Spectral control and temporal properties of resonant optical propulsion of dielectric microspheres in evanescent fiber couplers Yangcheng Li 1,a , Alexey V. Maslov 2 and Vasily N. Astratov 1,b 1 Department of Physics and Optical Science, Center for Optoelectronics and Optical Communications, University of North Carolina at Charlotte, Charlotte, NC 28223-0001, USA 2 Department of Radiophysics, University of Nizhny Novgorod, Nizhny Novgorod 603950, Russia ABSTRACT Resonant light pressure effects provide new degrees of freedom for optical manipulation of microparticles. In particular, they can be used for optical sorting of photonic atoms with extraordinary uniform resonant properties. These atoms can be used as building blocks of structures and devices with engineered photonic dispersions. To study the spectral shape of the force peaks, we developed a method to precisely control the wavelength detuning between the tunable laser emission line and central position of the whispering gallery mode (WGM) peaks in tapered fiber-to-microsphere water-immersed couplers. Our method is achieved by integrating optical tweezers to individually manipulate microspheres and based on preliminary spectral characterization of WGM peak positions followed by setting a precise amount of laser wavelength detuning for optical propulsion experiments. We demonstrated dramatic enhancement of the optical forces exerted on 20 μm polystyrene spheres under resonant conditions. Spectral properties of the resonant force enhancement were studies with controlled laser line detuning. In addition, we observed the dynamics of radial trapping and longitudinal propelling process and analyzed their temporal properties. Our studies also demonstrated a stable radial trapping of microspheres near the surface of tapered fiber for high speed resonant optical propulsion along the fiber. Keywords: Optical force, Optical propulsion, Optical trapping, Resonant enhancement, Evanescent fiber coupler, Spectral, Temporal, Microsphere 1. INTRODUCTION Light pressure was experimentally measured 1 more than a century ago by Russian physicist P. Lebedev and since that time it has been a subject of continuous interest of optics and photonics communities. Since the invention of optical tweezers 2 the use of optical forces for manipulating microparticles has become widely accepted in areas from physics 3 to biology. A significant achievement in this area was the observation of resonant peaks in the spectrum of the light force exerted on oil droplets. 4 However, the resonant force peaks have been relatively weekly pronounced in these experiments since a focused laser beam illumination did not produce strong coupling to whispering gallery modes (WGMs) in spheres. Since that time the ways of exerting light forces using evanescent fields have been developed using prism couplers 5 , surface waveguide 6-9 and tapered fiber 10-12 . However, the resonant peaks of the light forces in these experiments have not been observed or they have been relatively weakly pronounced. Meanwhile, the theory of resonant forces exerted on microspheres in evanescent couplers was developed. 13-16 The resonant enhancement of the light forces have been demonstrated in evanescent couplers, however the fundamental limits of the peak forces have not been investigated. Resonant gradient force has also been studied between two coupled microresonators 17-19 and between resonator and waveguide 20 . More recently, we studied optical propulsion of polystyrene microspheres with diameters of 3 to 20 μm in tapered fiber couplers. We observed that some of large spheres of 15-20 μm diameters can be optically propelled at an extraordinary high velocity. 21-24 Giant optical propulsion velocities of ~450 μm/s were recorded for some of 20 μm polystyrene a Email: yli63@uncc.edu, b Email: astratov@uncc.edu Laser Resonators, Microresonators, and Beam Control XVI, edited by Alexis V. Kudryashov, Alan H. Paxton, Vladimir S. Ilchenko, Lutz Aschke, Kunihiko Washio, Proc. of SPIE Vol. 8960, 89600C · © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2043097 Proc. of SPIE Vol. 8960 89600C-1 Downloaded From: http://spiedigitallibrary.org/ on 12/26/2014 Terms of Use: http://spiedl.org/terms