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Optical Phonons (optical + phonon)

Distribution by Scientific Domains

Physics and Astronomy	76%
Chemistry	23%

Terms modified by Optical Phonons

optical phonon mode

Selected Abstracts

Optical phonons in millerite (NiS) from single-crystal polarized Raman spectroscopy

JOURNAL OF RAMAN SPECTROSCOPY, Issue 10 2008
François Guillaume
Abstract Although several fundamental physico-chemical aspects of nickel sulfides have been studied in detail, particularly for millerite (nickel(II) monosulfide), the most common nickel sulfide mineral, no satisfactory investigation of optical vibrational modes has been reported previously. In this paper, we provide a definitive assignment of the optical phonons in millerite, investigated by polarized Raman spectroscopy on an oriented single crystal. The impact of the power of the incident laser beam on the spectra has also been investigated, revealing evidence for degradation in the quality of the spectra at sufficiently high laser power. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Optical phonons in hexagonal GaN/AlN and GaN/AlGaN superlattices

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2005
V. Yu.
Abstract In this work, by using numerical simulation of the lattice dynamics of hexagonal GaN/AlN superlattices, the microscopic nature of the vibrational states corresponding to the most intense lines in Raman spectra has been established and the interpretation of experimental spectra has been suggested. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Confinement effects on optical phonons in spherical, rod-, and tetrapod-shaped nanocrystals detected by Raman spectroscopy

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 2 2007
Concetta Nobile
Abstract Spherical, rod- and tetrapod shaped CdSe nanocrystals are investigated by Raman spectroscopy and the longitudinal-optical and surface optical phonons are observed. We find that the position of the longitudinal-optical phonon slightly red-shifts with decreasing diameter, whereas the position of the surface optical phonon depends significantly on diameter and length of the rods or the tetrapod arms. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Photoluminescence properties of PbSe/PbS core-shell quantum dots

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2009
Georgy I. Maikov
Abstract The present study describes the investigation of the ground-state exciton emission in the PbSe/PbS core-shell colloidal quantum dots (CQDs) with different core-radius/shell-thickness ratio, by recording temperature dependence of ground-state exciton photoluminescence (PL) properties, including line-width, integrated PL intensity, and the temperature coefficient of energy band-gap, over a temperature range from 1.4 K to 300 K. The obtained data reveal a reduction of the temperature coefficient of the energy band-gap as well as slightly decrease of exciton and optical phonon coupling in the PbSe/PbS core-shell CQDs with respect to that in the corresponding PbSe core. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Energy relaxation in CdSe/ZnSe quantum dots under the strong exciton-phonon coupling regime

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2006
A. V. Platonov
Abstract Resonant photoluminescence and photoluminescence excitation spectroscopy have been applied to CdSe/ZnSe quantumdots where separation between the quantized levels is close to the optical phonon energy. Astrong relaxation enhancement induced by the optical phonon is observed. The fine structure of the phonon replicas is resolved. It is attributed to the optical phonons with large wave vectors from the symmetrical points of the Brillouin zone. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Optical phonons in millerite (NiS) from single-crystal polarized Raman spectroscopy

The pH effects on the growth rate of KDP (KH2PO4) crystal by investigating Raman active lattice modes

JOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2007
M. Badrouj
Abstract We report on the dependence of the pH value on the growth rates of KDP single crystals. Extensive experimental work has been carried out in order to find the optimum pH ranges for growing KDP single crystals with suitable sizes and high optical quality. Different techniques including micro-Raman back-scattering spectroscopy, UV/vis/IR transmission spectroscopy and X-ray diffraction have been employed for this investigation. Deuterated substituted single crystals of KDP and DKDP also have been grown for the investigation of growth rates and Raman active mode identification purposes. The molecular vibration modes of the grown crystals, including internal modes of PO4 tetrahedrons molecular vibrations, external modes of optical phonons and hydrogen bonding modes have been determined exactly by micro-Raman back-scattering spectroscopy. The best pH values of the solution for the KDP crystal growth with reasonably higher growth rates from aqueous solutions that have been supersaturated ata temperature range of 30,50 °C have been found to be in the pH range of 3.2,5.4. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Raman spectroscopic study on the structure, phase transition and restoration of zirconium tungstate blocks synthesized with a CO2 laser

JOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2007
E. J. Liang
Abstract Densely packed zirconium tungstate blocks were synthesized by rapid solidification with a CO2 laser. The structure and phase transition properties of the samples were studied by Raman spectroscopy and X-ray diffraction. Raman spectroscopic study reveals that zirconium tungstate solidifies with an orthorhombic structure. This is attributed to the pressure encountered by the samples during rapid solidification. Several Raman bands change discontinuously at about 390 K, indicating a phase transition from the ,- to the ,-phase occurring at this temperature. In the ,-phase, most of the Raman modes give rise to negative Grüneisen parameters, suggesting contribution of the corresponding optical phonons to the negative thermal expansion coefficient of the material, at least for the ,-phase. A recovery of the ,-phase was observed when the samples were cooled to room temperature. This suggests that the cubic structure is metastable only at temperatures above 390 K, and at room temperature the ,-phase is preferred. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Raman spectroscopy of optical phonon confinement in nanostructured materials

JOURNAL OF RAMAN SPECTROSCOPY, Issue 6 2007
Akhilesh K. Arora
Abstract If the medium surrounding a nano-grain does not support the vibrational wavenumbers of a material, the optical and acoustic phonons get confined within the grain of the nanostructured material. This leads to interesting changes in the vibrational spectrum of the nanostructured material as compared to that of the bulk. Absence of periodicity beyond the particle dimension relaxes the zone-centre optical phonon selection rule, causing the Raman spectrum to have contributions also from phonons away from the Brillouin-zone centre. Theoretical models and calculations suggest that the confinement results in asymmetric broadening and shift of the optical phonon Raman line, the magnitude of which depends on the widths of the corresponding phonon dispersion curves. This has been confirmed for zinc oxide nanoparticles. Microscopic lattice dynamical calculations of the phonon amplitude and Raman spectra using the bond-polarizability model suggest a power-law dependence of the peak-shift on the particle size. This article reviews recent results on the Raman spectroscopic investigations of optical phonon confinement in several nanocrystalline semiconductor and ceramic/dielectric materials, including those in selenium, cadmium sulphide, zinc oxide, thorium oxide, and nano-diamond. Resonance Raman scattering from confined optical phonons is also discussed. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Confinement effects on optical phonons in spherical, rod-, and tetrapod-shaped nanocrystals detected by Raman spectroscopy

Strong anharmonicity and phonon confinement on the lowest-frequency Raman mode of nanocrystalline anatase TiO2

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2007
Kun Gao
Abstract We calculated and analyzed the effects of temperature and crystallite size on the Raman properties of nanocrystalline anatase TiO2 by using the related models mentioned previously [Tang and Herman, Phys. Rev. B 43, 2299 (1991); Richter et al., Solid State Commun. 39, 625 (1981)]. The temperature dependence of the Raman spectra of two different crystallite sizes of anatase TiO2 were obtained. Careful measurements of the frequency shifts of Raman modes were carried out at different temperatures. Through fitting the experimental data, pure-volume and the pure-temperature contributions to the frequency shifts of the lowest- and highest-frequency Eg modes, respectively, have been calculated. The results indicated that strongly intrinsic anharmonicity arising from optical-phonon couplings was the origin of hardening upon increasing temperature for the lowest-frequency Raman mode. The phonon-confinement effect on this mode was analyzed and anharmonic shifts of optical phonons with two different kinds of crystallite sizes were also compared. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Hot carrier energy losses in conducting layers of AlGaN/GaN heterostructures grown on SiC and Al2O3 substrates

