Transverse Optical Modes Research Articles

Microscopic and spectroscopic characterization of chemo-mechanically polished, furnace-annealed, nitrogen-doped 4H:SiC

The effect of chemical–mechanical polishing and high-temperature furnace annealing at temperatures ranging from 1000 °C to 1600 °C on nitrogen-doped crystalline 4H:SiC was investigated. Techniques used to characterize the samples included environmental scanning electron microscopy (ESEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy. The ESEM micrographs and EDS data indicated that there were structural defects on the unannealed sample that did not propagate into the sample or vary in composition from the bulk. The sample annealed at 1000 °C showed oxygen-rich and carbon-depleted surface defects. Annealing at temperatures above 1200 °C introduced defects that grew out of the sample surface. These were carbon and oxygen rich, but depleted in silicon. This supported the XPS data, which showed an increase in the surface C bonding with annealing temperatures above 1200 °C. The XPS data also suggested that the oxycarbide content may be increasing on annealing above 1200 °C. Raman micro-probe data from the defects on the sample annealed at 1200 °C showed the maximum shift in the transverse optical phonon mode at 776 cm-1, indicating that the beginning of carbon out-diffusion may be accompanied with structural changes. Optimal annealing temperatures are thus below 1200 °C.

GROWTH AND OPTICAL PROPERTIES OF ZnSe NANOPARTICLES VIA THERMAL EVAPORATION METHOD

ZnSe nanoparticles were synthesized via a simple, effective thermal evaporation method by a vapor phase reaction of Zn and Se powder. Particle characterization was performed by X-ray diffraction (XRD), transmission electron microscopy (TEM). It is found that the higher the growth temperature, the larger the nanoparticles. Obvious blue shift was observed in the photoluminescence (PL) spectra at room temperature with decreasing particle size. Raman spectra of the as-grown ZnSe nanoparticles were compared with that of the bulk ZnSe crystal. Though no obvious peak shift was observed for both the transverse optical (TO) and longitudinal optical (LO) phonon modes within the apparatus spectral resolution, non-symmetrical broadening of the LO phonon mode is obvious, indicating that the phonon lifetime is shorter in ZnSe nanoparticles than in bulk material.

Cubic and hexagonal boron‐nitride (c‐BN/h‐BN) thin films deposited in situ by r.f. magnetron sputtering

AbstractCubic boron‐nitride (c‐BN)/hexagonal boron nitride (h‐BN) thin films were grown in situ on (100) oriented silicon substrates by r.f. (13.56 MHz) magnetron sputtering technique. In order to obtain the highest fraction of the c‐BN phase, a negative d.c bias voltage, varying from 0 to –200 V was applied to the substrate during deposition. Another set of boron nitride thin films was deposited in situ on (100) oriented silicon substrates under r.f. bias voltage. The substrate holder was biased from 0 to –350 V by connecting such to an auxiliary r.f. generator (operated at 13.56 MHz). Films were characterized by Fourier Transformed Infrared Spectroscopy (FTIR) and Atomic Force Microscope (AFM). Well‐defined peaks at 787 cm–1, 1100 cm–1 and 1387 cm–1, corresponding to the 2Au (out‐plane bending of B–N–B bond) h‐BN vibration mode, the F2 (stretching) c‐BN Transversal Optical (TO) mode and the E1u (in‐plane stretching of B–N bond) vibration mode of the h‐BN, respectively, were observed in the FTIR spectra. A maximal fraction of the c‐BN phase close to 85% was obtained under a bias voltage of –150 V at substrate temperature of 300 °C and a total pressure of 4 × 10–2 mbar. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman Spectroscopy under Extreme Conditions

We report the results of Raman measurements of various materials under simultaneous conditions of high temperature and high pressure in the diamond anvil cell (DAC). High temperatures are generated by laser heating or internal resistive (ohmic) heating or a combination of both. We present Raman spectra of cubic boron nitride (cBN) to 40 GPa and up to 2300 K that show a continuous pressure and temperature shift of the frequency of the transverse optical mode. We have also obtained high-pressure Raman spectra from a new noble metal nitride, which we synthesized at approximately 50 GPa and 2000 K. We have obtained high-temperature spectra from pure nitrogen to 39 GPa and up to 2000 K, which show the presence of a hot band that has previously been observed in CARS measurements. These data have also allowed us to constrain the melting curve and to examine changes in the intramolecular potential with pressure.

