Optically excited self-resonant microbeams

Abstract

Optically excited self-resonance of polysilicon microbeams sealed in a cofabricated vacuum enclosure has been achieved. Unmodulated low-power laser diodes from 650 to 840 nm have been used to excite resonances ranging from 65 to 750 kHz on microbeams ranging from 0.79 to 2.38 μm in thickness. The photovoltaic excitation mechanism uses a p-n junction photodiode underneath the microbeam. The structure forms an effective optomechanical modulator at the microbeam resonance frequency, and the resonance can be readily detected with the reflected laser light, which is modulated at levels that can approach 100%. Analysis of the conditions for self-resonance gives predictions of minimum Q-values for self-resonance. Observed Q-values (20 000 to 130 000) are well in excess of the required values. Thicknesses of the microbeam and vacuum gaps above and below it are critical for achieving low oscillation thresholds, which may be as low as 1 μW of optical power. The clamped—clamped microbeams are sensitive strain transducers with high gage factors, low temperature sensitivity, and wide dynamic range. These are the first optically powered active devices to achieve gain by interchanging optical, electrical, and mechanical energy in a merged structure. They uniquely combine silicon microfabrication technology with optoelectronic technology and can form the basis for a new class of fiber-optic sensors for pressure, temperature, acceleration, and other variables that can be converted to a strain using an appropriate silicon microstructure.