Micro-particle transport and switch governed by guided-wave optical interference are presented. The optical interference, occurring in a directional coupler and a multi-mode interferometer made by inverted rib waveguides, results in a specific evanescent field dependent on wavelength. Through a detailed theoretical analysis, the field of induced optical force shows a correlative pattern associated with the evanescent field. Experimental results demonstrate that 10 μm polystyrene beads are propelled with a trajectory subject to the interference pattern accordingly. By launching different wavelengths, the polystyrene beads can be delivered to different output waveguide ports. Massive micro-particle manipulation is applicable.

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A design of microfluidic devices is presented to integrate single-mode, liquid-core waveguides with microfluidic channels that generate and deliver disklike emulsion microdroplet cavities doped with an organic dye. The microcavity modes can be directly coupled to the liquid waveguide. Cavity-enhanced spontaneous emission was observed at the waveguide with low pump pulse energy. ings [4-7] were reported with a wide range of applications in molecule diagnosis or laser emission. Most of the cavity structures are made by microfabrication processes and the optical performance is affected by the fabrication quality, including etching-induced surface roughness. Among the reported microfluidic cavities, liquid microspheres implemented by microdroplets showed an extremely high quality factor, usually more than 10 6 , due to a perfect sphere naturally developed by surface tension. Cavity-enhanced fluorescence, nonlinear optics, and quantum electrodynamics with low pump threshold energy were demonstrated. However, to tap resonant modes of the droplet cavity by optical fibers or waveguides is challenging. In this Letter, a microfluidic platform is proposed by monolithically integrating microfluidic channels with liquid-core waveguides. The microfluidic channels are used to generate and deliver disklike microdroplets to the liquid-core waveguide for mode coupling. Compared with microsphere cavities, these disklike microdroplets have smaller mode volume. Additionally, because the waveguide is made by liquid, there is no physical gap defined between the microdroplet and the waveguide. Weak or strong coupling is applicable. The disklike microdroplet cavity, resembling a microdisk resonator, can support multiple whispering gallery modes (WGM) [4,8]. The radial-dependent electromagnetic field of WGM can be approximately described by [9]:where J m , k, n d , α d , and r are the mth Bessel function, the wavenumber, the refraction index of the resonator, the evanescent decay constant along the radial direction, and the radial coordinate, respectively. m is the azimuthal mode number and l is the radial mode index. The WGM can be coupled to a waveguide mode once they are close to each other.