Electrically Tunable Transparent Displays for Visible Light Based on

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Electrically Tunable Transparent Displays for Visible Light Based on Dielectric Metasurfaces Chengjun Zou, Andrei Komar, Stefan Fasold, Justus Bohn, Alexander A Muravsky, Anatoli A Murauski, Thomas Pertsch, Dragomir N. Neshev, and Isabelle Staude ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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Electrically Tunable Transparent Displays for Visible Light Based on Dielectric Metasurfaces Chengjun Zou,∗,† Andrei Komar,‡ Stefan Fasold,† Justus Bohn,† Alexander A. Muravsky,¶ Anatoli A. Murauski,¶ Thomas Pertsch,† Dragomir N Neshev,‡ and Isabelle Staude∗,† †Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena, 07745 Jena, Germany ‡Nonlinear Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra, 2601 ACT Australia ¶Institute of Chemistry of New Materials, National Academy of Science of Belarus, 220141, Minsk, Belarus E-mail: [email protected]; [email protected]

Abstract Tunable dielectric metasurfaces able to manipulate visible light with high efficiency are promising for applications in displays, reconfigurable optical components, beam steering, and spatial light modulation. Infiltration of dielectric metasurfaces with nematic liquid crystals (LCs) is an attractive tuning approach, which is highly compatible with existing industrial platforms for optical and electronic devices. Here, we demonstrate electrically tunable transparent displays based on nematic LC-infiltrated tunable dielectric metasurfaces at visible frequencies. Importantly, the technique of photoalignment of LCs is adopted to improve the LC prealignment quality and thus the tuning accuracy and contrast in the visible. By applying a voltage across the infiltrated

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metasurface cell, we observe resonance shifts that are more than twice larger than their linewidth. We track the spectral shifts of the electric and magnetic dipole resonances as they move into and out of the so-called Huygens’ regime of high transparency originating from spectrally overlapping electric and magnetic dipole resonances. Furthermore, we realize a switchable metasurface display with a measured modulation depth of 53% at 669 nm operation wavelength for an applied voltage of 20V. The novel LC tuning platform demonstrated in our work may lead to the development of next-generation LC display devices that are able to overcome current limitations of minimal pixel size and speed of operation.

Introduction Liquid crystal (LC) displays and screens are ubiquitous in modern consumer electronics. Conventional LC displays rely on an adiabatic polarization conversion effect, which takes place upon propagation through an LC cell over at least several micrometers. This operation principle and the relatively large required thickness of the LC cells lead to several severe limitations associated with conventional LC displays, namely slow response times, large minimum pixel size, and difficulties to obtain a deep black color impression. Therefore, the exploration of new operation principles for LC displays is highly desirable. All-dielectric optical metasurfaces may offer a new avenue to overcome these limitations. During the last few years, dielectric metasurfaces were established as an efficient platform for the manipulation of light fields. 1,2 Owing to the low absorption losses of suitable dielectrics in the visible and near-infrared spectral range, dielectric metasurfaces allow for the realization of efficient and flat optical devices including beam deflectors, 3,4 lenses, 5–8 holograms, 9–11 and polarization converters. 12,13 As such, they offer intriguing opportunities for replacing conventional bulk optical components as well as for implementing new optical functionalities not accessible with conventional optics. To harness dielectric metasurfaces for practical applications that require a dynamic and programmable functionality, such as displays, beam scanners, 2


