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In this paper, a comparative analysis and numerical simulation of operation of two types of amplitude binary gratings (conventional and fork), both in the focal plane and near-field diffraction under illumination by mode beams with different polarization states, were performed. The simulation of the field formation in the focal plane was performed using the Richards–Wolf formalism. The diffraction calculation in the near-field diffraction was performed based on the FDTD method, considering the 3D structure of optical elements. The possibility of multiplying the incident beam in different diffraction orders of binary gratings and the polarization transformation associated with spin–orbit interaction at tight focusing were shown. In this case, various polarization transformations were formed in ±1 diffraction orders of the fork grating due to different signs of the introduced vortex-like phase singularity. The obtained results can be useful for the laser processing of materials and surface structuring. [ABSTRACT FROM AUTHOR]

Direct bandgap semiconductors possess a unique attribute: the presence of a free exciton state with a high total oscillator strength. This property makes them highly promising for applications in ultrafast optical signal processing and optical computing. One such protocol for optical computing is based on four‐wave mixing (FWM). In this study, the nonlinear optical effect in polycrystalline thin films of halide perovskite MAPbI3 (MA+ = CH3NH3+$_3^+$) at low temperatures is demonstrated. Through analyzing the spectroscopy of the FWM signal, studying the photoluminescence excitation spectra, and comparing the findings with results from MAPbI3 single crystals, it has been discovered that the strongest nonlinear response is observed at the free exciton resonance and in the region of shallow defect states. Surprisingly, the presence of FWM in cross‐linear excitation geometry has been observed, indicating potential involvement of other nonlinear or many‐body effects. The observations of FWM with free excitons even in highly defective MAPbI3 thin films demonstrate the robustness of the exciton resonance and highlight the practical prospects for utilizing this material in optical computing. [ABSTRACT FROM AUTHOR]

The spin degree of freedom of charge carriers in halide‐perovskite semiconductors can be highly useful for information photonics applications. The Faraday effect is known to be the best indicator of paramagnetism of the material and of the spin‐light interaction. In this work, the Faraday effect is demonstrated, for the first time, in a hybrid organic–inorganic halide perovskite MAPbI3 (MA+ = CH3NH3+$_3^+$). The Faraday rotation and birefringence are measured across the tetragonal‐cubic phase transition at 327 K. The Faraday rotation is strongly suppressed below the phase transition temperature due to anisotropy (linear birefringence) of the tetragonal crystal phase. The situation changes drastically above the phase transition temperature, when the crystal becomes optically isotropic. The emerging Faraday rotation obeys the Curie law, demonstrating its population‐related paramagnetic nature. This observation opens new prospects for application of these systems and for their investigations using methods of the polarization noise spectroscopy applicable to optically anisotropic materials. [ABSTRACT FROM AUTHOR]

We investigate vibrations of the pyridinium cation PyH+ = C5H5NH+ in one-dimensional lead halide perovskites PyPbX3 and pyridinium halide salts PyHX (X− = I−, Br−), combining infrared absorption and Raman scattering methods at room temperature. Internal vibrations of the cation were assigned based on density functional theory modeling. Some of the vibrational bands are sensitive to perovskite or the salt environment in the solid state, while halide substitution has only a minor effect on them. These findings have been confirmed by 1H, 13C and 207Pb solid-state nuclear magnetic resonance (NMR) experiments. Narrower vibrational bands in perovskites indicate less disorder in these materials. The splitting of NH-group vibrational bands in perovskites can be rationalized the presence of nonequivalent crystal sites for cations or by more exotic phenomena such as quantum tunneling transition between two molecular orientations. We have shown how organic cations in hybrid organic–inorganic crystals could be used as spectators of the crystalline environment that affects their internal vibrations. [ABSTRACT FROM AUTHOR]

Internal vibrations of organic cations in halide perovskites and their analogues could be used to study the crystal structure of these novel semiconductor materials. In this work, we have studied the vibration properties of the 3‐cyanopyridinium (3cp+ = [3‐CN‐C5H5NH]+) cation in the hybrid organic–inorganic halide post‐perovskite (3cp)PbBr3. For DFT modeling of the experimental Raman spectrum, we have constructed three different models: free cation, minimal stoichiometric cluster and nanocluster. Calculations of a free cation adequately describe most of the internal vibrations. To describe high‐wavenumber hydrogen stretching vibrations, and first of all N–H vibrations, it is necessary to use sufficiently large clusters. We show in the cluster approach for crystal field description that it is necessary to include in the cluster not only halogens but also their nearest environment. In this case, agreement with experiment is reached, and further considerations can be put forward about the strength of the hydrogen bond and its role in stabilising the crystal. [ABSTRACT FROM AUTHOR]

Resonant diffractive optical elements can be used to create the desired spatial and angular distribution of scattered light in a narrow wavelength range. Exciton resonance could help to make such elements active. Here an exciton diffraction grating made from fully inorganic CsPbBr3 halide perovskite is demonstrated. Spatial modulation of excitonic properties of the single crystal is achieved using local defect formation by 30 keV Ga+ focused ion irradiation with low irradiation dose 1014 cm−2. Such a low dose prevents the appearance of non‐resonant diffraction since no sample milling occurs. The observed diffraction reflex has a maximum efficiency of 2.3 × 10−3 at the free exciton resonance and 1.7 meV spectral width. The temperature, angular, and spectral properties of resonant diffraction are studied. The possibility of creating exciton diffraction gratings based on halide perovskites is one more confirmation of their applicability as a new material for information photonics. [ABSTRACT FROM AUTHOR]

