Femtosecond laser generation and detection of high-frequency acoustic phonons in GaAs semiconductors

Abstract

The experimental detection of short picosecond photoacoustic response induced by femtosecond laser pump radiation deeply penetrating in GaAs and generating long acoustic strain pulse is reported. It is demonstrated that it is possible, in this case, to achieve high-frequency coherent acoustic phonon monitoring in semiconductors as efficiently as in the case of metals where the penetration depth of pump radiation is shorter and the generated acoustic strain pulse is short itself. The physical origin of such monitoring of high-frequency acoustic phonons is discussed thanks to a detailed analysis of the spectral transformation functions of both the generation process ͑opto-acoustic transformation͒ and the detection process ͑acousto-optic transformation͒. We show that it is possible to tune the detection process from a narrowband to a broadband spectrum detection process. In particular, the broadband detection process is achieved with a proper choice of the probe wavelength permitting efficient coupling between the acoustic field and the probe electromagnetic wave only in a nanometric thin region near semiconductor surface. Efficient broadband acousto-optic detection results in the short pulsed photoacoustic responses in transient optical reflectivity even when long acoustic pulses are generated, because of sufficient sensitivity to high-frequency coherent phonons even weakly contributing to the total acoustic field. These results indicate the opportunity to broaden the range of the substrate materials and pump laser frequencies that can be used in high-frequency coherent phonon spectroscopy of the materials and nanostructures.

Microscale acoustofluidics: Microfluidics driven via acoustics and ultrasonics

“…They observed the generation of THz electromagnetic radiation as the solitons passed from GaN to AlN and back to GaN, coining the term nanoseismology for the propagation of such waves in thin films. Babilotte et al (2010) described the formation of acoustic waves in GaAs with a thin, optically transparent layer of either ZnO or SiO 2 propagating from a point where a femtosecond laser irradiates the surface of the GaAs under the thin film, along with a brief review of the field. They viewed the Brillouin mode at 44 GHz and other phonons from 100 to 300 GHz and exercised extraordinary care in the measurement technique which proved to be critical to their findings in the propagation of the waves in the transparent thin layer.…”

Photoexcitation of gigahertz longitudinal and shear acoustic waves in BiFeO3 multiferroic single crystal

“…As a matter of fact and in addition to magnetic and/or electric field, light appears as a powerful tool to probe and tune BFO multiferroic properties as well as an original way for the discovery of unrevealed yet phenomena. More specificaly, ultra-fast optical pump-probe spectroscopy can bring insights into the fundamental microscopic dynamics, important in many applications, that are at the origin of functional properties of various materials such as metals, 14 semiconductors, 15 collossal magnetoresistive manganites, 16 superconductors, 17 or multiferroics. 12,13,18 Such technique was used to investigate the model multiferroic BFO.…”

“…The setup used in our study follows the classical scheme of picosecond acoustics. 14,15,21,22 In the particular case of our study, the BFO sample is excited by a UV pump beam with 3.1 eV photon energy (see inset of Fig. 1).…”

Study of confined coherent acoustic phonon modes in a free-standing cubic GaN membrane by femtosecond spectroscopy

“…Our experimental results confirmed earlier existing theoretical predictions and experimental observations that the hydrostatic deformation potential of cubic GaN is positive. Time-resolved pump-probe spectroscopy has been applied to study ultrafast carrier dynamics and phononic properties of various semiconductors, such as GaN, [1][2][3][4] GaAs, [5][6][7] and Si. [8][9][10][11] Among these, GaN is of great importance in optoelectronic and short-wavelength devices because of its widebandgap.…”