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adaptive optical systems, or spatial light modulators, a growing amount of research recently focussed on tunable dielectric metasurfaces 14–19 and metadevices. 20–22 Dielectric metasurfaces composed of high refractive index Mie-resonant building blocks are highly suitable for achieving tunable functionalities, due to the strong spectral dispersion associated with the resonant response. Importantly, both electric and magnetic Mie-like resonances can be efficiently excited in high refractive index dielectric nanoresonators in the optical frequency range. By designing the metasurface such that the electric and magnetic dipole resonances appear at the same spectral position, one reaches the so-called Huygens’ regime which is characterized by high transmittance efficiency and a 2π phase variation as the wavelength is swept over the width of the resonances. 23–25 Furthermore, the resonant near-field enhancement of Mie-resonant dielectric metasurfaces can be exploited for enhancing and manipulating light matter interactions, such as nonlinear frequency generation 26 and spontaneous emission. 27,28 Various tuning mechanisms were suggested for Mie-resonant dielectric metasurfaces, including the use of phase-changing materials, 15,29 mechanical stretching, 16,21,30 and nonlinear-optical effects. 17,18,31 Furthermore, a promising approach for tuning the response of Mie-resonant dielectric metasurfaces is their integration into an LC cell. 14,19,32,33 Nematic LCs are particularly attractive for tunable metasurfaces owing to their large birefringence. Both temperature 14,32,33 and applied voltage 19 can be used as external control parameters. As a first metadevice based on this tuning approach, dynamic beam deflectors 22 were recently demonstrated, making use of the phase transition of the LC from nematic to isotropic for heating of the sample over the LC transition temperature. However, for applications requiring spatially variant tuning of the metasurface response, such as displays, applied voltage is the more practical control parameter. It provides control over the LC director orientation, allowing for an on-demand rotation of the refractive index tensor of the LC medium surrounding the metasurface building blocks. This results in pronounced changes of the optical responses of the dielectric metasurfaces. Using this approach, tuning of silicon metasurfaces operating at near-infrared

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frequencies was recently demonstrated by Komar et al. 19 Here we apply this concept to realize a new type of LC display based on spatially variant active control of the transmittance of a large-area dielectric metasurface operating in the visible spectral range. The major challenge in achieving this goal lies in the realization of a large tuning range of transmitted or reflected amplitude in the visible range, where the feature sizes of the metasurface design become small. Crucially, in all demonstrations of LC tuning of Mie-resonant dielectric metasurfaces so far, the alignment layer for defining the LC prealignment direction was coated only onto the surface of the upper cell window. No LC alignment could be applied on the metasurface itself since the commonly used alignment materials such as polyvinyl alcohol (PVA) or Nylon-6 require mechanical brushing, which would damage the metasurface. This limits the accuracy of the LC alignment near the metasurface. Metasurfaces operating in the visible spectral range are especially affected by these alignment inaccuracies, since their resonant evanescent near-fields are more strongly localized at the metasurface due to the shorter wavelength as compared to the near-infrared range. To overcome these issues, here, for the first time to our knowledge, we employ a photoalignment material, which does not rely on mechanical brushing, to control the LC prealignment direction in an LC metasurface cell. Specifically, the azo-dye photoalignment material potassium 3,7-bis[1(4-hydroxy-3-carboxylate)phenylazo]-5,5’-dioxodibenzothiophene (AtA-2) 34 is applied both onto the upper electrode of the LC cell as well as onto the metasurface itself. For application of a voltage on the LC metasurface cell, we observed pronounced spectral shifts of the metasurface resonances, resulting in a large transmittance modulation around 670 nm. Based on the LC-infiltrated dielectric metasurface cell, we realize a switchable transparent display operating at 669 nm wavelength. By using a pre-patterned upper electrode and tuning the applied voltage from 0 V to 20 V, a 53% modulation contrast of a pre-defined display pattern is achieved in transmission. The materials and methods employed for fabricating the suggested LC metasurface displays are well established in conventional LC display and silicon photonics technologies. Altogether, the demonstrated concept is promising for next-

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generation thin transparent displays, imaging and communication systems based on resonant subwavelength dielectric metasurfaces. Most importantly, the tuning concept does not rely on an adiabatic polarization conversion upon propagation through an LC over an extended distance, as the case for conventional LC displays. Instead, the functionality stems from the local interaction of light with the metasurface resonances. Consequently, the LC cells required for such metasurface displays could potentially be much thinner than conventional LC display cells, on the order of few hundreds of nanometers, thus holding the potential for a reduced response time, smaller pixel size, and reduced diffuse scattering as compared to conventional LC displays.

