Generation of subterahertz coherent acoustic phonon pulses using a femtosecond pulsed light source in a Ni/GaN interface with strong electron-phonon coupling

We present the generation and detection of coherent acoustic phonons in a thin layer of Ni over GaN using a high-fluence femtosecond pump and broad-spectrum probe technique. The generated phonons, spanning the sub-THz frequency range, are driven by both thermoelastic stress and deformation-potential mechanisms. This intense pumping generates hot electrons and induces significant electronic stress, leading to the observed phenomena. Both diffusive and ballistic regimes of hot electrons and heat conduction are observed. Various characteristics of the generated phonon pulses, including fluence dependency, broadening, interfacial reflection, phase decoherence from anharmonic effects, and wavelength dependency, are investigated. The coherent nature of the generated acoustic phonon modes suffers from decoherence from incoherent phonons, temperature rise, and higher-order nonlinear effects. The electron-phonon coupling coefficient for Ni is extracted, matching well with the experimental observations. These findings have significant implications for studying metals on semiconductors and understanding interfacial properties in metal-semiconductor contacts, symmetry breaking, and noise decoupling in quantum optical systems, offering a pathway to prevent decoherence. A two-temperature model coupled with wave equations can explain electron-phonon energy exchange and reproduce experimental observations.