Optical phonon production by upconversion: Heterojunction-transmitted versus native phonons

High-energy optical phonons are preferred in phonon-absorbing transitions, and regarding their production we analyze the phonon upconversion processes under nonequilibrium created by heterojunction transmission. For heterojunctions, steady phonon flux from a low-cutoff-frequency layer (e.g., Ge) is transmitted to a high cutoff layer (e.g., Si), creating a nonequilibrium population of low-energy phonons for upconversion. Using quantum spectral phonon transmission and first-principles calculations of the phonon interaction kinetics, we identify the high-conversion efficiency channels, i.e., modes and wave vectors. Junction-transmitted phonons, despite suffering from the interface reflection and from spreading interactions with equilibrium native phonons, have a high upconversion rate to Brillouin zone-boundary optical phonons, while nonequilibrium native phonons are efficiently upconverted over most of the zone. So, depending on the harvested optical phonon, one of these nonequilibrium phonons can be selected for an efficient upconversion rate.