Results and Discussion Metasurface Design and Fabrication. In this work, we use an all-dielectric metasurface composed of silicon nanocylinders integrated into a nematic LC cell. The silicon nanocylinders dimensions are chosen such that their dipolar Mie-type resonances occur in the red part of the visible spectrum after LC infiltration. Large area (5 × 5 mm) fabrication of the metasurfaces is performed using character-projection electron-beam lithography on custom 200 nm Si SiO2

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Figure 1: The fabricated metasurface and transmittance spectra before LC infiltration. (a) Photograph of fabricated sample of seven silicon metasurfaces (dark squares), each with a size of 5 × 5 mm2 . The blue dashed square denotes the ITO electrical contact region (see Methods). The blue solid square highlights the metasurface investigated in this work. (b) Scanning electron micrograph (SEM) image showing a focused-ion-beam cross-section of the fabricated sample. An ITO layer is buried at 1 µm beneath the array of Si disks for electrical contacting. (c) Measured and numerically calculated transmittance spectra for xand y-polarized light impinging at normal incidence. (d) Sketch of the metasurface unit cell.

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amorphous hydrogenated silicon (a-Si:H) wafers in combination with inductively coupled plasma etching. The details of the fabrication are summarized in the Methods section. Figure 1(a) shows the fabricated sample comprising seven metasurfaces. In the following, we concentrate on the metasurface marked by the blue solid square in Figure 1(a). A scanningelectron micrograph (SEM) of a focused-ion-beam cross-section of the metasurface is shown in Fig. 1(b). A 10 nm thick indium-tin-oxide (ITO) layer is embedded at 1 µm beneath the nanocylinder array for electrical contacting. The distance is chosen such that possible damages to the ITO layer during the fabrication process due to over-etching are avoided. Note that this choice of substrate allows for voltage tuning while preserving the transparency of the sample in the visible spectral range, which is in contrast to previous demonstrations where the metasurface samples were fabricated from silicon-on-insulator (SOI) wafers, resulting in a thick, opaque silicon substrate. 19 In order to characterize the optical response of the fabricated sample before LC infiltration, we performed linear optical transmittance measurements of the metasurface using a customized microscope setup coupled to a visible-wavelength spectrometer (iHR320, Horiba Jobin Yvon). The measured transmittance spectra for linearly polarized light impinging at normal incidence are presented in Fig. 1(c). Two resonance dips are observed at around 625 nm and 653 nm, respectively. For the short wavelength dip, we observe a slight difference between the x- and y-polarized transmittance, which is likely caused by a slight structural anisotropy of the fabricated sample. To further understand the origin of the two observed resonances, we perform electromagnetic simulations using the frequency-domain solver of the commercial software package CST Microwave Studio. In these simulations, we employ Floquet port excitation and unitcell boundaries. To optimize the agreement with experimental spectra, the diameter and the height of the nanocylinder, as well as the height of the silica pedestal beneath each nanocylinder, which originates from slight overetching into the substrate, are varied within the fabrication accuracy limits around values taken from SEMs. The periodicity of the

