Whispering gallery mode carousel – a photonic mechanism for enhanced nanoparticle detection in biosensing

Abstract

Individual nanoparticles in aqueous solution are observed to be attracted to and orbit within the evanescent sensing ring of a Whispering Gallery Mode micro-sensor with only microwatts of driving power. This Carousel trap, caused by attractive optical gradient forces, interfacial interactions, and the circulating momentum flux, considerably enhances the rate of transport to the sensing region, thereby overcoming limitations posed by diffusion on such small area detectors. Resonance frequency fluctuations, caused by the radial Brownian motion of the nanoparticle, reveal the radial trapping potential and the nanoparticle size. Since the attractive forces draw particles to the highest evanescent intensity at the surface, binding steps are found to be uniform.

Giant resonant light forces in microspherical photonics

“…In addition, the average size of this gap should depend on P 0 , D, and on the concentration of ions in a suspension. It is likely that the average size of these gaps in our experiments was on the scale of few tenths of nanometers; 22,27 however, additional studies are required for more precise characterization of the gap sizes. This plays a critical role in achieving steady propelling along the fiber, because the spheres were covered with a sticky surfactant layer by the manufacturer and physical contact with the fiber would retard their motion.…”

“…22 The origin of this gap has been studied in experiments on a WGM carousel, a photonic mechanism for trapping polystyrene nanoparticles in a circular motion around silica microspheres. 27 It has been demonstrated that the particle is radially trapped due to a combination of a long-range attractive interaction and a short-range repulsive interaction. The attractive optical force originates from the radial gradient of the evanescent fields.…”

“…The average gap sizes have been estimated to be around 35 nm. 27 In our work, we used significantly larger polystyrene microspheres. It is likely that in the course of propulsion the radial gap sizes can vary in a certain range that can lead to a variation of the optical force.…”

“…It is connected with the use of internal optical resonances in microparticles for enhancing optical forces. Recently, interesting experiments on manipulating polystyrene nanoparticles in a circular motion around silica microspheres have been performed by Arnold et al 27 The optical forces have been resonantly enhanced due to whispering gallery modes (WGMs) in the microspheres; however, the recipient of the optical force, the polystyrene nanoparticle, has been too small to possess resonant properties.…”

Detection of nanoparticles with a frequency locked whispering gallery mode microresonator

“…2b). We believe these events arise from the electrostatic repulsion between the microtoroid and nanorod surfaces, resulting in brief trapping of the nanorod followed by diffusion away from the resonator 14 .…”