Optical Phonon Spectra Research Articles

Observation of Raman scattering in GaAs photonic-crystal slab waveguides

We have observed Raman spectra of optical phonons at 1.34 mum in an air-bridge type of GaAs photonic-crystal (PC) slab waveguides (WGs) of 0.6 mm in length. Unlike the bulk GaAs case, both longitudinal (LO)- and transverse (TO) phonons were observed not only in the forward direction, but also in the backward direction. This anomalous feature can be well interpreted as arising from strong confinement of light in PC WGs. The scattering efficiency of LO phonon at 292 cm(-1) is estimated as 1.9 x 10(-7) cm(-1)-sr(-1) in the forward scattering. Based on the present information, the possibility of developing an optical amplifier with use of stimulated Raman scattering in a future ultrafast and ultrasmall PC WGs-based optical circuit is discussed.

Confinement of polar optical phonons in AlN/GaN superlattices

We study the optical-phonon spectra in heterojunctions fabricated from III to V nitride materials (GaN and AlN). We are concerned with the quaternary superlattice structure, namely, /substrate/AlN/Al x Ga 1− x N/GaN/Al x Ga 1− x N/…/, where the substrate is here considered to be a transparent dielectric medium like sapphire. We make use of a model based on the Frölich Hamiltonian, taking into account the macroscopic theory developed by Loudon, known as the continuum dielectric model. The optical-phonon modes are modelled considering only the electromagnetic boundary conditions (including retardation effects), in the absence of charge transfer between ions. Numerical results of the confined optical-phonon spectra are presented, characterizing three distinct optical-phonon classes designated as propagate (PR), interface (IF) and half-space (HS) modes. Furthermore, due to the dielectric anisotropy presented in the nitrides, some additional peculiarities will be presented, like dispersive confined modes.

Optical phonon modes confinement in quasiperiodic semiconductor superlattice

The dispersion relation and localization profile of confined optical phonon modes propagating in quasiperiodic Fibonacci nitride semiconductor structures are analyzed through a transfer-matrix approach. This substitutional sequence is described in terms of a series of generations that obey peculiar recursion relations and/or inflation rules. We present a quantitative analysis of the localization and magnitude of the allowed band widths in the optical phonon spectra, as well as how they scale as a function of the number of generations of the Fibonacci sequence.

Fuchs–Kliewer phonon excitations in GaNAs alloys

The surface optical phonon spectra of GaNxAs1−x alloys have been measured using high-resolution electron-energy-loss spectroscopy. Multiple excitations of both GaAs-like and GaN-like Fuchs–Kliewer phonons were observed. Comparison is made with surface optical phonon spectra obtained from GaAs and GaN, semiclassical dielectric theory calculations and previous Raman scattering measurements of bulk phonon spectra of GaNxAs1−x alloys.

Growth of germanium nanoislands and nanowires on singular and vicinal Si(111) surfaces prior to the formation of a wetting layer

The growth of germanium nanoislands and nanowires on singular and vicinal Si(111) surfaces is investigated by scanning tunneling microscopy (STM). It is shown that the formation of a Ge wetting layer on the Si(111) surface at germanium deposition rates of approximately 10−3 BL/min and epitaxial temperatures of 350–500°C occurs through the island or multilayer growth mechanism. This makes it possible to prepare arrays of Ge islands with a height of 3 BL and a density of 109–1012 cm−2. The growth of Ge nanowires with a constant height and a width dependent on the Ge coverage is observed on vicinal Si(111) surfaces. It is found that the surface diffusion coefficients of Ge adatoms on the Ge(5×5) surface are severalfold larger than those on the Si(7×7) surface. The Raman spectra of optical phonons on the Si(111) surface with Ge nanoislands 3 BL in height contain a number of lines associated with the quantization of the phonon spectrum along the [111] growth direction.

Role of electronic correlations on the phonon modes of MnO and NiO

The possibility of magnetic-order induced phonon anisotropy in single crystals of MnO and NiO is investigated using inelastic neutron scattering. Below Tn both compounds exhibit a splitting in their transverse optical phonon spectra of approximately 10%. This behavior illustrates that, contrary to general assumption, the dynamic properties of MnO and NiO are substantially non-cubic.

Effect of ZnS shell thickness on the phonon spectra in CdSe quantum dots

The evolution of the optical phonon spectra of colloidal core/shell CdSe/ZnS quantum dots with an increase of the shell thickness from 0.5 to 3.4 monolayers has been studied by resonant Raman spectroscopy. The results obtained suggest that the ZnS shell changes its structure from amorphous to partly crystalline as the thickness increases. Simultaneously, an increase in Raman scattering by surface (core/shell interface) phonons and the redshift of the corresponding phonon band have been observed and assigned to variations in the shell structure. The disorder present in the shell provides a major contribution to the line shape of the Raman spectra at higher ZnS coverage. A method to control the quality of quantum dots based on Raman spectroscopy is proposed.