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nanocylinder array, in constrat, is fixed at 380 nm, since it can be very precisely controlled in the experiment. The numerically simulated results for the optimized geometry are also presented in Fig. 1(c) alongside the experimentally measured spectra. The corresponding geometrical parameters are a = 380 nm, h = 98 nm, he = 81 nm, dx = 192 nm, and dx = 197 nm (see Fig. 1(d) for definitions). From the numerically calculated near-fields (see Section S3.1 of Supporting Information), we can further identify the resonances at 625 nm and 653 nm as the electric dipole (ED) and the magnetic dipole (MD) resonances of the nanocylinders, respectively. LC Cell Assembly Using Photoalignment Technique. For LC cell assembly, we sandwich the nematic LC (Licristal E7 from Merck) in between the metasurface sample and a top electrode formed by an ITO covered glass slide. While the general assembly scheme is the same as for previous works, 14,32,33 here, as a crucial advance, we deploy the photoalignment material AtA-2 for improving the LC prealignment quality and thus the metasurface tuning characteristics. During exposure of the photoalignment material, a surface anisotropy is induced leading to the alignment of the LC molecules in a direction defined by the polarization direction. 35 In order to achieve thin photoalignment layers, the AtA-2 is dissolved in dimethylformamide (DMF) and then spin-coated onto both the metasurface sample and the top electrode, resulting in a layer thickness of approximately 15 nm (see Methods for details). The AtA-2 layers are then exposed by linearly polarized blue light (λ = 455 nm). As illustrated in Fig. 2(a), during exposure the intermolecular bonds of azo-dye molecules oriented along the polarization direction of the illuminated light are relaxed and re-formed in the perpendicular direction, thus inducing a preferential orientation of the bonds over time. 36 This results in an in-plane prealignment of the LC molecules in the direction perpendicular to the polarization direction, with strong azimuthal anchoring energy of more than 2 × 10−4 J/m2 . 37 This strong anchoring ability has been experimentally verified to show a long-time stability at 100 ◦C and is important for achieving long-term device operations. 36 The prealignment direction can be readjusted by re-exposure with blue light with rotated

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0.4 0.2

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Figure 2: Principle of photoalignment. (a) Alignment principle of AtA-2. The images illustrate how the chemical bonds between the azo-dye molecules are reoriented upon exposure with polarized blue light. The red spheres denote the AtA-2 molecules, blue ellipses denote the bonds between them. (b,c) Measured and simulated transmittance spectra of the sample before and after spin-coating with a 15 nm AtA-2 layer. polarization vector. Next, in order to examine the influence of the photoalignment layer on the optical responses of the metasurface, we measure and compare the transmittance spectra of the metasurface before and after spin-coating with AtA-2. These results are presented in Fig. 2(b). Exposed AtA-2 has a measured anisotropic refractive index of nk = 2 and n⊥ = 1.5 in the considered wavelength range. Taking the refractive index of AtA-2 into account, we numerically simulate the transmittance spectra for the metasurface with and without a 15 nm layer of AtA-2. The results are presented in Fig. 2(c). The simulated results are in good agreement with our measurements. As seen in both measured and simulated spectra, coating the AtA-2 layer onto the metasurface only induces a small resonance redshift (∼4 nm), which does not impact the metasurface optical performance. Next, LC metasurface cell is assembled according to the schematic shown in Fig. 3(a). A custom-designed metallic sample clip is used to clamp the cell. The prealignment directions on the metasurface and on the upper electrode are chosen to be parallel to each other. To protect the metasurface from scratching, the minimum cell thickness is controlled by lithographycally defining poly8


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Top electrode (ITO coated) LCs ITO

Pre-alignment direction

V AtA-2 aSi:H resonators y

1 m

10 nm SiO2

x

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Figure 3: LC metasurface cell assembly. (a) Schematic of the cross-section of the assembled LC metasurface cell with an applied voltage. (b) Assembled and LC-infiltrated sample cell observed through two parallel linear polarizers. The seven dark squares correspond to individual metasurfaces with different geometrical parameters. The white square highlights the sample studied in this work. methyl-methacrylate (PMMA) pedestals on the top electrode, which are situated next to the metasurface fields in the assembled cell and serve as spacers. We further measure the cell thickness by recording the transmittance spectra through the uninfiltrated cell and fit it with a Fabry-Pérot model. The fitted cell thickness is typically around 5 µm, with slight variations for different cell assemblies. It is worth noting that the optical response of the metasurface itself is not sensitive to the microscale LC cell thickness. Figure 3(b) shows a photograph of the assembled LC metasurface cell observed through two parallel polarizers and back-illuminated with blue light. Visual inspection already shows that the cell is homogeneous and highly transparent within the entire sample area of 20 × 20 mm2 . Note that it is usually not possible to achieve such a high homogeneity by using only a single alignment layer, 19,33 as illustrated in Fig. S1 of the Supporting Information. This improvement already suggests that the use of photoalignment materials indeed brings LC tunable metasurfaces closer to practical applications. We also conducted alignment quality (AQ) measurements (see Section S1 in the Supporting Information for details), achieving a value of AQ = 0.99 in unstructured sample areas and AQ = 0.97 directly at the metasurface field. Finally, it can be seen in Fig. 3(b) that part of the top electrode and part of the bottom substrate are extending outside of the sample clip. In these areas, the ITO coating, which is buried underneath a silica layer and the photoalignment layer everywhere else on the sample, is exposed by suitable masking during application of these layers. This allows for convenient 9


