Giant resonant light forces in microspherical photonics

Abstract

Resonant light pressure effects can open new degrees of freedom in optical manipulation with microparticles, but they have been traditionally considered as relatively subtle effects. Using a simplified two-dimensional model of surface electromagnetic waves evanescently coupled to whispering gallery modes (WGMs) in transparent circular cavities, we show that under resonant conditions the peaks of the optical forces can approach theoretical limits imposed by the momentum conservation law on totally absorbing particles. Experimentally, we proved the existence of strong peaks of the optical forces by studying the optical propulsion of dielectric microspheres along tapered microfibers. We observed giant optical propelling velocities ,0.45 mm s 21 for some of the 15-20 mm polystyrene microspheres in water for guided powers limited at ,43 mW. Such velocities exceed previous observations by more than an order of magnitude, thereby providing evidence for the strongly enhanced resonant optical forces. We analyzed the statistical properties of the velocity distribution function measured for slightly disordered (,1% size variations) ensembles of microspheres with mean diameters varying from 3 to 20 mm. These results demonstrate a principal possibility of optical sorting of microspheres with the positions of WGM resonances overlapped at the wavelength of the laser source. They can be used as building blocks of the lossless coupled resonator optical waveguides and various integrated optoelectronics devices.

Spectral control and temporal properties of resonant optical propulsion of dielectric microspheres in evanescent fiber couplers

“…2(a), and coupled the fiber output to a photodiode connected to an oscilloscope. In a weak coupling regime, which is usually the case for polystyrene spheres in water, 22,37 the minimal values of transmitted signal can be achieved under contact conditions between the sphere and taper. Under these conditions, the intensity measured by photodiode can be used as a qualitative measure of the gap separating the microsphere from the taper.…”

“…Under resonant conditions (Δλ =0), we observed that the spheres tend to be optically attracted to the fiber. However, at very short separations the sphere is likely to be electrostatically repulsed from the fiber, 22 thus forming a potential minimum at a certain nanometer-scale distance from the taper required for stable propulsion. Once radially trapped, the resonant …”

“…We observed that some of large spheres of 15-20 μm diameters can be optically propelled at an extraordinary high velocity. [21][22][23][24] Giant optical propulsion velocities of ~450 μm/s were recorded for some of 20 μm polystyrene a microspheres in water with guided powers of only 43 mW. 22 These velocities were more than an order of magnitude higher than that observed in previous studies.…”

“…[21][22][23][24] Giant optical propulsion velocities of ~450 μm/s were recorded for some of 20 μm polystyrene a microspheres in water with guided powers of only 43 mW. 22 These velocities were more than an order of magnitude higher than that observed in previous studies. [6][7][8][9][10][11][12] Theoretically, we showed that the peak force can approach the level that an absorbing particle would experience based on the total momentum transfer from light to microparticle.…”

“…[6][7][8][9][10][11][12] Theoretically, we showed that the peak force can approach the level that an absorbing particle would experience based on the total momentum transfer from light to microparticle. 22,25 These experiments were carried out with an assembly of microspheres interacting with the tapered fiber. In these experiments, the laser emission line has been fixed, and different spheres from the suspension have been trapped and propelled along the taper.…”