Optical phase-gradient metasurfaces (PGMs)[
Chinese Optics Letters, Volume. 19, Issue 4, 042602(2021)
Optical beam splitting and asymmetric transmission in bi-layer metagratings On the Cover
In this work, inspired by advances in twisted two-dimensional materials, we design and study a new type of optical bi-layer metasurface system, which is based on subwavelength metal slit arrays with phase-gradient modulation, referred to as metagratings (MGs). It is shown that due to the found reversed diffraction law, the interlayer interaction that can be simply adjusted by the gap size can produce a transition from optical beam splitting to high-efficiency asymmetric transmission of incident light from two opposite directions. Our results provide new physics and some advantages for designing subwavelength optical devices to realize efficient wavefront manipulation and one-way propagation.
1. Introduction
Optical phase-gradient metasurfaces (PGMs)[
Inspired by the concept of PGMs, recently, subwavelength metal slit arrays with phase-gradient modulation, referred as metagratings (MGs)[
Alternatively, angularly asymmetric diffraction was observed theoretically and experimentally in MGs[
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Although great progress has been made with regard to single-layer MGs, the study of the multilayer system composed of the single-layer MG is still relatively rare. In recent years, in condensed matter physics, the interaction between layers of two-dimensional (2D) materials[
2. Models and Theories
In this work, as a concrete example, we design and study a bi-layer MG with a relatively simple structure operating at the visible frequencies, and each layer of MG contains only two units (
Before exploring the bi-layer MGs, we first consider two single-layer MGs (i.e., MG-1 and MG-2, see Figs. 1(a) and 2(a), respectively) used to form the bi-layer system and illustrate their diffraction characteristics. The working wavelength of interest is
Figure 1.(a) Structure of designed MG-1, a periodic metallic slit array (gray region) filled with two different kinds of media (colored blue regions) alternatively, forming a supercell containing two unit cells (i.e., m = 2). (b) Iso-frequency diagram indicating all possible diffraction orders. (c) and (d) are the magnetic field patterns for incident light with two different incident angles. The working wavelength is
Figure 2.(a) Structure of designed MG-2, a periodic metallic slit array (gray region) filled with two different kinds of media (colored blue regions) alternatively, forming a supercell containing two unit cells (i.e., m = 2). (b) Iso-frequency diagram indicating all possible diffraction orders. (c) is the magnetic field patterns for normally incident light.
When a TM polarized light is incident from up down onto the designed MG-1, it achieved an abrupt phase shift of
Alternatively, Fig. 2(a) displays schematically the geometry of MG-2 designed based on the binary unit structure of MG-1. Its supercell can still be considered to contain two unit cells (i.e.,
3. Results and Discussions
Next, we consider the case of bi-layer MGs by simply putting two single-layer MGs together (see Fig. 3). By suitably adjusting the air gap, asymmetric transmission could be obtained. In particular, for PI, most incident light is reflected due to the feature of MG-1 [see Fig. 3(a)], while for NI, beam splitting with high efficiency occurs when the incident wave passes through MG-2 and then through MG-1 [see Fig. 3(b)]. Figure 4(a) presents the numerically calculated relationship between the transmission/reflection and the gap size
Figure 3.Structure of designed bi-layer MG system based on MG-1 and MG-2. (a) and (b) schematically show the scattering process for (a) positive incidence (PI) and (b) negative incidence (NI), respectively.
Figure 4.(a) When the TM wave is incident to the bi-layer MGs, which are filled with impedance-matched material, the relationship curve between the transmission and reflection efficiency and the size of the air gap for PI and NI, respectively. Magnetic field diagram when the air gap with
Numerical simulations using Gaussian beams were carried out to demonstrate the performance of asymmetric transmission in bi-layer MGs. For comparison, two cases of
Although the impedance-matched material is used in the above discussions, similar results can be obtained by using nonmagnetic dielectrics. To demonstrate this point, here we employ the concept of local Fabry–Perot (FP) resonances[
Figure 5.(a) Geometric structure of nonmagnetic unit cell for the design of magnetic MGs based on the local Fabry–Perot (FP) resonances. (b) Transmission and phase shift of the unit cell versus the height d of filled dielectric with εd
Such a designed bi-layer MG exhibits good performance. Figure 6(a) presents the numerically calculated relationship between the transmission/reflection and the gap size for both PI and NI. Likewise, as
Figure 6.Performance demonstrations. (a) Relationship between the transmission/reflection efficiency and the size of the air gap for PI and NI. (b) Magnetic field pattern for PI when
4. Conclusion
In summary, we have designed and studied a bi-layer MG at the visible frequencies, with each MG consisting of subwavelength metal slit arrays with phase-gradient modulation. Based on the found reversed diffraction law, we have shown that the interlayer interaction can produce a transition from optical beam splitting to high-efficiency asymmetric transmission of incident light from two opposite directions. Numerical simulations fully confirm our findings. Due to the tolerance[
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Qiangshi Shi, Xia Jin, Yangyang Fu, Qiannan Wu, Cheng Huang, Baoyin Sun, Lei Gao, Yadong Xu, "Optical beam splitting and asymmetric transmission in bi-layer metagratings," Chin. Opt. Lett. 19, 042602 (2021)
Category: Physical Optics
Received: Aug. 26, 2020
Accepted: Oct. 19, 2020
Posted: Oct. 21, 2020
Published Online: Feb. 22, 2021
The Author Email: Baoyin Sun (bysun@suda.edu.cn), Yadong Xu (ydxu@suda.edu.cn)