Search Results (19)

KTaO₃ (KTO) is a quantum paraelectric perovskite oxide which belongs to the cubic space group Pm$\overline{3}$m in a large temperature range. Polar optical modes with a T_1u symmetry of KTO are infrared-active and Raman-inactive according to the centrosymmetric exclusion principle of the selection rule. In general, the soft modes responsible for ferroelectric instability are infrared-active and Raman-inactive in the paraelectric phase. Therefore, there are still not enough studies on Raman-inactive soft modes and related phonon polaritons. In the present study, Raman-inactive polar modes and related polaritons of KTO crystals are studied by Terahertz Time-Domain spectroscopy (THz-TDS) and FTIR. The real and imaginary parts of a dielectric constant along the [100] axis are uniquely determined by transmission and reflection THz-TDS without any fitting in the low-frequency range between 6 and 225 cm⁻¹, which covers the two lowest-frequency polar modes. The reflectivity is determined by reflection FTIR in the range between 50 and 1200 cm⁻¹, and the complex dielectric constant is also estimated by the fitting in the range between 6 and 1200 cm⁻¹. The phonon–polariton dispersion relations of the real and imaginary parts of the polariton wavevector are also studied in the range between 6 and 1200 cm⁻¹. The crossover from photon-like to phonon-like polaritons and related polariton decay are observed, while no anomaly related to polariton scattering and coupling to other elementary excitations is observed in the polariton dispersion. Full article

Both ZnO and BeO semiconductors crystallize in the hexagonal wurtzite (wz), cubic rock salt (rs), and zinc-blende (zb) phases, depending upon their growth conditions. Low-dimensional heterostructures ZnO/Be_xZn_1-xO and Be_xZn_1-xO ternary alloy-based devices have recently gained substantial interest to design/improve the operations of highly efficient and flexible nano- and micro-electronics. Attempts are being made to engineer different electronic devices to cover light emission over a wide range of wavelengths to meet the growing industrial needs in photonics, energy harvesting, and biomedical applications. For zb materials, both experimental and theoretical studies of lattice dynamics ${\mathsf{\omega }}_{\mathrm{j}}\left(\overrightarrow{\mathrm{q}}\right)$ have played crucial roles for understanding their optical and electronic properties. Except for zb ZnO, inelastic neutron scattering measurement of ${\mathsf{\omega }}_{\mathrm{j}}\left(\overrightarrow{\mathrm{q}}\right)$ for BeO is still lacking. For the Be_xZn_1-xO ternary alloys, no experimental and/or theoretical studies exist for comprehending their structural, vibrational, and thermodynamical traits (e.g., Debye temperature ${\mathsf{\Theta }}_{\mathrm{D}}\left(\mathrm{T}\right);$ specific heat ${\mathrm{C}}_{\mathrm{v}}\left(\mathrm{T}\right))$. By adopting a realistic rigid-ion model, we have meticulously simulated the results of lattice dynamics, and thermodynamic properties for both the binary zb ZnO, BeO and ternary Be_xZn_1-xO alloys. The theoretical results are compared/contrasted against the limited experimental data and/or ab initio calculations. We strongly feel that the phonon/thermodynamic features reported here will encourage spectroscopists to perform similar measurements and check our theoretical conjectures. Full article

