Propagation of high amplitude strain pulses in sapphire

The propagation of acoustic pulses in sapphire has been investigated using bolometric time-of-flight techniques. Excitation of a thin chromium film by an amplified Ti:sapphire laser pulse resulted in the generation of high-amplitude acoustic pulses. The propagation time of these across the 1-cm-thick sapphire crystal was measured using a superconducting aluminum bolometer. By increasing the excitation power, the growth of an intense fast pulse on the leading edge of the longitudinal heat pulse was observed. This was detected as an additional, separate, signal component, with a propagation time shorter than the heat pulse. This pulse is attributed to the formation and propagation of a high-amplitude coherent strain pulse. The bolometric detection technique allows high resolution images of the acoustic flux over macroscopic propagation distances, and measurements show the strain pulse propagates as a strongly collimated beam. These results demonstrate the applicability of bolometric detection techniques, in addition to Brillouin scattering and pump-probe measurements, to study the propagation of ultrashort strain pulses in crystals.

Figure 1

A typical bolometer trace, obtained for a pulse energy density of about $0.09\mathrm{mJ}{\mathrm{cm}}^{-2}$. The boxed area is shown in more detail in Fig. 2.

Figure 2

The LA components of the bolometer signals for a range of excitation energy densities. The signals have been normalized to the LA peak signal intensity and offset for clarity. Also marked on the figure is the range of arrival times which would correspond to published values of both measured and calculated sound velocities for the longitudinal mode in sapphire (the center line is the average of these).

Figure 3

An image of the supersonic pulse intensity over a $4.75\times 4.75{\mathrm{mm}}^2$ area. The outline of the bolometer is indicated by a dashed line. The lower panel shows the bolometer signals obtained for different positions of the laser spot.