Spatial coherence filtering of normal incidence light through leaky mode engineering

Filtering light by coherence by metasurfaces would enable compact, integrable coherence filters. A recent such proposal centered on a metasurface of zigzags. Here, we show that spatial coherence filtering can be achieved at normal incidence with a similar zigzag metasurface and can be traced to the band structure of the leaky modes of the thin film structure. We fabricate a metasurface of zigzags etched through a gold film deposited on a GaAs substrate and measure the transmittance of long wave infrared radiation. Selectivity matches predictions based on the band structure of fully coherent radiation. Furthermore, the degree of coherence filtering is shown to be controlled by the Q-factor of leaky modes.