Frequency- and momentum-resolved view of non-thermal acoustic phonons in semiconducting nanomembranes*

Frequency- and momentum-resolved detection of thermal acoustic phonons can be carried out with spontaneous Brillouin Light Spectroscopy (BLS) but many interesting applications of nanophononics occur far from thermal equilibrium [1]. In this talk, I will show how BLS, combined with femtosecond laser excitation, can provide frequency-, momentum- and space-resolved view of non-thermal acoustic phonons in semiconducting nanomembranes [2]. Using this technique, we find that the population of photoexcited GHz phonons displays a hundred-fold enhancement compared to thermal equilibrium, while their spectra reveal Stokes-anti-Stokes asymmetry due to transport, and strongly asymmetric Fano resonances due to coupling between the electron-hole plasma and the phonons.
[1] M. Sledzinska, et al. Adv. Funct. Mater. vol. 30, no. 8, 2020.
[2] Th. Vasileiadis et al. Science Advances 2020 (accepted).