Non‐Interleaved Four‐Channel Metasurfaces for Simultaneous Printing and Holographic Imaging

Simultaneous realization of printing and holographic images has become an emerging and promising technology for optical storage and anti‐counterfeiting, which can significantly enhance the information capacity and security of an optical system. Herein, a non‐interleaved four‐channel metasurface based on opposite‐chirality‐coexisted meta‐atoms is proposed, which can simultaneously support four independent circular polarization (CP) information channels, including two near‐field printing channels and two far‐field holography channels. In addition, due to the chiral resonance properties, the operation frequencies of these four polarization channels can also be flexibly designed to simultaneously achieve polarization multiplexing and frequency multiplexing. As a proof of concept, two four‐channel metasurfaces, one of which works at a single frequency and the other works at two different frequencies, are designed and validated numerically and experimentally. It is shown in the results that two printing and two holographic images with different handedness can be stored at one frequency or two different frequencies through the flexible design of metasurface. This four‐channel metasurface provides a new avenue for the design of future photonic devices with highly integrated and multiple functionalities, and may have an advantage in diverse potential applications, such as advanced anti‐counterfeits, optical data storage, and image displays.


Introduction
3][4][5][6][7][8][9][10][11][12][13][14] Due to their capacity of tailoring EM waves pixel by pixel, metasurfaces have also been extensively employed in the holographic imaging based on phase modulation, which overcomes the shortcomings (such as bulky frame and low efficiency) of conventional holographic displaying techniques. [15,16]Metasurfaces with both amplitude and phase controls provide more powerful ability to modulate the wave fronts, [17][18][19][20][21][22][23] which can recover holographic images with higher fidelity and sharpness.34][35] By applying Malus' law to the vectorial compound metasurface, the researchers successfully realized multichannel independent grayscale image displays with enhanced confidentiality and concealment. [36,37]ombining printing and holographic images into a single meta-device has become an emerging technology for optical storage and anti-counterfeiting, which has exhibited unprecedented superiorities in high-capacity storage, information encryption, and system integration.40][41] However, such approach is equivalent to the simple combination of several subarrays with different functionalities, and the information density has not been increased essentially.By taking use of the orientation degeneracy of Malus' law, a single-cell design approach can be applied to simultaneously engineer the Pancharatnam-Berry (PB) phase profile for holography and continuous intensity distribution for nanoprinting with high resolution and high fidelity. [42,43]In addition, to further improve the storage capacity of metasurface, three-channel metasurfaces with passive and non-interleaved architecture were proposed by combining intensity modulation controlled by Malus' law with phase manipulation, [44][45][46][47] which can simultaneously and independently project holographic and nanoprinting images in three channels.However, as far as we have known, these previously reported single-celled multichannel metasurfaces can only realize at most three channels for simultaneous near-field nanoprinting and far-field holography, and do not have frequency-multiplexing capacity.
In this work, we propose a method to design a four-channel metasurface for simultaneous printing and holographic imaging by using an opposite-chirality-coexisted meta-atom, which is composed of a composite split-ring resonator (CSRR) structure.Due the chiral resonance absorption of meta-atom combined with the decoupling of propagation phase and geometric phase, the amplitude and phase responses of the reflected left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) waves can be independently controlled by designing metaatoms, and thus a four-channel metasurface can be constructed to simultaneous and independent control of a printing and a holographic image for LCP and RCP waves, respectively.In addition, benefit from the chiral resonance properties, the operating frequencies of LCP and RCP channels can be flexibly and independently customized to achieve frequency multiplexing.As a proof of concept, two kinds of four-channel metasurfaces are designed, fabricated, and measured, one of which can simultaneously store four independent image information at a single operating frequency, and the other can store the printing and holographic images of LCP and RCP waves at two different operating frequencies, respectively.The measured results are in good agreement with the theoretical calculation.The proposed four-channel metasurfaces provide a new scheme in designing integrated optical information systems, especially for the development of spin-optical devices.

