title: Strong Amplification of Coherent Acoustic Phonons by Intraminiband Currents in a Semiconductor Superlattice.
abstract: Sound amplification in an electrically biased superlattice (SL) is studied in optical experiments with 100 fs time resolution. Coherent SL phonons with frequencies of 40, 375, and 410 GHz give rise to oscillatory reflectivity changes. With currents from 0.5 to 1.3 A, the Fourier amplitude of the 410 GHz phonon increases by more than a factor of 2 over a 200 ps period. This amplification is due to stimulated Čerenkov phonon emission by electrons undergoing intraminiband transport. The gain coefficient of 8×10^{3}  cm^{-1} is reproduced by theoretical calculations and holds potential for novel sub-THz phonon emitters.
authors.affiliation: Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Shinokita
authors.affiliation: Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany.
authors.forename: Klaus
authors.initials: K
authors.lastname: Reimann
authors.affiliation: Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Woerner
authors.affiliation: Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Elsaesser
authors.affiliation: Paul-Drude-Institut für Festkörperelektronik, 10117 Berlin, Germany.
authors.forename: Rudolf
authors.initials: R
authors.lastname: Hey
authors.affiliation: Laboratoire Pierre Aigrain, École Normale Supérieure, 75231 Paris, France.
authors.forename: Christos
authors.initials: C
authors.lastname: Flytzanis