In the past decade, sound amplification by the stimulated emission of (acoustic phonon) radiation (saser) devices for generating coherent terahertz (THz) acoustic waves have been demonstrated [1 – 3]. The devices exploit the electron-phonon interactions in periodic semiconductor nanostructures known as superlattices (SLs) to amplify acoustic phonons. In addition, the particular acoustic properties of SLs can be exploited to make mirrors and cavities for THz phonons. Thus SLs can provide the two essential elements of a saser: the acoustic gain medium and the acoustic cavity.
In this presentation I will describe experimental studies of the THz phonon dynamics in a weakly-coupled GaAs/AlAs saser SL, which is DC electrically biased into the Wannier-Stark regime. Picoseconds-duration pulses of coherent THz acoustic phonons were generated using pump light pulses from a femtosecond laser and injected into the SL device. These phonon pulses seeded the saser cavity modes at about 220 and 440 GHz, which were amplified within the device. The phonons were detected using two methods: reflection of femtosecond probe light pulses, in a conventional pump-probe arrangement, and through the transient electrical response of the device itself.
When the DC bias conditions for saser were achieved in the device, the amplitude and lifetime of the seeded modes were both increased, analogous to the threshold and spectral line narrowing effects seen in laser devices.
 R P Beardsley et al., Phys. Rev. Lett. 104, 085501 (2010).
 W Maryam et al., Nature Communications 4:2184 (2013).
 K Shinokita et al., Phys. Rev. Lett. 116, 075504 (2016).
Anthony J. Kent, Caroline L. Poyser, Andrey V. Akimov, William York, Mohamed Henini, and Richard P. Campion, "Decay of coherent acoustic phonons generated by femtosecond pulsed optical excitation and injected in a Wannier-Stark superlattice (Conference Presentation)," Proc. SPIE 10102, Ultrafast Phenomena and Nanophotonics XXI, 101021H (Presented at SPIE OPTO: February 02, 2017; Published: 19 April 2017); https://doi.org/10.1117/12.2249035.5391659185001.
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