Low-Power Photothermal Self-Oscillation of Bimetallic Nanowires

We investigate the nonlinear mechanics of a bimetallic, optically absorbing SiN-Nb nanowire in the presence of incident laser light and a reflecting Si mirror. Situated in a standing wave of optical intensity and subject to photothermal forces, the nanowire undergoes self-induced oscillations at low incident light thresholds of <1 mu W due to engineered strong temperature-position (T-z) coupling. Along with inducing Self-oscillation, laser light causes large changes to the mechanical resonant frequency omega(0) and equilibrium position z(0) that cannot be neglected. We present experimental results, and a theoretical model for the motion under laser illumination. In the model; we solve-the governing nonlinear differential equations by perturbative means to show that self-oscillation amplitude is set by the competing effects of direct T-z coupling and 2 omega(0) parametric excitation due to T-omega(0) coo coupling. We then-study the linearized equations of motion to show, that the optimal thermal time constant tau for photothermal feedback is tau -> infinity rather than the previously reported omega(0) tau = 1. Lastly, we demonstrate photothermal quality factor (Q) enhancement of driven motion as a means to counteract air damping. Understanding photothermal effects on nano- and micromechanical devices, as well as nonlinear aspects of optics-based motion detection) can:enable new device applications as oscillators or other-electronic elements with smaller device footprints and less stringent ambient vacuum requirements.