Longitudinal Optical Phonon Frequency Research Articles

Spectral narrowing of a phonon resonance in time-domain sum-frequency spectroscopy

Sum-frequency generation (SFG) spectroscopy provides a versatile method for the investigation of noncentrosymmetric media and interfaces. Here, using tunable picosecond infrared (IR) pulses from a free-electron laser, the nonlinear optical response of 4H-SiC, a common polytype of silicon carbide, has been probed in the frequency and time domain by infrared-visible vibrational SFG spectroscopy. In the SFG spectra we observe a sharp resonance near the longitudinal optical phonon frequency, arising from linear optical effects due to the epsilon-near-zero regime of the IR permittivity. In the time domain, the buildup of the SFG intensity is linked to the free-induction decay of the induced coherent IR polarization. When approaching the frequency of the phonon resonance, a slower polarization dephasing is observed as compared to off-resonant IR excitation. Thus, by introducing a temporal delay between the IR and the visible up-conversion pulse we are able to demonstrate spectral narrowing of the phonon SFG resonance, as corroborated by model calculations. Published by the American Physical Society 2024

Infrared dielectric function of GaAs1−xPx semiconductor alloys near the reststrahlen bands

The infrared dielectric function of thick GaAs1−xPx alloy layers grown on (001) GaAs substrates by hydride vapor phase epitaxy was investigated in the reststrahlen region using Fourier-transform infrared ellipsometry. The spectra are influenced by the Berreman artifact at the longitudinal optical phonon frequency of the GaAs substrate and by interference fringes due to the finite layer thickness. The ellipsometric angles were analyzed to determine the dielectric function of the alloy layer. Two-mode behavior, including strong GaAs-like and GaP-like optical phonons, was observed, confirming the results of Verleur and Barker [Phys. Rev. 149, 715 (1966)]. Due to the increased sensitivity of ellipsometry in the reststrahlen region, several weak phonon features could also be seen. The lattice absorption peaks are asymmetric and show side bands at the lower and higher frequencies. A single additional peak, as suggested by the percolation model, does not describe the spectra. The cluster model proposed by Verleur and Barker is a better fit to the data. Due to the broadening of the phonon absorption peaks, the authors were unable to find a unique decomposition into multiple components.

A Theoretical Estimation of Optical, Vibrational and Structural Properties of II–VI Quaternary Alloy Zn0.5Cd0.5SySe1–y

We present a theoretical estimation of optical, vibrational, and structural properties of II–VI semiconducting quaternary alloy Zn0.5Cd0.5SySe1−y for 0 < y < 1 giving total 10 compositions. The estimation of refractive index, elastic constants, bulk modulus, and vibrational frequencies are performed using the important input parameters provided by the empirical pseudopotential method. In this method, the bandgaps are computed, and the alloying effects are modeled through the modified virtual crystal approximation. We have computed the static refractive index, static and high-frequency dielectric constants, longitudinal and transverse optical phonon frequencies, elastic constants, bulk modulus, and cohesive energy for 10 compositions of the alloy. The results are compared to other experimental and theoretical values wherever available.

INFRARED REFLECTION SPECTRA OF MnxFe1–xIn2S4 SOLID SOLUTIONS

Large-block crystals of FeIn2S4, MnIn2S4 ternary compounds and MnxFe1–xIn2S4 solid solutions are grown by the method of directional crystallization (horizontal Bridgman method). The structure of the obtained crystals is determined by X-ray diffraction analysis. It is shown that both the starting compounds and solid solutions based on them crystallize in the cubic spinel structure. The IR reflection spectra in the frequency range 50–500 cm–1 of crystals of ternary compounds FeIn2S4, MnIn2S4 and solid solutions MnxFe1–xIn2S4 are studied. The frequencies of transverse (ωTO) and longitudinal (ωLO) optical phonons are determined. The concentration dependences of these parameters are plotted and the nature of their behavior is established.