Concept Description and Unit Design
The schematic diagram of the proposed four-channel metasurface is shown in Figure 1a, which can simultaneously record four independent image information into four different channels, including two near-field printing channels and two far-field holography channels.As an example, when the metasurface is illuminated by LCP/RCP wave, a near-field printing images of letter "L"/"R" (Channel-1/Channel-2) composed of continuous stroke and a far-field spin-locked holographic images of letter "L"/"R" (Channel-3/Channel-4) composed of discrete focusing spots can be generated, respectively.It is worth mentioning that the images in these four channels, no matter the printing images or holographic images, are completely independent.The metaatom of metasurface is an opposite-chirality-coexisted structure, as demonstrated in Figure 1b, which is composed of a metallic CSRR and a metallic ground spaced by an FR-4 (epoxy glass cloth laminate) dielectric substrate with the relative permittivity of 4.3 and the loss tangent of 0.025.The thickness of the dielectric substrate is t = 1.2 mm, and the period of structure is p = 5 mm.The CSRR contains two enantiomers of asymmetric split-ring resonator (ASRR) with opposite chiral geometry, where two enantiomers are spaced by the included angle γ = 12°.The outer radiuses of the outer and inner arc-shaped metallic structures are r 1 = 2.2 mm and r 2 = 1.9 mm, respectively, and the line width of metallic strip is w = 0.17 mm.Two outer arc-shaped metallic structures are completely mirror symmetrical with same fixed arc angle of θ = 174°.The structural lengths of two inner arc-shaped metallic structures (Arc-L and Arc-R) are represented by their corresponding angle parameters θ L and θ R , respectively.Because two ASRRs are typical chiral structures that exhibit opposite geometric chirality, the spin-selective reflection amplitudes for LCP and RCP waves can be independently controlled by altering the sizes of Arc-L and Arc-R (θ L and θ R ), respectively.In addition, the counterclockwise rotation angle of CSRR with respect to the x-axis is defined as α, which can be used to adjust the additional PB phase shift of LCP and RCP waves.It is worth mentioning that the circular polarization (CP) rotation direction is determined according to the rotation direction of electric-field vector of EM wave.When EM waves propagate toward the observer, clockwise rotation and anticlockwise rotation are defined as LCP and RCP waves, respectively.
The reflection amplitudes of the meta-atom under the normal incidence of LCP and RCP waves with different angle parameters of θ L are shown in Figure 2a, in which θ R is fixed as 138°.When θ L decreases from 153°to 127°, the absorption frequency of metaatom for LCP wave can be continuously adjusted from 18 to 22 GHz, while it has no influence on the absorption frequency for RCP wave, and the cross-polarized reflection coefficients (r RL and r LR ) always keep at a very small level.The similar results can be achieved when changing θ R and keeping θ L unchanged, that is, the absorption frequency of meta-atom for RCP wave can be continuously adjusted while has no influence on the absorption frequency for LCP wave.To further explore the underlying physical mechanisms, the simulated surface currents of the CSRR under the normal incidence of LCP and RCP waves at 20 GHz are demonstrated in Figure 2b,c, respectively, in which both θ L and θ R are all fixed as 138°.As shown in Figure 2b, a pair of strong antiparallel currents with similar intensities is excited on the left part of the CSRR under the illumination of LCP wave, while the current on the right part is very small and can be ignored.The similar results can be observed under the illumination of RCP wave, another pair of strong antiparallel currents is excited on the right part of the CSRR, while the current on the left part is very small, as shown in Figure 2c.These strong antiparallel currents on the left and right parts of CSRR lead to high absorptions of LCP and RCP waves, respectively, and thus the absorption frequencies of LCP and RCP waves can be independently designed by changing θ L and θ R , respectively.