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2006
B. A. Danilchenko
Abstract The energy relaxation rate for hot electrons in AlGaN/GaN heterostructures was measured over the temperature range 4.2,300 K. Samples grown on sapphire and 4H-SiC substrates were studied. The most important result is that the hot carrier energy dissipation differs for samples grown on sapphire and SiC substrates. In the case of sapphire substrate, the dissipation can be described by the emission of optical phonons with an energy of 90 meV and relaxation time of 25 fs. In the case of SiC substrate, both activation energy and relaxation time exceed the values characteristic of the electron-LO-phonon dissipation process. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Spectroscopic studies of 2H-PbI2(Mn) layered crystals

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 12 2005
F. V. Motsnyi
Abstract For the first time we studied Raman, exciton and EPR spectra of PbI2 layered crystals of 2H polytype doped with high concentration of Mn impurity. A novel type of Fermi resonance was found. The anomalous temperature shift of the exciton band n = 1 was registered. It was shown that this shift can be explained by means of anharmonic vibrations of layered lattice at T < 40 K and low-frequency optical phonons at T > 40 K. The manifestation of single and exchange-bound Mn2+ ions in EPR spectra was observed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Interface phonons and polaron effect in II-VI quantum wells

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2010
Alexander Yu.
Abstract The theory of large radius polaron in II-VI quantum wells is developed. The analytic expressions for polaron binding energy are obtained for the electron, the hole and the exciton polarons. The numerical results obtained show that the strong electron-phonon interaction conditions can be realized in the quantum wells based on a number of II-VI compounds. The influence of the interface optical phonons on the polaron states is accounted for. The extent of interface phonon influence depends on the properties both of the quantum well and of the barrier materials. It is shown that the interaction of charge particles with the interface phonons can lead to the enhancement as well as to the reduction of the polaron effects. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Ultrafast decay of non-equilibrium (hot) phonons in GaN-based 2DEG channels

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 12 2009
Arvydas MatulionisArticle first published online: 27 OCT 200
Abstract Fast and ultrafast decay of non-equilibrium longitudinal optical phonons (hot phonons) is discussed on examples of two-dimensional electron gas (2DEG) channels confined in nominally-undoped nitride and selectivelydoped arsenide heterostructures subjected to pulsed dc electric power. At a low,moderate power, a nonmonotonous dependence of the hot-phonon lifetime on the 2DEG density is found. The fastest decay takes place at ,2.5 × 1012 cm,2 in GaInAs 2DEG channel and at ,7 × 1012 cm,2 in GaN 2DEG channel. The result is discussed in terms of plasmon,LO-phonon resonance. The hot-phonon lifetime decreases when the supplied power is increased if the 2DEG density exceeds the resonance density. The shortest hot-phonon lifetime of 60 fs is obtained at/above 50 nW/electron for the lattice-matched AlInN/AlN/GaN 2DEG channel (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Phonons in InAs quantum dot structures

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2009
Alexander Milekhin
Abstract We present a Raman study of the phonon spectra of periodical structures with (In,Ga)As QDs in (Al,Ga)As matrix as well as AlAs QDs embedded in InAs grown by molecular beam epitaxy. Raman scattering by optical, interface and acoustic phonons was observed in the QD structures. TO and LO phonons in the QDs are strongly affected by both strain and confinement. The Raman study reveals a two-mode behavior of optical phonons in the whole composition range for both InGaAs QDs and the AlGaAs matrix. Raman scattering by InAs- and GaAs-like LO phonons in InGaAs QDs shows a size-selective resonant behaviour. Interface phonons were investigated in InGaAs QDs and the AlGaAs matrix. Their frequency positions were analyzed as a function of the alloy content within the dielectric continuum model. The positions of IF phonons in the QD structures observed in the experiment agree well with calculated ones assuming that the QDs have the shape of oblate ellipsoids. Multiple phonon Raman scattering involving both pure overtones of the first-order InAs, GaAs and AlAs optical and interface phonons and combination of phonons from the materials is observed in the vicinity with E0 resonance in QDs. Possible mechanisms of these processes are discussed. Low frequency resonant Raman scattering by acoustic phonons was observed in the QD structures. The periodic oscillations seen in the Raman spectra are well described by the elastic continuum model. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Room temperature intraband Raman emission and ultrafast carrier relaxation in GaN/AlN quantum dots

