Thermally induced ultrasonic: emission from porous silicon

The most common mechanism(1) for generating ultrasound in air is via a piez oelectric transducer, whereby an electrical signal is converted directly in to a mechanical vibration. But the acoustic pressure so generated is usuall y limited to less than 10 Pa, the frequency bandwidth of most piezoelectric ceramics is narrow and it is difficult to assemble such transducers into a fine-scale phase array with no crosstalk(2,3), An alternative strategy usi ng micromachined electrostatic diaphragms is showing some promise(4,5), but the high voltages required and the mechanical weakness of the diaphragms m ay prow problematic for applications. Here we show that simple heat conduct ion from porous silicon to air results in high-intensity ultrasound without the need for any mechanical vibrational system. Our non-optimized device g enerates an acoustic pressure of 0.1 Pa at a power consumption of 1 W cm(-2 ), and exhibits a flat frequency response up to at least 100 kHz. We expect that substantial improvements in efficiency should be possible, Moreover, as this material lends itself to integration with conventional electronic c ircuitry it should be relatively straightforward to develop finely structur ed phase arrays of these devices, which would give control over the wavefro nt of the acoustic emissions.