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Abstract
We report on the direct excitation of 246 GHz longitudinal acoustic phonons in silicon doping superlattices by the resonant absorption of nanosecond-pulsed far-infrared laser radiation of the same frequency. A longitudinally polarized evanescent laser light field is coupled to the superlattice through a germanium prism providing total internal reflection at the superlattice interface. The ballistic phonon signal is detected by a superconducting aluminum bolometer. The sample is immersed in low-temperature liquid helium.
Highlights
The optical generation of coherent, monochromatic acoustic phonons has been a long-standing goal in solid-state physics
It is has been demonstrated that it is possible to generate coherent pulses of longitudinal acoustic (LA) phonons in a gallium arsenide/aluminium arsenide compositional superlattice by femtosecond laser excitation when the excitation energy matches the E1-HH1 transition in the superlattice [1]. This excitation mechanism used to generate the coherent longitudinal acoustic pulse undoubtedly generates a burst of incoherent phonons by carrier relaxation in the gallium arsenide
The shorter time-of-flight of the measured phonon pulse could result from the elastic constants at 246 GHz being substantially different, or from a stiffening of the silicon due to stress caused by the phonons
Summary
The optical generation of coherent, monochromatic acoustic phonons has been a long-standing goal in solid-state physics. The possibility of direct action of the electric field of a FIR light wave on the layers of bound space charge to resonantly generate single polarization sound, was discussed in the context of doped superlattices, as early as 1983 [2]. In this vein, Ruden et al [3] have studied the effects on the lattice dynamical properties of an otherwise homogeneous semiconductor, stemming from impurity charges associated with a microstructured one-dimensional periodic n and p-doping sequence, a so-called nipi doping superlattice (DSL).
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