Observation of Phonon Propagation in Germanium Nanowires Using Femtosecond Pump–Probe Microscopy

Abstract

The excited-state dynamics in individual Ge nanowires (NWs) are imaged using ultrafast pump–probe microscopy with high spatial (∼600 nm) and temporal (∼500 fs) resolution. Photoexcitation of the NW by a focused femtosecond laser pulse promotes electrons from the valence band to the conduction band within a 400 nm segment of the 20–30 μm long NW. The localized excitation is then probed by a delayed femtosecond pulse, whose position with respect to the pump pulse is precisely controlled. The pump–probe signals contain contributions from free carriers, thermal excitation, and impulsively excited acoustic phonons, the latter of which are detected in the time domain as a coherent oscillation in the pump–probe signal. Examination of an ensemble of NWs with diameters (d) ranging from 50 to 300 nm shows that the coherence frequency is inversely proportional to d, consistent with excitation of a radial breathing mode. In addition, experiments performed with spatially separated pump and probe beams show that the vibrational motion is not confined to the excitation region but, rather, spreads as much as 2.5 μm along the NW axis during the first 3 ns after excitation. Spatially separated pump–probe microscopy also reveals the creation of a longitudinally propagating wave that travels along the NW at ∼6500 m/s or ∼20% faster than the speed of sound in bulk Ge.