Optical phonon spectra of GaP nanoparticles prepared by nanochemistry

Gallium phosphide (GaP) nanoparticles have been synthesized by colloidal nanochemistry with two different surfactants: trioctylphosphine oxide and dodecylamine. Transverse optical (bulk) and surface optical phonons associated with the GaP nanoparticles were observed and studied experimentally by infrared transmission spectroscopy of a solid dispersion of these nanoparticles in cesium iodide pellets. These vibrational properties of the nanoparticles were used to obtain information about the crystallinity and surface interactions. The crystallinity and the stoichiometry of the samples were also examined and characterized by transmission electron microscopy, electron diffraction, and energy dispersive x-ray spectroscopy.

Composition dependence of optical phonon properties and dielectric functions of group-III arsenide ternary and quaternary mixed crystals

Optical properties of group-III arsenide ternary and quaternary alloys are investigated theoretically by a generalized mixed crystal theory. On the basis of a modified random element isodisplacement approach, long-wavelength optical phonon energies, phonon mode strengths and dielectric functions of AlxGa1−xAs, AlxIn1−xAs, and GaxIn1−xAs ternary alloys, and also of AlxGayIn1−x−yAs quaternary alloys are calculated numerically in the whole compositional range. The results are discussed and compared with experimental results. It is confirmed that the optical phonon spectra of AlxGayIn1−x−yAs alloys will exhibit three optical phonon modes in most of the compositional range.

Optical-Phonon Precursory Behavior towards Semiconductor–Metal Transition in Ba1-xKxBiO3

Optical reflectivity measurements in a frequency range of 50–1000 cm -1 have been carried out for semiconducting Ba 1- x K x BiO 3 (BKBO) single crystals. Optical-phonon spectra exhibit the single-mode behavior in contrast to the two-mode one of BaPb x Bi 1- x O 3 (BPBO). Frequencies plot for transverse-optical (TO) and longitudinal-optical (LO) phonons in BKBO almost coincides with the upper-mode frequencies for the two-mode BPBO derived from the terminal BaBiO 3 . The Szigeti-effective-charge difference Δ e s * between inequivalent Bi sites is estimated from the oscillator strength of the third-highest-frequency TO-phonon mode for each concentration. The results show that Δ e s * is anomalously large as compared with the nominal one and tends to go to zero as x goes to a critical concentration of the semiconductor–metal transition. We conclude that the charge difference arises from a strong dynamical charge transfer during the anti-parallel vibrational motion of Bi ions.

Study of infrared phonons in the La $\mathsf{_{0.7}}$ Sr $\mathsf{_{0.3}}$ MnO $\mathsf{_{3}}$ manganite by means of reflectance measurements on epitaxial films

The optical phonon spectrum of the La0.7Sr0.3MnO3 pseudocubic manganite has been investigated by means of far infrared reflectance measurements on high-quality films of different thickness grown by Pulsed Laser Deposition on LaAlO3 and SrTiO3 substrates. The manganite phonon spectrum, as obtained by properly modelling the measured reflectances, does not strongly depend on substrate and film thickness d. Measurements on thick-films (100 < d < 200 nm) on SrTiO3 allow to precisely determine line profile and peak-frequency of bending (\(\nu_{B} = 336\pm 3\) cm-1) and stretching (\(\nu_{S} = 588\pm \)3 cm-1) phonons of the bulk manganite. The bending phonon is broadened by an extra component around 365 cm-1, which strongly decreases in intensity on decreasing d below 100 nm. Although this finding is not yet fully understood, the unambiguous conclusion is that thick-films on SrTiO3 are ideal samples to get detailed information on the phonon spectrum of a bulk manganite.

Raman scattering by magnetic excitations and phonons in diluted magnetic structures formed by self-organized quantum disks of CdZn(Mn)Se in a Zn(Mn)Se matrix

Photoluminescence and resonant Raman scattering by magnetic excitations and optical phonons were studied in diluted magnetic structures that consisted of nominally two monolayer depositions of CdSe between ZnSe barriers. Mn was incorporated either into the CdSe layers or into the barriers. Self-organized quantum islands of composition CdZnSe are formed in the ``well'' layers during growth. Luminescence bands of excitons localized at the islands showed under applied magnetic fields strong effects of $s,p\ensuremath{-}d$ exchange interaction of band carriers with ${\mathrm{Mn}}^{2+}$ ions. Different types of magnetic excitation were observed in resonant Raman scattering spectra, such as spin-flip scattering of the donor-bound magnetic polarons at the CdZnSe islands, Raman transitions between ground and first excited states of antiferromagnetically coupled nearest neighbor ${\mathrm{Mn}}^{2+}$ ions, and collective multiple spin-flip Raman scattering within a Zeeman split ${\mathrm{Mn}}^{2+}$ ground state. The Raman spectra of optical phonons were studied under resonance and off-resonance excitation.

Far IR excitation of longitudinal optical phonons in triglycine sulphate

In the parts of the Far IR (FIR) spectrum where Ferroelectric Triglycine Sulphate (TGS) is transparent for a given direction of the electric vector , the forced transverse vibrations that can be excited in that direction are shown to couple slightly with the Longitudinal Optical (LO) vibrations propagating along the wave vector (wave normal) of the FIR wave. Very clear LO phonons FIR transmission spectra at quasi-normal incidence with zero transmission sharp lines can thus be obtained at low temperature with thick samples, while classical spectra of Transversal Optical (TO) phonons are obtained with thin crystal plates. This coupling seems a second order effect, quite new, applicable to all low frequencies LO phonons of TGS and probably other crystals of low symetry. On the opposite, no coupling has been detected with an other direction than the vector.

