Transverse Optical Research Articles

Raman activity of the longitudinal optical phonons of the LiNbO3 crystal: Experimental determination and quantum mechanical simulation

AbstractIn this study, the and the two equivalent / polarized Raman spectra of a LiNbO3 single crystal have been recorded and used as a benchmark test for the density functional theory (DFT) calculation of the longitudinal modes and of their Raman activity. The theoretical approach, based on periodic boundary conditions and a linear combination of atomic orbitals (LCAO), provides excellent predictions of phonon wavenumbers and relative bands intensities for both A1 and E Longitudinal Optical (LO) modes and complements a previous paper limited to the study of Transverse Optical (TO) modes. Overall, the present investigation demonstrates that the LCAO approach, as implemented in the CRYSTAL software, gives results of similar accuracy for the TO and the LO phonons features. By means of a band deconvolution scheme applied to the experimental spectra, we present, for the first time, a quantitative comparison between experimental and theoretically predicted polarized Raman band intensities of LiNbO3 LO modes. This analysis highlights the role of the suitable determination of the static and high‐frequency dielectric matrices that are needed for the prediction of the TO/LO frequency split but also the first nonlinear electric susceptibility tensor for an accurate description of the Raman intensity pattern.

Anharmonic phonon coupling and decay of optical phonons in polycrystalline CdSe thin films

The lifetime of longitudinal optical (LO) phonon is a key parameter that significantly affects the performance of semiconductor optoelectronic devices. Herein, in situ temperature-dependent resonance Raman scattering experiment has been performed to study the phonon anharmonicity in sputter-deposited CdSe thin films. Strong multiphonon scattering processes involving 1LO + TO, 2LO-TO, 2LO + TO, 2LO+ E2low and TO-E2low are well resolved and allow us to investigate the anharmonic decay of 1LO, 2LO, and 1LO + TO phonon modes. Fundamental 1LO and its overtone 2LO phonon modes asymmetrically decay into a pair of transverse optical (TO) and transverse acoustical (TA) phonon at the ‘A’ point of the Brillouin zone along Γ−A direction. With increasing temperature, enhanced anharmonic phonon coupling and phonon decay led to an almost linear decrease in the wavenumber of both modes and a nearly linear increase in their full-width half maxima. The composite 1LO + TO mode exhibited a ∼ 30% faster decay rate compared to the decay rate of the fundamental 1LO phonon mode.

Lattice Dynamics and Structural Phase Transitions in Two-Dimensional Ferroelectric Methylhydrazinium Lead Bromide Investigated Using Raman and IR Spectroscopy

Two-dimensional (2D) lead halide perovskites are promising photovoltaic, light-emitting, nonlinear optical, and ferroelectric materials. Their key optoelectronic properties and polar order of organic cations are strongly affected by the dynamic response of the ionic lattice. Herein, we report polarized Raman and IR spectroscopy studies of methylhydrazinium lead bromide (MHy2PbBr4) exhibiting ferroelectric, photoluminescence, and multiple nonlinear optical phenomena. We determined the wavenumbers of longitudinal optical (LO) and transverse optical (TO) modes and their symmetries. These studies reveal that the dielectric response of MHy2PbBr4 is dominated by vibrations of the inorganic sublattice, which exhibits a large LO-–TO splitting (up to 44 cm–1). We also performed temperature-dependent Raman spectroscopy studies to monitor the effect of temperature on lattice dynamics and the effect of structural phase transitions on phonon modes. Raman spectroscopy data reveal that Raman bands are very narrow at 80 K, indicating a well-ordered structure. However, the increase of temperature leads to a very pronounced broadening of bands, indicative of a strong increase of lattice anharmonicity due to increased motional freedom of MHy+ cations and the inorganic network. Although former X-ray diffraction analysis showed that MHy2PbBr4 undergoes an order–disorder phase transition at 351 K, this transition leads to weak wavenumber shifts and broadening of Raman bands, more pronounced for modes related to the inorganic subnetwork. Furthermore, we do not observe any clear anomalies at 371 K, where the second phase transition into another disordered phase occurs. Our results indicate, therefore, that in contrast to many 2D lead halide perovskites, the dynamics of MHy+ cations and the inorganic network in MHy2PbBr4 do not exhibit pronounced slowing down at the order–disorder phase transition temperature but rather decrease gradually over a very large temperature range.

