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Indium (III) selenide is currently one of the most actively studied materials in the two-dimensional family due to its remarkable ferroelectric and optical properties. This study focuses on the luminescent properties of few-layer In$_2$Se$_3$ flakes with thicknesses ranging from 7 to 100 monolayers. To explore the photoluminescence features and correlate them with changes in crystal symmetry and surface potential, we employed a combination of techniques, including temperature-dependent micro-photoluminescence, time-resolved photoluminescence, Raman spectroscopy, atomic force microscopy, and Kelvin probe force microscopy. X-ray diffraction and Raman spectroscopy confirmed that the samples studied possess the $\alpha$-polytype structure. The micro-photoluminescence spectrum consists of two bands, A and B, with band B almost completely disappearing at room temperature. Temperature-dependent photoluminescence and time-resolved measurements helped us to elucidate the nature of the observed bands. We find that peak A is associated with emission from interband transitions in In$_2$Se$_3$, while peak B is attributed to defect-related emission. Additionally, the photoluminescence decay times of In$_2$Se$_3$ flakes with varying thicknesses were determined. No significant changes were observed in the decay components as the thickness increased from 7 to 100 monolayers, suggesting that there are no qualitative changes in the band structure.

Transition metal dichalcogenides (TMDCs) have emerged as highly promising materials for nanophotonics and optoelectronics due to their exceptionally high refractive indices, strong excitonic photoluminescence (PL) in monolayer configurations, and the versatility to engineer van der Waals (vdW) heterostructures. In this work, we exploit the intense excitonic PL of a MoSe$_2$ monolayer combined with the high refractive index of bulk WS$_2$ to fabricate microdisk cavities with tunable light emission characteristics. These microdisks are created from a 50-nm-thick WS$_2$/MoSe$_2$/WS$_2$ double heterostructure using frictional mechanical scanning probe lithography. The resulting cavities achieve a 4-10-fold enhancement in excitonic PL from the MoSe$_2$ monolayer at wavelengths near 800 nm. The excitonic PL peak is modulated by sharp spectral features, which correspond to whispering gallery modes (WGMs) supported by the cavity. A microdisk with a diameter of 2.35 $\mu$m demonstrates WGMs with a quality factor of up to 700, significantly surpassing theoretical predictions and suggesting strong potential for lasing applications. The spectral positions of the WGMs can be finely tuned by adjusting the microdisk's diameter and thickness, as confirmed by theoretical calculations. This approach offers a novel route for developing ultra-compact, all-TMDC double heterostructure light sources with record-small size.

Rare-earth orthoferrites are a promising platform for antiferromagnetic spintronics with a rich variety of terahertz spin and lattice dynamics phenomena. For instance, it has been experimentally demonstrated that the light-driven optical phonons can coherently manipulate macroscopic magnetic states via nonlinear magnetophononic effects. Here using TbFeO$_{3}$ as an example, we reveal the origin of the mode mixing between the LO and TO phonons, which is important for understanding of nonlinear phononics. We performed a comprehensive study of the lattice dynamics of the TbFeO$_{3}$ single crystal by polarized infrared and Raman scattering spectroscopic techniques, and experimentally obtained and carefully analyzed the spectra of anisotropic complex dielectric functions in the far-infrared spectral range. This allowed us to reliably identify the symmetries and parameters of most infrared- and Raman-active phonons. Next, the experimental studies were supplemented by the lattice dynamics calculations which allowed us to propose the normal mode assignments. We reveal that the relation between LO and TO polar phonons is complex and does not strictly follow the ``LO-TO rule'' due to the strong mode mixing. We further analyze how displacements of different ions contribute to phonon modes and reveal that magnetic Fe ions are not involved in Raman-active phonons, thus shedding light on a lack of spin phonon coupling for such phonons. The obtained results establish a solid basis for further in-depth experimental research in the field of nonlinear phononics and magnetophononics in rare-earth orthoferrites., Comment: 23 pages, 15 figures, 6 tables

