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4Gradient optical metasurfaces for analog image processing
P.D. Kiryanov1*, V.V. Iushkov1, A.S. Shorokhov1, A.A. Fedyanin1
1- Lomonosov Moscow State University, Moscow, Russia
Optical computing based on the metasurfaces is currently an actively developing area of research. Metasurfaces are flat arrays of resonant nanoparticles of various geometries, arranged in the required way. Metasurfaces allow unprecedented control over the wavefront spatial distribution with subwavelength resolution [1]. It allows the implementation of various optical signal processing functions, including differentiation, integration, etc. One of the fundamental approaches for the simplest analog optical calculations is a method using the spatial Fourier transform which is carried out by two lenses in the 4f-correlator configuration [2]. One of the disadvantages of previously used for this purpose metasurfaces is the pixel architecture. Gradient metasurfaces have already been demonstrated [3,4] to significantly improve the optical response and lead to smoothly varying transmission and shift phases.
Here we expect to smooth out the discreteness of the metasurface, obtaining gradient transmission, and phase shift profiles by using several control parameters. The metasurface consists of square tori made of gallium arsenide immersed into glass surrounding medium and located at the same distance from each other, forming a 2D lattice. Thus, the main control parameters are the outer and inner diameter of the tori. By varying these two parameters, it is possible to smoothly control the wave front.
Numerical modeling of a gradient metasurface realizing the first and second differentiation over a two-dimensional image is performed. A comparison is made with a similar pixelized metasurface and an improvement in the quality of the processed image is shown.
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[3] Z. Li, M.H. Kim, C. Wang, et al, Controlling propagation and coupling of waveguide modes using phase-gradient metasurfaces, Nature nanotechnology, vol.12, pp. 675-683, (2017).
[4] S. Abdollahramezani, O. Hemmatyar, A. Adibi, Meta-optics for spatial optical analog computing, Nanophotonics, vol.9, pp. 4075-4095, (2020).
