Invited Article: Real-time sensing of flowing nanoparticles with electro-opto-mechanics

High-Q optical resonators allow label-free detection of individual nanoparticles through perturbation of optical signatures but have practical limitations due to reliance on random diffusion to deliver particles to the sensing region. We have recently developed microfluidic optomechanical resonators that allow detection of free-flowing particles in fluid media with near perfect detection efficiency, without requiring labeling, binding, or direct access to the optical mode. Rapid detection of single particles is achieved through a long-range optomechanical interaction in which modification of the resonator vibrational modes during particle transits influences the scattered light spectra from the resonator. Here, we present a hybrid electro-opto-mechanical technique for substantially increasing the bandwidth of these opto-mechano-fluidic sensors, enabling real-time operation. The demonstrated improvements are obtained through high bandwidth lock-in measurement of the optical modulation that is induced by actuating the vibrational mode electrostatically at a fixed frequency. The presented system demonstrates temporal resolution of better than 20 μs (50 000 events/s) with particle sensing resolution (i.e., the particle size noise floor) down to 490 nm, operating in the air without any stabilization or environmental control. Our technique significantly enhances the sensing capabilities of high-Q optical resonators into the mechanics domain and allows extremely high-throughput analysis of large nanoparticle populations