Temperature sensors based on the GeV color center in diamond are gaining considerable attention in both scientific and industrial fields. For widespread industrial adoption, however, these sensors need a design that is as simple and cost-effective as possible. The original sensor design relied on measuring the spectral characteristics of the zero-phonon line. Recently, a modified approach was introduced, which involves splitting the GeV emission with a dichroic mirror and determining temperature based on the ratio of the two resulting signals. In this analysis, we provide a detailed comparison of both methods. At room temperature, the two methods show comparable performance, with slight variations depending on component quality. However, at temperatures around 300 {\deg}C, the new method's performance is estimated to be nearly twice that of the original, provided optimal filter parameters are used. Additionally, the sensitivity of the new method remains roughly consistent with its performance at room temperature.

We have modified the metal-assisted transfer technique to obtain large-area few-layer flakes from transition metal dichalcogenides bulk crystals by introducing an initial stage - exfoliation of the bulk crystal onto an intermediate substrate, specifically a silicon wafer coated with polyvinyl alcohol. Following this, we thermally evaporate silver onto the sample and transfer the top layers of the crystal along with the silver layer to the target substrate. This technique allows the production of visually non-corrugated single- and few-layer flakes with high yield. A direct comparison of the micro-Raman and micro-photoluminescence spectra of flakes exfoliated using our method with the spectra of those exfoliated from scotch tape reveals differences in their properties. We identify signatures of deformations in the flakes exfoliated from the intermediate substrate, indicating the presence of static friction between the substrate and the flake. Our findings thus suggest a novel method to induce intrinsic deformation in 2D materials., Comment: 6 pages 4 figures; Supplementary available on request

The critical behaviour of laser-induced changes in magnetic ordering is studied experimentally in two-dimensional zigzag antiferromagnets $XY$-like NiPS$_3$ and Ising-like FePS$_3$. To examine laser-induced dynamics in flakes of these compounds, we employ time-resolved exchange linear dichroism effect sensitive to zigzag magnetic ordering and independent of the orientation of the antiferromagnetic vector. In both compounds laser excitation in the vicinity of the absorption edge induces partial quenching of the antiferromagnetic ordering manifested by exchange linear dichroism reduction. The amplitude of the effect varies with temperature as the derivative of the antiferromagnetic vector and exhibits a critical behaviour with the exponents corresponding to $XY$- and Ising-models for NiPS$_3$ and FePS$_3$, respectively. Critical slowing down of the demagnetization in the vicinity of N\'eel temperature is found, however, only in FePS$_3$. In contrast, the increase of the demagnetization time near the ordering temperature in NiPS$_3$ is minor. We show that the difference in the demagnetization times correlates well with the spin specific heat in both compounds. Beyond the range of slowing down, the demagnetization times in NiPS$_3$ and FePS$_3$ are comparable, about 5 - 10 ps, and are longer than those reported earlier for CoPS$_3$ and considerably shorter than for MnPS$_3$. This points to the importance of the unquenched angular momentum of transition-metal ions in laser-induced demagnetization process., Comment: 12 pages, 6 figures

Core-shell nanorods (NRs) with InGaN/GaN quantum wells (QWs) are promising for monolithic white light-emitting diodes and multicolor displays. Such applications, however, are still a challenge because intensity of red band is too weak as compared with blue and green ones. To clarify the problem, we have performed power and temperature dependent, as well as time-resolved measurements of photoluminescence (PL) in NRs of different In content and diameter. These studies have shown that the dominant PL bands originate from nonpolar and semipolar QWs, while a broad yellow-red band arises mostly from defects in the GaN core. Intensity of red emission from the polar QWs at the NR tip is fatally small. Our calculation of electromagnetic field distribution inside the NRs shows a low density of photon states in the tip that suppresses the red radiation. We suggest a design of hybrid NRs, in which polar QWs, located inside the GaN core, are pumped by UV-blue radiation of nonpolar QWs. Possibilities of radiative recombination rate enhancement by means of the Purcell effect are discussed., Comment: 11 pages, 7 figures

