We present a protocol for fabrication of spin- and direction-multiplexed visible metaholograms, then conduct an optical experiment to verify their function. These metaholograms can easily visualize encoded information, so they can be used for projective volumetric display and information encryption.
The optical holography technique realized by metasurfaces has emerged as a novel approach to projective volumetric display and information encryption display in the form of ultrathin and almost flat optical devices. Compared to the conventional holographic technique with spatial light modulators, the metahologram has numerous advantages such as miniaturization of optical setup, higher image resolution and larger field of visibility for holographic images. Here, a protocol is reported for the fabrication and optical characterization of optical metaholograms that are sensitive to the spin and direction of incident light. The metasurfaces are composed of hydrogenated amorphous silicon (a-Si:H), which has large refractive index and small extinction coefficient in the entire visible range resulting in high transmittance and diffraction efficiency. The device produces different holographic images when the spin or direction of incident light are switched. Therefore, they can encode multiple types of visual information simultaneously. The fabrication protocol consists of film deposition, electron beam writing and subsequent etching. The fabricated device can be characterized using a customized optical setup that consists of a laser, a linear polarizer, a quarter waveplate, a lens and a charge-coupled device (CCD).
Optical metasurfaces composed of sub-wavelength nanostructures have enabled many interesting optical phenomena, including optical cloaking1, negative refraction2, perfect light absorption3, color filtering4, holographic image projection5, and beam manipulation6,7,8. Optical metasurfaces that have appropriately-designed scatterers can modulate the spectrum, wavefront and polarization of light. Early optical metasurfaces were mainly fabricated using noble metals (e.g., Au, Ag) due to their high reflectivity and ease of nanofabrication, but they have high Ohmic losses, so the metasurfaces have low efficiency at short visible wavelengths.
Development of nanofabrication techniques for dielectric materials that have low losses in visible light (e.g., TiO29, GaN10, and a-Si:H11) has enabled realization of highly efficient flat optical devices with optical metasurfaces. These devices have applications in optics and engineering. One intriguing application is optical holography for projective volumetric display and information encryption. Compared to conventional holograms that use spatial light modulators, the metahologram has numerous advantages such as miniaturization of optical setup, higher resolution of holographic images and larger field of visibility.
Recently, encoding of multiple holographic information in a single-layered metahologram device has been achieved. Examples include metaholograms that are multiplexed in spin12,13, orbital angular momentum14, incident light angle15, and direction16. These efforts have overcome the critical shortcoming of metaholograms, which is a lack of design freedom in a single device. Most conventional metaholograms could only produce single encoded holographic images, but multiplexed device can encode multiple holographic images in real time. Hence, the multiplexed metahologram is a crucial solution platform towards real holographic video display or multifunctional anticounterfeiting holograms.
Reported here are protocols to fabricate spin- and direction-multiplexed all-dielectric visible metaholograms, then to optically characterize them13,16. To encode multiple visual information in a single metasurface device, metaholograms are designed which show two different holographic images when the spin or direction of incident light are changed. To fabricate highly efficient holographic images in a manner comparable with CMOS technology, a-Si:H is used for the metasurfaces and dual magnetic resonances and antiferromagnetic resonances induced inside them are exploited. The fabrication protocol consists of film deposition, electron beam writing, and etching. The fabricated device is characterized using a customized optical setup composed of a laser, a linear polarizer, a quarter waveplate, a lens and a charge-coupled device (CCD).
1. Device fabrication
NOTE: Figure 1 shows the fabrication process of a-Si:H metasurfaces17.
2. Scanning electron microscope characterization
3. Optical characterization of the spin-multiplexed metahologram
4. Optical characterization of the direction-multiplexed metahologram
The a-Si:H metasurfaces enable high cross-polarization efficiency and can be fabricated using a method (Figure 1) that is compatible with CMOS; this trait may enable scalable fabrication and near-future commercialization. The SEM image shows the fabricated a-Si:H metasurfaces (Figure 2). Furthermore, a-Si:H has a larger refractive index than TiO2 and GaN, so even with low aspect ratio nanostructure of around 4.7, an a-SiH meta-hologram with high diffraction efficiency can be realized. The calculated efficiency at 633 nm wavelength was 74% and the measured efficiency was 61%.
A spin-multiplexed metahologram can switch the projected holographic images by simply flipping the handedness of the incident circularly polarized light (Figure 3a). To design such a spin-multiplexed metahologram, two kinds of metasurfaces were used; they can produce different responses depending on whether the light is circularly polarized to the left or to the right. The Gerchberg-Saxton algorithm was used to calculate a phase map that corresponds to the off-axis holographic images. As a result, depending on the input beam polarization states, ‘ITU’ and ‘RHO’ holographic images (Figure 3c−e) can be switched in real-time with high fidelity.
A direction-multiplexed metahologram can switch the projected holographic images by changing the incident light direction (Figure 4a). For instance, if the light comes in from the substrate side (forward direction), the holographic ‘RHO’ images can be observed (Figure 4b,d), and if the light comes from the metasurface side (backward direction), the holographic ‘ITU’ images can be seen (Figure 4c,e). The hologram device that operates in both directions has the advantages of extending the area in which information can be transmitted, and of transmitting and receiving different visual information according to the position of the observer.
Figure 1: Flow chart of a-Si:H metasurface fabrication. The fabrication starts with a double-side-polished fused silica substrate. Using PECVD, 380 nm thick a-Si:H is deposited and followed by spin-coating of the e-beam resist, PMMA A2. Electron beam lithography (EBL) scanning draws nanorod patterns on the resist, which are transferred onto the a-Si:H layer by the Cr lift-off process. A dry etching process transfers the Cr pattern onto the a-Si:H layer, then the Cr etch mask is removed using a Cr etchant. Please click here to view a larger version of this figure.