Thermal radiation resonating with longitudinal optical phonon from surface micro-stripe structures on metal-gallium nitride and sapphire

Thermal emissions from metal/dielectric-material stripe structures on the surface of dielectric materials are observed. These emissions are attributed to electric-dipole emissions induced by coherent thermal lattice vibrations in the surface structures of a few micrometers. Thermal emission based on coherent lattice vibrations has been reported via surface phonon polariton (SPhP), where metal ingredients have been excluded, whereas the present emission is independent of SPhP. The structures on undoped GaAs shows the emission close to the longitudinal optical (LO) phonon energy, while the peak energy of the emission from the structures on GaN is located in between the LO and transverse optical (TO) mode energies. The present studies on several vibration modes of α-Al2O3 and the dependence of the emission peak energy on geometrical conditions of surface structures reveal that modification of electric permittivity by the local depolarization field induces additional zero points of the real part of the permittivity, which yields additional local LO-like modes and the shift of the main emission peak energy from the original LO mode. This shift is significant for materials with a large energy difference between the LO and TO modes, such as 270 cm−1 for a mode of α-Al2O3 and 170 cm−1 for GaN. This is contrasted with the case of GaAs with 30 cm−1 difference.

Large-area long-wave infrared broadband all-dielectric metasurface absorber based on markless laser direct writing lithography.

Scalable and low-cost manufacturing of broadband absorbers for use in the long-wave infrared region are of enormous importance in various applications, such as infrared thermal imaging, radiative cooling, thermal photovoltaics and infrared sensor. In recent years, a plethora of broadband absorption metasurfaces made of metal nano-resonators with plasmon resonance have been synthesized. Still, their disadvantages in terms of complex structure, production equipment, and fabrication throughput, limit their future commercial applications. Here, we propose and experimentally demonstrate a broadband large-area all-dielectric metasurface absorber comprised of silicon (Si) arrys of square resonators and a silicon nitride (Si3N4) film in the long-wave infrared region. The multiple Mie resonance modes generated in a single-size Si resonator are utilized to enhance the absorption of the Si3N4 film to achieve broadband absorption. At the same time, the transversal optical (TO) phonon resonance of Si3N4 and the Si resonator's magnetic dipole resonance are coupled to achieve a resonator size-insensitive absorption peak. The metasurface absorber prepared by using maskless laser direct writing technology displays an average absorption of 90.36% and a peak absorption of 97.55% in the infrared region of 8 to 14 µm, and still maintains an average absorption of 88.27% at a inciedent angle of 40°. The experimentally prepared 2 cm × 3 cm patterned metasurface absorber by markless laser direct writing lithography (MLDWL) exhibits spatially selective absorption and the thermal imaging of the sample shows that the maximum temperature difference of 17.3 °C can exist at the boundary.

Raman scattering studies of the ambient atmospheric thermal stability of Be in periodic Be/Mo and Be/W multilayer mirrors

The ambient atmospheric thermal stability of beryllium (Be) layers in Be/Mo and Be/W multilayer mirrors was investigated by Raman scattering. The physical characteristic of the transverse optical (TO) mode was considered for structural analysis of the Be layers in the multilayers. With an increase in thermal annealing temperature, two important modifications of this mode were noticed: the TO mode of Be was found to shift to a lower frequency and the peak width became wider. These two facts are related to the deterioration of the crystalline quality of the Be layers upon thermal annealing. The TO mode of the crystalline Be phase completely vanished and high-intensity peaks at the shoulder were detected for the Be/Mo multilayer thermally annealed at 723 K. This evidence is associated with the transformation of polycrystalline into an oxidized amorphous Be phase. In this case, the diffusion of oxygen to the inner period and the destruction of the modulation of the periodic structure of the multilayer was investigated by a secondary ion mass spectrometer. However, the TO mode of Be embedded within W in Be/W multilayers was stable in similar annealing conditions, which revealed the thermal stability.

