Broadband high-NA laplacian differentiator via metasurfaces for multi-illumination conditions

Optical metasurfaces to perform optical analog spatial differentiation operations and image edge detection processing is a currently hot topic. However, some metasurface differentiators are limited by polarization dependence, narrow operating bandwidth, low numerical aperture (NA), requiring for additional polarization elements or digital processing, and under coherent light illumination conditions. So, an all-optical two-dimensional image processing device with large NA and broadband working spectrum that can work under coherent and incoherent illumination conditions is highly desired.

In this study, a metasurface composed of dielectric cylinders with hexagonal lattice period is demonstrated experimentally to realize second-order two-dimensional (2D) image edge detection directly in the real space. This type of differentiator achieves the broad operating bandwidth by detuning the electric toroidal dipole resonance and the magnetic toroidal dipole resonance, rather than relying on geometric-phase metasurfaces or individual resonance-mode metasurfaces that require additional structural components. At the same time, it maintains a higher numerical aperture and resolution.

Experimentally, we demonstrate that azimuthal-insensitive Laplace differential operations and dual-polarization second-order 2D edge detection with NA up to 0.64 and spectral bandwidths of nearly 100 nm from 750 nm to 850 nm. Notably, broadband incoherent and unpolarized edge detection experiments are also carried out with satisfactory performance. Such metasurface differentiator will pave the way for free-space realization of high-efficiency, broadband parallel optical-computation and image-processing in machine-vision, biomedical, and optical microscopy. Moreover, the metasurface differentiator operates in transmission mode, which is advantageous for direct integration with commercially available optical imaging or sensing systems.