Reinforced design method for moiré metalens with large spacing

Abstract

Although metalens has attracted many research interests for its advantages of light weight, ultrathin size, and high design freedom in realizing achromatic and aberration-free optical devices, it still lacks adjustability in zoomable optical systems. Moiré metalens, which consists of two cascaded metasurface layers, can realize large focus tuning range by the mutual rotation of the two layers, and becomes a possible solution to realize real application of reconfigurable metalenses. However, due to the spacing between the two metasurface layers, it suffers from aberration caused by diffraction, leading to a dramatically decreased efficiency with the spacing. In this paper, we propose a reinforced design method for moiré metalenses with large spacing based on diffraction optics. Simulation results demonstrate that at the wavelength of 810 nm, when the spacing of the two metasurfaces is 10λ, the focusing efficiency of the reinforced moiré metalens is 3.4 times larger than the traditional moiré metalens. Furthermore, in order to consider the situation that the spacing between the two metasurfaces cannot be controlled precisely, we also propose a reinforced design method for multiplex spacings, which can make the device maintain a high focusing efficiency (3 times larger than the traditional moiré metalens) for the spacing in a range of 6λ∼10λ. The new design method is anticipated to be applied in realizing tunable metalenses in integrated continuously zoomable optical systems.

Moiré metasurfaces for dynamic beamforming

“…Moiré effect has been recently introduced to metasurfaces for the purpose of investigating unusual dispersion properties and band structure (46,47) and interpreting the beam steering mechanism for the variant of leaky-wave antenna designs (48). However, we should remark that our moiré metasurface is very different from the multiple stacked metasurfaces (49)(50)(51) and the variforcal metalens doublet designs (52)(53)(54)(55)(56). In those design, the phase profiles, or functions, of each individual layer must be designed separately to allow the overall function as the simple combination of the function of individual layers.…”

“…All layers are tightly assembled in a nylon holder with the help of a 5-mm-thick foam cover on the top side, which ensures precise and uniform separation between the moiré pattern and back plate. Unlike the multiple stacked metasurfaces (49)(50)(51) and the varifocal metalens (52)(53)(54)(55)(56), in which all the layers must be perfectly aligned to guarantee the designed phase profiles, the radiation performance of our moiré metasurface will not be affected by in-plane misalignment errors (section S4), making it more robust in practical applications.…”

Highly Tunable Cascaded Metasurfaces for Continuous Two‐Dimensional Beam Steering

“…[ 30 , 39 ] The resonant nature of metasurfaces makes it possible that even fine‐tuning of the topology and geometry of the resonant structure can give rise to great changes in the device's light modulation function. In the far‐field coupling configuration, [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 31 , 32 , 33 , 34 ] each individual layer of the cascaded metasurfaces modulates the propagation of light separately, and the overall modulation function is a simple combination of individual layer functions. Thus, any mechanical motion that alters the overall modulation function will cause changes in the final output light beam.…”

“…Thus, any mechanical motion that alters the overall modulation function will cause changes in the final output light beam. Based on this configuration, varifocal Moiré lenses [ 31 , 32 , 33 ] and Alvarez lenses, [ 22 , 23 , 24 , 25 ] 1D tunable beam steering, [ 26 , 27 , 34 ] etc., have been previously demonstrated.…”

“…However, for cascaded metasurfaces tuned by lateral motion and based on the far‐field coupling scheme, [ 22 , 23 , 24 , 25 , 26 , 27 , 31 , 32 , 33 , 34 ] one major assumption often made in research is that the phase profiles of each layer are added directly to perform the overall desired function, [ 22 , 23 , 24 , 25 , 26 , 27 , 33 , 34 ] while this phase accumulation assumption is only accurate when the gap between the layers is infinitely small or equal to the Talbot length, [ 40 ] which is defined as L Talbot = 2 P 2 / λ 0 , where P is the lattice constant of metasurfaces and λ 0 is the free‐space wavelength. L Talbot is smaller than λ 0 as long as P < λ 0 /2 ≈ 0.7 λ 0 .…”