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application of an external voltage to the LC cell. Electrical Tuning of the Dielectric Metasurface. Next, we examine the electric tunability of the assembled metasurface LC cell. To this end, we apply an 1 kHz AC voltage Vrms across the cell. While varying the voltage from 0 to 60V root-mean-square (RMS), we measure the transmittance spectra of the metasurface for normally incident white-light illumination. The incident light is linearly polarized either along or perpendicular to the prealignment direction, as illustrated in Fig. 4(a). The measurement is performed with the same custom-built microscope setup used for obtaining the transmittance spectra shown in Fig. 2(b,c). The measured voltage-dependent spectra for incident light polarized perpendicular (y-direction) and parallel (x-direction) to the LC prealignment direction are presented in Fig. 4(b,c), respectively. A direct comparison of the transmittance at 0V and 60V is also shown. Note that irrespective of the applied voltage and incident polarization the resonance features are red shifted for the LC infiltrated case compared to the uninfiltrated case. For y-polarized incident light, we observe only one major transmittance minimum. As we continuously increase the applied voltage, the spectral position of this transmittance minimum shifts only slightly from 675 nm to 681 nm and is simultaneously getting more pronounced. For x-polarized incident light, we observe two major resonance minima. The resonance minimum at lower wavelength experiences a significant resonance shift from 659 nm to 677 nm as the applied voltage is increased from 0 to 60V. The resonance minimum at a higher wavelength of around 690 nm remains approximately stationary, however, the transmittance value at the resonance position gradually increases from 3% (693 nm) to 25% (689 nm) when increasing the applied voltage from 0 to Vrms = 30V. When further increasing the voltage to 60V, the transmittance at the resonance minimum decreases again to 11% (687 nm). In order to compare our experimental findings with theoretical expectations, we performed numerical simulations where we represented the LC as an uniaxial-anisotropic dispersive dielectric material. Importantly, in these simulations, we take the voltage-dependent spatially varying orientation of the LC molecules (i.e. LC director orientation) into account,

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thus providing a more realistic model of the experimental situation than previous works. 19 We first calculate the spatial z-dependence of the LC director for the case that the LC is infiltrated in between two parallel and smooth silica surfaces separated by 5 µm with an RMS voltage ranging from 0 to 60V applied across the cell. Details of the calculation are summarized in the Section S2 of the Supporting Information. These calculations allow us to present numerically calculated spectra as a function of applied voltage instead of just as a function of the LC director tilt angle for direct comparison with experimental data. The knowledge of the LC director distributions allows us to model the LC as a spatially inhomogeneous uniaxial-anisotropic dielectric material whose anisotropy axis orientation depends on the position inside the cell as well as on the applied voltage. The dispersive anisotropic refractive index tensor of the E7 LC mixture is described by a Cauchy model. 38 To simulate the voltage-dependent optical response of the LC cell incorporating the metasurface, we implement the obtained anisotropic and inhomogenous material distribution inside the LC cell in the frequency-domain solver of CST Microwave Studio. In these simulations, we again use unit cell boundaries and Floquet port excitation. Note that the model neglects any influence of the nanoscale metasurface topography on the LC alignment. For further details on the implementation, please refer to Section S3.1 in the Supporting Information. The voltage-dependent simulated transmittance spectra are shown in Fig. 4(d,e). A direct comparison for zero and the maximum applied voltage is also presented. A good overall qualitative agreement with experimental results is obtained. For illumination with y-polarized light (polarization perpendicular to the LC prealignment direction), the transmittance at the resonance minimum drops from 61% to 3% as the applied voltage is increased. Simultaneously, the spectral position of the transmittance minimum shifts from 672 nm to 682 nm wavelength. By examining the simulated near-field characteristics (see the Supporting Movie), we can now also identify the origin of the observed behaviour. For Vrms = 0V, the ED and MD resonances are spectrally very close to overlap around 672 nm wavelength, thus partially entering the Huygens’ regime of high transmission. 23 However, as the voltage