In this paper, the optical properties and lattice thermal conductivity of Ti₂AlB₂ were studied by first-principles calculations. The real part of the dielectric constant, ε₁, attains a significant value of 47.26 at 0.12 eV, indicating strong polarization capabilities and energy storage capacity. Regarding optical properties, Ti₂AlB₂ exhibits significant absorption peaks at photon energies of 4.19 eV, 6.78 eV, and 10.61 eV, and 14.32 eV, with absorption coefficients of 184,168.1 cm⁻¹, 228,860.8 cm⁻¹, 366,350.8 and 303,440.6 cm⁻¹, indicating a strong absorption capacity. The loss function exhibits peaks at 19.80 eV and the refractive index reaches a maximum of 8.30 at 0.01 eV. Reflectivity is notably higher in the 0–5 eV range, exceeding 44%, which demonstrates excellent reflective properties. This suggests that Ti₂AlB₂ has potential as an optical coating material across certain frequency bands. The lattice thermal conductivity of Ti₂AlB₂ is obtained at 27.2 W/(m·K). The phonon relaxation time is greater in the low-frequency region, suggesting that phonons have a longer duration of action during the heat transport process, which may contribute to higher thermal conductivity. Although the phonon group velocity is generally low, several factors influence thermal conductivity, including phonon relaxation time and Grüneisen parameters. The high Grüneisen parameter of Ti₂AlB₂ indicates strong anharmonic vibrations, which may enhance phonon scattering and consequently reduce thermal conductivity. However, Ti₂AlB₂ still exhibits some lattice thermal conductivity, suggesting that the contributions of phonon relaxation time and group velocity to its thermal conductivity may be more significant. The unique optical properties and thermal conductivity of Ti₂AlB₂ indicate its potential applications in optical coatings and high-temperature structural materials. Full article

Silicon-based optical waveguides exhibit high Brillouin gain, enabling the realization of Brillouin lasers directly on silicon substrates. These lasers hold significant promise for applications such as tunable-frequency laser emission, ultrafast pulse generation via mode-locking techniques, and other advanced photonic functionalities. However, a key challenge in silicon-based Brillouin lasers is the requirement for long waveguide lengths to achieve sufficient optical feedback and reach the lasing threshold. This study proposes a novel floating waveguide architecture designed to significantly enhance the Brillouin gain in silicon-based systems. Furthermore, we introduce a breakthrough method for achieving wide-range phonon frequency tunability, enabling precise control over stimulated Brillouin scattering (SBS) dynamics. By strategically engineering the waveguide geometry (shape and dimensions), we demonstrate a tunable SBS phonon laser, offering a versatile platform for on-chip applications. Additionally, the proposed waveguide system features adjustable operating frequencies, unlocking new opportunities for compact Brillouin devices and integrated microwave photonic signal sources. Full article

Quaternary (Al_xGa_1−x)_yIn_1−yP alloys grown on GaAs substrates have recently gained considerable interest in photonics for improving visible light-emitting diodes, laser diodes, and photodetectors. With two degrees of freedom (x, y) and keeping growth on a lattice-matched GaAs substrate, the (Al_xGa_1−x)_0.5In_0.5P alloys are used for tuning structural, phonon, and optical characteristics in different energy regions from far-infrared (FIR) → near-infrared (NIR) → ultraviolet (UV). Despite the successful growth of (Al_xGa_1−x)_0.5In_0.5P/n⁺-GaAs epilayers, limited optical, phonon, and structural characteristics exist. Here, we report our results of carefully examined optical and vibrational properties on highly disordered alloys using temperature-dependent photoluminescence (TD-PL), Raman scattering spectroscopy (RSS), and Fourier-transform infrared reflectivity (FTIR). Macroscopic models were meticulously employed to analyze the TD-PL, RSS, and FTIR data of the (Al_0.24Ga_0.76)_0.5In_0.5P/n⁺-GaAs epilayers to comprehend the energy-dependent characteristics. The Raman scattering and FTIR results of phonons helped analyze the reflectivity spectra in the FIR region. Optical constants were carefully integrated in the transfer matrix method for evaluating the reflectivity $\mathrm{R}\left(\mathrm{E}\right)$ and transmission $\mathrm{T}\left(\mathrm{E}\right)$ spectra in the NIR → UV regions, validating the TD-PL measurements of bandgap energies $\left({\mathrm{E}}_{\mathrm{g}}^{\mathrm{P}\mathrm{L}}\right)$. Full article