Hyper‐Raman scattering study of the soft mode in relaxor ferroelectric (Ba0.8Sr0.2)0.925Bi0.05Ti0.95(Zn1/3Nb2/3)0.05O3

AbstractWe report hyper‐Raman scattering measurements of the lowest transverse optic (TO) phonon branch in the relaxor Bi0.05‐BSTZN in the paraelectric phase from 300 to 973 K. The results evidence a displacive‐like behavior from a highly damped soft mode at high temperature to an overdamped (quasi‐elastic‐like) component on cooling close to Tc. A fit with a mean field law gives a temperature T0 = 102 K significantly lower than T0 = 188 K found by dielectric measurements. Reconciling the two experiments likely deserves considering a coupled modes analysis involving a second mode at slightly higher wavenumber. This behavior is very similar to that previously observed in PMN. Considering the rather complex chemical composition of the present compound as compared with PMN, these results emphasize a universal low‐wavenumber dynamics of ferroelectric relaxors.

Lattice Dynamics in Organic Ferroelectrics Using Neutron Spectroscopy and Ab-Initio Modeling

Lattice dynamics in organic ferroelectric PhMDA (2-phenylmalondialdehyde C9H8O2) has been investigated using inelastic neutron scattering (INS) spectroscopy and first principles based calculations. Most of the prominent features of the INS spectrum originated from the normal modes of hydrogen bonded malondialdehyde (MDA) units rather than from phenyl rings. It is also found that carbon-hydrogen bonds in the MDA unit are tighter than these in phenyl rings. From the calculated splitting of LO (longitudinal optical) and TO (transverse optical) normal modes, it is predicted that hydrogen bonds in MDA units, which hold the solid state structure, contribute more to the ferroelectric polarisation of PhMDA. Any distortion of H-ions in these hydrogen bonds thus affects ferroelectric properties of this material.

Effect of Ag doping on structural, morphological and optical properties of CdO nanostructured thin films

Pure CdO nanostructured thin films and Ag doped with 1.0, 2.0 and 4.0 wt% doping concentrations were fabricated successfully on glass substrates with a cost-effective spin coating technique. The prepared films exhibit a cubic crystal structure oriented along (111) plane. The surface morphology of the pure CdO films shows a cauliflower shape structure, becomes granular, and compact at high Ag doping concentration. Raman spectra of the films reveal LO (longitudinal optical) and TO (transverse optical) vibration modes at 406.80 cm−1 and 566.54 cm−1. The absorption of the films significantly varies with Ag doping concentrations and as a result, the electronic structure of the films is tailored. Moreover, the optical band gaps vary in the range of 1.42–1.70 eV. In addition, the optical parameters n, k were studied in correlation with Ag doping concentrations. The obtained room temperature photoluminescence spectra of the prepared films show a broad peak at ∼350 nm. The third order nonlinear optical parameters of the films were also examined systematically.

Synchrotron based x-ray absorption spectroscopy investigation and temperature dependent ferroelectric properties of Ni doped BaTiO3 nanostructures

