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.”

Figure 1

Top: the calculated double quantum-well potential and wave functions; bottom: an illustration of the Raman lasing characteristics. When the pump frequency is varied within the linewidth of the 1-to-3 transition, the emission frequency changes by the same amount so that the difference is kept the same.

Figure 2

Emission peak position versus pump position at 80 K. Each type of symbol represents one sample. The inset shows the transmission spectrum for one sample (indicated by the arrow) at room temperature under a multipass $45°$ geometry.

Figure 3

Difference between pump and emission photon energies (i.e., the Stokes Raman shift) versus level 1 and level 2 separation. The curves (solid for GaAs phonon and dashed for AlAs-like mode of AlGaAs) are calculated from the coupled-mode model. The two horizontal lines indicate the positions of the LO phonons, while the two vertical dotted lines show the expected anticrossing positions, 32.1 and 44.4 meV shifted from the phonon energies (36.3 and 47.5 meV) by the depolarization effect. All experimental data points have an error bar of about 1 meV in the $x$ axis, whereas the uncertainty in the $y$ axis is negligible (less than 0.1 meV).