INFRARED REFLECTION SPECTRA OF SOLID SOLUTIONS (FeIn2S4)х·(In2S3)1–х

On homogeneous monocrystals of solid solutions (FeIn2S4)х·(In2S3)1–х, grown by the method of directional melt crystallization (vertical Bridgman method), IR reflection spectra in the frequency range of 50—450 cm–1 were studied. The frequencies of transverse (ωTO) and longitudinal (ωLO) optical phonons, as well as their damping coefficients, were determined. The concentration dependences of the mentioned parameters were constructed and the nature of their behavior was established.

Super-Planckian thermal radiation between 2D phononic hBN monolayers

Near-field radiation heat transfer (NFRHT) in two-dimensional (2D) hexagonal boron nitride (hBN) monolayer and its composite symmetric-based structure was investigated. Contrary to three-dimensional (3D) materials, the splitting of modes (in 2D materials) into the longitudinal optical (LO) and transverse optical (TO) phononic modes disappears at the Γ-point even in some polar materials. The conductivity of atomic thin hBN monolayer depends on the LO phonon frequency under the long wavelength limit. Exploiting such concept, the electromagnetic local density of states (EM-LDOS) accounting for the near-field radiation spectrum was evaluated in free-space close to an interface with the hBN monolayer as well as composite structures. An intense narrow peak of LDOS was found in the case of hBN monolayer caused by the surface phonon polariton (SPhP) resonance. This peak was modified in the case of hBN-graphene composite due to hybridization, which strongly depends on two factors – the distance of observer (in vacuum) from interface and graphene Fermi energy. It was found that the flux density between monolayer hBN gets 2.8-fold enhanced as compared to the monolayer graphene configuration under 0.11 eV Fermi energy applied at a vacuum gap of 15 nm. The possibility of tuning Fermi energy has great potentials in designing thermally controlled devices.

Use of interface phonon-polaritons for the alloy determination in ZnO/(Zn,Mg)O multiple quantum wells

A method based on infrared reflectance spectroscopy is presented by which the Mg content in ZnO/(Zn,Mg)O multiple quantum wells with very thin barriers can be determined. The method relies on the observation of interface phonon-polaritons which appear as sharp dips in the p-polarized reflectance spectra at oblique incidence near the longitudinal optical (LO)-phonon frequencies of both QW and barrier materials. By fitting the reflectance spectra to a dielectric function model, the LO phonon frequency of the (Zn,Mg)O barrier layers can be determined. The LO phonon frequency depends on the Mg content. Comparing to Mg content calibration via cathodoluminescence, a linear relationship between the reflectance dip frequency and the Mg content is obtained. The presented method serves as a rapid means to determine the Mg content on final structures with very thin (Zn,Mg)O layers -such as device structures- where alternative, destructive methods cannot be used.

Critical Coupling and Perfect Absorption Using α‐MoO3 Multilayers in the Mid‐Infrared

Abstractα‐Phase molybdenum trioxide(α‐MoO3) is a biaxial van der Waals semiconductor that can naturally support anisotropic phonon polaritons with elliptic and hyperbolic in‐plane dispersion. α‐MoO3 can meet critical coupling (CC) condition for both transverse magnetic (TM) and transverse electric (TE) polarizations along the x and y directions and exhibit high absorption in the mid‐infrared region due to strong phonon polaritons. In this study, an absorber composed of few‐layer α‐MoO3 and one‐dimensional photonic crystal separated by a cavity is proposed. Perfect absorption for TE and TM polarized light is achieved at the longitudinal and transverse optical phonon frequencies, respectively. The underlying mechanism of perfect absorption is investigated by using coupled mode theory and simulating electric field and power dissipation profiles. The results prove that CC is responsible for the perfect absorption. Moreover, the influence of the thickness of α‐MoO3 layer, width of cavity between α‐MoO3 and photonic crystal, and angle of incident light on the CC and absorption performance are examined. Accordingly, the rules for realizing perfect absorption at different operation frequencies are established. Overall, this study provides useful insights on designing a perfect absorber based on α‐MoO3 for detection and sensing applications in the mid‐infrared region.

