Generation Of Coherent Phonons Research Articles

Generation and detection of coherent optical phonons in germanium.

Coherent optical phonons are generated impulsively in Ge by irradiation with femtosecond laser pulses. They are detected via reflectivity modulations in the time domain in a pump-probe configuration. The deformation potential of the optical phonon is responsible both for the generation and for the detection process. The excitation of the coherent optical phonons is explained by anisotropic weakening of bonding valence orbitals.

Optical generation of continuous-wave accumulated coherent phonons

We report on the generation of continuous-wave coherent phonons (ultrasonic waves) in ethanol by the laser-induced transient-grating method with a continuous-wave mode-locked argon-ion laser. We detected the phonons phase sensitively by monitoring the Bragg diffraction of a probe beam. Sharp resonance enhancement of the induced phonons was observed when the repetition rate of the excitation pulses coincided with the phonon frequency (the sound velocity divided by the grating period). The signal intensity was much increased as a result of interference with the co-induced thermal grating.

Generation mechanism for coherent LO phonons in surface-space-charge fields of III-V-compounds

We report on details of coherent LO phonon generation in surface-space-charge regions of III-V-compounds by optical injection of free carriers with laser pulses of 50 fs duration at 2 eV. Both the dynamics of the transient surface field, as well as the coherent lattice vibration, are measured via electro-optic sampling techniques under different experimental conditions. The driving force for the coherent phonon vibration is the sudden depolarization of the crystal lattice due to ultrafast screening of the intrinsic electrical surface field by photoexcited free carriers.

Coherent nonequilibrium phonons: An induced Peierls distortion.

A new approach to the Peierls distortion is described, where the lattice distortion is induced by some means external to the system. This nonequilibrium technique removes the temperature limitations on the Peierls distortion and the limitation of phonon localization, and so can generate charge-density waves at an arbitrary temperature. A method is considered for coherent phonon generation and subsequent use with a quantum wire array to induce a charge-density-wave transition

Subpicosecond generation and decay of coherent phonons in III-V compounds

Photoexcitation of III-V compounds with a 50 fs laser pulse leads to ultrafast longitudinal field changes of up to 100 kV cm-1. These are generated via screening and transport effects of the photoexcited carriers in the surface space-charge region. The field rise is sufficiently short to impulsively launch coherent LO phonons. A detailed study of the frequency and dephasing behaviour of these phonons as a function of excitation density and excess optical excitation energy is performed in GaAs and AlGaAs.

Generation of coherent phonons in condensed media

The generation of coherent phonons by absorption of femtosecond laser pulses through interband transitions in different materials is summarized. Starting with layered III-VI semiconductors, where optical excitation is performed with amplified pulses, the generation mechanism via strong interband transitions is investigated. In Ge, stimulated resonant Raman scattering is found to be the decisive driving force. In GaAs, however, the ultrafast screening of surface space charge fields launches LO-phonons via electrostatic coupling. In the metallic state of high-temperature superconductors, the generation of highly symmetric A/sub g/ modes is assigned to displacive processes induced by a nonequilibrium electronic carrier distribution. In the superconducting state the amount of coherent displacement is strictly correlated to the number of optically broken superconducting pairs. In addition to the comparison of various generation processes, the dephasing of coherent phonons is addressed. >

Mechanism for displacive excitation of coherent phonons in Sb, Bi, Te, and Ti2O3

Coherent phonons in Sb, Bi, Te, and Ti2O3 can be generated impulsively, and detected in the time domain through reflectivity modulation using 60 fs pulses of laser light at 2 eV. Experimental data for these opaque solids suggest that a direct Raman excitation mechanism is not responsible for coherent phonon generation. Rather, the excitation is attributed to an electronically induced displacement of the ion equilibrium coordinates.

Stimulated two-quantum photon–phonon transitions in indirect-gap semiconductors

An analysis is made of two-quantum photon–phonon transitions in indirect semiconductors in which either the optical or the acoustic components of the transition, or both may be stimulated simultaneously. A study is reported of possible generation of coherent transverse acoustic phonons in the 1012–1013 Hz frequency range by indirect photon–phonon interband transitions.

Enhanced coherent phonon generation by periodic excitation with Q-switched and mode-locked laser pulses

Enhanced coherent phonons (ultrasonic waves) produced by periodic excitation of transient grating with light pulses from a Q-switched and mode-locked laser were observed when the resonance condition v= nf was satisfied, where v is the phonon frequency, f the repetition frequency of the pulses and n an integer. The enhancement was observed for phonons with frequencies from 81.8 MHz to 654 MHz, and the maximum enhancement was about 100 in intensity. The dynamics involved was analyzed by developing the theory conforming to our experimental condition. Sound velocities and phonon intensities were measured in ethanol etc. containing a small amount of dye.