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Figure 4: Electrical tuning of the LC metasurface cell. (a) Schematic illustration of the different sample and illumination configurations considered. The LC prealignment direction is chosen to coincide with the x-direction of our coordinate system. (b, c) Measured and (d, e) simulated normal-incidence transmittance spectra for a variation of the applied voltage from 0 to 60V for the incident light polarized (b, d) perpendicular and (c, e) parallel to the LC prealignment direction. The black dashed lines indicates the wavelength used for the demonstration of spatially inhomogeneous switching, see Fig. 5.

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increases, the MD resonance shows a clear red shift while the ED remains almost stationary, such that the resonances are moved out of spectral overlap leading to the observed decrease of the transmission. Note that the resonances are nevertheless still spectrally close to each other, so that only a single resonance feature is observed even for the highest applied voltage of 60V. The stationary behavior of the ED resonance can be explained by its near-field characteristics. Specifically, the electric fields of the ED are mainly oriented along the y-direction, perpendicular to the rotation axis of LC molecules in the x-z plane. Therefore, the ED resonance does not experience a significant change of the embedding environment. The electric field of the MD resonance, in contrast, has a strong Ez component (see Supporting Movie). It therefore experiences an effective increase of the refractive index, leading to a redshift of the resonance position. For the x-polarized incident light, when Vrms = 0V, we find two pronounced transmittance minima at 681 nm and 704 nm. Based on an inspection of the near-field properties, we can identify them as the MD and ED resonances (see Section S3.2 of the Supporting Information) of the nanocylinders, respectively. They shift in opposite spectral direction as the voltage increases, reaching the Huygens’ regime of overlapping resonances at Vrms = 50V (at 693 nm) with fairly high transmitted power of 60%. As we further increase the applied voltage, the ED and MD move out of the Huygens’ regime, resulting again in a decreased transmission. The opposite shifts of the two resonances are expected, since the electric fields of the ED (MD) resonance experience a decrease (increase) in the effective refractive index of their surrounding environment. In Fig. 4(d,e), black and white arrows indicate the respective directions of the resonance shifts. Furthermore, in accord with experimental findings, a third resonance minimum is observed for x-polarized incident light at Vrms = 0V. As the applied voltage is gradually increased to 60V, it strongly shifts from 651 nm to 672 nm while simultaneously getting more pronounced. By examining its calculated near-field distribution, we can identify this resonance as an electric dipole resonance with its dipole moment pointing out of the x − y plane (see Section S3.3 in the Supporting Information). While such so-called dark modes cannot usually couple to the far-field for nor-