Dielectric and vibrational properties of CdTe studied by first-principles and terahertz time-domain spectroscopy

The optical phonon frequency and damping factor of Cadmium telluride (CdTe) have been investigated by using both the first-principles calculation and terahertz time-domain spectroscopy (TDS) system. The temperature dependence of transverse and longitudinal optical phonon frequency obtained by using the quasi-harmonic approximation is consistent with experimental results at the lower temperature range. Finally, the damping factors estimated from the three phonon contributions deviate from the extracted experimental data in the higher temperature range, which indicates the scattering process of four phonon plays an important role in anharmonic phenomena.

Elastic constants and optical phonon frequencies of BX (X= P, As, and Sb) semiconductors: Semi-empirical prediction

Based on some simple empirical formulas established by Adachi in, Properties of group-IV, III-V and II-VI semiconductors, John Wiley & Sons, Chichester (2005), and the experimental lattice constants reported in the literature; the present work aims to predict the elastic constants and some other significant properties of cubic zinc-blende boron compounds (BP, BAs and BSb). The obtained values of C12and C44 are in general good agreement with other data of the literature, while C11 and B are slightly lower. The zone-center longitudinal optical (LO) and transverse optical (TO) phonon frequencies are also obtained. The LO and TO phonon frequencies of BP compound were found at 866.6and 834.5 cm–1, respectively; these of BAs were found at 731.3 and 727.1 cm–1, respectively; while for the BSb narrow-gap semiconducting compound were found at around 598.3and 586.2 cm–1, respectively. These two later values are in general slightly lower than the calculated values, and the observed Raman spectroscopy values reported in the literature.Â

Light scattering by a charged infinite cylinder in a transparent medium

Although charged particles exist widely in a plasma environment or an electrolyte, the problem of electromagnetic scattering by a charged cylinder has remained unsolved. In this paper, using the tangential boundary conditions with a surface current induced by excess surface charges, we calculate the extinction, scattering and absorption efficiencies for a charged infinite cylinder upon which a plane wave is normally incident. For two polarizations of the incident light, we show the different effects of the excess surface charges on the optical efficiencies. For p- polarization, the extinction efficiencies own a platform structure where the extinction is in proportion to the density of surface charges, which corresponds to the light absorption by the excess surface charges. For s- polarization, the excess surface charges will induce a new resonance at the dielectric function of the cylinder larger than − 1 and a shift on the anomalous resonance which can only exist for the dielectric function near and smaller than − 1. The increasing surface charge density can shift these resonances to a larger size parameter and broaden the widths of the anomalous resonances. Furthermore, considering a specific case of a charged MgO cylinder, we identify a blue shift of the resonances near the longitudinal optical phonon frequency.

Enhancing Spectral Reflection through Controlled Phase Distribution Using Doped Polar-Dielectric Metasurfaces.

Controlling the phase distribution of wavefronts using optical metasurfaces has led to interesting optical properties and applications. Here, we explore the control of phase distribution through polar-dielectric metasurfaces composed of doped SiC nanosphere arrays. We investigate the impact of doping concentration on the optical properties of SiC nano-spheres. Our results indicate that increasing the doping of SiC nanoparticles influenced electric dipolar resonances, whereas it did not change the dipolar resonances. Using this concept, we numerically studied the extension of this idea to form metasurface arrays of single, dimer and linear trimers of the doped SiC nano-spheres. Using different doping schemes, we studied the improvement of the reflectivity at frequencies greater than the longitudinal optical phonon frequency.