The co-polarized reflection amplitude and phase of orthogonal CP varying with α at 20 GHz are demonstrated in Figure 2d, in which both θ L and θ R are fixed as 132°.The results show that the reflection amplitudes of both r LL and r RR are almost not affected by the change of α, but the reflection phases (known as geometric phase or PB phase) are gradually increased for RCP waves and decreased for LCP waves with the trend of 2α and À2α, respectively.In addition, the co-polarized reflection amplitude and phase of LCP wave varying with θ L at 19, 20, and 21 GHz are demonstrated in Figure 2e.Take 20 GHz as an example, it is observed that the reflection amplitude decreases from 0.78 to 0.25 as θ L varies from 132°to 138°, and then increases from 0.25 to 0.78 as θ L varies from 138°to 145°(red-dashed line), while the reflection phase of LCP wave gradually decreases from 90°to À180°as θ L varies from 132°to 145°(red solid line), which is known as the propagation phase.The similar results can be achieved at 19 GHz (blue lines) and 21 GHz (green lines), implying that the operating frequencies can be customized flexibly and independently by controlling the lengths of Arc-L.It should be noted that the reflection phase responses at 19, 20, and 21 GHz have been normalized to the range from À180°to 90°.In addition, because the CSRR is a mirror-symmetric structure, the same results can be obtained for RCP incidence.Therefore, the spin-independent amplitude controls of LCP and RCP waves can be achieved by varying the lengths of Arc-L and Arc-R, respectively, while their spin-decoupled phase control can be achieved by combining propagation phases (φ PGL and φ PGR ) with geometric phase (φ PB ) where φ PGL and φ PGR are the propagation phases of LCP and RCP waves, respectively, which can be independently controlled by θ L and θ R , while the geometric phases (φ PB ) of LCP and RCP waves are interrelated and only related to the rotation angle α.Therefore, according to previous discussions, two kinds of representative 1-bit meta-atoms were designed and optimized to construct different four-channel metasurfaces, one of which can work at a single frequency of 20 GHz, and the other can work at two operating frequencies of 19 and 21 GHz, respectively.For the single-frequency metasurface, the meta-atoms with θ L = 145°, 138°, 132°and θ R = 145°, 138°, 132°are selected to construct its basic 1-bit units.As shown in Figure 3a, when θ L = 145°(blue solid line) or 132°(green solid line), the LCP co-polarized reflection amplitudes of meta-atoms at 20 GHz are larger than 0.75 and encoded as amplitude code "1," and when θ L = 138°(red solid line), the LCP co-polarized reflection amplitude of meta-atoms at 20 GHz is smaller than 0.25, which is encoded as amplitude code "0."As shown in Figure 3b, the phase difference of LCP co-polarized reflection between meta-atoms with θ L = 145°(blue-dashed line) and θ L = 132°(green-dashed line) at 20 GHz is about 270°, which is usually referred to as their propagation phase difference (Δφ PGL ).According to Equation (1) in the main text, 1-bit meta-atoms with 180°phase difference for LCP wave can be achieved by using this 270°propagation phase difference combining with the PB phase obtained by structure rotation (φ PB = 2α).It should be noted that the reflection phase of θ L = 138°at 20 GHz (red-dashed line) can be ignored in the design of metasurface due to the low reflection amplitude.The same strategy can be applied for the 1-bit meta-atom design of RCP wave, whose amplitude and phase responses varying with θ R can be seen in Figure 3c,d, respectively.Finally, 16 basic single-frequency 1-bit meta-atoms are obtained, as demonstrated in Figure 3e, which can realize spin-independent two-step amplitude and phase modulation at 20 GHz.It should be noted that the meta-atoms are encoded in the form of "C A /C P ," where "C A " indicates amplitude code and "C P " indicates phase code.
For dual-frequency metasurface, the meta-atoms with θ L = 151°, 145°, 139°and θ R = 138°, 132°, 126°are selected to construct its basic 1-bit units, as shown in Figure 4.For LCP incidence at 19 GHz, as shown in Figure 4a, when θ L = 151°( blue solid line) or 139°(green solid line), the reflection amplitudes of meta-atoms are larger than 0.75 and encoded as amplitude code "1," and when θ L = 145°(red solid line), the reflection amplitude of meta-atom is lower than 0.25, which is encoded as amplitude code "0."While the propagation phase difference of meta-atoms with θ L = 151°(blue-dashed line) and θ L = 139°(green-dashed line) is about 270°, as shown in Figure 4b, the reflection phase of meta-atom with θ L = 145°at 19 GHz (red-dashed line) can also be ignored in the design of the second metasurface due to the low reflection amplitude.The similar strategy can be applied for the 1-bit meta-atom design of RCP incidence, where the amplitude and phase responses of meta-atoms with θ R = 138°, 132°, and 126°are shown in Figure 4c,d, respectively.Finally, 16 basic dualfrequency 1-bit meta-atoms are achieved, as demonstrated in Figure 4e, which can realize spin-independent two-step amplitude and phase modulation for LCP wave at 19 GHz and RCP wave at 21 GHz, respectively.