Linear in the wave-vector terms of an electron Hamiltonian play an important role in topological insulators and spintronic devices. Here we demonstrate how an external electric field controls the magnitude of a linear-in-K term in the exciton Hamiltonian in wide GaAs quantum wells. The dependence of this term on the applied field in a high quality sample was studied by means of the differential reflection spectroscopy. An excellent agreement between the experimental data and the results of calculations using semi-classical non-local dielectric response model confirms the validity of the method and paves the way for the realisation of excitonic Datta-and-Das transistors. In full analogy with the spin-orbit transistor proposed by Datta and Das [Appl. Phys. Lett. {\bf 56}, 665 (1990)], the switch between positive and negative interference of exciton polaritons propagating forward and backward in a GaAs film is achieved by application of an electric field with non-zero component in the plane of the quantum well layer., Comment: 30 pages, 6 figures

Complex systems with coexisting polarity, chirality and incommensurate magnetism are of great interest because they open new degrees of freedom in interaction between different subsystems and therefore they host a plethora of intriguing physical properties. Here we report on optical properties and lattice and spin dynamics of Ni$_2$InSbO$_6$ single crystals studied with the use of polarized optical microscopy and micro-Raman spectroscopy in the temperature range 10-300 K. Ni$_2$InSbO$_6$ crystallizes in a polar structure described by the noncentrosymmetric space group R3 and two types of structural domains were visualized due to natural optical activity of opposite chirality. Raman tensor elements of most A and E phonons along with their symmetry were determined. The manifestation of LO-TO splitting was observed for the A modes. By tracking the temperature dependencies of phonon frequencies the well pronounced spin-phonon interaction was observed for several modes below and above the N\'eel transition temperature TN = 76 K. In antiferromagnetic phase a wide excitation centred at 247 cm-1 was detected and assigned to the two-magnon mode and this value was used for estimating exchange parameters through linear spin-wave theory calculations., Comment: 10 pages, 7 figures

Nanotubes (NTs) of transition metal dichalcogenides (TMDs), such as MoS2 and WS2, were first synthesized more than a quarter of a century ago; nevertheless, many of their properties have so far remained basically unknown. This review presents the state of the art in the knowledge of the optical properties of TMD NTs. We first evaluate general properties of multilayered TMD crystals, and analyze available data on electronic band structure and optical properties of related NTs. Then, the technology for the formation and the structural characteristics of TMD NTs are represented, focusing on the structures synthesized by chemical transport reaction. The core of this work is the presentation of the ability of TMD NTs to emit bright photoluminescence (PL), which has been discovered recently. By means of micro-PL spectroscopy of individual tubes we show that excitonic transitions relevant to both direct and indirect band gaps contribute to the emission spectra of the NTs despite the presence of dozens of monolayers in their walls. We highlight the performance of the tubes as efficient optical resonators, whose confined optical modes strongly affect the emission bands. Finally, a brief conclusion is presented, along with an outlook of the future studies of this novel member of the family of radiative NTs, which have unique potential for different nanophotonics applications., Comment: 38 pages, 11 fugures, 106 references

We study the optical properties of MoS$_2$ nanotubes (NTs) with walls comprising dozens of monolayers. We reveal strong peaks in micro-photoluminescence ($\mu$-PL) spectra when detecting the light polarized along the NT axis. We develop a model describing the optical properties of the nanotubes acting as optical resonators which support the quantization of whispering gallery modes inside the NT wall. The experimental observation of the resonances in $\mu$-PL allows one to use them as a contactless method of the estimation of the wall width. Our findings open a way to use such NTs as polarization-sensitive components of nanophotonic devices., Comment: 7 pages, 4 figures

We present optical study of MAPbBr3 single crystals grown from solution by inverse temperature crystallization method. Photoluminescence temperature dependence of narrow and isolated exciton resonance showed encouragingly low inhomogeneous broadening {\Gamma} ~ 0.5 meV, which is comparable to those of MBE-grown III-V heterostructures, indicating high quality of the solution-grown perovskite. 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. In addition, for the first time phonon replicas were resolved in MAPbBr3 photoluminescence spectra and attributed to the known lattice vibrational modes by comparison with Raman scattering spectra.

