Angle dependence of acoustic phonon-assisted tunneling in a weakly coupled superlattice: Evidence for terahertz phonon amplification

We investigate nonequilibrium acoustic phonon-assisted tunneling in a weakly coupled GaAs/AlAs superlattice (SL). Previously it had been predicted that, in such structures, the conditions for terahertz acoustic phonon amplification can be achieved [B. A. Glavin, V. A. Kochelap, and T. L. Linnik, Appl. Phys. Lett. 74, 3525 (1999)]. We present experimental measurements of the change in current Delta I due to an incident nonequilibrium phonon pulse generated by thermalizing a laser pulse in a metal film. In particular, we measure the dependence on the angle of incidence theta of the phonons relative to the SL growth direction. The results are compared with numerical calculations made using the two-well model. It is found that good agreement between the measurements and calculations can be obtained for all theta if we assume that amplification is occurring for phonons propagating in a direction near theta = 0. Due to the amplification of phonons with energy h omega q less than or similar to Delta, where Delta is the energy drop per period of the SL, the nonequilibrium phonon distribution in the vicinity of the SL is different from the expected Planck spectrum of the phonon source. We show that this can explain the experimental measurements of Delta I at theta = 0. We present these results as evidence for phonon amplification in the 400-GHz-1-THz frequency range. (c) 2005 American Institute of Physics.