We report the first experimental evidence for the direct excitation of coherent high-frequency acoustic phonons in semiconducting doping superstructures by electromagnetic fields of the same frequency. Nanosecond pulses of acoustic phonons have been detected by a superconducting bolometer at the appropriate time-of-flight across a (100) silicon substrate for ballistic longitudinal phonons when a silicon doping superlattice is illuminated with grating-coupled nanosecond-pulsed 246-GHz laser radiation with power density of ∼1 kW/mm2. The absorbed phonon power density in the microbolometer is estimated to be ∼ 10/Wmm2, in agreement with theory. The phonon pulse duration matches the laser pulse duration. The absence of any detected transverse acoustic phonon signal by the superconducting bolometer is particularly striking. We believe these observations provide the first evidence that coherent zone-folded longitudinal acoustic phonons can be generated in a silicon doping superlattice and with negligible associated heat pulse generation, through the resonant absorption of grating-coupled pulsed far-infrared laser radiation.
Wilson TE. (2011) Evidence for the generation of coherent longitudinal acoustic phonons through the resonant absorption of pulsed far-infrared laser radiation in silicon doping superlattices. Chin. J. Phys. 49(1):118-126.