High Q optical resonances of polystyrene microspheres in water controlled by optical tweezers

Abstract

Using evanescent field coupling via frustrated total internal reflection, we measured whispering-gallery optical resonances of polystyrene microspheres held in aqueous buffer by optical tweezers, as a function of the distance from the planar coupling substrate. An intrinsic Q factor of 4×106 was found for a microsphere of 30μm diameter, indicating the potential of such microresonators for highly sensitive biomolecular detection. These measurements provide an upper limit of 2×10−7 for the imaginary part of the complex refractive index of polystyrene at 770nm wavelength.