Simulation and Experiment Verification
Figure 5 shows a design flowchart, in which two target printing images (such as letters "L" and "R" with continuous stroke) and two target holographic images (such as letters "L" and "R" with discrete focusing spots) are stored into four channels sequentially.First, the amplitude distributions of two printing images (A L and A R ) are prestored into Channel-1 (LCP) and Channel-2 (RCP), respectively, as shown in Figure 5a.We use binary codes "0" and "1" of amplitude to represent the low and high reflection, respectively, as shown in Figure 5b.Then, A L and A R are set as initial input intensities in the modified GS algorithm (see Section S1, Supporting Information) to calculate the 1-bit phase distributions of two target holographic images (φ L and φ R ) stored in Channel-3 (LCP) and Channel-4 (RCP), as shown in Figure 5c,d.At last, the configuration of metasurface can be determined according to the required amplitude distributions (A L and A R ) and phase distributions (φ L and φ R ), as depicted in Figure 5e.
Based on the aforementioned design principles, two different four-channel metasurfaces are designed, one with all channels operating at the same frequency and another at two different frequencies.The size of metasurface is 465 Â 465 mm, containing 93 Â 93 meta-atoms.All the far-field holographic images are predesigned on the same focusing plane of 450 mm away from the metasurface, and the printing images exist just above the surface of the metasurface.
For the first metasurface, all four channels are designed at 20 GHz, where two printing images of letters "L" and "R" with continuous stroke are stored in Channel-1 (LCP printing image) and Channel-2 (RCP printing image), and two holographic images of letters "L" and "R" with discrete focusing spots are stored in Channel-3 (LCP holographic image) and Channel-4 (RCP holographic image), as illustrated in Figure 6a-d.The calculated 1-bit amplitude and phase distributions of the two target printing images and two target holographic images are shown in Figure 6e-h, respectively, in which amplitude codes "0" and "1" represent the low and high reflection, and phase codes "0" and "1" represent two reflection phases with 180°phase difference.Figure 6i,j illustrates the numerically calculated near-field printing images of letters "L" and "R" on the plane of z = 20 mm under LCP and RCP incidences, respectively, showing that the printing images can be clearly observed at 20 GHz. Figure 6k, l illustrates the numerically calculated far-field holographic images of letters "L" and "R" on the focal plane of z = 450 mm under LCP and RCP incidences, respectively, and the results also show that the clear holographic images can be observed at 20 GHz.The corresponding measurement results are shown in Figure 6m-p, which are in good agreement with the calculations.In addition, both numerically calculated and measured results show that while the observation plane is closer to the metasurface, the clearer printing images will be, while the observation plane is closer to the designed focal plane z = 450 mm, the clearer holographic images will be (see Figure S2, Supporting Information), which means the accurate information of both printing and holographic images can only be revealed under specific incident polarization, observation distance, and detection frequency, thus showing good performance in encrypting information.It is worth mentioning that the metasurface was designed by using single-frequency 1-bit meta-atoms as shown in Figure 3e.Moreover, to reduce the diffraction effect of reflection waves and achieve clear printing images and holographic images at the same time (see Section S3, Supporting Information for more details), super-cell strategy is adopted in constructing the metasurface and each super-cell contains 3 Â 3 identical basic meta-atoms.
For the second metasurface, two LCP channels and two RCP channels are designed at two different frequencies, as shown in Figure 7c-f shows the calculated results of the metasurface under LCP and RCP incidences, among which Figure 7c-f are the results of the near-field printing images (z = 20 mm) and far-field holographic images (z = 450 mm), respectively.For near-field printing images, Arabic numerals "1" and "2" can only be clearly revealed at 19 GHz under LCP incidence and at 21 GHz under RCP incidence, respectively, as shown in Figure 7c,e.For far-field holographic images, the holograms of three-focal points and four-focal points can only be observed distinctly at 19 GHz under LCP incidence and at 21 GHz under RCP incidence, respectively, as shown in Figure 7d,f.Otherwise, when the detection frequency deviates from the corresponding operating frequency, only the mixed holograms composed of seven-focal points can be observed, such as the holograms at 20 and 21 GHz in Figure 7d and the holograms at 19 and 20 GHz in Figure 7f, where the image information of both original two holographic images is mixed together and cannot be distinguished.Figure 7g,i,h,j shows the corresponding measurement results of near-field printing images and far-field holographic images, which are in good agreement with the calculations.The dark shadows on the left edge of each measured printing images are caused by the blocking of the probe bracket, which blocks the incident EM waves during the measurement.It is worth mentioning that the metasurface was designed by using dual-frequency 1-bit meta-atoms shown in Figure 4e, and also adopts 3 Â 3 super-cell strategy.

