Animation for a metasurface hologram was achieved using a cinematographic approach. Time-lapsed images were reconstructed using sequentially arranged metasurface hologram frames. An Au rectangular nanoaperture was adopted as a meta-atom pixel and arrayed to reproduce the phase distribution based on the help of a Pancharatnam–Berry phase. We arrayed 48 hologram frames on a 2-cm2 substrate and measured and assessed the retardation of fabricated meta-atoms to reconstruct the holographic image, successfully demonstrating the movie with a frame rate of 30 frames per second.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Metasurface is a planar branch of metamaterials consisting of metallic or dielectric unit cells (meta-atoms) with a subwavelength dimension. It can change the amplitude, polarization, and/or phase of an incident electromagnetic wave. Moreover, it can be used to modify and tailor optical wavefronts into designed distribution , for example, the parabolic phase for focusing as a lens or complex diffraction pattern for metasurface holography. To date, various applications have been studied, including very thin metalenses [2–4], waveplates [5–7], and polarization converters .
The phase delay mechanisms of meta-atoms can be categorized into metallic and dielectric from the viewpoint of the material. Metallic metasurfaces have sufficient phase shift, reaching up to π rad, based on plasmon resonance with a very thin antenna pattern. However, their efficiency is limited  due to the intrinsic ohmic loss. The dielectric meta-atoms have superior efficiency owing to their lossless nature, and fabrication difficulty arises when achieving a relatively high aspect ratio.
Holography is a promising metasurface application, especially in the visible wavelength range. It can minimize visual fatigue compared with other three-dimensional (3D) imaging methods using the stereoscopic approach, for example, virtual reality glasses and 3D cinema [10–12]. Metasurface holograms consisting of metallic or dielectric meta-atom arrangements are actively studied owing to their compactness and wide viewing angle [13,14] when reconstructing a holographic image. The viewing angle $\phi $ of a hologram is defined as:15], and when the period is smaller than the wavelength, the viewable range covers the whole hemisphere. Therefore, the subwavelength nature of metasurface is an advantage for holographic applications.
The holographic movie is one of the most expected frontiers. Several methods have been proposed to achieve a holographic movie based on spatial light modulator devices, for example, the acousto-optic modulator, liquid crystal display, liquid crystal on silicon, digital mirror device, and photoreactive materials . However, the balance among the pixel period, the modulation depth of the phase or intensity, and the achievable frame rate is the key issue. Regarding the holographic movie, one of the expected solutions is the use of reconfigurable metasurfaces. Moreover, several methods have been reported, including polarization switching, chemical activation of meta-atoms, and strain multiplexing by stretchable substrates. In polarization switching, polarization-sensitive metasurfaces are used, and two holographic image frames can be switched by selecting one of the illuminating orthogonal polarization pairs (vertical/horizontal linear polarizations or right/left circular polarizations) [17–20]. Chemical activation uses a reversible shift of plasmonic responses of pixel material involved in the molecular phase change between the metallic and dielectric state caused by gas exposure. This method enables the on/off control of the reconstructed image, although the response time is limited regarding the frame rate . In strain multiplexing, holograms fabricated on a stretchable substrate are used, and three hologram frames can be selectively reconstructed with substrate strains of 0%, 12%, and 30% . Thus, as the number of reconstructable frames is limited, metasurface holographic movies consisting of more than 4 frames have not yet been reported.
Another advantage of metasurface holography is the compactness of the hologram size, which is enabled by the smallness of meta-atoms at the given resolution. This enables both wide projection width and high resolution holography because the sampling size Ls and sampling pitch s of the Fourier plane are expressed by Ls = λf/p, and s = λf/D, respectively [23,24]. Here f is the focal length of the Fourier transform lens, and D = np is the whole size of the hologram, where n is the pixel number. This compactness also allows sequential playback with both substantial image resolution and frame rate. Therefore, in this research, we have adopted an alternative, cinematographic approach toward a holographic movie. The cinematograph is the first instrument used to record and project a movie  and to play back the image frames exposed on a film sequentially. Although this approach have also been studied using holographic film [26,27], image quality of this method is limited because subwavelength pixel pitch cannot be achieved due to optical exposure. Therefore, in this research, we have adopted cinematographic approach based on metasurface holography. A set of 48 metasurface hologram frames are fabricated on a 2-cm2 substrate, and the movie was played back by sequentially reconstructing each frame with a substantial frame rate of 30 frames per second (fps).
