Observation of terahertz radiation coherently generated by acoustic waves

Over the past decade, pioneering and innovative experiments using subpicosecond lasers have demonstrated the generation and detection of acoustic and shock waves in materials with terahertz frequencies, the highest possible frequency acoustic waves(1-5). In addition to groundbreaking demonstrations of acoustic solitons, these experiments have led to new techniques for probing the structure of thin films(6-8). Terahertz-frequency electromagnetic radiation has been used in applications as diverse as molecular and material excitations(9,10), charge transfer(11,12), imaging(13) and plasma dynamics(14). However, at present, existing approaches to detect and measure the time dependence of terahertz-frequency strain waves in materials use direct optical probes-time-resolved interferometry or reflectrometry(2,15,16). Piezoelectric-based strain gauges have been used in acoustic shock and strain wave experiments for decades, but the time resolution of such devices is limited to similar to 100 ps and slower, the timescale of electronic recording technology. We have recently predicted that terahertz-frequency acoustic waves can be detected by observing terahertz radiation emitted when the acoustic wave propagates past an interface between materials of differing piezoelectric coefficients(17,18). Here, we report the first experimental observation of this fundamentally new phenomenon and demonstrate that it can be used to probe structural properties of thin films.