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.

Probing Carrier Transport in Layered Perovskites with Nonlinear Optical and Photocurrent Spectroscopies

“…Knowledge of carrier diffusion in layered perovskites is motivated by implications for electron and hole mobilities when subjected to an electric field . Transient absorption microscopy has been demonstrated as a powerful new approach for studying carrier diffusion in nanoparticles and films. − In these experiments, the pump beam is usually focused with a microscope objective and a probe beam is raster scanned to image carrier dynamics. Such point-by-point data acquisition is readily accomplished using laser systems with MHz repetition rates.…”

Leaving the Limits of Linearity for Light Microscopy

“…Investigations of the photodynamics of materials from low-dimensional nanostructures ,− to thin films − to heterojunctions have taken full advantage of the diversity in timescales accessible with TAM. A significant advancement for the technique came with the introduction of spatially offset/separated pump and probe pulses to facilitate the measurement of carrier diffusion in these systems. − Figure shows images of free carriers diffusing along a germanium nanowire . Here the pump beam is fixed to the center, and images are collected by raster scanning the probe over the length of the wire at different time delays .…”

“…A significant advancement for the technique came with the introduction of spatially offset/separated pump and probe pulses to facilitate the measurement of carrier diffusion in these systems. − Figure shows images of free carriers diffusing along a germanium nanowire . Here the pump beam is fixed to the center, and images are collected by raster scanning the probe over the length of the wire at different time delays . The ability to directly visualize the spatial propagation of energy, charge, and phonons provides crucial feedback for the development of improved materials.…”

“…The pump–probe delay is indicated in the lower-left corner of each image. Each image is then obtained by raster scanning the probe beam across the field of view and collecting its pump-induced changes in intensity (i.e., Δ I ) . Reprinted in part from ref with permission.…”

“…32−36 Figure 2 shows images of free carriers diffusing along a germanium nanowire. 37 Here the pump beam is fixed to the center, and images are collected by raster scanning the probe over the length of the wire at different time delays. 37 The ability to directly visualize the spatial propagation of energy, charge, and phonons provides crucial feedback for the development of improved materials.…”