Low-Power Photothermal Self-Oscillation of Bimetallic Nanowires

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 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...

Dueling dynamical backaction in a cryogenic optomechanical cavity

“…While the ability to cool below single phonon occupancy in the non-sideband-resolved regime is promising, reaching this limit in practice presents a significant challenge, largely due to residual heating from the photon absorption processes associated with photothermal damping [27]. However, as this device was not purposefully designed for photothermal coupling, it may be possible to engineer this effect to achieve the photothermal parameters detailed at the end of the previous section, perhaps by adding a metallic layer to the resonator to enhance its differential thermal contractions and optical absorption [42]. Furthermore, one could also imagine modifying the thermal time constant by changing the dimensions of the resonator, which would also affect the strength of the photothermal damping.…”

“…Though efforts have largely focussed on this radiationpressure-driven interaction, optomechanical coupling can be mediated by other means, such as the photothermal (or bolometric) force, whereby photon absorption in the mechanical element introduces a temperature gradient across the device, causing it to deflect due to differential thermal contractions [25][26][27][28][29]. Photothermal effects have historically been studied in optical cavities comprised of gold-plated cantilevers [30][31][32][33][34], but have also been observed in buckled microcavities [35,36], multilayered Bragg mirror beams [37], thin metallic mirrors [38], membranes [39,40], nanowires [35,41,42] and superfluid helium [43][44][45]. As in the case of radiationpressure-driven optomechanics, photothermal forces can also be used to manipulate the motion of mechanical resonators.…”

Current-induced restructuring in bent silver nanowires

“…As far as we could observe, it did not result in plastic deformation nor did it change the contact regions of the interconnect. It is known that resonance in nanostructures can be induced by electrostatic forces, 70,71 electromagnetic forces, 72 opto-thermal effects 73 or electron beam exposure. 74,75 During our experiments, the energy of the electron beam (300 keV) and the magnetic field created by the electromagnetic lenses in the TEM remained unaltered.…”