Characterization of self-assembled quantum dots using the phonon-induced features of PL spectra

We consider the photoluminescence spectra for self-assembled CdSe quantum dots on the ZnSe substrate. The spectra of optical phonons are derived for these structures. The PL features due to the LO-phonon-assisted transitions from the remnant-layer exciton state to the quantum-dot exciton state are analyzed. They compare well with the recently observed fine structure of PL in self-assembled CdSe quantum dots formed by thermally activated surface reorganization.

Correlations in optical phonon spectra of complex solids

Spectral correlations in the optical phonon spectrum of a solid with a complex unit cell are analysed using the Wigner-Dyson statistical approach. Despite the fact that all force constants are real, we find that the statistics are predominantly of the GUE type depending on the location within the Brillouin zone of a crystal and the unit cell symmetry. Analytic and numerical results for the crossover from GOE to GUE statistics are presented.

Energy-dependent electron scattering via interaction with optical phonons in wurtzite crystals and quantum wells

A formalism for the calculation of the scattering rate in wurtzite-type crystals and quantum wells (QW's) is developed taking into account features of the optical phonon spectra in an optically anisotropic medium. The electron-scattering rate due to the interaction with infrared/Raman-active polar optical phonons in GaN, bulk material, and heterostructures, is investigated. To determine the dependence of scattering rate on optical anisotropy and dimensionally induced transformation of the phonon spectra, three cases are considered: (a) bulk material with different orientations of the electron wave vector with respect to the optical axis; (b) a system in which bulk phonons interact with electrons confined in a QW; and (c) free-standing and embedded QW's where the effects of confinement of both electron and phonon subsystems are taken into account. It is found that the scattering rate depends weakly on the initial orientation of the electron wave vector. Exceptions are the energy intervals which correspond to the threshold values for emission of both TO-like and LO-like bulk phonons. Our results reveal a complex and strong dependence of the electron-scattering rate on the dispersion of a particular mode. Moreover, this dependence is found to be the main factor which determines electron-phonon scattering in wurtzite heterostructures, in particular, through the relation between phonon phase and group velocities. For the optically anisotropic media considered, the effect of spatial localization of the phonon modes on the scattering rate is found to be as strong as the effect of electron confinement.

Optical phonon spectra of PbF2 single crystals

The spectra of dipole-active optical phonons are measured for the cubic and orthorhombic phases of PbF2 single crystals. The frequencies and eigenvectors of normal modes in the Pnma orthorhombic phase are calculated. It is found that the spectrum of the cubic phase exhibits excess vibrational modes of the PbF2 orthorhombic phase.

Coherent phonon spectroscopy of GaAs surfaces using time-resolved second-harmonic generation

The general theory and experimental considerations are presented for a novel all-optical time-domain technique for measuring low frequency vibrational modes of surfaces (phonons or adsorbate vibrational modes). A pump-laser pulse impulsively drives an initial displacement of the surface atoms. The subsequent free-induction decay of the coherent phonon modes of the surface atoms is detected by time-resolved second-harmonic generation (SHG). The spectral features are recovered by fitting the time-domain data to exponentially decaying sinusoids. This all-optical probe has advantages over inelastic particle scattering techniques because it can be applied at buried interfaces. It has signal to noise advantages over linear and spontaneous Raman techniques. This technique is demonstrated by measurement of surface optical phonon spectra on the clean GaAs (110)–relaxed–(1×1), GaAs (100)–(1×6), and GaAs (100)–(4×1) in UHV and of a local monolayer-scale interfacial mode at the buried native-oxide-covered GaAs (100). The chemical sensitivity is demonstrated by in situ oxidation of the GaAs (100)–(4×6). The general mechanism of generating coherent surface phonons is discussed in light of the symmetry selection rules.

Density matrices for a chain of oscillators

We consider chains with an optical phonon spectrum and study the reduced density matrices which occur in density-matrix renormalization group (DMRG) calculations. Both for one site and for half of the chain, these are found to be exponentials of bosonic operators. Their spectra, which are correspondingly exponential, are determined and discussed. The results for large systems are obtained from the relation to a two-dimensional Gaussian model.

Optical phonon spectrum rearrangement in quasi-one-dimensional disordered systems

By employing the Green's-function expansion method, the optical spectrum rearrangement in the quasi-one-dimensional (1D) impurity vibrational systems has been investigated. Based on the validity criteria for the conventional coherent potential approximation (CPA), the dispersion of phononlike modes and main parameters for the rearranged spectra are determined. The most interesting rearrangement occurs when the bandwidth exceeds the limiting frequency of the optical branch. In this case, the spectrum rearrangement exhibits features characteristic of both the acoustic branch and high-frequency optical branch. Formations of the impurity optical branch and a well-defined limiting frequency are described.