Sol–Gel Synthesis and Photoluminescence of III-V Semiconductor InAs Nanocrystals Embedded in Silica Glasses

III-V semiconductor nanocrystals rarely exist as spherical inclusions inside glasses, due to the difficulties during their preparation, such as high toxic reagents or fast oxidation under usual glass technology temperatures. In this paper a sol-gel method for synthesis of InAs nanocrystals embedded in silica glasses was described. Gels were synthesized by the hydrolysis of a complex solution of Si(OC2H5)4, InCl3 x 4H2O, and As2O3. The gels were then heated at 200-450 degrees C in the presence of H2 gas to form fine cubic InAs crystallites. The X-ray diffraction patterns showed four strong peaks from InAs. The Raman spectrum showed InAs longitudinal-optic (233 cm(-1)) and transverseoptic modes (215 cm-(-1)). The size of InAs nanocrystals was found to be from 5 to 30 nm in diameter by transmission electron microscopy. A strong room temperature photoluminescence with peaks at 601 and 697 nm was observed from InAs nanocrystals embedded in silica glasses. The results suggest that it might be possible to synthesize other III-V semiconductor nanocrystals embedded in silica glasses through the sol-gel process.

Microstructural and defect population change in electron beam irradiated Ge:SiO 2 MCVD glasses in the conditions of refractive index change writing

Electron spin resonance (ESR) and ultra violet (UV) optical absorption spectra are measured in Ge:SiO 2 glasses elaborated by modified chemical vapor deposition (MCVD) and irradiated by an electron beam of medium energy (10–50 keV). The doses (a few C/cm 2) like the electron energy used, correspond to conditions for writing refractive index change for optical applications. These data yielded information on defect population changes. Raman spectroscopy was used for correlating these changes to microstructural changes. We point out a 5.1 eV absorption bleaching like surprisingly the one occurring under 5 eV laser irradiation correlated to an absorption increases around (4.4 and 5.8 eV). E′ centres are also produced. More specifically, we found in a previous work two domains of irradiation parameters for samples with large Ge content for which specific volume increases or decreases. We thus precise in this paper that (1) when compaction occurs, detectable by surface topography depression and D2 peak intensity increase, Si E′ centres are produced and a diamagnetic component absorbing strongly at 6.1 eV that may arise from sixfold coordinated Ge, (2) when expansion dominates, D2 peak does not decrease at all but long range order changes, detectable by modification of transverse optical (TO) and longitudinal optical (LO) mode intensity. In this case, Ge E′ and Ge(1) dominate and the previous absorption at 6.1 eV disappears. In addition, an absorption peak appears at 3.1 eV from an unknown defect.

Ab initiolinear response and frozen phonons for the relaxorPbMg1∕3Nb2∕3O3

We report first principles density functional studies using plane wave basis sets and pseudopotentials and all-electron linear augmented plane wave (LAPW) of the relative stability of various ferroelectric and antiferroelectric supercells of PMN $(\mathrm{Pb}{\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}{\mathrm{O}}_{3})$ for 1:2 chemical ordering along [111] and [001]. We used linear response with density functional perturbation theory as implemented in the code ABINIT to compute the Born effective charges, electronic dielectric tensors, long wavelength phonon frequencies and longitudinal optic-transverse optic (LO-TO) splittings. The polar response is different for supercells ordered along [111] and [001]. Several polar phonon modes show significant coupling with the macroscopic electric field giving giant LO-TO splittings. For [111] ordering, a polar transverse optic mode with $E$ symmetry is found to be unstable in the ferroelectric $P3m1$ structure and the ground state is found to be monoclinic. Multiple phonon instabilities of polar modes and their mode couplings provide the pathway for polarization rotation. The Born effective charges in PMN are highly anisotropic and this anisotropy contributes to the observed huge electromechanical coupling in PMN solid solutions.

