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Non-reciprocal asymmetric polarization encryption by layered plasmonic metasurfaces Daniel Frese, Qunshuo Wei, Yongtian Wang, Lingling Huang, and Thomas Zentgraf Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b01298 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 4, 2019

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Non-reciprocal asymmetric polarization encryption by layered plasmonic metasurfaces Daniel Frese1,#, Qunshuo Wei2,#, Yongtain Wang2,*, Lingling Huang2,*, and Thomas Zentgraf1,* 1

Department of Physics, Paderborn University, Warburger Straße 100, 33098 Paderborn, Germany

2

School of Optics and Photonics, Beijing Institute of Technology, 100081, Beijing, China

Abstract: As flexible optical devices that can manipulate the phase and amplitude of light, metasurfaces would clearly benefit from directional optical properties. However, single layer metasurface systems consisting of 2D nanoparticle arrays exhibit only a weak spatial asymmetry perpendicular to the surface and therefore have mostly symmetric transmission features. Here, we present a metasurface design principle for non-reciprocal polarization encryption of holographic images. Our approach is based on a two-layer plasmonic metasurface design that introduces a local asymmetry and generates a bidirectional functionality with full phase and amplitude control of the transmitted light. The encoded hologram is designed to appear in a particular linear crosspolarization channel, while it is disappearing in the reverse propagation direction. Hence, layered metasurface systems can feature asymmetric transmission with full phase and amplitude control and therefore expand the design freedom in nano-scale optical devices towards asymmetric information processing and security features for anti-counterfeiting applications. Keywords: Asymmetric transmission, metasurface, plasmonics, phase control, polarization control, metasurface holography

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1. Introduction Metasurfaces enable a high degree of freedom in tailoring optical properties of light1. The mostly lithographically fabricated planar structures allow a local phase and amplitude modulation due to their structural geometry2,3. A prominent example for meta-optics is metasurface-based holography4, where complex amplitude and phase distributions are encoded into 2D subwavelength structures5,4,6. Meanwhile, different meta-atom geometries and materials have been used to realize transmitting polarization multiplexed holograms5,7,4,6,8–10, reflection-only type holograms11,12, and vectorial displays9. An important feature of metasurfaces is the strong light-matter interaction and the possibility to introduce strong birefringence by properly designing building blocks of anisotropic resonators or nanoantennas, making them attractive candidates for polarization optics13,14. The outstanding advantage compared to conventional refractive optics is the high flexibility in wavefront shaping and the possibility of embedding meta-structures in integrated photonic devices due to their subwavelength dimensions15,16. However, due to the relatively low asymmetry in the propagation direction, metasurfaces show a strong reciprocal optical behavior, so that between any two points in the optical system the transmission is the same for the opposite propagation direction17,18. Breaking this symmetry is indispensable for many photonic applications in communication systems or for protecting laser systems of back reflections19. Classically, these non-reciprocal devices –optical isolators20- operate based on the Faraday rotation effect using magnetized materials21. However, in modern integrated optics, these materials are handicapped due to their large dimensions. Metasurfaces that break the symmetry in propagation direction can result in asymmetric transmission behavior. Recently, a chiral metasurface where slit-groves with different depths have been etched in a gold film, that introduces this asymmetry, have been realized to build direction-controlled bifunctional polarizers22. Also, layered metamaterial systems that break this symmetry have been presented, to


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either increase polarization conversion efficiencies23 or realize asymmetric transmission features for particular polarization states24–26. Here, we demonstrate a concept for a two-layer metasurface system that breaks the spatial symmetry for propagation, resulting in different optical properties. For our demonstration, we encode a Fourier hologram into the metasurface system and show that it supports different bidirectional holographic image generation properties. As a result, for forward and backward propagation of light, the metasurface behaves non-reciprocal. The encoded holographic image can be only observed in particular polarization and propagation direction encrypted channels. Recently, a similar functionality but for orthogonal propagation directions was demonstrated based on transformation optics for propagation within a nanostructured silicon waveguide27. Here, metasurfaces have the advantage that they only require a subwavelength propagation length. Our concept of asymmetric metasurface holography and non-reciprocal light propagation builds upon the polarization conversion of plasmonic L-shaped nanoantennas in combination with a plasmonic nanostructure dimer polarizer. Hereby, the holographic information is encoded as a pixel-by-pixel hologram in the combined L-dimer meta-atoms, whereas the directionality of the polarization channels can be attributed to the plasmonic dimer polarizer. Figure 1 illustrates the operation principle with the designed reconstructed holographic image showing the initials and logos of our institutions in the first line and the word META in the second line. If vertically polarized light first encounters the L-shaped nanoantenna layer, the holographic image is reconstructed in the horizontal output channel. On the other hand, by illuminating the flipped sample with the same polarization state, the holographic image disappears. Hence, such a double layer system can act as polarization and direction encrypted converter with full spatial phase and amplitude control.


