Figure 1
Illustration: Alan Stonebraker

Figure 1: Schemes for phonon amplification and lasing. (Top) In the experiment of Grudinin et al. [2], two coupled microcavities are excited by an optical pulse traveling through an optical fiber (blue). The two cavities are placed very close together so that they are coupled by evanescent waves and this coupling removes the degeneracy of the whispering gallery modes. The resulting energy level splitting is tuned to be equal to the energy of a phonon of the sound wave that can resonate within the same structure mechanically. (Center) Pump photons entering through the optical fiber are converted to lower energy photons and coherent phonons. At a threshold pump power, phonon gain exceeds phonon loss resulting in phonon lasing. (Bottom) Phonon amplification in a superlattice. Tunneling of electrons from one quantum well to the next is accompanied by phonon emission. When a strong phonon wave is applied, Beardsley et al. [3] observe phonon amplification. (Top and center panels adapted from [2], bottom panel adapted from [3].)