FTIR Study of CO2 and H2O/CO2 Nanoparticles and Their Temporal Evolution at 80 K

Fourier transform infrared (FTIR) spectroscopy combined with a long-path collisional cooling cell was used to investigate the temporal evolution of CO2 nanoparticles and binary H2O/CO2 nanocomposites in the aerosol phase at 80 K. The experimental conditions for the formation of different CO2 particle shapes as slab, shell, sphere, cube, and needle have been studied by comparison with calculated data from the literature. The H2O/CO2 nanoparticles were generated with a newly developed multiple-pulse injection technique and with the simpler flow-in technique. The carbon dioxide nu3-vibration band at 2360 cm(-1) and the water ice OH-dangling band at 3700 cm(-1) were used to study the evolution of structure, shape, and contact area of the nanocomposites over 150 s. Different stages of binary nanocomposites with primary water ice cores were identified dependent on the injected CO2 portion: (a) disordered (amorphous) CO2 slabs on water particle surfaces, (b) globular crystalline CO2 humps sticking on the water cores, and (c) water cores being completely enclosed in bigger predominantly crystalline CO2 nanoparticles. However, regular CO2 shell structures on primary water particles showing both longitudinal (LO) and transverse (TO) optical mode features of the nu3-vibration band could not be observed. Experiments with reversed nucleation order indicate that H2O/CO2 composite particles with different initial structures evolve toward similar molecular nanocomposites with separated CO2 and H2O regions.

Optical simulation of vertical-cavity surface-emitting lasers with non-cylindrical oxide confinement

Two extensions of weighted index method are presented in order to calculate the splitting of degeneracy for non-circular vertical-cavity surface-emitting lasers. Both of them automatically satisfy which transverse optical modes split, and give a reliable approximation for the resonant frequencies, threshold gains and confinement factors. The intuitive effective radius method traces back the splitting for different effective aperture radii which are defined according to the transverse intensity distributions. The hybrid analytical axial and numerical lateral method is more precise from a mathematical point of view, and provides the mode patterns as well as the optical data.

Ab initio study of the structure and dynamical properties of crystalline ice

We investigated the structural and dynamical properties of a tetrahedrally coordinated crystalline ice from first principles based on density functional theory within the generalized gradient approximation with the projected augmented wave method. First, we report the structural behaviour of ice at finite temperatures based on the analysis of radial distribution functions obtained by molecular dynamics simulations. The results show how the ordering of the hydrogen bonding breaks down in the tetrahedral network of ice with entropy increase, in agreement with the neutron diffraction data. We also calculated the phonon spectra of ice in a 3× 1× 1 supercell using the direct method. So far, due to the direct method used in this calculation, the phonon spectra are obtained without taking into account the effect of polarization arising from dipole–dipole interactions of water molecules, which is expected to yield the splitting of longitudinal and transverse optic modes at the Γ point. The calculated longitudinal acoustic velocities from the initial slopes of the acoustic mode are in reasonable agreement with the neutron scattering data. Analysis of the vibrational density of states shows the existence of a boson peak at low energy of the translational region, a characteristic common to amorphous systems.

Microwave dielectric properties of lanthanum aluminate ceramics and single crystal

Single crystal and ceramics of LaAlO 3 were prepared by a Czochralski method and conventional solid phase reaction, respectively, using high purity reagents. Far infrared reflectivity spectra for single crystals and ceramics of LaAlO 3 were measured and eigenfrequencies and damping constants of transverse and longitudinal optical modes were evaluated in order to discuss variations in the dielectric properties. The observed reflectivity spectra were fitted by four IR active modes (predicted by factor group analysis) in order to calculate the vibration eigenfrequencies and damping constants. Differences in dielectric loss were found between the two kinds of single crystals, having different crystal orientations. It was inferred that the difference in dielectric loss was due to difference in cut off frequencies of these single crystals. The loss obtained from the IR reflectivity of the ceramics was used to evaluate the extrinsic loss in the ceramics.