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mal incidence due to the emission characteristics of a dipole (no emission along the dipole axis), here a symmetry break is introduced through the asymmetric refractive index tensor of the LC. Therefore, this otherwise dark mode can be excited in the metasurface as it is integrated in the LC cell. The redshift of this resonance originates from the increase of the refractive index component in z-direction upon rotation of the LC molecules. For y-polarized incident light, two narrow resonance features are also observed at the short-wavelength side of the spectrum at no applied voltage. They correspond to a dark mode of the metasurface (shorter-wavelength feature) and to a mode supported by the LC slab (longer-wavelength feature). Altogether, all the resonance features and the shifting trends observed in the experimental results are well reproduced in the simulated voltage-dependent transmittance spectra. However, quantitative deviations between the simulated and the measured results can be noted. They are partly due to sample imperfections and partly originate from the approximations of the model. Most importantly, the calculations of the LC director distributions are based on a simplified scenario, which does not take the topography of the metasurface into account. The metasurface topography can lead to an out of x-z plane LC director distribution near the nanocylinders, which results in a reduced birefringence in the x- and y-direction compared to the ideal scenario (i.e. all LC director align in the x-z plane) in our simulation. This can explain the reduced difference between the measured transmittance spectra for the two incident polarizations at 0V compared to the simulated spectra in Fig. 4. Additionally, we expect to see increased scattering losses and spatial inhomogeneities of the resonance shifts. In particular, the relatively low measured transmittance in the Huygens’ regime is likely due to increased light scattering and local deviations from the perfect Huygens’ condition, preventing the metasurface to reach a highly transparent regime. Note that in comparison to previous demonstrations 19 of electrical tuning of silicon metasurfaces at near-infrared wavelengths, the metasurfaces for visible wavelengths have much smaller feature sizes and more strongly localized near-fields, and are thus expected to be more sensitive

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Figure 5: Demonstration of display functionality based on electrical tuning of a silicon metasurface. (a) Measured transmittance difference at applied voltages of 0V and 20V. (b) Real-color images of the metasurface at 0V, 5V, 10V, and 20V recorded in transmission. Note that the entire red area is occupied by the metasurface. The images are optimized for contrast and brightness to match the visual tuning perception observed with the bare eye. to LC misalignment near the sample surface. A Switchable Dielectric Metasurface Display. Based on the results discussed in the previous section, next we demonstrate a switchable transparent display operating for red light. As a simple demonstration of display functionality, we replace the upper electrode of the LC metasurface cell with a patterned electrode forming the letters "FSU-ANU". The electrode is prepared by area-selective etching of the deposited ITO layer. Using this electrode, the LC metasurface cell is assembled along the same procedure as described earlier. Again, the prealignment directions for the patterned electrode and the metasurface are chosen to coincide (x-direction). By changing the applied voltage across the cell, we can tune the power transmitted by the metasurface in the area of the shaped electrodes. From the transmittance measurement, the maximum transmittance modulation is found under x15


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polarized normal incidence for a variation of applied voltage from 0V to 20V at a wavelength of 669 nm. The difference of the transmitted power in the ‘on’ and ‘off’ state is presented in Fig. 5(a). Using a simple custom-built imaging setup (See Section S4 in the Supporting Information for details), we furthermore measure the spatially resolved transmission at 669 nm wavelength for a passband width of 2.3 nm, x-polarized incident light, and a variation of the applied voltage from 0 to 20V in steps of 5V (details of the setup are included in the Supporting Information). The recorded images are shown in Figure 5(b). At 0V, no pattern can be seen. As we increase the voltage, the letters appear and gradually become darker. A maximum modulation contrast of 53% is achieved when increasing the applied voltage from 0V to 20V.

Conclusions In conclusion, we have experimentally realized, for the first time, an electrically controlled, dielectric metasurface LC display for visible frequencies. The device operates at a wavelength of 669 nm and reaches an absolute transmission modulation of 53%. This is achieved by integrating a Mie-resonant silicon metasurface into a nematic LC cell. By coating both the upper electrode and the metasurface with a photoalignment material, we achieve an excellent homogeneity of the LC prealignment over the entire area of 20×20 mm2 including seven 5×5 mm2 metasurfaces. When applying an external AC voltage across the LC cell, we observe significant spectral shifts of the metasurface resonance positions, leading to strong transmission changes around 670 nm wavelength. Furthermore, we performed numerical simulations of the metasurface transmittance, where we take into account the voltage-dependent spatially-varying orientation of the LC anisotropy axis and achieve a good qualitative agreement with experimental results. Importantly, the demonstrated metasurface display employs a different operation principle than conventional LC displays. As a crucial advance, the entire tuning action happens inside and near the very thin (

Electrically Tunable Transparent Displays for Visible Light Based on

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