Optomechanical Devices and Sensors Based on Plasmonic Metamaterial Absorbers
Surface plasmon resonance is the resonant oscillations of the free electrons at the interface between two media with different signs in real permittivities, e.g. a metal and a dielectric, stimulated by light. Plasmonics is a promising field of study, because electron oscillations inside a subwavelength space at optical frequencies simultaneously overcome the limit of diffraction in conventional photonics and carrier mobilities in semiconductor electronics. Due to the subwavelength confinement, plasmonic resonances can strongly enhance local fields and, hence, magnify light-matter interactions. Optical absorbers based on plasmonic metamaterials can absorb light resonantly at the operating wavelengths with up to 100% efficiency. We have explored plasmonic absorbers at infrared wavelengths for thermal detectors, e.g. a gold nanostrip antenna absorber that can absorb 10-times light using only 2% of material consumption comparing to a uniform gold film. In an optomechanical device, the optical mode and mechanical mode are mutually influenced, through the optical forces exerted on the mechanical oscillator and the detuning of optical resonance by the mechanical oscillator, so that the mechanical oscillations are either amplified or suppressed by light. We designed an optomechanical device integrated with plasmonic metamaterial absorber on a membrane mechanical oscillator, wherein a tunable Fano-resonant absorption in the absorber arises from the coupling between the plasmonic and Fabry-Perot reonsances. The absorber traps the incident light and heat up the membrane, causing an increase in thermal stress and a normal plasmomechanical force on it. This is a light-absorption-dependent elastic force arising from the opto-thermo-mechanical interactions. Due to the finite thermal response time in the membrane, the elastic plasmomechanical force is delayed and, consequently, generates a viscous component modifying the damping rate of the mechanical oscillator. We have observed optomechanical amplification and cooling in the device at designed detuning conditions. In particular, on the condition that the optomechanical gain beats the intrinsic mechanical damping, the oscillation becomes coherent, i.e. phonon lasing. We successfully demonstrated phonon lasing with a threshold power of 19 μW. This device is promising as an integration-ready coherent phonon source and may set the stage for applications in fundamental studies and ultrasonic imaging modalities.
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