Multifunctional and Transformative Metaphotonics with Emerging Materials.

Future technologies underpinning multifunctional physical and chemical systems and compact biological sensors will rely on densely packed transformative and tunable circuitry employing nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of resonant metaphotonics may provide a versatile practical platform for nanoscale science by employing resonances in high-index dielectric nanoparticles and metasurfaces. Here, we discuss the recently emerged field of metaphotonics and describe its connection to material science and chemistry. For tunabilty, metaphotonics employs a variety of the recently highlighted materials such as polymers, perovskites, transition metal dichalcogenides, and phase change materials. This allows to achieve diverse functionalities of metasystems and metasurfaces for efficient spatial and temporal control of light by employing multipolar resonances and the physics of bound states in the continuum. We anticipate expanding applications of these concepts in nanolasers, tunable metadevices, metachemistry, as well as a design of a new generation of chemical and biological ultracompact sensing devices.

Giant and Tunable Excitonic Optical Anisotropy in Single-Crystal Halide Perovskites.

During the last years, giant optical anisotropy has demonstrated its paramount importance for light manipulation. In spite of recent advances in the field, the achievement of continuous tunability of optical anisotropy remains an outstanding challenge. Here, we present a solution to the problem through the chemical alteration of halogen atoms in single-crystal halide perovskites. As a result, we manage to continually modify the optical anisotropy by 0.14. We also discover that the halide perovskite can demonstrate optical anisotropy up to 0.6 in the visible range─the largest value among non-van der Waals materials. Moreover, our results reveal that this anisotropy could be in-plane and out-of-plane depending on perovskite shape─rectangular and square. As a practical demonstration, we have created perovskite anisotropic nanowaveguides and shown a significant impact of anisotropy on high-order guiding modes. These findings pave the way for halide perovskites as a next-generation platform for tunable anisotropic photonics.

Polaron-Enhanced Polariton Nonlinearity in Lead Halide Perovskites.

Exciton-polaritons offer a versatile platform for realization of all-optical integrated logic gates due to the strong effective optical nonlinearity resulting from the exciton-exciton interactions. In most of the current excitonic materials there exists a direct connection between the exciton robustness to thermal fluctuations and the strength of the exciton-exciton interaction, making materials with the highest levels of exciton nonlinearity applicable at cryogenic temperatures only. Here, we show that strong polaronic effects, characteristic for perovskite materials, allow overcoming this limitation. Namely, we demonstrate a record-high value of the nonlinear optical response in the nanostructured organic-inorganic halide perovskite MAPbI3, experimentally detected as a 19.7 meV blueshift of the polariton branch under femtosecond laser irradiation. This is substantially higher than characteristic values for the samples based on conventional semiconductors and monolayers of transition-metal dichalcogenides. The observed strong polaron-enhanced nonlinearity exists for both tetragonal and orthorhombic phases of MAPbI3 and remains stable at elevated temperatures.

Enhanced Multiphoton Processes in Perovskite Metasurfaces.

Multiphoton absorption and luminescence are fundamentally important nonlinear processes for utilizing efficient light-matter interaction. Resonant enhancement of nonlinear processes has been demonstrated for many nanostructures; however, it is believed that all higher-order processes are always much weaker than their corresponding linear processes. Here, we study multiphoton luminescence from structured surfaces and, combining multiple advantages of perovskites with the concept of metasurfaces, we demonstrate that the efficiency of nonlinear multiphoton processes can become comparable to the efficiency of the linear process. We reveal that the perovskite metasurface can enhance substantially two-photon stimulated emission with the threshold being comparable with that of the one-photon process. Our modeling of free-carrier dynamics and exciton recombination upon nonlinear photoexcitation uncovers that this effect can be attributed to the local field enhancement in structured media, a substantial increase of the mode overlap, and the selection rules of two-photon absorption in perovskites.

Directional Lasing from Nanopatterned Halide Perovskite Nanowire.