In this report, the local structure geometry of BaTi1-xNixO3 (0.0 ≤ x ≤ 0.06) ceramics has been studied through synchrotron based x-ray-absorption spectroscopy, measured at the Ti K, Ba L3 and Ni K-edges at room temperature. The x-ray absorption near edge spectroscopy (XANES) at Ti K-edge confirms the TiO6 octahedron geometry of Ti atom due hybridization between Ti(3d)-O(2p) orbitals and signifies the non-centrosymmetric nature of BaTiO3 samples. However, Ti–O off-center displacement (non-centrosymmetric) is disturbed under the effect of Ni doping as a result of oxygen vacancies formation. More interestingly, XANES studies at Ni K-edge ensure successful substitution of Ni in BaTiO3 as Ni2+ ions. Extended x-ray absorption fine structure (EXAFS) data at Ni K-edge have been fitted to estimate pertinent local structural parameters of the Ni–O, Ni–Ba, Ni–Ti and Ni–Ni shells (viz. bond lengths and disorder parameters) and it also reveal that the structural disorders around the Ti sites in the doped BaTiO3 expand with Ni doping. The mixed-valence states of titanium ion, i.e., Ti4+ and Ti3+ in the doped samples were established with the x-ray photoelectron spectroscopy (XPS). Moreover, XPS divulges the creation of oxygen vacancies due to Ni doping in BaTiO3 matrix. The complementary information about the lattice vibration is analyzed through Raman studies that approve the softening of the transverse optical (TO) mode present at 515 cm−1. The temperature dependent ferroelectric studies affirm that the ferroelectricity vanishes in the doped samples due to the decrement in the off-center displacement between the Ti4+ and O2− ions in the TiO6 octahedral geometry. Differential thermal analysis (DTA) exhibits a ferroelectric tetragonal to para-electric cubic phase transition in the pristine as well as Ni doped BaTiO3 samples.

Investigation of Be doped GaSb with varying hole concentrations using single magnetic field Hall measurement and Raman spectroscopy

Raman scattering results on Beryllium doped Gallium antimonide (GaSb) epi-layers with varying hole concentrations (3 × 1017 cm−3 to 5 × 1019 cm−3) are presented in this study. The hole concentrations are determined from single magnetic field Hall measurements and corrected taking into consideration two band conduction. The Raman spectra show four distinct features: Longitudinal optical (LO) phonon mode from the surface accumulation/depletion region, disorder activated forbidden transverse optical (TO) phonon mode, LO phonon coupled Plasmon (LOPC) mode and the uncoupled LO phonon mode from the bulk region. A detailed Raman line-shape analysis using deformation potential and electro-optic contribution has been carried out to reproduce the experimental results. The effect of plasmon damping on the LO phonon coupled Plasmon (LOPC) mode is discussed. Free hole concentrations are derived from the Raman line-shape analysis and compared with that from the Hall measurements.

Tuning the onset pressure of pressure-induced phase transition in indium phosphide by extrinsic doping

In-situ angle-dispersive X-ray diffraction (ADXRD) and Raman scattering were carried out to delineate the phase transitions under pressure in p-type zinc-doped indium phosphide (InP:Zn) at ambient temperature. The ADXRD measurements with pressure up to 17.3(2) GPa reveal that InP:Zn transforms from zinc-blende (ZB) to rock-salt (RS) structure in a pressure range of 8.1(2) - 8.6(1) GPa. The Raman scattering measurements indicate that the phase transition proceeds continuously over a wide range of pressure and reaches completion at the highest pressure applied (14.5(2) GPa) with significant metallization, as suggested by the disappearance of the transverse optical (TO) and longitudinal optical (LO) modes. In contrast, originally absent at ambient pressure, additional Raman shift of Zn local vibrational modes (LVMs), appeared at 276 cm-1 and 420.7(1) cm-1 within the pressure range of 2.2(1) up to 10.1(2) GPa, which can be attributed to Zn3P2 and VPZnInVP LO modes, respectively. The zero-pressure isothermal bulk moduli and its first order pressure derivatives for the zinc-blende InP:Zn are 79.44(2) GPa and 3.92(1), respectively. A total enthalpy change of about 45.45 KJ•mole-1 was derived for the ZB to RS structure transformation occurring at the pressure of 8.6(1) GPa. The present results suggest that diffusion of Zn in InP:Zn may have been governed by the modified substitution and interstitial (S–I) mechanism. Moreover, comparisons on the pressure-dependent behaviors in InP:Zn, InP:S, and InP:Fe extrinsically doped systems revealed that the exact type of the defects introduced into the lattice upon doping may have played crucial roles in the pressure-induced phase transitions.

