Department of Physics, ‡Department of Mathematics, §Sibley School of Mechanical and Aerospace Engineering, and ∥School of Applied and Engineering Physics, Cornell University , Ithaca, New York, 14853, United States.
Nano Lett. 2017 Jul 12;17(7):3995-4002. doi: 10.1021/acs.nanolett.6b04769. Epub 2017 Jun 12.
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 ω and equilibrium position z 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 2ω parametric excitation due to T-ω 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 previously reported ω τ = 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.
我们研究了在入射激光和反射 Si 镜存在下，具有双金属、光吸收特性的氮化硅-铌纳米线的非线性力学行为。由于工程上强的温度-位置（T-z）耦合，纳米线处于光强的驻波中，并受到光热力的作用，在低入射光阈值<1 μW 下就会发生自激振荡。激光不仅会导致机械共振频率ω和平衡位置 z 发生大的变化，而且这些变化不能被忽略。我们提出了实验结果和激光照射下运动的理论模型。在模型中，我们通过微扰方法求解控制非线性微分方程，表明自激振荡幅度由直接 T-z 耦合和 T-ω 耦合引起的 2ω 参数激励的竞争效应决定。然后，我们研究了运动的线性化方程，以表明光热反馈的最佳热时间常数 τ 是 τ→∞，而不是以前报道的 ωτ=1。最后，我们展示了驱动运动的光热品质因数（Q）增强，这是一种抵消空气阻尼的方法。了解光热效应对纳米和微机械器件的影响，以及基于光学的运动检测的非线性方面，可以为振荡器或其他具有更小器件足迹和更低环境真空要求的电子元件等新器件应用提供支持。