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Figure 1. Schematic illustration of the non-reciprocal functionality of the two-layer polarization sensitive meta-hologram. Left: Illuminating the sample from the front side (Lshaped on top) with V-polarized light, the holographic image is reconstructed in the crosspolarization state (H). Right: By flipping the sample so that the back side is illuminated (plasmonic dimers first), the holographic image is hidden in the cross-polarization state (H).

2. Metasurface design and working principle Our designed and fabricated metasurface consists of two stacked layers of plasmonic meta-atom arrays. Both layers exhibit different functionalities that in combination introduce a spatial symmetry breaking (non-reciprocity), which allows asymmetric transmission and additional full spatial phase encoding. The non-reciprocal functionality of the metasurface can be understood in a simple model using the Jones formalism. The upper metasurface layer is made of plasmonic Lshaped nanoantennas (Figure 2a) that can convert the linear polarized light states into both their co-


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Figure 2. Double-layer metasurface structure for non-reciprocal holography. (a-b) Schematic illustrations of the unit-cell of the hologram layer and the polarizer layer. The hologram is encoded in an L-shaped nanoantenna array. Each L-shape antenna is determined by its arm lengths

and

. Two dipole antennas for the polarizer layer form a pair (dimer) centered in each unit-cell, separated by a 100 nm gap. (c) In the final design, the polarizer layer is placed on a quartz glass substrate. The L-shaped nanoantennas are centered above two dipole antennas, separated by a 50nm-thick SiO2 spacer to prohibit a direct contact between the two layers. Each unit-cell has an inplane dimension of 550 nm by 550 nm. (d-f) Corresponding scanning electron microscopy images of the fabricated structures as a single-layer L-structure, single layer dimer and, double layer directional hologram, respectively.

polarization and cross-polarization state. In the most general case, the transmission matrix an L-structure illuminated in forward direction consists of diagonal and off-diagonal elements: =

.


for

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Here, we use the superscripts for the propagation direction (f – forward, b – backward). Note, that the off-diagonal matrix elements

and

are complex transmission coefficients that

describe the cross-polarization conversion by the L-structure from linear vertically to linear horizontally polarized light and vice versa. If we investigate the backward propagation of the same structure, we will find that =

=

.

While the diagonal elements remain unchanged, the off-diagonal elements exchange and undergo a sign change28. Thus, in general, one cannot expect non-reciprocal behavior from isotropic single layer metasurfaces. To break this symmetry, we add a second layer of plasmonic dimer antennas to achieve optical non-reciprocity (Figure 2c). The second metasurface layer acts as a plasmonic polarizer, which ideally reflects vertically polarized light and ideally transmits horizontally polarized light by neglecting absorption losses. Hence, the transmission matrix can be written as: =

1 0 . 0 0

Note, that this transmission matrix is for the ideal case. A more detailed discussion can be found in the Supporting Information. If we ignore any potential Fabry-Perot-effects due to the spacing of the two layers, the transmission of the total system can be calculated by simple matrix multiplication. We note that illuminating the sample from the backside reverses the order of the matrix multiplication. In our experiment, we specifically consider the cross-polarization transmission from the initial V-polarization state |

to target H-polarization state

idealized system, this results for the forward propagation direction in =

and in analogy for the backward propagation direction in


|. For an

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=0

One can see that the transmission is defined by the off-diagonal transmission coefficient when vertically polarized light first encounters the L-shaped structure. However, if the sample is flipped, the vertically polarized light is blocked by the dimer layer. Furthermore, by rotating the initial and the target polarization by 90°, the transmission is determined by

for the opposite

propagation direction, resulting in

=

=0

Note, that the complex off-axis transmission coefficients can be altered by changing the geometry of the L-shape structure in this simple picture. For our design, we desire a tailored phase and amplitude modulation of the combined structure separated by 50 nm. Hence, coupling effects need to be considered by tailoring the total transmission coefficients (

) of each combined unit-cell.