Halide perovskites are novel photonics materials promising numerous applications in fields such as photovoltaics, LED light sources, microlasers, and radiation detectors. Many halide perovskites are direct-gap semiconductors, and Wannier–Mott excitons play a significant role in their optical properties near the fundamental absorption edge. The high oscillator strength of these states favors applications where efficient interaction with light is required. In this work, to study excitonic states in CsPbBr 3 halide perovskite single crystals, the reflection spectroscopy at temperatures from 4 K was used. A reflection coefficient up to 70% was observed for the n = 1 exciton state, followed by weak excited states of excitons with n = 2 and n = 3 . It should be noted that the Sommerfeld enhancement factor should be considered for a correct description of the behavior of the dielectric constant, taking into account excitonic effects. [ABSTRACT FROM AUTHOR]

The MAPbI3 halide perovskite single crystals are studied at 5 K temperature using the photoluminescence excitation spectroscopy. Two noninteracting types of states are determined: bound excitons and defect‐related states. Excitation of the crystal with light energy below the bound exciton resonance reveals the complex low‐density defect emission, otherwise hidden by the emission of bound excitons. A way to separate these defect‐related luminescence spectra is proposed, and the thorough study of this emission regime is carried out. The results of this study open an area of low‐density defect and dopant exploration in halide perovskite semiconductors. [ABSTRACT FROM AUTHOR]

Halide perovskites are a promising optical gain medium with high tunability and simple solution synthesis. In this study, two gain regimes, namely, amplified spontaneous emission and random lasing, are demonstrated in the same MAPbBr3 halide perovskite single crystal. For this, photoluminescence is measured at a temperature of 4 K with pulsed femtosecond pumping by UV light with an 80 MHz repetition rate. Random lasing is observed in areas of the sample where a random resonator is formed due to cracks and crystal imperfections. In more homogeneous regions of the sample, the dominant regime is amplified spontaneous emission. These two regimes are reliably distinguished by the line width, the mode structure, the growth of the intensity after the threshold, and the degree of polarization of the radiation. The spectral localization of the stimulated emission well below the bound exciton resonance raises a question concerning the origin of the emission in halide perovskite lasers. [ABSTRACT FROM AUTHOR]

Chloride perovskites are semiconductors with a near-ultraviolet bandgap that are promising for applications in optoelectronics and photonics. One of the most studied representatives of this family is the methylammonium lead chloride MAPbCl 3 (MA + = CH 3 NH 3 + ). Low-temperature luminescence spectroscopy of this material demonstrates a complex emission structure. In this work, we have studied the cathodoluminescecne of the MAPbCl 3 halide perovskite single crystal at 70 K. Excitation by an electron beam was used to localize different emitters: excitons, defect-related states, and inclusions, previously assigned to the material itself. Exciton luminescence is observed from an undamaged sample, while the defect band is emitted from regions with dislocations, growth defects, and crystal damage. Defect formation under electron beam irradiation was studied. It was found that MAPbCl 3 is resistant to irradiation, which supports the defect tolerance of halide perovskites and paves the way for their electron-beam modification for applications.

Heterovalent CsPbBr 3 doping with Bi results in a significant red shift of the optical absorption of both single-crystal and powdered samples. The results of low-temperature (3.6 K) photoluminescence studies of perovskite single crystals indicate that the position of the excitonic luminescence peak remains unaffected by Bi doping that, in turn, infers that the band gap of Bi-doped perovskite is not changed as well. The position and state density distribution of the valence band and Fermi level of single-crystal perovskites were determined by another direct method of ultraviolet photoelectron spectroscopy. The obtained results show that Bi 3+ doping causes no changes in the valence band structure but an increase in the Fermi level by 0.6 eV. The summary of the obtained results directly demonstrates that the concept of the band-gap engineering in Bi 3+ -doped CsPbBr 3 halide perovskite is not valid.

We present an optical study of MAPbBr 3 single crystal grown from solution. The crystal Pm3m symmetry was confirmed by electron backscatter diffraction. Our major attention was focused on optical effects related to the excitonic states in MAPbBr 3 . Photoluminescence temperature dependence of narrow exciton resonance showed encouragingly low inhomogeneous broadening Γ ≈ 0.5 meV that allows one to distinguish the signals from free excitons and those arising from recombination of excitons localized on defects. Excitonic origin of the resonance was proved by its superlinear pump intensity dependence, in contrast to the linear behavior of the defect-assisted recombination bands. For the first time, the phonon replicas originating from free exciton recombination accompanied by partial energy transfer to the phonons were observed in high-resolution PL spectra and confirmed by independent low-temperature Raman scattering experiments. In turn, low-temperature, low-frequency Raman scattering studies let us resolve the structure of the low-frequency phonon spectrum.