Integration of optical components into microfluidic devices can enhance particle manipulations, separations, and analyses. We present a method to fabricate annular nanoscale lenses within glass microchannels to precisely position optical tweezers within microfluidic devices. Integrated thin-film nanolenses perform the laser focusing required to generate sufficient optical forces to trap particles without significant off-device beam manipulation. Moreover, the ability to trap particles with unfocused laser light allows multiple optical traps to be powered simultaneously by a single input laser. We have optically trapped polystyrene particles that are 0.5, 1, 2, and 4 μm in diameter over nanolenses fabricated in both chromium and gold films. Optical forces generated by these nanolenses are capable of capturing particles flowing in fluid streams up to 25 μm/s. Quantitative trapping experiments under flow conditions demonstrate size-based differential trapping of particles, which matches theoretical values quantitatively. The optical forces on these particles are identical to traditional optical traps, but the addition of drag forces and inertia from the microfluidic flow introduces a size selectivity that is absent in a traditional optical tweezer apparatus. Thus, the combination of optic tweezers and the microfluidic platform enhance the capabilities of both individual technologies.

Size-selective optical trapping of particles with nanoscale lenses integrated into microfluidic channels

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