Fermi level pinning at the oxidized (110) surfaces of III-As nanowires (GaAs, InAs, InGaAs, AlGaAs) is studied. Using scanning gradient Kelvin probe microscopy, we show that the Fermi level at oxidized cleavage surfaces of ternary Al$_{x}$Ga$_{1-x}$As (0$\le$x$\le$0.45) and Ga$_{x}$In$_{1-x}$As (0$\le$x$\le$1) alloys is pinned at the same position of 4.8$\pm$0.1 eV with regard to the vacuum level. The finding implies a unified mechanism of the Fermi level pinning for such surfaces. Further investigation, performed by Raman scattering and photoluminescence spectroscopy, shows that photooxidation of the Al$_{x}$Ga$_{1-x}$As and Ga$_{x}$In$_{1-x}$As nanowires leads to the accumulation of an excess arsenic on their crystal surfaces which is accompanied by a strong decrease of the band-edge photoluminescence intensity. We conclude that the surface excess arsenic in crystalline or amorphous forms is responsible for the Fermi level pinning at oxidized (110) surfaces of III-As nanowires., Comment: 7 pages, 3 figures

Copper borate Cu$_3$(BO$_3$)$_2$ is a complex compound with a layered crystallographic structure in which the Jahn-Teller active and magnetic copper Cu$^{2+}$ ions occupy sixteen nonequivalent positions in the unit cell displaying controversial magnetic behavior. In this paper, we report on the infrared and Raman spectroscopic studies of the lattice dynamics and the electronic structure of 3$d^9$ copper states below the fundamental absorption band. The lattice dynamics is characterized by a large number of phonons due to a low $P\overline{1}$ space group symmetry and a large unit cell with Z=10. Unusually rich set of phonons was found in the low-energy part of the infrared and Raman spectra below 100 cm$^{-1}$, which we tentatively assign to interlayer vibrations activated by a crystal superstructure and/or to weak force constants for modes related to some structural groups. Several phonons show anomalous behavior in the vicinity of the magnetic phase transition at $T_N$=10 K thus evidencing magnetoelastic interaction. No new phonons were found below $T_N$, which excludes the spin-Peierls type of the magnetic transition. In the region of electronic transitions, a strong broad absorption band centered at $\sim$1.8 eV is observed, which we assign to overlapping of transitions between the 3$d^9$ states of Cu$^{2+}$ ions split by the crystal field in nonequivalent positions. The fundamental charge-transfer absorption band edge has a complex structure and is positioned around $\sim$2.8-3.0 eV., Comment: 11 pages, 8 figures, 2 tables, 68 references

Lattice and magnetic dynamics of NiWO$_4$ single crystals were studied with the use of polarized Raman spectroscopy in a wide temperature range of 10-300 K including the antiferromagnetic ordering temperature $T_N$=62 K. Static magnetic measurements were used for characterizing the single crystals. All Raman-active phonons predicted by the group theory were observed and characterized. Magnetic symmetry analysis was used to determine possible magnetic space groups for NiWO$_4$ which can be also applied to any other isostructural crystal with the same magnetic propagation vector k=(1/2,0,0). Though the magnetic structure of NiWO$_4$ is relatively simple, a rich set of narrow and broad magnetic excitations with different polarization properties and temperature behavior in the very broad frequency range of 10-200 cm$^{-1}$ was observed, with some modes surviving at temperatures much higher than $T_N$ up to 220 K. Part of the magnetic excitations was identified as acoustic and optical spin-wave branches which allow us to construct exchange structure and estimate exchange and anisotropy constants with the use of linear spin-wave theory. Since the magnetic structure can be described as exchange-coupled AFM chains of $S=1$ ions, previously unobserved magnetic excitation at 24 cm$^{-1}$ is tentatively assigned to a Haldane gap mode.