Tight confinement of photons and phonons in nano-scale optical waveguides or cavities has revealed a rich landscape of optoacoustic phenomena. We have continued to study the "dual-nanoweb" fibre, first proposed at MPL [Butsch (2012)]. It consists of two closely-spaced ultra-thin nanowebs, suspended in a capillary, that act as optically coupled planar waveguides, supporting eigenmodes with symmetric and anti-symmetric transverse field distributions. Optical gradient forces deflect the mechanically compliant nanowebs and cause an increase in modal refractive index with launched optical power [Butsch (2012a)]. This optomechanical nonlinearity is strongly enhanced when the fundamental flexural resonance of the nanowebs is driven. Simulations show that the frequencies of upper and lower nanoweb are slightly different due to structural asymmetry.
An evacuated dual-nanoweb fibre was observed to self-oscillate when the launched power of single-frequency light at 1.55 µm exceeded several mW. Simultaneously a frequency comb spaced by 5.611 MHz, the self-oscillation frequency, appeared in the transmitted optical spectrum. The underlying mechanism turns out to be stimulated Raman-like scattering (SRLS) by the flexural resonance in one nanoweb, initiated by thermal phonons and providing unprecedentedly high gain (of the order ~105 W‑1m‑1). The nanoweb vibration mediating SRLS is almost entirely transverse, so that, close to flexural cut-off, the phonon wavevector can be freely chosen while keeping the frequency fixed. We studied this effect using a novel interferometric scanning technique to probe nanoweb vibrations at high spatial resolution from the side of the fibre, perpendicularly to its axis. After these highly detailed experimental measurements, an extensive theory showed that self-oscillation is the result of stimulated intermodal scattering (SIMS) unbalancing SRLS gain suppression [Koehler (2016)]. This novel mechanism allows the design of single-pass optomechanical resonators requiring only a few mW pump power, without electronics or any optical resonator. The design could also be implemented in silicon or any other suitable material.
We have explored the frequency response of the optomechanical nonlinearity for different gas pressures, finding that the gas, trapped in the gap between the oscillating nanowebs, contributes "squeezed-film" damping involving both viscous damping and a significant increase in mechanical stiffness [Koehler (2013)]. As a consequence of the unusually narrow slot between the nanowebs (22 µm by 550 nm), the gas-spring effect causes a pressure-dependent frequency shift that is some 15 times larger than typically measured in micro-electro-mechanical devices. In vacuum the quality factor of the mechanical nanoweb resonance (at ~6 MHz) reaches values of several thousand. When the structure was driven using an intensity-modulated laser beam, the optomechanical nonlinearity at resonance exceeded the Kerr-related nonlinearity by a factor of ~60,000. Moreover, we obtained very good quantitative agreement between experimental data and a free-molecular “energy transfer model”.