Tunable output directivity of thermally induced ultrasound generator based on nanocrystalline porous silicon

To confirm the usefulness of thermally induced ultrasonic emission from nanocrystalline porous silicon (nc-PS) as a probing source for object sensing in air, its characteristics have been investigated in terms of the input pulse width dependence of both the sound pressure amplitude and the ultrasound emission directivity. The device is composed of a silicon wafer substrate, an nc-PS layer, and a thin metal'tilm surface heater electrode. When the device is driven under a pulse input with various widths, the output sound pressure amplitude remains almost constant, whereas the emission directivity changes from the isotropic mode to the directional mode: the measured directivity angles at pulse widths of 50 and 10 mu s are 180 and 71.6', respectively. These results can be explained consistently with theoretical analyses. The observed controllability of the emission angle is attractive for the development of a three-dimensional ultrasonic image sensor, with an area-selective function by a simple pulse-width modulation.