Interaction and energy transfer between acoustic phonons and free carriers have attracted wide attention because of their fundamental importance for transport properties and for the generation and amplification of sound waves at sub-terahertz (sub-THz) frequencies. Different interaction schemes of acoustic phonons with electric currents have been analyzed in the literature, mainly for bulk materials. The artificial periodicity of semiconductor superlattices (SLs) allows for tailoring acoustic phonon dispersion by back-folding into a narrow mini-Brillouin zone, resulting in sub-THz SL phonon states at the zone center (q = 0) . Coherent SL phonons have been generated and manipulated by ultrashort optical pulses . Moreover, a limited amplification of optically generated coherent SL phonons has been achieved by interaction with small electric tunneling currents through SLs . Here, we report a novel approach for acoustic phonon amplification at sub-THz frequencies by interaction with strong intra-miniband currents in a GaAs/AlGaAs superlattice. Phonon amplification is mapped in a time-resolved way in femtosecond pump-probe measurements of the transient reflectivity of the SL sample. We find strong phonon amplification by up to 200% at 410 GHz and identify the underlying coupling mechanism between electrons and phonons .
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