Coupled Electron-Phonon Modes in Optically Pumped Resonant Intersubband Lasers

Intersubband lasing at $12–16\mu \mathrm{m}$ based on a ${\mathrm{C}\mathrm{O}}_2$ laser pumped stimulated resonant Raman process in $\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ three-level double-quantum-well structures is reported. The presence, or lack of, lasing action provides evidence for resonantly coupled modes of collective electronic intersubband transitions and longitudinal optical phonons. An anticrossing behavior of these modes is clearly seen when the difference between the pump and lasing energies (i.e., Stokes Raman shift) is compared with the subband separation. This work reveals the significance of the strong coupling between intersubband transitions and phonons and raises a new possibility of realizing a phonon “laser.”