Nonlinear optics in cold, bunched atoms
In the fields of nonlinear optics and quantum information science, there exists a broad interest in learning ways to enhance the nonlinear optical response of atoms to optical fields. Many systems implement optical cavities or employ electromagnetically induced transparency to enhance the light-atom interaction strength. We present an alternative method and show that the nonlinear susceptibility can be enhanced even for two-level atoms in free space by cooling them to sub-Doppler temperatures and allowing them to spatially bunch in an optical lattice. We show theoretically that the nonlinearity that arises due to atomic bunching gives rise to more than two orders of magnitude enhancement in the third-order nonlinear optical susceptibility over the case of a homogeneous gas. With this enhanced material response, we experimentally observe ultra-low powers required for the nonlinear optical phenomenon of optical pattern formation. We also discuss the synergistic interplay between the optical patterns and the spatial distribution of the atoms.