Halide perovskite nanowire-based lasers have become a powerful tool for modern nanophotonics, being deeply subwavelength in cross-section and demonstrating low-threshold lasing within the whole visible spectral range owing to the huge gain of material even at room temperature. However, their emission directivity remains poorly controlled because of the efficient outcoupling of radiation through their subwavelength facets working as pointlike light sources. Here, we achieve directional lasing from a single perovskite CsPbBr3 nanowire by imprinting a nanograting on its surface, which provides stimulated emission outcoupling to its vertical direction with a divergence angle around 2°. The nanopatterning is carried out by the high-throughput laser ablation method, which preserves the luminescent properties of the material that is typically deteriorated after processing via conventional lithographic approaches. Moreover, nanopatterning of the perovskite nanowire is found to decrease the number of the lasing modes with a 2-fold increase of the quality factor of the remaining modes.

Lasing Action from Anapole Metasurfaces.

We study active dielectric metasurfaces composed of two-dimensional arrays of split-nanodisk resonators fabricated in InGaAsP membranes with embedded quantum wells. Depending on the geometric parameters, such split-nanodisk resonators can operate in the optical anapole regime originating from an overlap of the electric dipole and toroidal dipole Mie-resonant optical modes, thus supporting strongly localized fields and high-Q resonances. We demonstrate room-temperature lasing from the anapole lattices of engineered active metasurfaces with low threshold and high coherence.

Stimulated Raman Scattering from Mie-Resonant Subwavelength Nanoparticles.

Resonant dielectric structures have emerged recently as a new platform for subwavelength nonplasmonic photonics. It was suggested and demonstrated that magnetic and electric Mie resonances can enhance substantially many effects at the nanoscale including spontaneous Raman scattering. Here, we demonstrate stimulated Raman scattering (SRS) for isolated crystalline silicon (c-Si) nanoparticles and observe experimentally a transition from spontaneous to stimulated scattering manifested in a nonlinear growth of the signal intensity above a certain pump threshold. At the Mie resonance, the light gets confined into a low volume of the resonant mode with enhanced electromagnetic fields inside the c-Si nanoparticle due to its high refractive index, which leads to an overall strong SRS signal at low pump intensities. Our finding paves the way for the development of efficient Raman nanolasers for multifunctional photonic metadevices.

Light-Emitting Nanophotonic Designs Enabled by Ultrafast Laser Processing of Halide Perovskites.

Nanophotonics based on resonant nanostructures and metasurfaces made of halide perovskites have become a prospective direction for efficient light manipulation at the subwavelength scale in advanced photonic designs. One of the main challenges in this field is the lack of large-scale low-cost technique for subwavelength perovskite structures fabrication preserving highly efficient luminescence. Here, unique properties of halide perovskites addressed to their extremely low thermal conductivity (lower than that of silica glass) and high defect tolerance to apply projection femtosecond laser lithography for nanofabrication with precise spatial control in all three dimensions preserving the material luminescence efficiency are employed. Namely, with CH3 NH3 PbI3 perovskite highly ordered nanoholes and nanostripes of width as small as 250 nm, metasurfaces with periods less than 400 nm, and nanowire lasers as thin as 500 nm, corresponding to the state-of-the-art in multistage expensive lithographical methods are created. Remarkable performance of the developed approach allows to demonstrate a number of advanced optical applications, including morphology-controlled photoluminescence yield, structural coloring, optical- information encryption, and lasing.

Electronic structure of CsPbBr3-xClx perovskites: synthesis, experimental characterization, and DFT simulations.