Polar van der Waals (vdW) crystals, composed of atomic layers held together by vdW forces, can host phonon polaritons—quasiparticles arising from the interaction between photons in free-space light and lattice vibrations in polar materials. These crystals offer advantages such as easy fabrication, low Ohmic loss, and optical confinement. Recently, hexagonal boron nitride (hBN), known for having hyperbolicity in the mid-infrared range, has been used to explore multiple modes with high optical confinement. This opens possibilities for practical polaritonic nanodevices with subdiffractional resolution. However, polariton waves still face exposure to the surrounding environment, leading to significant energy losses. In this work, we propose a simple approach to inducing a hyperbolic phonon polariton (HPhP) waveguide in hBN by incorporating a low dielectric medium, ZrS₂. The low dielectric medium serves a dual purpose—it acts as a pathway for polariton propagation, while inducing high optical confinement. We establish the criteria for the HPhP waveguide in vdW heterostructures with various thicknesses of ZrS₂ through scattering-type scanning near-field optical microscopy (s-SNOM) and by conducting numerical electromagnetic simulations. Our work presents a feasible and straightforward method for developing practical nanophotonic devices with low optical loss and high confinement, with potential applications such as energy transfer, nano-optical integrated circuits, light trapping, etc. Full article

Organic–inorganic halocuprates based on monovalent copper are promising luminescent compounds for optoelectronic applications; however, their relaxation processes in the excited electronic state are severely underexplored. In this contribution, we prepare thin films of bis(tetraethylammonium) di-µ-bromo-dibromodicuprate(I) [N(C₂H₅)₄]₂[Cu₂Br₄], abbreviated (TEA)₂Cu₂Br₄, which features a “molecular salt” structure containing discrete [Cu₂Br₄]²⁻ anions. This compound, which has an absorption peak at 283 nm, displays a blue, strongly Stokes-shifted emission with a peak at 467 nm. Transient photoluminescence (PL) experiments using broadband emission detection and time-correlated single-photon counting (TCSPC) both find an excited-state lifetime of 57 μs at 296 K. UV–Vis transient absorption experiments at 296 K covering time scales from femto- to microseconds provide evidence for the formation of the T₁ state through intersystem crossing from S₁ with a time constant of 184 ps. The triplet state subsequently decays to S₀ predominantly by phosphorescence. In addition, the time constants for carrier–optical phonon scattering (1.8 ps) and acoustic phonon relaxation (8.3 ns and 465 ns) of (TEA)₂Cu₂Br₄ are provided. Full article

Experimental and theoretical assessments of phonon and optical characteristics are methodically accomplished for comprehending the vibrational, structural, and electronic behavior of InP_1−xSb_x/n-InAs samples grown by Gas-Source Molecular Beam Epitaxy. While the polarization-dependent Raman scattering measurements revealed InP-like doublet covering optical modes (${\omega }_{\mathrm{L}\mathrm{O}}^{\mathrm{I}\mathrm{n}\mathrm{P}}~$350 cm⁻¹, ${\omega }_{\mathrm{T}\mathrm{O}}^{\mathrm{I}\mathrm{n}\mathrm{P}}~$304 cm⁻¹) and phonons activated by disorders and impurities, a single unresolved InSb-like broadband is detected near ~195 cm⁻¹. In InP_1−xSb_x, although no local vibrational (InSb:P; x → 1) and gap modes (InP:Sb; x → 0) are observed, the Raman line shapes exhibited large separation between the optical phonons of its binary counterparts, showing features similar to the phonon density of states, confirming “two-mode-behavior”. Despite the earlier suggestions of large miscibility gaps in InP_1−xSb_x epilayers for x between 0.02 and 0.97, our photoluminescence (PL) results of energy gaps insinuated achieving high-quality single-phase epilayers with x ~ 0.3 in the miscibility gap. Complete sets of model dielectric functions (MDFs) are obtained for simulating the optical constants of binary InP, InSb, and ternary InP_1−xSb_x alloys in the photon energy (0 ≤ E ≤ 6 eV) region. Detailed MDF analyses of refractive indices, extinction coefficients, absorption and reflectance spectra have exhibited results in good agreement with the spectroscopic ellipsometry data. For InP_0.67Sb_0.33 alloy, our calculated lowest energy bandgap E₀ ~ 0.46 eV has validated the existing first-principles calculation and PL data. We feel that our results on Raman scattering, PL measurements, and simulations of optical constants provide valuable information for the vibrational and optical traits of InP_1−xSb_x/n-InAs epilayers and can be extended to many other technologically important materials. Full article