Figure 2: The SEM image of the fabricated device. A tilted view of the SEM image of 380 nm thick a-Si:H metasurfaces is presented. During the etching process, a slanted side-wall profile occurred. Please click here to view a larger version of this figure.
Figure 3: A spin-multiplexed metahologram. (a) Schematic of operation of the proposed spin-multiplexed metahologram. (b) Optical microscope and SEM images. The total size of the fabricated metahologram device is 400 µm x 400 µm. A single nanorod has a length of 200 nm, a width of 80 nm, and a height of 380 nm. (c) Experimentally obtained ‘ITU’ holographic images with the left circular polarization working at a wavelength of 633 nm. (d) Experimentally obtained ‘RHO’ holographic images with the right circular polarization captured with a CCD camera. (e) Experimentally obtained both holographic images using the elliptically polarized light. This figure was modified from Ansari et al.13. Please click here to view a larger version of this figure.
Figure 4: A direction-multiplexed metahologram. (a) Schematic of operation of the proposed direction-multiplexed metahologram. (b,c) Fresnel-type metahologram finite-different time-domain simulation results. A left circular polarized light illuminated in forward and backward directions. (d,e) Experimentally obtained holographic images captured with a CCD camera. This figure was modified from Ansari et al.16. Please click here to view a larger version of this figure.
The a-Si:H metasurfaces were fabricated in three major steps: a-Si:H thin film deposition using PECVD, precise EBL, and dry etching. Among these steps, the EBL writing process is the most important. First, the pattern density on metasurfaces is quite high, so the process requires precise control over the electron dose (energy) and scanning parameters such as number of dots per unit area. The development condition should also be chosen carefully. The density of the pattern is very high, so when the development process is done instantaneously, the nanorod-shaped patterns are not defined well, but are connected to each other. To prevent this problem and to provide an appropriate negative slop of photoresist, which enables easy lift-off, a cold-development technique was used in which the development process is conducted at 2−4 °C. Furthermore, a bi-layer resist method can be used for easy lift-off process, where two different kinds of resists having different molecular weights and solubility in a development solution are used. Additionally, the side wall profile during the etching process should be made as close to 90° as possible by adjusting the etching process.
SEM and optical characterization of the fabricated metasurfaces should be rigorously conducted. By observing SEM images of the fabricated structures, exact geometric parameters and side-wall profile should be checked to predict the efficiency of metahologram. For the optical experiment, to produce and obtain high quality holographic images, the incident laser beam shape and focusing should be adjusted accurately. Hence, the optical component should be well aligned with each other and properly positioned according to the component specifications such as focal length of lens and angle of polarizer and waveplate.
In this work, we presented a detailed fabrication and characterization method for spin- and direction-multiplexed metaholograms. Increasing the number of functionality on single-layer metasurface is a useful technique for expanding the applications of metasurface. At the same time, however, active functions that can change diverse functions imposed in real time should also be studied. In this experiment, passive methods, such as changing the polarizer angle or optical components, were used to switch holographic images. However, if active material systems such as phase change materials or liquid crystals are combined with the multifunctional metahologram, the holographic video display and anticounterfeiting display technology with metahologram can be commercialized in the near future18. Furthermore, advanced nanoimprinting method will be of great help for scalable manufacturing of metahologram devices.19 Also, new design methodology, such as wavelength-decoupled metasurface design methodology, will enable full-color hologram devices.20
The authors have nothing to disclose.
This work was financially supported by the National Research Foundation (NRF) grants (NRF-2019R1A2C3003129, CAMM-2019M3A6B3030637, NRF-2019R1A5A8080290) funded by the Ministry of Science and ICT of the Korean government. I.K. acknowledges the NRF Global Ph.D. fellowship (NRF-2016H1A2A1906519) funded by the Ministry of Education of the Korean government.
Aceton | J.T. Baker | 925402 | |
Beam splitter | Thorlabs | CCM1-BS013/M | |
Chromium etchant | KMG | Cr-7 | |
Chromium evaporation source | Kurt J. Lesker | EVMCR35D | |
Clamp | Thorlabs | CP175 | |
Conducting polymer | Showa denko | E-spacer | |
Diode laser | Thorlabs | CPS635 | |
E-beam evaporation system | Korea Vacuum Tech | KVE-E4000 | |
E-beam resist | Microchem | 495 PMMA A2 | |
Electron beam lithography | Elionix | ELS-7800 | |
Half-wave plate | Thorlabs | AHWP05M-600 | |
Inductively-coupled plasma reactive ion etching | DMS | – | |
Iris | Thorlabs | SM1D12 | |
Isopropyl alcohol | J.T. Baker | 909502 | |
Kinematic mirror mount | Thorlabs | KM100/M | |
Lens | Thorlabs | LB1630 | |
Lens Mount | Thorlabs | LMR2/M | |
Linear polarizer | Thorlabs | GTH5-A | |
Mirror | Thorlabs | PF10-03-G01 | |
Neutral density filter | Thorlabs | NDC-50C-4 | |
Plasma enhanced chemical vapor deposition | BMR Technology | HiDep-SC | |
Post | Thorlabs | TR75/M | |
Post holder | Thorlabs | PH75E/M | |
Quarter-wave plate | Thorlabs | AQWP10M-580 | |
Resist developer | Microchem | MIBK:IPA=1:3 | |
Rotational mount | Thorlabs | RSP1/M | |
Scanning electron microscopy | Hitachi | Regulus8100 | |
XY translation mount | Thorlabs | XYF1/M | |
1-inch adapter | Thorlabs | AD11F | |
1-inch lens mount | Thorlabs | CP02/M |