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009
L. Nevou
Abstract We report on the intraband emission at room-temperature from GaN/AlN quantum dots grown by plasma-assisted molecular beam epitaxy as well as on the measurements of GaN quantum dot intraband relaxation using femtosecond spectroscopy. The quantum dots exhibit s-pz intraband absorption peaked at 1.55 ,m. The pz -s intraband luminescence at , = 1.48 ,m is observed under optical excitation at , = 1.34 ,m. The population of the pz state arises from Raman scattering by GaN A1 longitudinal optical phonons. Based on the emission spectral shape, we estimate that the homogeneous linewidth of the s-pz intraband transition is less than 4 meV. The carrier lifetime of the pz state (T1) is measured by femtosecond pump-probe spectroscopy. T1 = 165 fs while the thermalization of electrons in the ground state proceeds in 1.5 ps. The saturation intensity of the s-pz intraband absorption is estimated to be in the 15 to 137 MW/cm2 range taking into account the uncertainty on the pump beam waist. These results demonstrate the promising capabilities of GaN/AlN QDs for application to telecom intraband lasers as well as multi-Tbit/s all-optical switching devices. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Understanding quantum dots: overheating of the LO-phonon modes

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2007
K. Král
Abstract Longitudinal optical phonons have been used recently to explain the electronic energy relaxation in quantum dots. In this theory they served as a reservoir, on which the electron executes multiple scattering acts. Rather naturally such a phonon subsystem is expected to be passive, namely, in a long-time limit of development the whole system should be able to achieve such a stationary state in which statistical distributions of both subsystems, electron and phonons, do not change in time. Here we remind briefly that the recent approach to the relaxation theory in quantum dots displays a non-passivity of such a reservoir. We remind briefly the method of a partial elimination of this phonon overheating effect by using the Lang-Firsov transformation. We apply the modified relaxation theory to the electronic relaxation at low electronic densities in quantum dot systems and come to conclusions about the role of e-LO scattering mechanism in these situations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Energy relaxation in CdSe/ZnSe quantum dots under the strong exciton-phonon coupling regime

Photoluminescence, Raman scattering and vertical transport of CdSe/CdMgSe superlattices

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2006
I. I. Reshina
Abstract Photoluminescence and photoluminescence excitation spectra, vertical transport of photoexcited charge carriers and excitons and Raman scattering by optical phonons have been studied for the first time in low-strained superlattices CdSe/CdMgSe grown by molecular-beam epitaxy on InAs substrates. The vertical transport was studied by purely optical means involving an enlarged quantum well built into the superlattice. The enlarged quantum well served as a sink for the photoexcited carriers and excitons that have tunnelled through the superlattice. The measurements conducted in temperature range 2,150 K and also under in-plane strong magnetic fields show that vertical transport occurs mainly by free heavy-hole excitons but in superlattices with 5.9 nm and 7.3 nm periods it is not of the Bloch type. A comparison of the calculated energies of the interband transitions with the experimental data provides the valence-band ofsett in the range 0.4,0.5. The Raman spectra indicate a two-mode behavior of the optical phonons in CdMgSe barriers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Detection of non-equilibrium longitudinal optical phonons in InN and its consequences

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2005
W. Liang
Abstract Picosecond Raman spectroscopy has been used to study properties of non-equilibrium longitudinal optical phonons in a high quality, single crystal wurtzite structure InN sample. Our experimental results show that the bandgap of InN cannot be around 1.89 eV; but are consistent with a bandgap of around 0.8 eV. Our results provide a concrete evidence that 0.8 eV-luminescence observed recently in InN cannot be due to deep level radiative emission in InN. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Is polaron effect important for resonant Raman scattering in self-assembled quantum dots?

