Photoexcited hypersonic surface acoustic waves propagating under periodic metal grating

摘要

Photoinduced strain gradients in nanometer length scales using periodically patterned grating on a substrate is an attractive way of producing coherent surface acoustic waves (SAW). Ultrafast laser pulses incident on this structure produce constrasting thermal strain in the grating and the substrate thus launching surface acoustic waves. The frequency of the SAW is inversely dependant on the period of the grating and thus the generation of high frequency SAW (~1-100 GHz) requires sub-micron scale gratings. Hypersonic SAW are scientifically significant for photoacoustic spectroscopy and metrology of nanostructures in addition to investigations of phonon mediated heat transport. In this work, we study the propagation of hypersonic surface acoustic waves (~10-20 GHz) in silicon with aluminum gratings fabricated on them for varying periods and duty cycles of the grating (coverage ratio of Al on Si). Modeling the optical absorption of the laser and the resultant thermoelastic strain reveals requirements on the elastic and the thermal properties of the grating and the substrate for efficient SAW generation. Using the time resolved reflectivity measurements, we show that the SAW frequency shift with the duty cycle departs from the widely used perturbation theory by square of sinusoid in duty cycle with highest deviation around 0.5. A similar finding for attenuation of SAW suggests that mass loading on SAW by Al grating places limitation on the duty cycle in design of hypersonic phononic crystals. Further, we conduct finite element based eigenmode analysis on the Si-Al periodic composite which show a good agreement with the experimental data. Modal analysis further reveals that higher attenuation in the duty cycle regime 0.3 to 0.6 is due to radiation of the energy from the surface modes into the bulk due to mass loading from the Al grating.