Effect of Magnesium (Mg) to the Optical and Absorption Gamma-Ray Properties of Composite Shield Cassava Starch /Fe3O4/Mg

Optical and absorption properties of composite shield cassava-starch/Fe3O4/Mg have been studied for various amounts of Mg (0.05 gr, 0.075 gr, and 0.1 gr) by using Fourier transform infra-red (FTIR) spectroscopy and an IBT 103 Irradiator equipped with a Cs-137 radioactive source (emitting γ-rays with an energy of 662 keV), respectively. The quantitative analysis of the FTIR spectra has allowed to determine the refractive index (n), extinction coefficient (k), dielectric functions (ε), and energy loss function (Im (−1/ε)) by applying the Kramers-Kronig (K–K) relations. The transversal optical (TO) phonon frequency is shifted by 1 cm−1 to higher wave numbers by increasing the amount of Mg in the composite. This indicates a small effect on the optical properties of the lattice when O atoms are partially replaced by Mg ones in the bonding with Fe (probably forming MgxFeO1-x). The plasma frequency of the composite cassava starch/Fe3O4/Mg, for various amounts of Mg, remains constant at 1126 cm−1 indicating the formation of a stable bonding between the MgxFeO1-x filler and the cassava starch matrix. The highest linear attenuation coefficient and tensile strength are 4.633 cm−1 and 73.19 Mpa, respectively, for the highest amount of Mg in the composite (APMG 0.1). These results indicate that the composite cassava starch/Fe3O4/Mg shows a high potential as a new type of apron in several applications with γ-rays.

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.

Ultrafast nonlinear phonon response of few-layer hexagonal boron nitride

Ultrafast optical-phonon dynamics and the related nonlinear lattice response of few-layer hexagonal boron nitride ($h$-BN) are elucidated in a combined experimental and theoretical study. The lifetimes of transverse optical (TO) phonons and interlayer modes are 1.2 and 20 ps. In-plane TO phonon displacements are shown to couple to interlayer shear and breathing motions. In the nonlinear phonon response, a transient reduction of the TO phonon frequency arises from this anharmonic coupling, which is a particular property of few-layer $h$-BN.

TO-phonon anisotropies in a highly doped InP (001) grating structure

For zinc blende semiconductors, such as InP, the Raman selection rules for a backscattering configuration from the (001) surface forbid the transversal optical (TO) phonon mode, whereas the longitudinal optical mode is allowed. However, when InP is highly doped with Si atoms, InP-Si clusters with the reduced C3v symmetry allow TO modes in the Raman spectrum with the backscattering configuration. Here, we demonstrate that the amplitude of the TO modes can be modulated spatially by using a highly doped InP grating. By exciting the sample with a laser linearly polarized parallel and perpendicular to the grating grooves, we observe a change in amplitude of the phonon optical response for the TO mode.

Influence of crystallinity on the corrosion rate of chemically vapor‐infiltrated SiCf/SiC composites under 310°C hydrothermal condition

AbstractThe rationale of this work was to investigate the effect of crystallinity on corrosion rate of chemically vapor‐infiltrated (CVI) SiCf/SiC composites. SiCf/SiC composites were fabricated from fabric preform with densifying by CVI. To evaluate the effect of crystallinity on the behavior of SiCf/SiC composite, the infiltration temperature was increased from 1000 to 1100°C with 25°C intervals. X‐ray diffraction and Raman spectroscopy results indicated the diffraction peaks, as the infiltration temperature elevated, become sharp and the transverse optical (TO) and longitudinal optical (LO) phonon peak of the composite increased, respectively. Hydrothermal corrosion tests were performed on the polished surface in a static autoclave at a DI water for 7 and 30 days to evaluate the relation of crystallinity and the microstructure of the composite. Low infiltration temperature revealed columnar growth structure at the matrix after corrosion while weight loss behavior had a linear decreasing tendency from 1000 to 1075°C. These findings are particularly important for determining the infiltration temperature which changes the crystallinity of the SiC phase, and hence the corrosion rate of SiCf/SiC composites.