All-inorganic lead halide perovskites of various compositions have emerged as a prospective family of materials for light-emitting devices and photonic applications. However, a comprehensive study of their structural and electronic properties is still missing. Moreover, thin film fabrication of these perovskites comprising heterohalide anions by wet chemistry approaches also remains challenging. Here we fabricate high-quality CsPbBr3-xClx perovskite thin films using a wet chemical method accompanied by a chemical vapor anion exchange procedure, which allows rigorously studying their optical and structural properties at different compositions. Namely, we present both the numerical and experimental studies of the electronic properties of all-inorganic mixed-halide perovskites, showing their optical absorption, excitonic photoluminescence and exciton binding energy, phase, chemical composition, and band structure and the band gap evolution with a gradual change in x in CsPbBr3-xClx. The results reveal that as Cl ions substitute for Br ones in the perovskite crystal lattice the room-temperature phase does not change its orthorhombic symmetry, whereas the energy of the direct electronic transition from the valence to conduction band at the Γ-point increases nonlinearly. By using the experimentally derived nonlinear dependence it is easy to accurately predict the band gap for any CsPbBr3-xClx perovskite thin film consisting of grains with sizes beyond the quantum confinement regime.

Tunable Hybrid Fano Resonances in Halide Perovskite Nanoparticles.

Halide perovskites are known to support excitons at room temperatures with high quantum yield of luminescence that make them attractive for all-dielectric resonant nanophotonics and meta-optics. Here we report the observation of broadly tunable Fano resonances in halide perovskite nanoparticles originating from the coupling of excitons to the Mie resonances excited in the nanoparticles. Signatures of the photon-exciton (" hybrid") Fano resonances are observed in dark-field spectra of isolated nanoparticles, and also in the extinction spectra of aperiodic lattices of such nanoparticles. In the latter case, chemical tunability of the exciton resonance allows reversible tuning of the Fano resonance across the 100 nm bandwidth in the visible frequency range, providing a novel approach to control optical properties of perovskite nanostructures. The proposed method of chemical tuning paves the way to an efficient control of emission properties of on-chip-integrated light-emitting nanoantennas.

Resonant Nonplasmonic Nanoparticles for Efficient Temperature-Feedback Optical Heating.

We propose a novel photothermal approach based on resonant dielectric nanoparticles, which possess imaginary part of permittivity significantly smaller as compared to metal ones. We show both experimentally and theoretically that a spherical silicon nanoparticle with a magnetic quadrupolar Mie resonance converts light to heat up to 4 times more effectively than similar spherical gold nanoparticle at the same heating conditions. We observe photoinduced temperature raise up to 900 K with the silicon nanoparticle on a glass substrate at moderate intensities (<2 mW/µm2) and typical laser wavelength (633 nm). The advantage of using crystalline silicon is the simplicity of local temperature control by means of Raman spectroscopy working in a broad range of temperatures, that is, up to the melting point of silicon (1690 K) with submicrometer spatial resolution. Our CMOS-compatible heater-thermometer nanoplatform paves the way to novel nonplasmonic photothermal applications, extending the temperature range and simplifying the thermoimaging procedure.

Efficient Second-Harmonic Generation in Nanocrystalline Silicon Nanoparticles.

Recent trends to employ high-index dielectric particles in nanophotonics are motivated by their reduced dissipative losses and large resonant enhancement of nonlinear effects at the nanoscale. Because silicon is a centrosymmetric material, the studies of nonlinear optical properties of silicon nanoparticles have been targeting primarily the third-harmonic generation effects. Here we demonstrate, both experimentally and theoretically, that resonantly excited nanocrystalline silicon nanoparticles fabricated by an optimized laser printing technique can exhibit strong second-harmonic generation (SHG) effects. We attribute an unexpectedly high yield of the nonlinear conversion to a nanocrystalline structure of nanoparticles supporting the Mie resonances. The demonstrated efficient SHG at green light from a single silicon nanoparticle is 2 orders of magnitude higher than that from unstructured silicon films. This efficiency is significantly higher than that of many plasmonic nanostructures and small silicon nanoparticles in the visible range, and it can be useful for a design of nonlinear nanoantennas and silicon-based integrated light sources.

Experimental study of fs-laser induced sub-100-nm periodic surface structures on titanium.