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2005
M. I. Vasilevskiy
Abstract While the diagonal (or intra-level) interaction of a confined exciton with optical phonons in self-assembled quantum dots (SAQD's) is rather weak, the non-diagonal one can lead to a considerable change of the exciton spectrum and the formation of a polaron. An impact of this effect on resonant inelastic light scattering is studied theoretically. The polaron spectrum is obtained by numerical diagonalisation of the exciton,phonon interaction Hamiltonian in a truncated Hilbert space of the non-interacting excitons and phonons. Based on this spectrum, the probability of the multi-phonon Raman scattering is calculated, which is compared to that obtained within the standard perturbation theory approach (where phonon emission and absorption are irreversible). It is shown that there are two major effects of the polaron formation: (i) the intensity of the two-phonon (2 LO) peak, relative to that of the fundamental 1 LO one is strongly increased and (ii) the resonant behaviour of the 1 LO peak differs considerably from the perturbation theory predictions. With the correct theoretical interpretation, resonant Raman scattering in SAQD's opens the possibility of accessing the (renormalised) exciton spectrum and exciton,phonon coupling constants. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Interface and confined polar optical phonons in spherical ZnO quantum dots with wurtzite crystal structure

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004
Vladimir A. Fonoberov
Abstract We derive analytically the interface and confined polar optical-phonon modes for spherical quantum dots with wurtzite crystal structure. While the frequency of confined optical phonons in zincblende nanocrystals is equal to that of the bulk crystal phonons, the confined polar optical phonons in wurtzite nanocrystals are shown to have a discrete spectrum of frequencies different from those in bulk crystal. The calculated frequencies of confined polar optical phonons in wurtzite ZnO quantum dots are found to be in excellent agreement with experimental resonant Raman scattering data. The derived analytical expression for phonon modes can facilitate interpretation of experimental data obtained for ZnO quantum dots. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Coherence length and time of excitons in ZnSe quantum wells

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2004
B. Dal Don
Abstract We investigate the in-plane transport of excitons in quantum wells by nano-photoluminescence. The experimental method is based on a confocal microscope with an enhanced resolution given by the introduction of a solid immersion lens. In combination with pulsed laser excitation and streak-camera detection, we have access to transport phenomena on a timescale faster than the time of scattering with acoustic phonons and a length scale of the light wavelength. We use ZnSe-based quantum wells as a model system since hot excitons with well defined excess energy can be formed assisted by the emission of optical phonons. This results e.g. in a periodic quenching of the excitonic transport length as function of excitation excess energy which, in comparison, is not found in GaAs quantum wells. Monte Carlo simulations of the nonlinear expansion of the luminescence spot observed as a function of time reveal the difference between the spatial profiles of the luminescence and the exciton density. The latter shows an oscillatory behaviour in time due to the dominant backscattering, when the first acoustic phonon is emitted. From this oscillation we can determine simultaneously the coherence time and length of the excitonic transport in ZnSe quantum wells. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Extension of the time-dependent dynamical diffraction theory to `optical phonon'-type distortions: application to diffraction from coherent acoustic and optical phonons

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 1 2003
Peter Sondhauss
An extension of the time-dependent Takagi,Taupin theory to `optical phonon'-type distortions is presented. By splitting the susceptibility into the contributions from each atom in a unit cell, modifications to the structure factor as well as lattice parameter are taken into account. The result is a compact, surprisingly simple, equation with a strong formal similarity to the classical Takagi,Taupin equation, with the latter included as a special case. Time dependence is explicitly retained and thus the analysis is applicable to situations where the crystal is modified on time scales comparable with that for the X-rays to traverse an extinction depth. A comparison is made between the influence of coherent acoustic and optical phonons on the diffraction of X-rays. Numerical and perturbative analytical solutions of the generalized Takagi,Taupin equation are presented in the presence of such phonons. [source]