Low-power photothermal self-oscillation of bimetallic nanowires

TL;DR

This paper explores the nonlinear photothermal self-oscillation behavior of bimetallic nanowires, revealing low-threshold oscillations, frequency shifts, and enhanced quality factors, with implications for miniaturized optomechanical devices.

Contribution

It provides the first combined experimental and theoretical analysis of photothermal self-oscillation in bimetallic nanowires, highlighting the role of thermal coupling and feedback dynamics.

Findings

Self-oscillation occurs at incident laser power below 1 μW.

Large shifts in resonant frequency and equilibrium position are observed.

Optimal thermal time constant for feedback is infinite, not unity.

Abstract

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.