Ultrafast Modulation of Optically Driven GHz–THz Coherent Acoustic Phonons in an AlGaN/GaN HEMT Heterostructure under High Excitation Density

We demonstrate the modulation of coherent acoustic phonons (CAPs) and their response to high excitation densities in AlGaN/GaN heterostructures under ultrafast near-band gap optical excitation. Displacively excited CAPs induce a strain-modulated piezoelectric field, modulating the transmission of the probe beam through the bulk Franz-Keldysh effect, and manifest as a phase flip when photon energy approaches the intrinsic bandgap. A large initial amplitude of CAP is observed in GaN, followed by a rapid decay arising from screening by the piezoelectric field. In the AlGaN/GaN heterostructure, the top strained layers enhance CAP-piezoelectric coupling. Additionally, the higher thermal conductivity of the top layers and the intrinsic nature of the GaN layer in the heterostructure play a significant role in maintaining phonon coherence due to reduced phonon-phonon scattering and phonon-impurity scattering, respectively. The excited acoustic phonons are long-lived, with high amplitude, exhibiting cosinusoidal displacive behavior that persists due to the interplay between strain waves, piezoelectric field modulation, deformation potential coupling, and thermoelastic expansion. Observed phonon frequencies between 40 and 80 GHz are governed by the acoustic transit time and pump wavelength. We also observe Brillouin frequency components in the range 100-120 GHz, with a significantly long decay time. Furthermore, inhomogeneous plasma distribution modifies the phonon frequency and induces coupled phonon oscillations, which are observed as a beating pattern with frequency content in the sub-THz and THz regimes. These experimental conditions are analogous to high-field transport under electrical injection, where the carrier excitation mechanism differs but the energy release to the phonon subsystem resembles that under optical excitation. This work presents a pathway for the development of on-demand coherent phonon sources without a superlattice structure, thereby opening up the possibility of understanding plasmon-phonon interactions in AlGaN/GaN heterostructures for solid-state applications.