Study of optical constants and dielectric properties of nanocrystalline α-cordierite ceramic

ABSTRACT The nanocrystalline 2 Mg.2Al2O3.5SiO2 as amorphous-type α-cordierite was prepared by the modified sol-gel (or Pechini) technique. After calcination at 800ºC, the pressed powder as disk pellets was sintered at 1150ºC, 1250ºC, and 1350ºC. The structural information of α-cordierite was obtained by X-ray diffraction (XRD), scanning electron microscopy (SEM), micro-Raman scattering spectra, and Fourier transform infrared (FTIR) spectra. The XRD and Raman spectra showed that the nanocrystalline α-cordierite phases from pellets of glass-ceramic powders appeared when the temperature increased. The SEM results confirmed the rounded shape of the nanocrystalline α-cordierite. Using Kramers–Kronig (KK) method, the FTIR spectroscopy was evaluated. The increasing synthesized temperature increased the size of nanocrystalline. Furthermore, by increasing the synthesized temperatures the optical constants of nanocrystalline, both real and imaginary parts of the refractive index and dielectric function decreased. Consequently, the transversal optical (TO) and longitudinal optical (LO) phonon frequencies shifted to red frequencies by increasing reaction temperature.

Optical response and structural properties of Fe-doped Pb(Zr0.52Ti0.48)O3 nanopowders

In this paper, the Pb(Zr0.52Ti0.48)1−xFexO3 (FePZT, x = 0.05) nanopowders were prepared by dry and wet sol–gel methods in the morphotropic phase boundary (MPB) region. The effect of Fe concentration on the structural, morphological, and optical properties of PZT nanopowders was investigated using X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Fourier-transform infrared (FTIR), and Ultraviolet–Visible (UV–Vis) analysis. XRD results showed that the nanopowders have a perovskite structure with the tetragonal phase. The lattice parameters and average crystallite size of samples decreased from 33 to 21 nm with increasing Fe incorporation due to the substitution of Fe atoms instead of Ti and Zr atoms. FESEM images showed that all average diameters of the nanopowders decreased with the Fe concentration. The optical properties of the pure and FePZT nanopowders such as longitudinal optical (LO) and transverse optical (TO) phonon frequencies, refractive index, extinction coefficient, and the real-imaginary parts of dielectric function were examined by the Kramers–Kronig model. As a result, the TO and refractive index of nanopowders are increased by substituting Ti and Zr with Fe atoms due to their different ionic radius. Also, while the crystallite size increases from 20.95 to 33.52 nm, the LO–TO splitting increases too. The optical band-gap values of the pure and FePZT nanopowders were estimated using UV–Vis spectroscopy and Kubelka–Munk model. The band-gap increased from 3.52 to 3.60 eV with a decrease in the crystal size of nanopowders.

Temperature Dependence of Raman and Photoluminescence Spectra of Ternary Alloyed CdSe0.3Te0.7 Quantum Dots

The Raman and photoluminescence spectra of ternary alloyed CdSe0.3Te0.7 quantum dots (QDs) have been studied at temperatures between 84 K and 293 K. The average diameter of QDs is about 5.1 nm. The temperature dependence of the longitudinal optical (LO) phonon frequencies and the phonon band width was analyzed and the anharmonic constants relating to various high-order phonon processes was determined. While the three-phonon processes plays a dominant role in the temperature-dependent shift of the LO-phonon frequencies, the four-phonon processes contribute to the increase of the phonon linewidth with increasing temperature. The photoluminescence (PL) spectra were used to study the temperature dependence of the bandgap, the linewidth and the integrated PL intensity. At low temperatures below about 120 K, the PL linewidth decreases and the PL intensity increases as temperature increases. This temperature behavior can be ascribed to the exciton fine structure.

Ultrafast Active Tuning of the Berreman Mode

The Berreman effect, by which thin films of polar dielectric materials exhibit strong, narrow resonances near their longitudinal optic (LO) phonon frequency, results in strong material interactions...