Figures 1(a) and (b) present the schematic illustration of a metasurface holographic movie, which is inspired by the sequential playback of cinematography. A He–Ne laser with a 633-nm wavelength was used as the light source. The 48 frames of a metasurface hologram were arranged on a substrate to form a sequential loop actuated by an external two-axis stage. Each frame consists of rectangular metallic aperture meta-atoms. The wide viewing angle, high resolution, and fast frame rate are essential requirements for a holographic movie. We adopted the pixel period p of 300 nm to achieve whole hemisphere coverage at 633-nm illumination. The resolution of each frame was determined as 2048 × 2048 pixels to achieve high image quality beyond the 1080 p full HD (1920 × 1080 pixels) and the applicability to the fast Fourier transform. Therefore, the size of each frame is 0.6 × 0.6 mm2. Because we used a Gaussian beam He–Ne laser with a full-width half-maximum beam diameter of 0.6 mm, the frame period on the glass substrate was calculated as 1 mm. Thus, the stage actuation speed of 30 mm/s is required to achieve 30 fps.
We selected 48 images of the earth from different angles to express the holographic movie. All images were captured from a public domain video, which was downloaded from a website . These images were converted into the phase distribution using a free software library of iterative Fourier transform algorithm (IFTA) (WFL ver. 3.5.0), which was developed by Matsushima . We assumed the homogeneous intensity distribution of planar incident light and used a random phase distribution as the initial phase input.
An Au rectangular nanoaperture meta-atom with the length, width, and thickness of 200, 100, and 120 nm, respectively, was adopted (Fig. 1(c)) . This meta-atom should produce a π phase shift between linear polarizations parallel and perpendicular to the length direction and work as a half-wave plate (HWP). To achieve full 0–2π phase coverage, we adopted the Pancharatnam–Berry phase (P-B phase) or geometric phase . The output Jones vector Eout from an incident left circular polarization (LCP) Ein = (1, i) through an HWP with the rotation angle θ to the x-axis is expressed as:
2.2 Fabrication and characterization
Figure 2 presents a schematic of the fabrication process. At first, Cr, Au, and Si layers were deposited on a glass substrate by using a facing target sputtering apparatus (FTS Co., NFTS-3S-R0601). The Cr, Au, Si, and glass substrates were 2-nm, 120-nm, 20-nm, and 500-µm thick, respectively. The hologram pattern was then drawn by electron beam (EB) lithography (JEOL Co., JBX-6300SL). The drawing time of the sample (including 48 hologram frames, aperture array patterns, and global alignment marks) was 6.5 hours. We used a positive e-beam resist (ZEON Co., ZEP520A-7) with a thickness of 300 nm and performed oxygen plasma ashing after resist development for descam. Then, the resist pattern was transferred to the Si layer by using SF6 reactive ion etching (RIE), followed by Au layer milling by Ar plasma with an RIE apparatus (Tateyama Machine Co., TEP-Xd). Finally, the Si mask layer was removed by SF6 RIE again.
Figure 3 presents the SEM images of the fabricated hologram and aperture array patterns. Table 1 presents the average dimensions of these meta-atoms. The height H of the apertures is measured using a surface profiler (Bruker, Dektak XT) after Au deposition. The averaged dimensional error of fabricated apertures to the design was up to 7%.
3. Results and discussion
3.1 Optical characterization
We used the spectroscopic rotating analyzer method (RAM) to characterize the optical performances of fabricated metasurface . Figure 4(a) presents the measurement setup constructed on polarization microscopy. The aperture array pattern sample was set on the stage of microscopy, of which the fast axis was fixed at an orientation of φ = 45° at each pattern by rotating the sample substrate. The sample was sandwiched between the polarizer and analyzer at orientations of 90° and ψ (0°–180°), respectively. The light transmitted through the sample was expanded and guided to the USB spectrometer (QE65000, Ocean Optics) through an objective lens (TU Plan Fluor 10×/0.3A Pol) and an optical fiber (P50-2-IV-VIS) with a 50-µm core diameter.
The Stokes vector S of the transmitted light is expressed by Eq. (4):
The transmittance spectra of the aperture array patterns were measured by rotating the analyzer angle ψ with a glass substrate reference. The retardation Δ is then derived by parameter fitting to the transmittance spectra using Eq. (5).
