The influence of thin helium films on the phonon spectrum of optically excited silicon

Abstract

We have studied the phonon spectra resulting from laser pulse excitation of a silicon surface We used a phonon spectrometer based on pressure tuned boron levels in silicon (Si: B-spectrometer). A new design allowed us to use the spectrometer in vacuum for the first time. We observed phonon frequencies up to 1 THz. The pulse shapes indicated both, quasidiffusion and a long exciton lifetime. On adding helium films to the surface we have found dramatic changes of the spectrum. Full thermalisation was reached already at a thickness of 1.5 monolayers. This indicates a strong anharmonic interaction in the very first monolayers at these high frequencies and, in addition, a strong confinement of the phonons to the surface by quasidiffusion.

Single Phonon Detection for Dark Matter via Quantum Evaporation and Sensing of $^3$Helium

“…However, those experiments used a polished, rather than cleaved, crystal. It is known that even well-polished surfaces covered with helium lead to enhanced phonon thermalization [19] and inefficient transport of phonons across the interface into a film [20]. The efficiency of quantum evaporation from a van der Waals film of liquid helium on a well-cleaved crystal is not yet known.…”