Discussion of Imaging Efficiency and Imaging Strategy
To evaluate the metasurface performance, the efficiency of each channel for two metasurfaces is calculated, where the efficiency of each printing channel is the ratio of the total electric-field energy of the LCP or RCP waves at the plane of z = 20 mm to the electric-field energy incident to the metasurface, and the efficiency of each holography channel is the ratio of the total electricfield energy within the effective region (the region with bright spots in the target holographic image) at the plane of z = 450 mm to the electric-field energy incident to the metasurface.For the first metasurface, based on the results shown in Figure 6i-l According to previous discussions, it can be easily known that the efficiency of all channels will be lower and lower as the proportion of black pixels (code "0", low reflection) of the printing image becomes greater and greater.It should be noted that the efficiency reduction caused by the increase of black pixels may not affect the imaging effect of near-field printing image, but the brightness of the far-field holographic images will be significantly affected, especially for the case of printing image with large-area black pixels.To solve this problem, we can adjust imaging strategy of printing image according to the indicator of ratio of black (RoB), which is the ratio of the black area of the printing image in the entire image.To be specific, when RoB is greater than 50%, we can reverse the areas of black and white pixels to ensure the brightness of far-field images without affecting the near-field imaging effect.More details can be seen in Section S4, Supporting Information.

Conclusions
In summary, we propose a design method for single-layered and non-interleaved four-channel metasurfaces, which contain two printing channels and two holography channels with independent coding freedom.The metasurface is composed of opposite-chirality-coexisted meta-atoms, which can simultaneously realize spin-independent two-step amplitude and phase modulations, and thus achieving quadruple information channels without using conventional spatial multiplexing technique.In addition, the operating frequencies for different CP incidences can be customized flexibly and independently to simultaneously achieve frequency multiplexing and polarization multiplexing, which is difficult to be realized by conventional spin-decoupled metasurfaces.The proposed method has been validated by designing two metasurfaces working at a single frequency and two different frequencies, respectively, and the calculation and measurements results agree very well.The proposed fourchannel metasurface may stimulate a great potential in various applications, such as anti-counterfeiting, information multiplexing, optical data storage, multichannel image displays, and so on.In addition, the proposed four-channel metasurface is promising to be manufactured at the nanoscale by using electron-beam lithography technology to make it operate in the terahertz or optical frequency band.

Experimental Section
The photographs of two fabricated metasurfaces are illustrated in Figure S5a and S5b, Supporting Information, respectively.The measurements were performed in a microwave anechoic chamber, as shown in Figure S5c,d, Supporting Information, in which Figure S5c, Supporting Information, is the experimental setup for measuring near-field printing image and Figure S5d, Supporting Information, is the experimental setup for measuring far-field holograms, respectively.An emitting horn was placed about 1 m away from the metasurface, which could emit LCP wave or RCP wave according to the requirement.A rectangular waveguide probe was placed above the metasurface to detect the electric-field distributions, which could be moved horizontally and vertically by the control of an automatic test system, and the distance between the metasurface and probe could be adjusted flexibly.For measuring the near-field printing image, the probe should be placed as close to metasurface as possible, and we chose an appropriate distance z = 20 mm in this case, as shown in Figure S5c, Supporting Information, while the probe was placed z = 450 mm (focal plane) away from the metasurface for measuring the far-field holograms, as shown in Figure S5d, Supporting Information.It should be noted that, to avoid the blocking of probe, the emitting horn was placed in front of metasurface at an oblique incident angle of about 10°, and thus the incident wave was an oblique incidence with an angle of 10°in measurement, it was not quite the same with the normal incidence in the simulations, which might lead to the slight deterioration of measurement.In addition, a vector network analyzer linking both the emitting horn and rectangular waveguide was used to acquire the electric-field data by measuring the transmission coefficients (S 21 ).Because the x-and y-polarized components of electric field could be detected, respectively, by adjusting the rectangular waveguide to make its aperture electric field perpendicular to and horizontal with the ground, and then the LCP and RCP images were obtained by synthesizing the measured results of x-and y-polarized components, respectively.

Figure 1 .
Figure 1.Schematic of the proposed single-layered and non-interleaved four-channel metasurface.a) The four-channel metasurface possesses two nearfield printing channels and two far-field holography channels.b) The proposed opposite-chirality-coexisted meta-atom.