Figure 4(b) presents the transmittance spectra. Figure 4(c) presents the angular plot of the transmittance at a wavelength of 633 nm. Figure 4(d) presents retardation spectra. Dashed lines in Figs. 4(b) and 4(d) represents electromagnetic simulation results calculated with a commercial finite element method software COMSOL Multiphysics 5.1 (COMSOL Inc., USA), with the measured aperture dimensions summarized in Table 1. Simulation results agree well to the measurements, although some peak shift in transmittance spectra and peak broadening in retardation spectra were observed. These shifts are considered to be due to fabrication error including cross-sectional tapering and connection of adjacent apertures. From Fig. 4(c), we calculated the ellipticities of the transmitted light for aperture array patterns of θ = 0°, 45°, 90°, and 135° as 0.28, 0.54, 0.36, and 0.55, respectively. The averaged transmittance of aperture array pattern samples is approximately 5%, and the retardation performance peaks were observed at the wavelength range from 630 to 700 nm, which correlates well with the previous research .
3.2 Projection of holographic movie
Figure 5 presents the experimental setup for the holographic movie projection. The fabricated substrate was fixed on a two-axis stage (Chuo Seiki, MMU-60X) and illuminated by left circular-polarized light. A He–Ne laser (λ = 633 nm) was used with a linear polarizer and a quarter-wave plate. A PC can control this stage and can be moved in the x-y plane with a maximum speed of 30 mm/s. The beam diameter was adjusted by an iris with a diameter of 1.1 mm to eliminate stray light. Then, each hologram frame is sequentially reconstructed on white cardboard as a screen, and a holographic movie was presented. Figure 6 presents clips (frame numbers of #10, #21, #35, and #45) of a movie frames taken using iPhone 6s Plus. The movie is also available online (see Visualization 1). The image angle of the globe was 62.6°. As the reconstructed image was taken at an angle as shown in Fig. 5, the appears to shrink horizontally.
Considering the maximum stage speed of 30 mm/s and frame pitch of 1 mm, we achieved a frame rate of 30 fps, although some frame lagging can be observed in the corner of the frame arrangement. This may be improved by optimizing the stage load or adopting rotational operation. Each reconstructed frame has enough resolution to recognize the earth (Fig. 6), and the 0th diffraction spot can be observed at the center of the reconstructed images. We can consider this spot as the non-modulated LCP component, whereas diffracted images correspond to the RCP component. Using a laser power meter, the intensities of LCP and RCP components were measured as 0.0748 and 0.2442 mW, respectively. From these values and the tan2Δ/2 tendency as described above, the average retardation Δ of frame #1 was estimated at 122°. Retardations of aperture array patterns were measured in this way and summarized in Table 2 together with the RAM results, and conversion efficiency (defined as RCP/(LCP + RCP)×100% ). The measured retardation and conversion efficiency of the hologram frame were the minimum, respectively. This is considered to be due to the connections between adjacent apertures as shown in Fig. 3, and the efficiency may be improved by optimizing fabrication parameters to avoid tapering and connection.
Contrary to the needs of the synchronization between sliding film frames and a blinking light in the traditional cinematograph, this holographic movie does not require light source blinking. Figure 7 presents the intermediate image when the light shines on the border between frames #3 and #4. Note that the two frames are superimposed and can be observed at the same time. Thanks to this characteristic, the holographic movie can be reconstructed by using a continuous wave light source.
On this occasion we have drawn 48 frames on a 2-cm-square chip. We believe this chip size is enough for a very short and looped movie. Considering the current drawing throughput of 6.5 hours, if a whole 4-inch wafer was used, approximately 10,000 frames (corresponding to a movie-playing time of 333 seconds with 30 fps) can be drawn within 833 hours. This throughput might be improved by using character-projection type EB lithography apparatus or stepper photolithography.
We have demonstrated a metasurface holographic movie based on a cinematographic approach. We used a metallic rectangular aperture meta-atom with an average retardation of 122° to express 2048 × 2048 pixels hologram frames, and a movie consisting of 48 frames was successfully reconstructed with a frame rate of 30 fps.
Japan Society for the Promotion of Science (17H02754).
The authors wish to thank Prof. K. Matsushima at Kansai University for the development of free software library for IFTA (WFL ver. 3.5.0).
The authors declare no conflicts of interest.
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