Low-Power Optical Actuation of n-GaAs Cantilevers via Surface Piezoelectric Coupling

The mechanical behavior of any semiconductor microstructure is inevitably coupled to light. In the case of micro mechanical resonators (MEMs), carrier generation can affect the quality factor of the resonance through electron-phonon scattering and ohmic losses, but it may also inject energy into the structure and induce movement. Thus, semiconductor MEMs may be regarded as intrinsically optomechanical systems. Here, we report on the optical actuation of a simple-clamped n-GaAs cantilever. This is achieved through modulated nanowatt LED illumination at the resonance frequency. We propose that the mechanism responsible for the coupling between light and movement is piezoelectrically induced stress in the surface depletion layer. Motion may be detected using two methods: by measuring the piezoelectric voltage generated due to deformation or the current arising due to the capacitance changes. In the latter case, a bias voltage must be applied to the device, which leads to nonlinear dynamics. Our results indicate that photothermal and electrostatic effects can be ruled out because i) we measure a very small drift of the resonance frequency with the light modulation offset which, together with finite-element simulations, indicates photothermal effects are not responsible for actuation, ii) we observe a phase shift under different bias voltage polarities, which rules out electrostatic actuation and iii), static measurements performed using an optical profilometer confirm the piezoelectric nature of the interaction with light. In summary, we find homogeneous low power LED illumination to be an effective, simple and convenient method of actuation for piezoelectric semiconductor based MEMs.