In this work the formation of laser-induced periodic surface structures (LIPSS) on a titanium surface upon irradiation by linearly polarized femtosecond (fs) laser pulses with a repetition rate of 1 kHz in air environment was studied experimentally. In particular, the dependence of high-spatial-frequency-LIPSS (HSFL) characteristics on various laser parameters: fluence, pulse number, wavelength (800 nm and 400 nm), pulse duration (10 fs - 550 fs), and polarization was studied in detail. In comparison with low-spatial-frequency-LIPSS (LSFL), the HSFL emerge at a much lower fluence with orientation perpendicular to the ridges of the LSFL. It was observed that these two types of LIPSS demonstrate different fluence, shot number and wavelength dependencies, which suggest their origin is different. Therefore, the HSFL formation mechanism cannot be described by the widely accepted interference model developed for describing LSFL formation.

Giant Ultrafast All-Optical Modulation Based on Exceptional Points in Exciton-Polariton Perovskite Metasurfaces.

Ultrafast all-optical modulation with optically resonant nanostructures is an essential technology for high-speed signal processing on a compact optical chip. Key challenges that exist in this field are relatively low and slow modulations in the visible range as well as the use of expensive materials. Here we develop an ultrafast all-optical modulator based on MAPbBr3 perovskite metasurface supporting exciton-polariton states with exceptional points. The additional angular and spectral filtering of the modulated light transmitted through the designed metasurface allows us to achieve 2500% optical signal modulation with the shortest modulation time of 440 fs at the pump fluence of ∼40 µJ/cm2. Such a value of the modulation depth is record-high among the existing modulators in the visible range, while the main physical effect behind it is polariton condensation. Scalable and cheap metasurface fabrication via nanoimprint lithography along with the simplicity of perovskite synthesis and deposition make the developed approach promising for real-life applications.

Photoinduced Transition from Quasi-Two-Dimensional Ruddlesden-Popper to Three-Dimensional Halide Perovskites for the Optical Writing of Multicolor and Light-Erasable Images.

Optical data storage, information encryption, and security labeling technologies require materials that exhibit local, pronounced, and diverse modifications of their structure-dependent optical properties under external excitation. Herein, we propose and develop a novel platform relying on lead halide Ruddlesden-Popper phases that undergo a light-induced transition toward bulk perovskite and employ this phenomenon for the direct optical writing of multicolor patterns. This transition causes the weakening of quantum confinement and hence a reduction in the band gap. To extend the color gamut of photoluminescence, we use mixed-halide compositions that exhibit photoinduced halide segregation. The emission of the films can be tuned across the range of 450-600 nm. Laser irradiation provides high-resolution direct writing, whereas continuous-wave ultraviolet exposure is suitable for recording on larger scales. The luminescent images created on such films can be erased during the visualization process. This makes the proposed writing/erasing platform suitable for the manufacturing of optical data storage devices and light-erasable security labels.

Perovskite Microlaser Integration with Metasurface Supporting Topological Waveguiding.

Halide perovskite nano- and microlasers have become a very convenient tool for many applications from sensing to reconfigurable optical chips. Indeed, they exhibit outstanding emission robustness to crystalline defects due to so-called "defect tolerance" allowing for their simple chemical synthesis and further integration with various photonic designs. Here we demonstrate that such robust microlasers can be combined with another class of resilient photonic components, namely, with topological metasurfaces supporting topological guided boundary modes. We show that this approach allows to outcouple and deliver the generated coherent light over tens of microns despite the presence of defects of different nature in the structure: sharp corners in the waveguide, random location of the microlaser, and defects in the microlaser caused by mechanical pressure applied during its transfer to the metasurface. As a result, the developed platform provides a strategy to attain robust integrated lasing-waveguiding designs resilient to a broad range of structural imperfections, both for electrons in a laser and for pseudo-spin-polarized photons in a waveguide.

Photoinduced Self-Gating of Perovskite Photovoltaic Cells in Ionic Liquid.

