Probing beam coherence via terahertz metasurface.

Motivated by the recent advancements in terahertz metaoptics, this work investigates a metasurface capable of probing the spatial coherence of incident THz beams. Traditionally, periodic boundary conditions are applied to simulate metasurface responses under coherent illumination, whereas under incoherent illumination, both the simulation and the experiment become more complex and challenging. In this study, we employ an angular spectrum averaging technique derived from the Van Cittert-Zernike theorem, which relates the wavevector distribution of the incident beam to the spatial coherence width. The experimental measurements show good agreement with the simulation results. Additionally, we demonstrate that the depth of the metasurface transmission dip increases with decreasing spatial coherence width. Further, on the application end, we briefly discussed a metasurface-based spatial low-pass filter. The designed THz metasurface has potential applications in THz imaging, spatial filtering, and improving the signal-to-noise ratio in optical communication.