Optically hyperpolarized $^{129}$Xe gas has become a powerful contrast agent in nuclear magnetic resonance (NMR) spectroscopy and imaging, with applications ranging from studies of the human lung to the targeted detection of biomolecules. Equally att
ractive is its potential use to enhance the sensitivity of microfluidic NMR experiments, in which small sample volumes yield poor sensitivity. Unfortunately, most $^{129}$Xe polarization systems are large and non-portable. Here we present a microfabricated chip that optically polarizes $^{129}$Xe gas. We have achieved $^{129}$Xe polarizations greater than 0.5$%$ at flow rates of several microliters per second, compatible with typical microfluidic applications. We employ in situ optical magnetometry to sensitively detect and characterize the $^{129}$Xe polarization at magnetic fields of 1 $mu$T. We construct the device using standard microfabrication techniques, which will facilitate its integration with existing microfluidic platforms. This device may enable the implementation of highly sensitive $^{129}$Xe NMR in compact, low-cost, portable devices.
