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VCSEL-based optical trapping for microparticle manipulation

In recent years, research into microfluidic devices has attracted much interest in the fields of biology and medicine, since they promise cheap and fast sample analysis with drastically reduced volume requirements. The combination of various analysis steps on one chip forms a small-sized biomedical system, where handling, fixing, and sorting of particles are major components. Here, it was demonstrated that optical manipulation is an efficient tool; in particular it is accurate, contactless, and biocompatible. However, the commonly required extensive optical setup contradicts the concept of a miniaturized system. We present a novel particle manipulation concept based on vertical-cavity surface-emitting lasers (VCSELs) as light sources. The small dimensions and the low power consumption of these devices enable a direct integration with microfluidic systems. The symmetric geometry of VCSELs leads to a high-quality, circular output beam, which we additionally shape by an etched surface relief in the laser output facet and an integrated photoresist microlens. Thus, a weakly focused output beam with a beam waist of some micrometers is generated in the microfluidic channel. With this configuration we were able to demonstrate particle deflection, trapping, and sorting with a solitary VCSEL with output powers of only 5mW. Furthermore, the surface emission of VCSELs allows a comparatively easy fabrication of two-dimensional laser arrays with arbitrary arrangement of pixels. Smart particle sorting and switching schemes can thus be realized. We have fabricated densely packed VCSEL arrays with center-to-center spacings of only 24 μm. Equipped with integrated microlenses, these arrays are integrated with microfluidic chips based on polydimethylsiloxane (PDMS), enabling ultra-compact particle sorting and fractionation.