The development of new strategies for the mass synthesis of SiC nanocrystals with high structure perfection and narrow particle size distribution remains in demand for high-tech applications. In this work, the size-controllable synthesis of the SiC 3C polytype, free of sp² carbon, with high structure quality nanocrystals, was realized for the first time by the pyrolysis of organosilane C₁₂H₃₆Si₆ at 8 GPa and temperatures up to 2000 °C. It is shown that the average particle size can be monotonically changed from ~2 nm to ~500 nm by increasing the synthesis temperature from 800 °C to 1400 °C. At higher temperatures, further enlargement of the crystals is impeded, which is consistent with the recrystallization mechanism driven by a decrease in the surface energy of the particles. The optical properties investigated by IR transmission spectroscopy, Raman scattering, and low-temperature photoluminescence provided information about the concentration and distribution of carriers in nanoparticles, as well as the dominant type of internal point defects. It is shown that changing the growth modes in combination with heat treatment enables control over not only the average crystal size, but also the LO phonon—plasmon coupled modes in the crystals, which is of interest for applications related to IR photonics. Full article

Stimulated Brillouin scattering (SBS) lasers based on silicon waveguides with large SBS gain have been widely used in frequency tunable laser emissions, mode-locked pulse lasers, low-noise oscillators, optical gyroscopes and other fields. However, among SBS lasers, the realization of Brillouin laser output often requires a longer waveguide length, which not only increases waveguide loss but also increase the size of the device. As a new medium, lithium niobate has been fabricated into a new type of hybrid structure. Meanwhile, the width of a suspended waveguide is adjusted to tune the phonon frequency of an SBS laser based on lithium niobate substrate. Simulation results show that the tunable forward SBS effect is realized in a lithium niobate-suspended optical waveguide, showing a larger forward stimulated Brillouin scattering gain of 0.31 W⁻¹m⁻¹. The tunable phonon frequency ranges from 1 to 15 GHz. Therefore, utilizing the photon–phonon conversion effect, the waveguide system with LiNbO₃ will pave a new way forward with better integration. Full article

Brillouin spectroscopy has recently gained considerable interest within the biomedical field as an innovative tool to study mechanical properties in biology. The Brillouin effect is based on the inelastic scattering of photons caused by their interaction with thermodynamically driven acoustic modes or phonons and it is highly dependent on the material’s elasticity. Therefore, Brillouin is a contactless, label-free optic approach to elastic and viscoelastic analysis that has enabled unprecedented analysis of ex vivo and in vivo mechanical behavior of several tissues with a micrometric resolution, paving the way to a promising future in clinical diagnosis. Here, we comprehensively review the different studies of this fast-moving field that have been performed up to date to provide a quick guide of the current literature. In addition, we offer a general view of Brillouin’s biomedical potential to encourage its further development to reach its implementation as a feasible, cost-effective pathology diagnostic tool. Full article