Femtosecond multiple pulse impulsive stimulated Raman scattering

Generation of coherent optic phonons through impulsive stimulated Raman scattering (ISRS) can occur whenever a sufficiently short optical pulse passes through a Raman-active medium.1-3 In this paper we report the application of specially shaped femtosecond pulse trains to achieve improved control over coherent vibrational motion. In particular, we demonstrate multiple-pulse ISRS, in which terahertz- rate trains of femtosecond pulses are used for repetitive impulsive excitation of optic phonons. By matching the pulse repetition rate to a particular phonon frequency, we are able to select and amplify individual vibrational modes.4 Each successive pulse in the sequence arrives at the sample in phase to amplify the coherent vibrational motion, analogous to repetitively pushing a child on a swing.

Proposal for the direct electromagnetic generation of coherent terahertz acoustic phonons in semiconductor superlattices at the University of California, Santa Barbara far-infrared free-electron-laser facility

We discuss the possibility of the generation of coherent packets of monochromatic acoustic waves in the far-infrared frequency range by electromagnetic radiation. The University of California, Santa Barbara free-electron laser could be used to excite resonantly transverse acoustic phonons in periodic space-charge layers formed in modulation-doped GaAs–Ga1−xAlxAs superlattice structures. The conversion efficiency for the process (acoustic power/incident electromagnetic power) is estimated to be of the order of 10−3 at 0.25 THz.

Infrared-laser-induced photodesorption: Analysis of theoretical models

In this paper, the theoretical models of laser-induced desorption are analyzed. (a) Laser-induced desorption via intramolecular energy transfer to the desorption mode. (b) Desorption via combined action of laser radiation and intermolecular energy transfer. (c) Energy transfer between isotope molecules. (d) Resonant heating. (e) Desorption via laser-induced coherent surface photons. Intermolecular energy transfer and generation of coherent surface phonons are important mechanisms which have to be taken into account in the analysis of the experimental data; these mechanisms are carefully examined in this paper.

Dynamics of coherent optical phonon generation and decay in GaP.

Coherent longitudinal optical (LO) phonons are generated through nonlinear excitation by laser light. It is observed that the vibronic sideband structure associated with coherent LO phonons, and that of the acoustic phonons, generated during the dephasing of the LO phonons have an unexpected dependence on the power of the excitation lasers. The effect is traced to the presence of an electron-hole plasma which is shown to influence the phonon generation process. Moreover, it is directly verified that the dephasing of coherent near-zone-center longitudinal optical phonons in GaP is dominated by anharmonic decay into two half-energy longitudinal acoustic phonons.

Acoustic phonon generation in the picosecond dynamics of dense electron-hole plasmas in InGaAsP films

Oscillations superposed on transient transmission and reflection from submicron films of InGaAsP (Eg=0.96 eV) and GaAs excited by subpicosecond optical pulses have been observed. The oscillations are due to coherent acoustic phonon generation in the thin films. Values for the velocity of sound, and the band-gap variation due to hydrostatic pressure and the difference in acoustic deformation potentials between conduction and valence bands are determined by analyzing the period and magnitude of the oscillations.

Coherent Phonon Generation and Detection by Picosecond Light Pulses

Using the picosecond pump and probe technique we have detected oscillations of photoinduced transmission and reflection in thin films of $a$-${\mathrm{As}}_{2}$${\mathrm{Te}}_{3}$ and cis-polyacetylene. These oscillations are due to the generation and propagation of coherent acoustic phonons in the film. We discuss the generation and detection mechanism, and we use this effect to measure the sound velocity in a film of $a$-Si${\mathrm{O}}_{2}$.

Ferroelectricity and coherent phonon generation in piezoelectric composition-modulated structures

In this paper we suggest an approach to making new metastable ferroelectric materials and piezoelectric transducers which should be efficient in generating coherent phonons with wavelengths approaching interatomic distances. This involves the fabrication of composition-modulated (layered) structures consisting of piezoelectric components. The coherency strain accompanying short wavelength composition modulation leads to ferroelectricity; the periodic modulation of the piezoelectric parameters leads to an anomalously strong coupling to far infrared light when the composition wavelength is an odd half-multiple of the wavelength of a phonon at the driving frequency. Such transducers can open a new dimension in the study of phonons in dielectrics at low temperatures.

Double magnetoacoustic resonances in crystals

The introduction contains an exposition of the basic principles of magnetic quantum acoutics and the possibility of its use to investigate the structure and the dynamics of solids. The distinguishing features of different double magnetoacoustic resonances (nuclear-nuclear, electron-nuclear, and electron-electron) are then considered. Results are presented of investigations of spin-phonon interactions, relaxation processes, and singularities of the structure in various classes of crystals, carried out with the aid of such resonances. This is followed by consideration of dynamic polarization of atomic nuclei with the aid of ultrasound. Particular attention is paid to the use of magnetoacoustic resonances for quantum amplification and generation of coherent phonons (acoustic masers) and for the development of quantum detectors for ultrasound.

Coherent phonon generation by optical mixing in a one-dimensional superlattice

With the help of the lattice momentum, phase-matched optical generation of coherent phonons with frequencies higher than 50 GHz appears feasible. We have considered two cases: (i) direct conversion of millimeter or submillimeter photons in a piezoelectric superlattice, (ii) optical mixing of two laser beams in a nonpiezoelectric superlattice.