Probing of ultrahigh optical Q-factors of individual liquid microdroplets on superhydrophobic surfaces using tapered optical fiber waveguides

Abstract

We report measurements of ultrahigh quality factors (Q-factors) of the optical whispering-gallery modes excited via a tapered optical-fiber waveguide in single glycerol–water microdroplets standing on a superhydrophobic surface in air. Owing to the high contact angle of the glycerol–water mixture on the superhydrophobic surface (>155°), microdroplets with the geometry of a truncated sphere minimally distorted by gravity and contact line pinning effects could be generated. Q-factors up to 2.3×106 were observed for such droplets with radii of 100–200 μm exposed to the ambient atmosphere in a closed chamber with controlled relative humidity. Placement of microdroplets in a constant humidity environment permitted prolonged characterization of Q-factors for individual microdroplets. We found that the Q-factors in air were stable over more than 1 h and their measured values were limited mostly by the thermally induced droplet shape fluctuations.