Brillouin scattering is a dominant inelastic scattering observed in optical fibers, where the energy and momentum transfer between photons and acoustic phonons takes place. Narrowband reflection (or gain and loss) spectra appear in the spontaneous (or stimulated) Brillouin scattering, and their linear dependence of the spectral shift on ambient temperature and strain variations is the operation principle of distributed Brillouin sensors, which have been developed for several decades. In few-mode optical fibers (FMF’s) where higher-order spatial modes are guided in addition to the fundamental mode, two different optical modes can be coupled by the process of stimulated Brillouin scattering (SBS), as observed in the phenomena called intermodal SBS (two photons + one acoustic phonon) and intermodal Brillouin dynamic grating (four photons + one acoustic phonon; BDG). These intermodal scattering processes show unique reflection (or gain and loss) spectra depending on the spatial mode structure of FMF, which are useful not only for the direct measurement of polarization and modal birefringence in the fiber, but also for the measurement of environmental variables like strain, temperature, and pressure affecting the birefringence. In this paper, we present a technical review on recent development of distributed Brillouin sensors on the platform of FMF’s. Full article

Resonance Raman analysis is performed in order to gain insight into the nature of impurity-induced Raman features in GaN:(Mn,Mg) hosting Mn-Mg_k cation complexes and representing a prospective strategic material for the realization of full-nitride photonic devices emitting in the infra-red. It is found that in contrast to the case of GaN:Mn, the resonance enhancement of Mn-induced modes at sub-band excitation in Mg co-doped samples is not observed at an excitation of 2.4 eV, but shifts to lower energies, an effect explained by a resonance process involving photoionization of a hole from the donor level of Mn to the valence band of GaN. Selective excitation within the resonance Raman conditions allows the structure of the main Mn-induced phonon band at ~670 cm⁻¹ to be resolved into two distinct components, whose relative intensity varies with the Mg/Mn ratio and correlates with the concentration of different Mn-Mg_k cation complexes. Moreover, from the relative intensity of the 2LO and 1LO Raman resonances at inter-band excitation energy, the Huang-Rhys parameter has been estimated and, consequently, the strength of the electron-phonon interaction, which is found to increase linearly with the Mg/Mn ratio. Selective temperature-dependent enhancement of the high-order multiphonon peaks is due to variation in resonance conditions of exciton-mediated outgoing resonance Raman scattering by detuning the band gap. Full article

A silicon photonic microresonator comprising two curved vertical grating couplers and a single suspended Si nanowaveguide (NWG) is developed to investigate the giant enhanced Brillouin scattering in subwavelength NWGs caused by photon-phonon interaction. Finite element modelling based on COMSOL Multiphysics is conducted to optimize the critical device parameters (e.g., waveguide width, height, and length). As the smallest structures that need to be resolved are down to ~15 nm in size, electron-beam nanolithography is employed. In this case, dosage tests are carried out to minimize proximity charging effects during the nanopatterning of the silicon-on-insulator (SOI) surface, resulting in appropriate adaptive current area dosage distributions for the periodic gratings, couplers peripheral areas, and NWG, respectively. Furthermore, an enhanced inductively coupled plasma dry reactive ion etching (ICP-DRIE) process at a cryogenic temperature is used to realize smooth vertical sidewalls. Finally, buffered hydrofluoric acid (BHF)-based wet chemical etching is carried out to remove the buried oxide resulting in a suspended Si waveguide. Full article

It is always a difficult task to assign the peaks recorded from a vibrational spectrum. Herein, we explored a new pathway of density functional theory (DFT) simulation to present three kinds of spectra of ice XIV that can be referenced as inelastic neutron scattering (INS), infrared (IR), and Raman experimental spectrum. The INS spectrum is proportional to the phonon density of states (PDOS) while the photon scattering signals reflect the normal vibration frequencies near the Brillouin zone (BZ) center. Based on good agreements with the experimental data, we identified the relative frequency and made scientific assignments through normal vibration modes analysis. The two hydrogen bond (H-bond) peaks among the ice phases from INS were discussed and the dynamic process of the H-bond vibrations was found to be classified into two basic modes. We deduced that two H-bond modes are a general rule among the ice family and more studies are ongoing to investigate this subject. Full article