Expanding ultrafast photoacoustics for investigation of mechanical properties and thermal transport

Abstract

To address the need for broadband mechanical spectroscopy, femtosecond laser pulses were used to generate and detect acoustic waves. To expand the acoustic phonon frequency bandwidth and range, a thin metal film, a strongly magnetostrictive galfenol film, and strained piezoelectric InGaN/GaN multiple quantum wells were used as transducers. Acoustic wave detection methods included monitoring of optical transmittance/ reflectance, polarization, and diffraction over time. A magnetostrictive material galfenol (Fe₁₋[subscript x] Ga[subscript x]) with 80 percent iron and 20 percent gallium was used as an acoustic transducer using demagnetostriction effect. Galfenol showed great potential as an optimal transducer for the ultrafast magnetostriction in both longitudinal and shear modes. Strained piezoelectric InGaN/GaN semiconductor superlattices were used to generate and to study longitudinal THz acoustic phonons in GaN based structures. During the investigation of the lifetime of up to a 1.4 THz frequency acoustic phonons, specular reflection from an air/GaN free surface was observed. The photo-excitation of THz acoustic phonons in layered structures was introduced as an effective noninvasive tool to investigate the integrity of the fabrication process. This study opened many possibilities for studying mechanical properties and thermal phonons. Next, thermal conductivity reduction due to carrier-phonon interactions was presented. Phonon contributions are critical in heat transport in semiconductors and insulators. To isolate the carrier contributions to the scattering events, photo-excited carriers were generated in silicon through pulsed laser excitation. To measure thermal conductivity changes, time-domain thermoreflectance and transient thermal grating techniques were employed with a, carefully timed additional excitation pulse for carrier generation.