Growth of “New Form” of Polycrystalline Silicon Thin Films Synthesized by Hot Wire Chemical Vapor Deposition

AbstractIn this work, we report on next-generation hot wire chemical vapor deposition technique, we call it ceramics hot-wire CVD. Using a new concept of rectangular ceramics filament holder and “confinement of thermal radiation from the filament”, a “new form” of polycrystalline silicon thin films has been developed at low temperature (˜ 250°C). The grains are found to be symmetrically distributed in array along the parallel lines, in (111) direction. On the surface of individual grains, “five-fold” and “six-fold” symmetries have been observed and we suspect that we developed “buckyball” type “giant silicon molecular solids” with different crystalline silicon lattice other than standard single-crystal silicon structure. We observed rarely found “icosaderal” symmetry in silicon thin films. This hypothesis has been supported by multiple Raman active transverse optical modes and the crystallographic structure analyzed by X-ray diffraction.

Synthesis of Uniform Titanium Nitride Nanocrystalline Powders via a Reduction–Hydrogenation–Dehydrogenation–Nitridation Route

A novel reaction of TiCl4 and NaNH2 at 450°–500°C for 12–24 h was performed for the preparation of TiN nanocrystalline powders. Powder XRD patterns indicated that the as‐synthesized powder was cubic‐phase TiN with a lattice constant a=4.225 Å. Transmission electron microscopy images showed that the TiN powders consisted of uniform particles with an average diameter of 10–20 nm. The binding energies of Ti2p3/2 and N1s core levels at the position of 455.50 and 396.96 eV, respectively, were detected by X‐ray photoelectron spectra. Four peaks, related to transverse acoustic, longitudinal acoustic, second‐order acoustic, and transverse optical (TO) modes of TiN, respectively, were observed in the Raman spectra of TiN particles. A very reasonable formation mechanism of TiN nanocrystalline powders was proposed.

Anomalous Dynamical Charge Change Behavior of Nanocrystalline 3C‐SiC upon Compression

Using diamond anvil cell (DAC) technique, in situ high‐pressure Raman scattering and energy‐dispersive X‐ray diffraction (EDXRD) experiments were used at room temperature to study 3C‐SiC with an average grain size of 30 nm. In contrast to its bulk counterpart, a decrease of the Born's transverse effective charge of these nanocrystals was observed with increasing pressure from measurements of the longitudinal and transverse optical phonon modes (longitudinal optical–transverse optical) splitting. This is therefore indicative of a diminishing ionicity of nanocrystalline 3C‐SiC on compression.

Indium phosphide nanocrystals formed in silica by sequential ion implantation

Fused silica substrates were implanted with: (1) phosphorus only, (2) indium only, and (3) phosphorus plus indium ions. Vibrational and electronic characterizations have been performed on the P only and In only samples to obtain an understanding of the thermal annealing behavior in order to obtain a meaningful guide for the fabrication of InP quantum dots (QDs) formed by sequential ion implantation of In and P in SiO 2. Thermal annealing procedures for InP synthesis have been established and InP quantum dots are confirmed by TEM, XRD and far infrared measurements. Far IR spectra show a single resonance at 323 cm −1 rather than two absorption peaks in its counterpart of bulk InP crystals. The single band absorption is attributed to the surface phonon of InP quantum dots which will appear between transverse optical (TO) and longitudinal optical (LO) phonon modes of the bulk.

