A GaAs/AlAs superlattice as an electrically pumped THz acoustic phonon amplifier

Abstract

Abstract. In a semiconductor superlattice (SL), phonon-assisted electron transitions can occur under a quasi-population inversion, brought about by electrical biasing. This paper demonstrates the amplification of an optically excited quasi-monochromatic phonon beam by stimulated emission of phonons. Coherent phonons are generated by ultrafast optical excitation of a generator SL and passed once through a dc biased, GaAs/AlAs gain SL. A 20% increase in the phonon flux is detected when pumping is applied to the gain superlattice, which corresponds to an acoustic gain coefficient of 3600 cm −1 . A theoretical model of the phonon amplification is presented that also includes the effects of disorder in the SL. It is found that the amplification process is robust in the presence of disorder and good agreement is obtained with the main features of the experimental observations.

A weakly coupled semiconductor superlattice as a harmonic hypersonic-electrical transducer

“…The current-voltage characteristics measured at the temperature, T = 5 K which was used throughout these experiments, are shown in figure 1 (b) for 7 mesas (denoted M1-M7) of the same design. All mesas demonstrate current-voltage characteristics typical of GaAs/ AlAs SLs studied in previous works [30,41]. The dash-dot-dot vertical lines indicate the values of the threshold voltages, V t , above which the space charge and negative conductivity effects become apparent for mesa M2.…”

Dynamics of a vertical cavity quantum cascade phonon laser structure

“…The maximum value of γ 0 , at pump amplitude 200 mV, is about an order of magnitude smaller than the measured gain of a similar SL at 650 GHz5. This difference can be largely attributed to the phonon frequency dependence of the transition rates in a realistic, disordered, system5 and at finite temperature10, which are larger for higher frequency phonons.…”

“…The width of the peak suggests that this threshold condition is satisfied for pump amplitudes in the range about 80–250 mV. This breadth of gain spectrum is probably due to disorder in the gain SL5.…”

“…The maximum gain for LA phonons when the Stark splitting is approximately equal to the phonon energy may be calculated51011, and for 325 GHz phonons is about 10% per round trip. Therefore, even after allowing for the uncertainty in the gain and additional losses due to phonon scattering in the cavity, the vertical cavity structure we have made is theoretically capable of achieving and sustaining phonon oscillation when pumped above a threshold.…”

“…Examples include phonon avalanches in optically pumped ruby at 50 GHz (ref. 1); optomechanical devices at megahertz frequencies2; optically pumped trapped ions at very low frequencies3; and acoustic amplification at about half a THz, due to electron–phonon interactions in semiconductor superlattices (SLs)45. The latter is particularly important as it addresses the generation of coherent acoustic waves in the THz frequency range with corresponding acoustic wavelengths in the nanometre range, useful in a number of applications6.…”