Acoustic Phonon Emission from a Weakly Coupled Superlattice under Vertical Electron Transport: Observation of Phonon Resonance

We report measurements of acoustic phonon emission from a weakly coupled $\mathrm{AlAs}/\mathrm{GaAs}$ superlattice (SL) under vertical electron transport. The phonons were detected using superconducting bolometers. A peak (resonance) was observed in emission parallel to the SL growth axis when the electrical energy drop per SL period matched the energy of the first SL mini-Brillouin zone-center phonon mode. This peak was mirrored by an increase of the differential conductance of the SL. These results are evidence for stimulated emission of terahertz phonons as previously predicted theoretically and suggest that such a SL may form the basis of a SASER (sound amplification by stimulated emission of radiation) device.

Figure 1

Illustration of phonon-assisted (via stimulated emission) hopping conduction between a pair of neighboring GaAs wells in a SL. In the bottom portion of the figure the 2D subbands corresponding to 2D electrons confined in the adjacent QWs are shown. The indirect phonon-assisted electron transitions characterized by the population inversion are indicated.

Figure 2

Experimental sample structure and $I(V)$ characteristics of the SL device.

Figure 3

(a) Folded phonon dispersion calculated for the SL used in the measurements; (b) acoustic reflection coefficient (as a function of phonon frequency) for the SL.

Figure 4

Phonon signal, normalized by the total power dissipated in the device, as a function of $\Delta$.

Figure 5

Differential conductance of the SL device as a function of $\Delta$.