  Metasurfaces provide the disruptive technology enabling miniaturization of
complex cascades of optical elements on a plane. We leverage the benefits of
such a surface to develop a planar integrated photonic beam collimator for
on-chip optofluidic sensing applications. While most of the current work
focuses on miniaturizing the optical "detection" hardware, little attention is
given to develop on-chip hardware for optical "excitation". In this manuscript,
we propose a flat metasurface for beam collimation in optofluidic applications.
We implement an inverse design approach to optimize the metasurface using
gradient descent method and experimentally compare its characteristics with
conventional binary grating-based photonic beam diffractors. The proposed
metasurface can enhance the illumination efficiency almost two times in on-chip
applications such as fluorescence imaging, Raman and IR spectroscopy and can
enable multiplexing of light sources for high throughput biosensing.