For that, we simulated the optical properties by rigorous coupled wave analysis (RCWA) for various L-shaped geometries together with the plasmonic dimer structure (Figure 2c) and selected eight suitable parameter sets. In such a way, we ensure that coupling effects are taken into account and high conversion efficiency with strong suppression of the unwanted polarization state is obtained. Figure 3 shows eight different combined designs (L-shape antenna and plasmonic dimer polarizer) that we selected from our simulations (for more details see Supporting Informations). The eight selected parameter sets achieve phase shifts between 0 and transmission coefficient

,

at the chosen design wavelength of

for the total target

= 1150 nm. The full phase

range from 0 to 2 can be obtained by considering the 90° rotated L-structures, which results in an

additional phase delay of . Therefore, we achieve in total 16 phase levels to cover the full phase range that we utilize to encode a Fourier hologram into our metasurface using the Gerchberg-


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Saxton algorithm. Note that the transmission amplitude

,

is almost uniform for the different

designs and also notably greater than the reverse transmission coefficient layer metasurface breaks the symmetry in the cross-polarization channel ,

, ,

,

and

,

,

,

. Hence, the double,

. To fulfill such condition, we specified

, ,

, as well as ,

and

as a target in the numerical optimization for the structure geometry.

Figure 3. Simulated transmission coefficients of the double layer meta-atoms. (a) Phase modulation of the cross- and co-polarized transmission coefficients for eight different L-shaped geometries (L1 - L8). The cross-polarization coefficients modulation over , resulting in a phase range over the final design. (b) The transmission amplitudes

and

provide each a phase

2 together with the rotated meta-atoms in

for the target polarization change from V to

H are similar for the different geometries and drop significantly for the opposite cross-polarization . 3. Experimental results The designed double-layer structure is fabricated in three steps by using e-beam lithography and lift-off processes. First, we structured a 200 x 200 m2 area with the polarizer design consisting of

dimer antenna pairs that are made of 3 nm chromium for better adhesion on the substrate and 50


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nm of gold for the plasmonic antennas. The chromium and the gold are deposited by using an ebeam evaporator. Second, after the lift-off process, a 50-nm-thick SiO2 spacer is deposited using an ion-assisted e-beam evaporator. In the third step, the L-shaped gold antennas are fabricated with the same procedure as the dimer layer using alignment marks to achieve a high in-plane positioning accuracy of ± 50 nm over the whole sample area. The precision in the alignment ensures that the antennas of the different layers are positioned in the same way as it was designed. Figures 2 (d) to

(f) show scanning electron microscopy images of the obtained structures for a single L-shaped layer, a single dimer layer, and the combined non-reciprocal structure, respectively. A more detailed study of the dependency of the double-layer metasurface properties with regard to fabrication errors and misalignment can be found in the Supporting Information. For the optical characterization, we used as a light source an optical parametric oscillator (OPO) with a wavelength of 1150 nm (Figure 4 (a)). The linearly polarized input state in front of the sample is set using a half-wave plate. In our case, we chose vertically and horizontally polarized input states and set the target output polarization using a linear polarizer as an analyzer in front of the imaging camera. Note, that in each case the linear polarizer is orientated 90° to the input polarization to measure the cross-polarization transmission

and

, for both forward and

backward propagation. To reconstruct the full holographic information, the input beam is slightly focused on the sample, illuminating the entire 200 by 200 m2 metasurface area. The transmitted

light is collected using a 40x microscope objective with a numerical aperture of NA=0.6. The

holographic information is obtained by imaging the Fourier plane of the microscope objective on a sCMOS camera (Zyla 4.2), using two lenses.


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Figure 4. Experimental setup and reconstructed holographic images. (a) Optical setup for measuring the transmission holograms. As a light source, we use a tunable optical parametric oscillator (OPO) and set the wavelength to = 1150 nm. The linear input polarization can be rotated by a half-wave plate. The output polarizer is always orientated perpendicular to the input polarization. The light from the metasurface is captured by a microscope objective (40x magnification, NA = 0.6) and the k-space hologram is imaged to a sCMOS camera. (b) Measured holograms for the different illumination directions and polarization states as illustrated by the schematics. For vertically polarized input and horizontally polarized output (V to H), the hologram is revealed for forward propagation and is hidden in backward propagation. For the opposite polarization configuration from H to V, the directionality changes and the hologram appears in backward propagation, but is hidden in forward propagation. At the same time, the images are mirrored with respect to the rotation axis for flipping the sample.


