Coherent Terahertz Sound Amplification and Spectral Line Narrowing in a Stark Ladder Superlattice

The bias voltage applied to a weakly coupled $n$-doped GaAs/AlAs superlattice increases the amplitude of the coherent hypersound oscillations generated by a femtosecond optical pulse. This bias-induced amplitude increase and experimentally observed spectral narrowing of the superlattice phonon mode with a frequency 441 GHz provides the evidence for hypersound amplification by stimulated emission of phonons in a system where the inversion of the electron populations for phonon-assisted transitions exists.

Figure 1

(a) Schematic representation of phonon-assisted tunneling between neighboring QWs in the SL by stimulated emission; (b) folded LA phonon dispersion in a GaAs/AlAs SL with a period $d_{\mathrm{SL}}=10\mathrm{nm}$, the Brillouin backscattering, $B$, and folded phonon, $FP$, modes are indicated; (c) the structure of the experimental device; (d) photograph of the optical MESA structure and (e) device current-voltage characteristics (negative bias to top contact).

Figure 2

Time evolution of the reflectivity changes $\Delta R(t)$ at zero bias. The inset shows a fragment of the time evolution for $\Delta R(t)$ in which the oscillations due to the FP modes are clearly seen.

Figure 3

Fourier spectra of the FP mode in a 50 ps window centered on $t_0=1\mathrm{ns}$ for zero bias and a constant bias of 145 mV. The inset shows the bias-induced signal change at the peak (compared to the peak at zero bias) as a function of the applied bias voltage.

Figure 4

The amplitude of the FP oscillations at zero bias (squares) and the amplitude of the bias-induced oscillations (circles). The solid lines are, respectively, the fits of Eqs. (1, 2) to the data.

Figure 5

The spectra of the FP modes at zero bias and the bias-induced signal.