Radiation-pressure-induced nonlinearity in microdroplets

Abstract

High quality (Q) factor whispering gallery modes (WGMs) can induce nonlinear effects in liquid droplets through mechanisms such as radiation pressure, Kerr nonlinearity, and thermal effects. However, such nonlinear effects, especially those due to radiation pressure, have yet to be thoroughly investigated and compared in the literature. In this study, we present an analytical approach that can exactly calculate the droplet deformation induced by the radiation pressure. The accuracy of the analytical approach is confirmed through numerical analyses based on the boundary element method. We show that the nonlinear optofluidic effect induced by the radiation pressure is stronger than the Kerr effect and the thermal effect under a large variety of realistic conditions. Using liquids with ultralow and experimentally attainable interfacial tension, we further confirm the prediction that it may only take a few photons to produce measurable WGM resonance shift through radiation-pressure-induced droplet deformation.

Comparative analysis of nonlinear optofluidic processes in microdroplets

“…Since the temperature varies along the fluid interface, the thermocapillary effect may arise and induce interfacial flow on the droplets. Such effect can therefore lead to another class of nonlinear optofluidic processes, which was not analyzed quantitatively in our previous publication [18]. The electric field intensity |E| 2 of the WGM in a a = 100 µm droplet within the x-z plane.…”

“…As shown in [18], the absorption of the WGM energy by the droplet may result in the temperature change in the fluid, which should increase the droplet volume and change its refractive index. In addition, the temperature gradient on the fluid interface can also induce the thermocapillary effect, i.e., a shear stress along the interface.…”

“…[18], the fluid motion induced by the optical radiation pressure, [9,17], is also governed by Eqs. (5a) and (7a) with the total stress on the interface f = (2σκ m − p opt )n.…”

“…[17,18], we analyzed a highly nonlinear optofluidic system that is comprised of light circulating in a liquid droplet in the form of a high quality (Q) factor whispering gallery mode (WGM). In such a system, the radiation pressure of the high-Q WGM can push the droplet surface outwards and form the bulge depicted in Fig.…”

“…Baroud et al [19] demonstrated that the thermocapillary force could induce a circulation of fluid in a droplet and thus control the motion of the droplet. In our work on microdroplets as whispering gallery resonators [18], we computed the temperature increase due to the WGM energy absorption, from which the volume expansion and refractive index change were estimated. Since the temperature varies along the fluid interface, the thermocapillary effect may arise and induce interfacial flow on the droplets.…”