Berreman effect in bimetallic nanolayered metamaterials

The visible and ultraviolet responses of a binary regular stack, composed of alternating Al and Ag nano-thin layers, are both experimentally and theoretically studied. It is found that the s- and p-polarization reflectivity spectra (Rs and Rp, respectively) of the inherently anisotropic bimetallic system are noticeably different due to the appearance of a prominent dip near the zero of the Ag permittivity. Such an effect is similar to the Berreman one, which is observed in thin polar-crystals films near the longitudinal-optical phonon frequency, as well as in thin metal films near plasma frequency. Using both the effective medium approach and a nonlocal homogenization formalism, being valid even beyond the long wavelength limit, it is demonstrated that the bimetallic multi-layer stack behaves as a uniaxial anisotropic metamaterial. As it is shown, the appearance of the dip in the p-polarization spectrum is associated to the zero of the longitudinal component of the effective permittivity (ϵz). The calculation of 45-degree reflectometry spectrum, namely the difference Rp−Rs2, from experimental and theoretical reflectivity data confirmed that the observed dip in Rp is connected to a plasma resonance of the energy-loss function ℑ(−1∕ϵz).

Micro-Raman Mapping of the Strain Field in GaAsN/GaAsN:H Planar Heterostructures: A Brief Review and Recent Evolution

Raman scattering is an effective tool for the investigation of the strain state of crystalline solids. In this brief review, we show how the analysis of the GaAs-like longitudinal optical phonon frequency allowed to map the strain behavior across interfaces in planar heterostructures consisting of GaAsN wires embedded in GaAsN:H matrices. Moreover, we recently showed how the evolution of the longitudinal optical frequency with increasing H dose strongly depends on polarization geometry. In a specific geometry, we observed a relaxation of the GaAs selection rules. We also present new results which demonstrate how laser irradiation intensity–even at low levels–may affect the line shape of the GaAs-like spectral features in GaAsN hydrogenated materials.

Effects of heat treatment on the structural and electrical conductivity of Fe2O3–P2O5–PbO glasses

Glasses having compositions xFe2O3–(80 − x)P2O5–20PbO with x = 20, 25, 30, 35 mol% were prepared by traditional glass melt-quenching method. The effects of the heat treatment of glasses on their structural and electrical conductivity were studied by X-ray diffraction (XRD), differential scanning calorimeter, density (ρ) and dc conductivity (σ). The glass transition temperature (Tg) was observed to increase with Fe2O3 content. The density of the prepared glass and corresponding glass–ceramics increases with Fe2O3 content. XRD of as-quenched glass samples confirm the amorphous nature. Triclinic lead iron phosphate PbFe2(P2O7)2 is the main crystalline ceramic phase formed after heat treatment at 973 K, of 20 and 25 Fe2O3 mol% with average crystallite size of 74 nm. Hexagonal iron phosphate FePO4 phases appeared with Fe2O3 exceeded 30 mol%. The high-temperature conductivity is well explained by polaronic hopping conduction model. It is observed that in this system non-adiabatic hopping conduction is present. The longitudinal optical phonon frequency (νο) and density of state N(EF) of the prepared glass and corresponding glass–ceramics are reasonable for the localized states.

Surface and Volume Phonon Polaritons in Boron Nitride Nanotubes.

Phonon polaritons (PhPs) are quasiparticles created by coupling of photons to polar lattice vibrations. Previously, PhPs have been observed as both surface and volume confined waves. The dispersion of the polariton depends strongly on the nature of the material. Volume polaritons show asymptotic behavior near the longitudinal optical phonon frequency of the material, whereas surface polaritons instead approach the surface phonon frequency. Boron nitride nanotubes (BNNTs) were found to exhibit the dispersion of volume modes below the surface phonon frequency. However, around and above the surface phonon frequency, the behavior becomes that of a surface wave with an amplified near-field response. These findings improve our understanding of photonics within BNNTs and suggest potential applications that take advantage of the high fields and density of states in that spectral region.