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  • Open Access

Spatial filtering with nonlocal non-Hermitian metasurfaces

Biao Chen*, Mikael Reichler, Radoslaw Kolkowski†, and Andriy Shevchenko‡

  • Department of Applied Physics, Aalto University, P.O. Box 13500, Aalto FI-00076, Finland

  • *Contact author: biao.chen@aalto.fi
  • †Contact author: radoslaw.kolkowski@aalto.fi
  • ‡Contact author: andriy.shevchenko@aalto.fi
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Phys. Rev. Applied 24, 024067 – Published 28 August, 2025

DOI: https://doi.org/10.1103/fsmd-wsnr

Abstract

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 1∘ 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.

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Outline

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Phys. Rev. Applied 24, 024067– Published 28 August, 2025
  • Received 5 March 2025
  • Revised 2 May 2025
  • Accepted 31 July 2025
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