Metasurface with Directional Controlled Asymmetric Transmissions

We present a dual-functional metasurface with an asymmetric bidirectional transmission phase. This proposed metasurface comprises bi-layered structures shaped like double arrows with a sub-wavelength metal grating. It enables both linear polarization (LP) conversion for waves incident from opposing directions and independent phase control achieved by introducing rotation and adjusting the geometrical parameters of the double-arrow-shaped structures.


I. INTRODUCTION
Metasurfaces, which are planar arrays made up of subwavelength artificial meta-atoms, have undergone rapid development in the past few decades, showcasing a multitude of advantages [1][2][3].The Janus metasurface, defying the notion of directional transmission symmetry and enhancing space utilization efficiency, has garnered significant attention in recent years.Fig. 1a and Fig. 1b illustrate the schematic principle of two typical methods for designing a Janus metasurface to achieve directional selective full-space wave manipulation.In the case of segmented metasurfaces [4], the metasurface is divided into distinct areas, each loaded with independent spatial phase distribution functions.This approach allows for the realization of multiple independent wavefront distributions and is presently one of the most common methods for achieving multifunctional metasurfaces.For example, this technique has been employed to create monolayer directional selective transmissive meta-holograms in the visible spectrum using an all-dielectric metasurface that records spin-dependent holograms [5].As for interleaved structures, a 3D Janus metamaterial has been reported, comprising two interlaced plasmonic helical nanoapertures with direction-controlled polarization sensitivity [6].In a recent study by Chen et al. [7], unidirectional propagating units arranged in an interleaved fashion within a Janus metasurface were utilized to achieve asymmetric transmission.The approaches mentioned above involve the integration of two metasurfaces into one, ensuring that only half of the elements exhibit scattering effects in each direction.In general, achieving asymmetric wavefront manipulation across the entire space solely based on the characteristics of unit structure in a metasurface, without a specific arrangement, remains a challenging task.
In this work, we present a concept for a non-interleaved Janus metasurface aimed at achieving directional control over the asymmetric transmission of linear polarization (LP) waves.Through adjustments in the angle and geometrical parameters of the unit structures, this metasurface is capable of simultaneously conveying two independent phases.When y-polarized waves propagate in both forward and backward directions, this metasurface facilitates wave deflection and focusing, as confirmed by the proof-of-concept prototypes.Notably, every meta-atom contributes to each channel, highlighting the comprehensive utilization of the metasurface.

II. STRATEGY AND META-ATOM DESIGN
To realize the proposed Janus metasurface design, a sophisticated meta-atom must be engineered to achieve independent polarization-regulated phase control in transmission for both forward and backward incident waves.We utilize a meta-atom comprising three layers of 18-µmthick copper patterns and two 3-mm thick dielectric substrates (ɛr =2.65, tan δ = 0.001).The schematic design of the proposed Janus metasurface is depicted in Fig. 2a This phase-shift can be achieved by modifying θ.Such characteristics establish the basis for realizing a bi-directional metasurface with distinct functionalities and significantly streamline the design process, enabling asymmetric transmission and distinct phase encoding.
To obtain the 16 coding elements, commercial software CST Microwave Studio is utilized, employing unit boundary conditions in the xand y-directions.The calculated phase distributions are discretized into "0," "π/2," "π," and "3π/2," represented by coding elements "00," "01," "10," and "11," respectively.The coding meta-atoms are extracted and described as follows:  The amplitude and phase spectra for bidirectional transmission of the design coding meta-atoms are depicted in Fig. 3.In Fig. 3a, the cross-polarized (x-polarized) transmission amplitude spectra for both forward and backward y-polarized incident waves are illustrated.Furthermore, an immediate observation from Fig. 3b is the existence of four operating states of the transmission phase, introducing a new degree of freedom to enable independent functionalities in the two propagating directions.

III. BIFUNCTIONAL METASURFACES FOR INDEPENDENT MANIPULATION OF WAVES IN OPPOSITE DIRECTIONS
By encoding 16 element atoms based on the desired phase, dual-functional metasurfaces can be constructed and controlled.We design and fabricate a passive Janus metasurface with focusing and deflection functionalities, which consists of 51×51 meta-atoms and occupies a total area of 306×306 mm 2 .The schematic principle for the two functionalities is depicted in Fig. 4a.The sample is produced using conventional printed circuit board (PCB) technology, and an electric-field distribution on the transmission imaging plane is measured using a near-field scanning system, as illustrated in Figs.4b and 4c.Fig. 4d illustrates a schematic of the experimental near-field scanning system.The spatial phase profiles enabling the realization of these two functionalities are as follows: 2 sin ( , ) where f and b express the forward and backward direction of the incident wave,  is the working wavelength and F is the focal length set as 80 mm.For the forward incident wave, the wavevector is set to be directed to an angle of  = 30° to show the deflection functionality.For the backward incident wave, the cross-transmitted wave is converged into a single focus in the z = 80 mm plane.Fig. 4e and 4f present the target phase distribution of the two functions extracted from (2) and (3).Simulation and measurement results of the metalens are displayed in Figs.4g-4j, displaying a notable agreement and affirming the exceptional deflection and focusing effects in both opposite directions.

IV. CONCLUSION
In summary, we have introduced a Janus metasurface, which can enable the simultaneous control of opposite directional transmissions.Both simulated and experimental results demonstrate consistent outcomes, highlighting the control capabilities of cross-polarized transmission components.
. It incorporates two bi-layered double-arrow-shaped structures (labeled I and III) in the top and bottom layers, while a metal grating (labeled II) structure serves as the middle layer.The coding meta-atoms maintain a periodicity of p = 6 mm with the following parameters kept constant: b = 0.6 mm, s = 1.2 mm, w = 0.4 mm, and l = 5 mm.As depicted in Fig.2b, with fixed parameters d1, d2, g, and 2, and an operating frequency set to 15 GHz, altering the parameter 1 (rotation angle) of structure I induces a change in the forward transmission phase under y-polarized wave forward incidence, i.e.However, for the backward transmission phase, a symmetrical change is observed, i.e.A similar pattern is observed by varying the rotation angle 2  of structure III for both forward and backward y-polarized incident waves, as illustrated in Fig.2c, i.e.

Fig. 3 .
Fig. 3. (a) The amplitude and (b) Phase of all 16 coding atoms under ypolarized forward and backward incident waves at 15 GHz.

Fig. 4 .
Fig. 4. (a) Schematic diagram of the proposed metasurface for refraction and focusing under y-polarized wave bidirectional illuminations.(b) and (c) The fabricated sample.(d) Measurement setup for extracting electric field data.(e) and (f) Phase of the refracted and focused beams.(g) and (i) Simulation and measurement results of wavefront refraction in the xoz plane.(h) and (j) Simulation and measurement results of focusing in the xoy plane.