Chip-scale sub-Doppler atomic spectroscopy enabled by a metasurface integrated photonic emitter

We demonstrate chip-scale sub-Doppler spectroscopy in an integrated and fiber-coupled photonic-metasurface device. The device is a stack of three planar components: a photonic mode expanding grating emitter circuit with a monolithically integrated tilt compensating dielectric metasurface, a microfabricated atomic vapor cell and a mirror. The metasurface photonic circuit efficiently emits a 130 micrometer-wide $1/e^2$ diameter) collimated surface-normal beam with only -6.3 dB loss and couples the reflected beam back into the connecting fiber, requiring no alignment between the stacked components. We develop a simple model based on light propagation through the photonic device to interpret the atomic spectroscopy signals and explain spectral features covering the full Rb hyperfine state manifold.