Propagation of laser-generated heat pulses in crystals at low temperature: Spatial filtering of ballistic phonons

Abstract

At low temperatures, energy deposited on a crystal surface by a laser pulse or an Ohmic heater can flow rapidly away from the excitation point by ballistic phonon propagation. However, at higher excitation levels, evidence has been reported that a portion of the absorbed energy is trapped near the excitation region, creating a "hot spot." Thin-film bolometers on the opposite crystal face have been used to detect phonons emitted from the excitation region. Unfortunately, a subset of the phonons that strike the detector has been scattered in flight so the usual heat-pulse signals are not a reliable measure of the local dynamics of the excitation region. To remedy this, we have developed a technique for investigating both the time and spatial dependence of a laser-induced heat pulse. The method uses the phonon-focusing effect to discriminate between phonons that propagate directly from the excitation region to the detector and those that are scattered in the bulk of the sample. Prior studies of heat pulses following pulsed laser excitation of Ge and Si revealed, at high powers, a long-lived (∼10-μs) flux of phonons following the sharp ballistic pulse. Our spatial filtering technique shows that a major portion of this delayed flux in Ge is spatially very diffusive, but a short-lived component of the delayed flux emanates directly from the excitation region. We conclude that in Ge at 1.8 K, a 0.1-μs laser pulse produces a local storage of energy decaying with a time constant ∼0.1 to 1.5 μs, depending on the excitation level. This local energy storage also appears in metal-coated Ge, heavily-doped Ge, and metal-coated GaAs and LiF, indicating that the observed energy storage is not electronic. A simple scattering model that uses local melting as a temperature calibration explains the data in terms of a localized heat-storage effect.