Classical simulation of quantum entanglement using optical transverse modes in multimode waveguides

We discuss mode-entangled states based on the optical transverse modes of the optical field propagating in multi-mode waveguides, which are classical analogs of the quantum entangled states. The analogs are discussed in detail, including the violation of the Bell inequality and the correlation properties of optical pulses' group delays. The research on these analogs may be important, for it not only provides useful insights into fundamental features of quantum entanglement, but also yields new insights into quantum computation and quantum communication.

Optical Spectroscopy of Plasmons and Excitons in Cuprate Superconductors

An introduction is given to collective modes in layered, Josephson coupled high Tc superconductors. An experimental demonstration is treated of the mechanism proposed by Anderson whereby photons travelling inside the superconductor become massive, when the U(1) gauge symmetry is broken in the superconductor to which the photons are coupled. Using the Ferrell-Tinkham sumrule the photon mass is shown to have a simple relation to the spectral weight of the condensate. Various forms of Josephson plasmons can exist in single-layer and bilayer cuprates. In the bilayer cuprates a transverse optical plasma mode can be observed as a peak in the c-axis optical conductivity. This mode appears as a consequence of the existence of two different intrinsic Josephson couplings between the CuO2 layers. It is strongly related to a collective oscillation corresponding to small fluctuations of the relative phases of the two condensates, which has been predicted in 1966 by A. J. Leggett for superconductors with two bands of charge carriers. A description is given of optical data of the high Tc cuprates demonstrating the presence of these and similar collective modes.

Infrared reflectance of GaN1−xPx ternary alloys grown by metalorganic chemical vapor deposition

Infrared (IR) reflection spectra of GaN1−xPx alloys, grown by light-radiation heating, low-pressure metalorganic chemical vapor deposition, have been investigated with phosphorus composition x=0–0.15. A multi-oscillator model has been used to fit the IR reflection spectra. In addition to the demonstration of a GaN-like transverse-optical (TO) mode, a GaP-like TO mode, located at about 355cm−1, has been identified for x>0, showing a typical two-mode behavior. With the increment of x the frequency of the GaN-like TO phonon decreases and the GaP-like TO phonon keeps its frequency almost unchanged within the measured composition range. A simple random-element-isodisplacement model has been employed to interpret the frequency change of both GaN-like and GaP-like phonons with the phosphorous composition x. The damping of the GaN-like TO mode increases significantly with x and is explained by the disorder effect resulting from the introduction of foreign P atoms.

Raman spectroscopy of cubic boron nitride under high temperature and pressure conditions: A new optical pressure marker

The pressure dependence of Raman peaks of cubic boron nitride (cBN) is determined at 100, 200 and 300 °C using pressure scales of ruby and gold. At pressures lower than 6 GPa, the pressure dependences of cBN Raman determined with the ruby pressure scale for transverse-optical (TO) and longitudinal-optical modes are 3.45±0.02 and 3.36±0.02 cm−1/GPa at 100 °C and 3.43±0.02 and 3.44±0.07 cm−1/GPa at 300 °C, respectively. These values are consistent with those in a previous study conducted at room temperature using the ruby pressure scale. Synchrotron x-ray diffraction experiments using a gold pressure marker also yield 3.45±0.03 cm−1/GPa for TO mode at 200 °C in a range of pressure up to 32 GPa. Under the present pressure and temperature conditions, the pressure dependence of Raman peaks of cBN seems to be independent of the temperature conditions. cBN can be used as an optical pressure marker under high temperature conditions.

Tunable resonant optical microcavities by self-assembled templating.

Micrometer-scale optical cavities are produced by a combination of template sphere self-assembly and electrochemical growth. Transmission measurements of the tunable microcavities show sharp resonant modes with Q factors of >300 and 25-fold local enhancement of light intensity. The presence of transverse optical modes confirms the lateral confinement of photons. Calculations show that submicrometer mode volumes are feasible. The small mode volumes of these microcavities promise to lead to a wide range of applications. in microlasers, atom optics, quantum information, biophotonics, and single-molecule detection.