malko, J.A. Hollingsworth, C.A. Leatherdale, H.J. Eisler, M.G. Bawendi, Science290, 314 (2000).
[2] V.I. Klimov, S.A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J.A. McGuire, A. Piryatinski, Nature 447, 441 (2007).
[3] V.I. Klimov, A.A. Mikhailovsky, D.W. McBranch, C.A. Leatherdale, M.G. Bawendi, Science 287, 1011 (2000).
[4] J. Lim, Y.-S. Park, V.I. Klimov, Nat. Mater. 17, 42 (2018).
[5] K. Wu, Y.-S. Park, J. Lim, V.I. Klimov, Nat. Nanotech. 12, 1140 (2017).
[6] O.V. Kozlov, Y.-S. Park, J. Roh, I. Fedin, T. Nakotte, V.I. Klimov, Science 365, 672 (2019).
[7] J. Roh, Y.-S. Park, J. Lim, V.I. Klimov, Nat. Comm., in press(2019).

Achieving room-temperature quantum-mechanical strong coupling, or vacuum Rabi splitting, between a single emitter and a plasmon resonance has been a longstanding goal. Recently, two peaks have been observed in the scattering spectra of plasmonic metal nanostructures coupled to single molecules and single quantum dots, and this was taken as evidence of strong coupling. However, a two-peak scattering structure can also arise at intermediate coupling strengths, below the strong-coupling threshold, due to Fano-like interference between the plasmon and emitter dipole.
We unambiguously distinguish between intermediate and strong coupling by measuring both the scattering spectra and the photoluminescence spectra of coupled plasmon-emitter structures. Specifically, we couple single colloidal quantum dots to a plasmon resonator by placing them in the gap between a gold nanoparticle and a silver film. We observe weak, intermediate, and strong coupling in these hybrid metal-semiconductor structures at room temperature, depending on the detailed nanoscale structure of the metal nanoparticle.
These structures have the potential to provide ultrafast, low-power optical nonlinearities on the nanoscale. Both induced transparency and strong coupling can be canceled by absorbing a photon in the quantum dot, leading to a strong change in extinction at the quantum dot transition frequency. Since only a single photon must be absorbed by the QD for this to happen, the energy needed for modulation has the potential to be extremely low, and the structure has the potential to enable all-optical information processing, possibly including neuromorphic computation and quantum information processing.
One limitation to the practical application of these structures, however, is the random nature of their assembly and the resulting low yield of structures exhibiting the desired transparency or strong coupling. One approach we have taken to this low yield is to replace the metal nanoparticle with a metal scanning-probe tip. In this way, we are able to achieve reversible, controllable strong coupling to several individual quantum dots, one at a time. Our current efforts are focused on developing bottom-up techniques of chemical synthesis and assembly for the high-yield production of strongly-coupled metal-semiconductor hybrid nanostructures.

In this talk, we will describe a materials self-assembly approach to controllably weld gold nanorods end-to-end, forming much higher aspect ratio dimers in macroscale quantities, linking the nanorods using unfocused femtosecond light. Electrostatic-based molecular assembly controls the discrete selection of end-to-end dimer assemblies, preventing larger structures. Large, high-quality yields of welded dimers can be produced by illuminating these dimer suspensions at their capacitively coupled plasmon resonance wavelength using femtosecond light pulses. In-situ absorbance measurements demonstrate an isosbestic point which results from the conversion of single nanorods to capacitively coupled plasmon dimers, indicative of direct population conversion. We will discuss the underlying physical mechanisms for the welding process as well as the optical properties of the newly formed high aspect ratio dimers.