We introduce a subwavelength thick (～ 200 nm) plasmofluidic microlens that effortlessly achieves objective-free focusingand self-alignment of opposing optical scattering and fluidic drag forces for selective separation of exosome sizebioparticles. Our optofluidic microlens provides a self-collimating mechanism for particle trajectories with a spatialdispersion that is inherently minimized by the optical gradient and radial fluidic drag forces. We demonstrate that thisfacile platform facilitates complete separation of small size bioparticles (i.e., exosomes) from a heterogenous mixturethrough negative depletion and provides a robust selective separation capability based on differences in chemicalcomposition (refractive index). Unlike existing optical chromatography techniques that require complicatedinstrumentation (lasers, objectives and precise alignment stages), our platform open up the possibility of multiplexed andhigh-throughput sorting of nanoparticles on a chip.
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