Generation Of Coherent Phonons Research Articles

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.

Coherent phonon generation mechanism in solids

Coherent phonon generation mechanism in solids

Amplification of transverse acoustic phonons in quantum well heterostructures with piezoelectric interaction

We have analyzed amplification of transverse phonons confined in quantum well (QW) heterostructures through piezoelectric electron–phonon interaction with drifting electrons. It was found that this mechanism of interaction couples the low-dimensional electrons and the shear-horizontal (SH) confined phonons. We have studied the electrostatic potential accompanying the SH waves and found that efficient interaction can be achieved for the lowest antisymmetric SH phonon branch in a narrow band of phonon frequencies. For AlGaAs QWs the amplification coefficient was calculated to be on the order of 100 cm−1 in the sub-THz phonon frequency range. These results suggest an electrical method for coherent phonon generation in the technologically well-developed AlGaAs QW heterostructures.

Generation of coherent off-diagonal raman-active phonons by femtosecond laser pulses in high-temperature superconductor YBa2Cu3O7−x

The time-resolved optical response from various crystallographic planes of a YBa2Cu3O7−x monocrystal is investigated. In such measurements of temporal behavior, the phonon system is driven by an ultrashort (subpicosecond) pulse into a coherent state and is probed by a second ultrashort pulse with a given temporal delay. A comparison of the Fourier transforms of the temporal responses with the spontaneous Raman scattering spectra shows that the contribution to a temporal response comes not only from fully symmetric phonons, but also from off-diagonal modes. The mechanism of generation of coherent phonons in high-temperature superconductors is discussed.

Cooperative Generation of Coherent Phonons by Localized Excitations in Glasses

Cooperative Generation of Coherent Phonons by Localized Excitations in Glasses

Coherent optical phonon generation by the electric current in quantum wells

This letter addresses the effect of generation of confined LO phonons by drifting electrons in quantum wells. We have derived a general formula for the phonon increment as a function of phonon wave vector, electron drift velocity, and structure parameters. Numerical estimates of the phonon increment and the phonon lifetimes have shown that AlAs/GaAs/AlAs and GaSb/InSb/GaSb quantum well structures can demonstrate the effect of coherent LO phonon generation by the electric current.

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.

A double-barrier heterostructure generator of terahertz phonons: many-body effects

In this paper we study the generation of coherent terahertz phonons in a double-barrier heterostructure (DBH) under the influence of an external applied bias. The system is characterized by an energy difference between the two lowest levels in the well, which resonates with the optical phonon energy, producing a high rate of emission of longitudinal optical (LO) phonons. The strong electron-phonon interaction in a polar semiconductor leads to the formation of a polaron that is relevant close to this resonance. Therefore the levels corresponding to the first excited state and the satellite of the ground state anticross in two polaronic branches. The LO phonon has a very short lifetime. It decays by stimulated emission of a pair of phonons (LO + TA) (where TA stands for transverse acoustic). As a consequence an intense beam of TA coherent phonons is produced, which could have several important applications. A rough model of this system has already been presented (Makler S S, Vasilevskiy M I, Weberszpil J, Anda E V, Tuyarot D E and Pastawski H M 1998 J. Phys.: Condens. Matter 10 5905). Several improvements to that model are presented here. Besides the more accurate treatment of the electron-phonon interaction, the phonon-phonon interaction is considered here taking into account the fact that the whole system is out of equilibrium. The results confirm that the proposed phonon laser is reliable.

Detection of ultrafast phenomena by use of a modified Sagnac interferometer

We describe a time-division interferometer based on the Sagnac geometry for monitoring ultrafast changes in the real and the imaginary components of the refractive index as well as phase changes that are due to surface displacement. Particular advantages of this interferometer are its simple common-path design and operation at normal incidence with a microscope objective for both pumping and probing. Operation is demonstrated by detection of temperature changes and coherent phonon generation in a gold film.