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Figure 4 (b) shows the obtained holographic images for various polarization configurations in forward and backward propagation direction for three different orientations of the sample. First, we measured the cross-polarization transmittance for the forward propagation direction with polarization input state |

. For the forward direction with the light passing the L-shaped layer

first, we reconstruct the holographic image in the target polarization

,

. Further, an 180° in-

plane rotation of the sample results in a 180° in-plane rotation of the image. Flipping the sample around the vertical axis significantly reduces the holographic image intensity, due to the dimer layer blocking the light. Note, that for the input state |

, the double-layer structure shows also

asymmetric transmission, if the output polarizer is removed (see Supp. Figure S10). The polarization-conversion efficiency for both propagation directions is measured using Fourier Transform Infrared spectroscopy (Supp. Figure S9). Here, the reference for each measurement is the total transmission through the bare substrate of horizontally or vertically linear co-polarized light, depending on the chosen input polarization state. Hence, the cross-polarization conversion efficiency is determined by choosing the linear polarizers in a perpendicular arrangement while measuring the total transmission through the metasurface with regard to the reference. We obtain an efficiency in the imaging channel backward propagation channel

,

,

of 3.8 % at the design wavelength (1150 nm). In the

, the efficiency drops to 0.6 %. The same effect is obtained in

the perpendicular cross-polarization channels

,

and

,

. Thereby, the hologram is

reconstructed with a polarization-conversion efficiency of 3.3 %, while in backward propagation the image is dampened to 0.4 %. Furthermore, from the transmission spectra one observes an increasing cross-polarization efficiency for longer wavelength. Thus, we measured the metasurface functionality 100 nm below and above the design wavelength (see Supp. Figure S11). For a wavelength of 1250 nm, the efficiency increases to 6.6 % for forward propagation imaging, while the dampened channel is reduced to 0.8 %. The higher conversion efficiency and better contrast at


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longer wavelengths is mainly due to the fabrication process and deviation of the material. Mostly, the lithographically fabricated structures are slightly larger than in the design. Therefore, the overall structure resonance shifts to longer wavelengths. In contrast to the simulations, our experimentally realized asymmetric layered metasurface show significantly lower crosspolarization conversion efficiencies in both forward and backward propagation direction. This can be addressed to the deviations in the nanofabrication processes. As shown in Figures 2 (d) to (f), the fabricated structures are not perfectly shaped and also the surfaces of the antennas and the spacer layer are not perfectly smooth. Thereby, unwanted scattering effects can lower the overall efficency of the layered metasurface structure. Nevertheless, the damping factor that influences the image contrast compared to the blocked imaged reaches up to 13.4.

4. Conclusion In conclusion, we designed a polarization encrypted non-reciprocal layered metasurface hologram with full phase and amplitude control in the near-infrared. The meta-structure is fabricated in a three-step e-beam lithography process. The design based on plasmonic L-shaped antennas combined with plasmonic dimers operates in the linear cross-polarization channels and

. Within these two channels, the bi-layer structure enables tailoring asymmetric

transmission properties with full phase and amplitude modulation. We experimentally show that the reverse propagation direction in both of the two operation channels can be suppressed by a factor of 13.4. The holographic images are measured for different wavelengths around the design wavelength, illustrating the broadband non-reciprocal behavior. We obtained a maximum crosspolarization conversion efficiency of 6.7 % at a wavelength of 1270 nm, while the opposite propagation direction shows only 0.5 % transmission. In contrast to previous works investigating layered metasurfaces with non-reciprocal transmission amplitude, we provide and demonstrate a


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concept for full spatial phase control of the transmitted wavefronts. By designing more diverse geometries to ensure complete amplitude/phase modulation capability for different linear or circular polarization channels (some brief discussion can be found in Supporting Informations), we can achieve various asymmetric information processing, security features for anti-counterfeiting applications and complex wavefront shaping.

Supporting Information: The Supporting Information is available free of charge on the ACS Publishing website at DOI:

Author Information: Corresponding Authors *Y.W.: Email: [email protected] *L.H.: Email: [email protected] *T.Z.: Email: [email protected] ORCID Lingling Huang: 0000-0002-3647-2128 Thomas Zentgraf: 0000-0002-8662-1101 Author Contributions #

D.F. and Q.W. contributed equally to this work.

Acknowledgments: This project has received funding from the Deutsche Forschungsgemeinschaft through the Collaborated Research Center TRR 142 (No. 231447078), the National Key R&D Program of


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China (No. 2017YFB1002900), the NSFC-DFG joint program (DFG No. ZE953/11-1, NSFC No. 61861136010), and the National Natural Science Foundation of China (No. 61775019) program.

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