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 µW due to engineered strong temperature-position (T -z) coupling. Along with inducing self-oscillation, laser light causes large changes to the mechanical resonant frequency ω0 and equilibrium position z0 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 2ω0 parametric excitation due to T -ω0 coupling. We then study the linearized equations of motion to show that the optimal thermal time constant τ for photothermal feedback is τ → ∞ rather than the widely reported ω0τ = 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.Micro-and nano-mechanical resonators are widely studied for applications including electro-mechanical circuit elements and sensing of ultra-weak forces, 1 masses, 2 and displacements.3 An integral part of these systems is the detection method employed to readout motion, which must itself be extremely sensitive and inevitably imparts its own force on the resonator, influencing the dynamics. The phase relation between mechanical motion and the resulting detector back-action determines whether this interaction will serve to dampen vibrations or amplify them, potentially leading to self-oscillation if the detector supplies enough energy per cycle to overcome mechanical damping.Feedback due to external amplifiers has been used to generate self-oscillation of micro-mechanical resonators;4-8 in such systems the oscillation amplitude R is set either by nonlinearity of the amplifier or of the resonator. Systems in which mechanical motion influences the amount of laser light circulating in an optical cavity 9-12 or magnetic flux through a Superconducting QUantum Interference Device 13,14 (SQUID) have also been shown to self-oscillate under the right experimental conditions. In these systems R is set largely by the periodicity of the detection scheme -either R ≈ λ/4 where λ is the laser wavelength or R ≈ Φ 0 /2 where Φ 0 is the displacement needed to change the SQUID flux by one flux quantum. In the case of a mechanical resonator coupled to an optical cavity, back-action can arise either from radiation pressure or photothermal force -that is, thermally-induced deflectio...