Observation of coherent folded acoustic phonons propagating in a GaAs/AlAs superlattice by two-color pump-probe spectroscopy

The coherent folded longitudinal acoustic (FLA) phonons in a GaAs/AlAs superlattice have been studied using a two-color pump-probe technique and a designed sample in which the superlattice layer is sandwiched by ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ cladding layers. The two-color pump-probe technique makes it possible to separate regions of generation and detection of coherent phonons, which provides a powerful tool to examine the spatial dynamics of coherent phonons. We have found two types of coherent FLA phonons: the first-order FLA phonons with finite wave vectors $(q\ensuremath{\ne}0)$ and the zone-center mode of the first-order FLA phonons $(q=0).$ When ballistic acoustic phonons are generated near the surface region of the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ cladding layer and propagate through the superlattice layer, the first-order FLA phonons with $q\ensuremath{\ne}0$ are observed in the superlattice layer. On the other hand, when the FLA phonons are generated within the superlattice layer, the zone-center FLA phonon extending from the superlattice layer is detected near the surface region of the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ cladding layer.

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.

Generating coherent THz phonons with light pulses

We present an overview of experimental work and discuss the theory of femtosecond-pulse generation of coherent optical phonons. Recent developments and differences between solids and molecular systems are emphasized. Theoretically, we focus on Raman mechanisms for the coherent driving force providing a comprehensive approach for the description of both below-gap and resonant excitation. The survey covers also polaritons and folded acoustic phonons in superlattices as well as a comparison with spontaneous Raman scattering.

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.

Superconducting tunnel junction as possible source of coherent photon and phonon generation

Abstract In this report pe analpse the possibilitp of utilising superconductors as active media for infrared lasers and as generators of high-Frequency coherent acoustic fields. The main problem in this direction is to obtain the inverse population in the charge-carrier spstem. Tunnel injection of electrons is one of the most promising methods to create nonepuilibrium states in superconductors. The deviation of electrons from epuilibrium changes the shape of single electron distribution function. Despite rather large observed shifts from epuilibrium, the inverse population of charge carriers in tunnel junctions is not pet registered experimentallp. After discoverp of oxide high-temperature superconductors (HTSC) nep possibilities in this field emerge. The analpsis shops that to obtain the inverse population is much more promising in HTSC, phich have anisotropic crpstalline structure and non-Debye phonon spectrum. pe present the results of theoretical investigation of non-epuilibrium distribution function both for “narrop” and for “pide” regimes of tunnel injection. pe shop that the inverse population can be obtained more easilp in the case phen the transverse Bose-fields are involved in the mechanism of Cooper pairing. In “dirtp” samples, containing sufficient amount of elastic scatterers, the inverse population map initiate the “phonon” instabilitp. Hence, the nonepuilibrium junction map serve as the generator of coherent phonon field. For crpstalline samples pith sufficientlp perfect structure the additional possibilitp emerges, because the absorption coefficient of high-Frequency electromagnetic radiation map become negative. This suggests a possibilitp to create nep family lasers: superconductor generators of far and middle range infrared radiation.

Ultrafast nonequilibrium stress generation in gold and silver.

The dynamics of coherent phonon generation by femtosecond optical pulses in thin gold and silver films is studied using a pump and probe scheme. Detection is achieved by monitoring ultrafast surface vibrations in real time using laser-beam deflection. The phonon strain pulse shapes can be explained through the nonequilibrium coupling of the electron and phonon distributions, suggesting a new method for measuring the electron-phonon coupling constant.

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.

Generation of coherent phonons and high-Tc superconductivity.

The generation of coherent phonons upon photon-induced pair breaking observed by Kutt, Albrecht, and Kurz in superconducting ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ may be taken as evidence of a role of static deformations in the binding of Cooper pairs, as recently proposed by the present authors. Quantitatively, the size of the lattice deformation brought on by superconductivity as observed by neutron scattering is consistent with the authors' hypothesis.

Subpicosecond electric field dynamics in low-temperature-grown GaAs observed by reflective electro-optic sampling

Abstract For the first time we investigate the subpicosecond electric field and coherent LO phonon dynamics in as-grown and post-growth annealed low-temperature (LT) GaAs by reflective electro-optic sampling (REOS) with femtosecond time-resolution. This technique allows a direct observation of the screening of built-in surface fields after pulsed optical excitation of free carriers and the built-up of an electric field due to the different mobilities of electrons and holes (photo Dember effect) in this high resistivity material. After trapping of mobile carriers the decay of the signal gives an accurate measure of the carrier lifetime in trapped states. The observed differences in annealed and as-grown LT GaAs are related to the microscopic form of excess arsenic and the density of point defects. The generation of coherent LO phonons allows the determination of the LO phonon frequency and lifetime in as-grown and annealed LT GaAs. Besides a red-shift of the LO phonon in as-grown LT GaAs we observe an additional local-mode below the LO frequency, which gives evidence for structural defects.