Abstract

Time-of-flight detection of ballistic phonons in solids at low temperatures has previously employed phase insensitive detectors such as bolometers, tunnel junctions, or changes in optically excited fluorescence orabsorption. We present a phase sensitive technique for time-of-flight detection of phonons using frequency-modulation spectroscopy (FMS) of persistent spectral holes (PSH). A PSH is burned into the 607 nm color center zero phonon line of a NaF crystal at 1.5 K using a dye laser. Then, ballistic phonon pulses are generated by the absorption of Nd:YAG laser pulses in a Cr film on a NaF surface. Local measurement of the propagating stress-strain field is achieved by detecting the PSH splitting with a frequency-modulated cw dye laser beam. By examining the phonon time-of-flight signal dependence on the probe light polarization, position within the sample, and FM detection phase, an identification of the types of propagating phonons is made. Along with the ability to determine the sign of the acoustic disturbance, this experiment features a strain detection limit of 4 × 10⁻⁹ in 1500 averages at a time resolution of 50 ns.

© 1990 Optical Society of America