Insights into the phase evolution-composition-structural aspect of silicon carbide powders preparing from nature silica sands of south Libya

The silicon carbide (SiC) powders were prepared from four types of silica sands taken as a silica source. These sand samples were collected from Alhasawneh Mountain, Murzuq city, Sebha city and Zalaf region located in the southern of Libya. Here, the carbon source was citrus sinensis tree branches and the sol-gel method was used for preparing the samples. The calcination process was performed by immersing SiC samples in a bath containing silica/carbon mixture (∼600 °C) for 6 h. The X-ray diffraction (XRD) results confirmed the formation of three polytypes of 6H, 3C and 21R–SiC. The cell parameter values of SiC samples were very close to the actual value. Excluding in the 21R phase, a slight increasing in the cell parameter c value of 53.08, 53.49, 53.82, 53.30 Å was observed for SiCAlhasawneh Mountain, SiCMurzuq sand, SiCSebha Sand and SiCZalaf sand samples, respectively. The particle morphology was measured using a field emission scanning electron microscopy (FE- SEM). The SiCAlhasawneh Mountain sample showed larger particles sizes with irregular shapes. The irregular particle with large holes (from 0.5 to few microns) was observed in the SiCMurzuq sand sample. The SiCSebha Sand sample contains a few hexagonal flakes ∼179 nm in diameter. The SiCZalaf sand sample showed irregular particles with an average size of 138 nm. Raman spectra of all samples showed D and G bands at 1360 cm−1 and 1600 cm−1, respectively. The weak peak at 798 cm−1 corresponded to the transverse optical (TO) mode that appeared in SiCZalaf sand sample. The results showed that it is possible to use natural silica sands and silica/carbon bath method for synthesis of SiC product with low cost and inexpensive materials to be used in many applications of industrials.

Phonons and Valence-band Splitting in Strained GaAs<SUB>1-x</SUB>N<SUB>x</SUB>/GaAs Epilayers

The strain effects in strained GaAs<SUB>1-x</SUB>N<SUB>x</SUB> epilayers are characterized by Raman spectroscopy and photocurrent spectra at various nitrogen composition. In addition, the nitrogen composition and the strain were determined by using high-resolution X-ray diffraction (HR-XRD). The Raman spectra are observed to be dominated by the GaAs-like longitudinal optical (LO) phonon mode as the strongest peaks show up around 289~294 cm<SUP>-1</SUP>. Moreover, the weak and broad peaks features in the range of 255~276 cm<SUP>-1</SUP> originate from the peaks of GaAs-like transverse optical (TO) phonon mode and disorder induced, GaN-like LO. And the Raman peak shifts toward lower wave number with increasing nitrogen compositions, which is indicates the presence of tensile strain in the strained GaAs<SUB>1-x</SUB>N<SUB>x</SUB> epilayers. The valence-band splitting of GaAs<SUB>1-x</SUB>N<SUB>x</SUB> are obtained from photocurrent spectra. As the nitrogen concentration increases, the tensile strain in strained GaAs<SUB>1-x</SUB>N<SUB>x</SUB> epilayers increases while the valence-band splitting increases.

Investigation of transverse optical phonon of thin Si films embedded in periodic Mo/Si and W/Si multilayer mirrors

Physical properties and stress analysis of Si layers embedded in bi-layer periodic Mo/Si and W/Si multilayer mirrors operating in extreme ultraviolet and X-ray region of wavelengths were investigated by Raman scattering. The study showed that alternate Si layers in these multilayers were exist in the amorphous phase. This was determined from the peak shift and linewidth of the transverse optical (TO) phonon mode. This mode was blueshifted with an increase in the thickness of amorphous Si (a-Si) layers embedded in the Mo/Si multilayers and redshifted upon thermal annealing. In contrast, the redshift of this mode in a-Si with an increase of thickness was observed for W/Si multilayers, and blueshifted upon thermal annealing. The behavior of the TO phonon mode of a-Si was also influenced by the microstructure of metallic layers and the composite stress of multilayers. Thermally annealed multilayers showed a reduction of the period thickness due to defect annihilation of a-Si layers which lead to the change of reflection properties of mirrors.