Figure 2 .
Figure 2. The simulated results of meta-atom.a) The amplitude responses of the meta-atom under left-handed circularly polarized (LCP) and righthanded circularly polarized (RCP) incidences as θ L increases from 127°to 153°with θ R fixed as 138°.b,c) The induced surface current distributions of meta-atom under the incidence of (b) LCP and (c) RCP waves at 20 GHz with θ L and θ R fixed as 138°.d) The co-polarization reflection amplitude and phase under LCP and RCP incidences at 20 GHz as α varying from 0 to 180°with θ L and θ R fixed as 132°.e) The co-polarization reflection amplitude and phase of the meta-atom at 19, 20, and 21 GHz under LCP incidence varying with θ L .

Figure 3 .
Figure 3.The basic meta-atoms for constructing single-frequency metasurface.a-d) The amplitude and phase responses of the meta-atoms with selected θ L and θ R for the design of single-frequency metasurface.e) The 16 basic single-frequency 1-bit meta-atoms.

Figure 4 .
Figure 4.The basic meta-atoms for constructing dual-frequency metasurface.a-d) The amplitude and phase responses of the meta-atoms with selected θ L and θ R for the design of dual-frequency metasurface.e) The 16 basic dual-frequency 1-bit meta-atoms.

Figure 7 ,
Figure 7, in which Channel-1 (LCP printing image) and Channel-3 (LCP holographic image) are designed at 19 GHz, while Channel-2 (RCP printing image) and Channel-4 (RCP holographic image) are designed at 21 GHz.In this case, the target printing images stored in Channel-1 and Channel-2 are Arabic numerals "1" and "2," respectively, and the target holographic images stored in Channel-3 and Channel-4 are three-focal points and four-focal points, respectively, as shown in Figure 7a.The calculated 1-bit phase distributions of the two target holographic images are shown in Figure 7b.The amplitude distributions are not given here because they are similar with the target printing images.Figure7c-f shows the calculated results of the metasurface under LCP and RCP incidences, among which Figure7c-f are the results of the near-field printing images (z = 20 mm) and far-field holographic images (z = 450 mm), respectively.For near-field printing images, Arabic numerals "1" and "2" can only be clearly revealed at 19 GHz under LCP incidence and at 21 GHz under RCP incidence, respectively, as shown in Figure7c,e.For far-field holographic images, the holograms of three-focal points and four-focal points can only be observed distinctly at 19 GHz under LCP incidence and at 21 GHz under RCP incidence, respectively, as shown in Figure7d,f.Otherwise, when the detection frequency deviates from the corresponding operating frequency, only the mixed holograms composed of seven-focal points can be observed, such as the holograms at 20 and 21 GHz in Figure7dand the holograms at 19 and 20 GHz in Figure7f, where the image information of both original two holographic images is mixed together and cannot be distinguished.Figure7g,i,h,j shows the corresponding measurement results of near-field printing images and far-field holographic images, which are in good agreement with the calculations.The dark shadows on the left edge of each measured printing images are caused by the blocking of the probe bracket, which blocks the incident EM waves during the measurement.It is worth mentioning that the metasurface was designed by using dual-frequency 1-bit meta-atoms shown in Figure4e, and also adopts 3 Â 3 super-cell strategy.

Figure 5 .
Figure 5.The design flowchart of the four-channel metasurface.a) Two target printing images.b) To determine amplitude distributions of LCP and RCP waves.c) Two target holographic images.d) To determine phase distributions of LCP and RCP waves.e) Meat-atom selection.

Figure 6 .
Figure 6.Characterization of the first four-channel metasurface operating at one single frequency.a-d) Four target images of (a,b) printing and (c,d) holography.e-h) The corresponding 1-bit amplitude and phase distributions for the target printing and holographic images.i-l) The calculated results in the (i,j) near field (z = 20 mm) and (k,l) far field (z = 450 mm) under LCP and RCP incidences at 20 GHz.m-p) The measured results in the (m,n) near field (z = 20 mm) and (o,p) far field (z = 450 mm) under LCP and RCP incidences at 20 GHz.

Figure 7 .
Figure 7. Characterization of the second four-channel metasurface operating at two different frequencies.a) Four target images of printing and holography.b) The corresponding two-step phase distributions for the target holographic image of the three-focal points and the four-focal points.c-f ) The calculated results in the near field (z = 20 mm) and far field (z = 450 mm) under the (c,d) LCP and (e,f ) RCP incidence at 19, 20, and 21 GHz.(g-j) The measured results in the near field (z = 20 mm) and far field (z = 450 mm) under the (g,h) LCP and (i,j) RCP incidence at 19, 20, and 21 GHz.