We demonstrate that the power conversion efficiency (PCE), photocurrent, and fill factor (FF) of perovskite solar cells (PSC) can be significantly improved by the photoinduced self-gating in ionic liquids (ILs) via n-doping of the carbon nanotube (CNT) top electrode on the fullerene electron transport layer (ETL). CNTs, graphene, and other carbon electrodes have been proven to be stable electrodes for PSC, but efficiency was not high. We have previously shown that the performance of PSCs with CNT electrodes can be improved by IL gating with gate voltage (Vg) applied from an external power source. Here we demonstrate that effective self-gating in ILs is possible by a photoinduced process, without an external source. The open circuit voltage (Voc) generated by the PSC itself can be applied to the CNT/C60 electrode as Vg leading to photogating. This self-gating with Voc is compared to photocharging of CNTs in ILs without any gating for two types of fullerene ETLs: C60 and C70, Two types of ILs, DEME-TFSI and BMIM-BF4, are tested for two types of nanotubes electrodes: single wall (SWCNT), and multiwall (MWCNT). The resulting improvements are analyzed using the effective diode-circuit (DC) and the drift-diffusion (DD) models. Self-gating allows the PCE improvement from 3-5% to 10-11% for PSCs with a thick ETL, while for optimal combination of a thin SWCNT/ETL with added layers for improved stability, the PCE reached 13.2% in DEME-TFSI IL.

Light-Controlled Multiphase Structuring of Perovskite Crystal Enabled by Thermoplasmonic Metasurface.

Halide perovskites belong to an important family of semiconducting materials with electronic properties that enable a myriad of applications, especially in photovoltaics and optoelectronics. Their optical properties, including photoluminescence quantum yield, are affected and notably enhanced at crystal imperfections where the symmetry is broken and the density of states increases. These lattice distortions can be introduced through structural phase transitions, allowing charge gradients to appear near the interfaces between phase structures. In this work, we demonstrate controlled multiphase structuring in a single perovskite crystal. The concept uses cesium lead bromine (CsPbBr3) placed on a thermoplasmonic TiN/Si metasurface and enables single-, double-, and triple-phase structures to form on demand above room temperature. This approach promises application horizons of dynamically controlled heterostructures with distinctive electronic and enhanced optical properties.

Solution-Processed OLED Based on a Mixed-Ligand Europium Complex.

An approach to increase the efficiency of europium-based OLEDs was proposed through the formation of a mixed-ligand complex. The design of a series of europium complexes, together with an optimization of the solution deposition, including the host selection, as well as the variation of the solvent and deposition parameters, resulted in a noticeable increase in OLED luminance. As a result, the maximum luminance of the Eu-based OLED reached up to 700 cd/m2, which is one of the highest values for an Eu-based solution-processed OLED. Finally, its stability was investigated.

Perovskite Nanowire Laser for Hydrogen Chloride Gas Sensing.

Detection of hazardous volatile organic and inorganic species is a crucial task for addressing human safety in the chemical industry. Among these species, there are hydrogen halides (HX, X = Cl, Br, I) vastly exploited in numerous technological processes. Therefore, the development of a cost-effective, highly sensitive detector selective to any HX gas is of particular interest. Herein, we demonstrate the optical detection of hydrogen chloride gas with solution-processed halide perovskite nanowire lasers grown on a nanostructured alumina substrate. An anion exchange reaction between a CsPbBr3 nanowire and vaporized HCl molecules results in the formation of a structure consisting of a bromide core and thin mixed-halide CsPb(Cl,Br)3 shell. The shell has a lower refractive index than the core does. Therefore, the formation and further expansion of the shell reduce the field confinement for experimentally observed laser modes and provokes an increase in their frequency. This phenomenon is confirmed by the coherency of the data derived from XPS spectroscopy, EDX analysis, in situ XRD experiments, HRTEM images, and fluorescent microspectroscopy, as well as numerical modeling for Cl- ion diffusion and the shell-thickness-dependent spectral position of eigenmodes in a core-shell perovskite nanowire. The revealed optical response allows the detection of HCl molecules in the 5-500 ppm range. The observed spectral tunability of the perovskite nanowire lasers can be employed not only for sensing but also for their precise spectral tuning.