Frequency-domain study of nonthermal gigahertz phonons reveals Fano coupling to charge carriers.

Telecommunication devices exploit hypersonic gigahertz acoustic phonons to mediate signal processing with microwave radiation, and charge carriers to operate various microelectronic components. Potential interactions of hypersound with charge carriers can be revealed through frequency- and momentum-resolved studies of acoustic phonons in photoexcited semiconductors. Here, we present an all-optical method for excitation and frequency-, momentum-, and space-resolved detection of gigahertz acoustic waves in a spatially confined model semiconductor. Lamb waves are excited in a bare silicon membrane using femtosecond optical pulses and detected with frequency-domain micro-Brillouin light spectroscopy. The population of photoexcited gigahertz phonons displays a hundredfold enhancement as compared with thermal equilibrium. The phonon spectra reveal Stokes-anti-Stokes asymmetry due to propagation, and strongly asymmetric Fano resonances due to coupling between the electron-hole plasma and the photoexcited phonons. This work lays the foundation for studying hypersonic signals in nonequilibrium conditions and, more generally, phonon-dependent phenomena in photoexcited nanostructures.