Acoustically Modulated Gap Plasmons

Among all techniques used to modulate light, mechanical modulation techniques hold high promise due to their ability to alter the optical permittivity to a greater extent than any other modulation technique. However, typical mechanical techniques employing acoustic waves and the photo-elastic effect, such as Acousto-Optic Modulators (AOMs), struggle to efficiently modulate light due to the weak refractive index modulation that acoustic waves impose on optical materials. Micro-Electromechanical Systems (MEMS) can be more efficient, but are limited in speed and spatial resolution. The ideal mechanical system would thus combine the advantages of both AOMs and optical MEMS. Here, we present and characterize a Nano-Electromechanical System (NEMS) acousto-optic resonator, which comprises a gap plasmon resonator with a deformable gap filler. The plasmonic gaps confine light to nanometer-sized regions, enabling small nanometer-scale movements to significantly and efficiently modulate the optical scattering spectra. Surface acoustic waves (SAWs) are emitted and shaped under an array of these gap plasmon resonators, facilitating spatially tunable optical scattering. We show time-resolved optical spectra and demonstrate high-frequency continuous optical beam steering by sweeping the input SAW frequency. This work represents a significant advancement in the field of acousto-optic modulation, and provides a new platform to investigate soft materials at the nanometer and nanosecond scale.