* Corresponding author. E-mail: ppxsac@nottingham.ac.uk. Physica B 263—264 (1999) 537—539 Stimulated phonon emission in superlattices S.A. Cavill*, A.V. Akimov, F.F. Ouali, L.J. Challis, A.J. Kent, M. Henini Department of Physics, Nottingham University, University Park, NG7 2RD, UK Ioffe Physical-Technical Institute, 26 Polytechnicheskaya str, 194021 St. Petersburg, Russia Abstract We present the first study of the effect of ballistic acoustic phonons generated by a heated metal film on the tunnel current in GaAs/AlAs superlattices. The phonon-induced increase of the tunnel current as a function of applied voltage has a maximum at a voltage which varies linearly with heater temperature both in zero and applied magnetic field. The behaviour is consistent with phonon-assisted tunnelling by stimulated phonon emission. The system acts as a phonon spectrometer in the 1THz frequency region with a resolution of approximately 200 GHz.  1999 Elsevier Science B.V. All rights reserved. Keywords: Superlattice; Stimulated phonon emission; Electron—phonon interaction The transport properties of double and multi- barrier tunnelling devices have been intensively stud- ied and are now qualitatively understood. However, the role of phonon scattering, and in particular that by acoustic phonons, has been investigated to a much lesser extent. The first experimental invest- igation of the effect of acoustic phonons on a double barrier resonant tunnelling device (DBRTD) was made by measuring the change in tunnel current induced by a pulse of non-equilib- rium acoustic phonons [1,5]. The measurements focused on tunnelling through donor levels in the quantum well. The signals were attributed to assist- ed tunnelling as a result of phonon absorption and stimulated phonon emission. This investigation has recently been extended to tunnelling through the ground state of the quantum well in magnetic fields [2] and the results were attributed to phonon assisted tunnelling between two Landau levels. These earlier experiments showed that DBRTDs have potential as phonon spectrometers. However, their resolution is rather modest because of level broadening (several meV). In this present work, we report the first measurements of assisted tunnelling in a superlattice (SL) produced by non-equilibrium phonons. The principle of using a SL as a phonon spec- trometer is illustrated in Fig. 1. Acoustic phonons induce assisted tunnelling transitions by absorption and stimulated emission of phonons which lead to changes of the tunnelling current through the SL. By changing the bias, », across the SL it is possible, in principle, to change the energy of phonons which are active in tunnelling processes so the SL be- comes a phonon spectrometer which can be tuned by changing the applied bias. 0921-4526/99/$ — see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 2 5 5 - 1