  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 $<1\, \rm{\mu W}$ 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 $T$-$\omega_0$ coupling. We then study the linearized
equations of motion to show that the optimal thermal time constant $\tau$ for
photothermal feedback is $\tau \to \infty$ rather than the widely 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 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.
