Generation Of Coherent Acoustic Phonons Research Articles

Coherent optical and acoustic phonon generation correlated with the charge-ordering phase transition inLa1−xCaxMnO3

We have observed coherent optical and acoustic phonon generation, which are strongly coupled to the charge-ordering (CO) transition in (, 0.58) using femtosecond optical pump-probe spectroscopy. Coherent optical phonons, observed at low temperatures, disappear above the charge-ordering temperature , while coherent acoustic phonons display the opposite behavior, disappearing gradually below . Coherent optical phonons are generated by the displacive excitation mechanism where their coupling to the photoexcited charge carriers is enhanced by the structural change corresponding to the CO phase transition. The oscillation frequency for the coherent acoustic phonon depends on the probe wavelength, which is consistent with the propagating strain pulse mechanism. The dramatic change of lattice constants across the charge-ordering transition explains the overall temperature dependence of the coherent acoustic phonon amplitude.

Preface

Preface

Generation of coherent acoustic phonons in GaN‐based p‐n junction

Coherent acoustic phonons were generated in the piezoelectric bulk semiconductor through inverse piezoelectric effect by femtosecond laser pulses. While the photocarriers are generated in the depletion region of the piezoelectric semiconductor, the electrons and holes sweep in counter-directions due to the built-in electric field and result in strain pulses through the piezoelectric effect. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Generation of coherent acoustic phonons in strained GaN thin films

Coherent acoustic phonon oscillations were generated and studied in strained GaN thin films. Inside the bulk GaN film, the longitudinal interference of an ultraviolet femtosecond pump pulse created periodic carrier distribution that screened out the strain-induced piezoelectric field and initiated the coherent longitudinal acoustic phonon oscillations corresponding to the carrier periods. The created coherent phonon oscillation modulated the piezoelectric field thus modified the absorption property of the GaN thin film through Franz–Keldysh effect. This time-dependent absorption modulation was reflected in the transmission variation of the followed probe pulses, resulting a long decay time ∼300 ps for the initiated coherent phonon oscillations.

Observation of coherent zone-folded acoustic phonons generated by Raman scattering in a superlattice

We have used pulse time-of-flight techniques to examine the phonon emission from an optically excited GaAs/AlAs superlattice structure. For laser excitation wavelengths shorter than 767 nm (the energy of E1HH1 transition), we detect a significant longitudinal acoustic phonon component directed in a narrow beam normal to the structure. Under identical excitation conditions, generation of coherent longitudinal acoustic phonons has previously been observed in this structure. We suggest that the excitation wavelength and angular characteristics of the longitudinal acoustic emission is consistent with those of propagating modes produced as coherent phonons “leak” from the superlattice structure.

Generation and amplification of sub-THz coherent acoustic phonons under the drift of two-dimensional electrons

This paper addresses the \ifmmode \check{C}\else \v{C}\fi{}erenkov emission of high-frequency confined acoustic phonons by drifting electrons in a quantum well. We have found that the electron drift can cause strong phonon amplification (generation). The spectra of the confined modes are calculated and their confinement properties are analyzed. The spectra consist of a set of branches, and for each branch, the confinement effect increases considerably when the phonon wave vector increases. We have studied the coupling between electrons and confined modes and proved that the coupling is a nonmonotonous function of the wave vector for each of the phonon branches. We have obtained a general formula for the gain coefficient as a function of the phonon frequency and the structure parameters. For each of the branches, the amplification takes place in a spectrally separated and quite narrow amplification band in the high-frequency range. For the example of p-doped Si/SiGe/Si heterostructures it is shown that the amplification coefficients of the order of hundreds of ${\mathrm{cm}}^{\ensuremath{-}1}$ can be achieved in the sub-THz frequency range.

Generation of high-frequency coherent acoustic phonons in a weakly coupled superlattice

In this work we analyze conditions of generation of high frequency coherent acoustic phonons in a biased semiconductor superlattice. We show that the electron perpendicular transport provides the electron population inversion for interwell phonon-assisted transitions and generation of phonons propagating along the superlattice axis. The generation rate is large and exceeds the phonon losses in perfect superlattices. Under such conditions, the phonon subsystem is unstable with respect to generation of the coherent phonon modes. Influence of the phonon spectra modification in the superlattice on the generation rate is examined. Effects restricting the intensity of the phonon generation are also pointed out.

Coherent Phonons in Ag-BaO Thin Films

Coherent acoustic phonons have been observed in Ag nanoscale particles (NSPs) embedded in BaO thin film by means of the femtosecond time-resolved pump-probe technique. The oscillatory component in the signal corresponds to a very low-frequency acoustic phonon mode of Ag NSPs. The generation of coherent acoustic phonons is attributed to the resonant excitation of localized surface plasmon.

Ultrafast acoustic phonon generation in gold

We investigate the ultrafast generation of coherent longitudinal acoustic phonons in gold films with a femtosecond optical pump and probe technique. The strain pulses observed are much broader than expected from simple thermoelastic generation. This is explained by the supersonic diffusion of electrons over distances much larger than the optical absorption depth. The electron-phonon coupling constant can be estimated directly from the phonon pulse duration. Using a novel analytical approach, we present an extension of the model to include electron-electron interactions.

Optical generation of high-frequency acoustic waves in GaAs/AlxGa1−xAs periodic multilayer structures

A picosecond study of ultrahigh-frequency acoustic phonons in specifically engineered GaAs/AlxGa1−xAs periodic multilayer structures is presented. The lattice-matched boundary conditions for photothermal acoustic generation and optical properties of these materials make these structures ideal for sound-wave generation in the 100 GHz to THz range. The acoustics are generated using ultrashort-laser-pulse excitation and detected in real time by measuring the strain-induced change in reflectivity with the pump-probe technique. By using 12 nJ, 90 fs pulses from a Ti:sapphire laser source, the generation and detection of ∼50 GHz acoustics in a 6-bilayer, [001]-oriented GaAs/Al0.4Ga0.6As structure, 500 Å thickness per layer, on a GaAs substrate, are successfully demonstrated. The structure was specifically designed to give the maximum sensitivity to the acoustics through étalon-induced modulations in the reflectivity spectrum. With similarly designed multilayer structures, the upper frequency limit can be achieved for the thermoelastic generation of coherent acoustic phonons, that is, ∼300 GHz in GaAs for ∼1 eV above band-gap-energy photons.

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.

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.

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${\mathrm{O}}_{2}$.