Spatial filtering with nonlocal non-Hermitian metasurfaces

Spatial filtering of optical fields has widespread applications, ranging from beam shaping to optical information processing; however, conventional spatial filters are bulky and alignment sensitive. Here, we present nonlocal non-Hermitian metasurfaces that can act as exceptionally effective optical spatial filters while being highly compact and insensitive to both lateral and longitudinal displacements. The design of these metasurfaces is based on a resonant waveguide grating in which radiative losses of the modes are tailored to realize a symmetry-protected bound state in the continuum in the middle of a non-Hermitian flat band. Using this design, we propose a compact spatial-filtering device operating over an angular range of approximately <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msup><a:mn>1</a:mn><a:mo>∘</a:mo></a:msup></a:math> around normal incidence. In addition to being ultrathin and robust against translational misalignment, the proposed metasurfaces are easy to manufacture, which makes them an attractive alternative to conventional spatial filters, with the potential to become a widely used optical component.