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Pragmatic metasurface hologram at visible wavelength: the balance between diffraction efficiency and fabrication compatibility Gwanho Yoon, Dasol Lee, Ki Tae Nam, and Junsuk Rho ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01044 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017
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Pragmatic metasurface hologram at visible wavelength: the balance between diffraction efficiency and fabrication compatibility Gwanho Yoon1, Dasol Lee1, Ki Tae Nam2 and Junsuk Rho1,3,4* 1
Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang
37673, Republic of Korea 2
Department of Materials Science and Engineering, Seoul National University, Seoul 08826, Republic of Korea
3
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang
37673, Republic of Korea 4
National Institute of Nanomaterials Technology (NINT), Pohang 37673, Republic of Korea
Abstract Metasurfaces have shown many interesting physical phenomena by designing the sub-wavelength antennas and thus controlling the complex amplitude of optical waves. Practicality is one of the biggest challenges of metasurfaces because practical applications have not been realized yet despite of well-demonstrated metasurfaces such as achromatic lenses, holograms and optical cloaks. Early metasurfaces composed of plasmonic resonators have a significant loss of optical power at visible wavelengths. Amorphous silicon which is easy to fabricate can overcome the optical loss only above the wavelength of 600 nm. Use of other dielectric materials such as crystalline silicon or titanium dioxide drastically increases the efficiency of the metasurfaces at whole visible wavelengths, but complex fabrication processes remain an ongoing challenge for practical applications. Here, we exploit polycrystalline silicon to achieve both fabrication compatibility and hologram functionality at the wavelength of 532 nm. Polarization-independent meta hologram is experimentally demonstrated to verify our approach, and our device shows the highest efficiency compared to other reported meta holograms which do not need complicated fabrication processes. We believe that our approach can provide a useful perspective on practicality improvement of metasurfaces.
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Keywords polycrystalline silicon, polarization independent, high-contrast, green laser, transmission
Metasurfaces consist of ultrathin sub-wavelength antennas can be used to control the complex amplitude of an incident optical wave. They have applications in light manipulation; e.g., as beamsteering devices,1-3 ultrathin lenses,4-5 holographic techniques6-11 and invisibility cloaks.12 Plasmonic metasurfaces that are composed of metallic nanorods usually cause considerable loss of optical power,13-15 but the loss can be reduced by replacing metallic components with dielectric materials.16-22 Furthermore, metasurfaces have the possibility of being combined with photonic integrated circuits to develop miniaturized optically-functional modules.23-24 Digital holography is one promising application of metasurfaces. Most conventional digital holograms are demonstrated using spatial light modulators, but they suffer from low resolution, narrow viewing angle, and production of twin images. These demerits are more a result of relatively large pixel size than of the operating wavelength.25 Moreover, such devices can be realized only as the reflection type because of their opaque components. Because the metasurface consists of sub-wavelength scale antennas, the metasurface-based hologram does not suffer from these aforementioned demerits. However, practical applications of conventional metasurface holograms are constrained by a dilemma between diffraction efficiency and fabrication compatibility. Amorphous silicon (a-Si) is widely used for metasurface holograms that use visible and near infrared (NIR) wavelength λ.17 a-Si thin film can be deposited easily by plasma-enhanced chemical vapor deposition on any kind of substrate; however, the operating wavelength of a-Si is limited to λ > 600 nm due to a high absorptivity at λ < 600 nm.11 This limitation means that metasurfaces made of a-Si are not appropriate for operation at visible λ = 532 nm, because optical power loss is severe. Achieving metasurface functionality in visible wavelengths is important for practical applications, e.g. holography, because visible wavelengths region is a unique range we can observe with our naked eyes.
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Other dielectric materials such as crystalline silicon (c-Si) and titanium dioxide (TiO2) can solve this problem because they are seldom absorptive at visible wavelengths. Metasurfaces based on these materials can overcome the optical loss over the entire range of visible wavelengths, but deposition of these materials requires additional complicated fabrication steps such as atomic layer deposition or wafer bonding.18-19 High diffraction efficiency and fabrication compatibility cannot be achieved simultaneously. Therefore, to achieve practicality of metasurfaces at visible wavelengths, a trade-off between diffraction efficiency and fabrication compatibility must be optimized. Here, we demonstrate a high-contrast metasurface hologram that is based on polycrystalline silicon (poly-Si) as an example of the balanced trade-off. The hologram operates at λ = 532 nm. The process combines use of poly-Si with appropriate metasurface design that considers its optical properties. The complex refractive index of poly-Si at λ = 532 nm is ~4.5 + 0.2i;16 the imaginary component is higher than in c-Si and lower than in a-Si.22 Although poly-Si has a lower diffraction efficiency than c-Si, poly-Si has the great advantage that it can be easily deposited as thin films by standard low-pressure chemical vapor deposition. The demonstrated metasurface is also compatible with complementary metal-oxide semiconductors (CMOSs) because poly-Si is widely used in them. To fully exploit the optical properties of poly-Si, Fourier holography and Dammann grating method are used to design the metasurface hologram. As a result, our metasurface can generate a high-contrast holographic image that can be observed without lenses or charge-coupled devices (CCDs). We use Lumerical FDTD commercial software to derive structural parameters of a metasurface that can enable 2π phase modulation. The unit cell contains a circular post; its diameter differed among designs. Metasurfaces composed of these circular post structures can work for any incident polarization.26 To optimize the device functionality, height of the posts is set to 270 nm, and two sweep parameters are selected: (1) distance (“pitch”, 160 - 250 nm) between adjacent unit structures; (2) ratio (0.3 to 0.8) of post diameter to pitch (Figure 1). The phase modulation effect caused by circular posts is based on a strong resonance, so transmission amplitude is also strongly affected by the absorptivity of the material. Near the region in which the phase varies rapidly, the relative
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amplitude drops drastically to < 0.25. Compared to c-Si,22 the diffraction efficiency of poly-Si is low, but still enough for generation of high-contrast holographic images. Furthermore, poly-Si is compatible with CMOS fabrication processes. Other materials such as c-Si and TiO2 are seldom compatible with CMOS fabrication due to their complicated fabrication processes. Therefore, poly-Si, as one of the widely used materials in CMOS devices, can be promising material for further applications. Based on the FDTD simulation result, and considering diffraction efficiency and fabrication compatibility, we determine optimal structure designs. Six unit structures that have different diameters with constant phase variation from 0 to 2π are selected (Figure 2). Diffraction efficiency and image fidelity improve as the number of phase-variation steps increases, but the improvement is asymptotic;27 after six phase steps, the increase in diffraction efficiency become negligible, so this number of phase variations is enough. We also consider the feasibility of fabricating the metasurface. Electron beam lithography (EBL) is used for patterning. Theoretically, EBL can achieve beam diameter < 10 nm, but the resolution is strongly affected by other factors such as the type of resist, the developing solution and the mask material, so the final resolution that can be achieved easily is a few tens of nanometers. Therefore, choice of structural parameters must consider the fabrication capability of the EBL system. Computer-generated phase-only Fourier holography with 2 × 2 Dammann grating method is used to generate high-contrast holographic images.28-29 A conventional Fourier hologram is encoded from objective light that is already a Fourier-transformed image. Therefore, to produce original images, the outgoing optical wave from the hologram should be Fourier-transformed again. A convenient way to do this is to let the optical wave propagate to an infinitely-distant plane; in theory, this process works the same as a thin lens. The practical position of the infinitely-distant plane is determined by the ratio of the absolute distance to the hologram size, not by the absolute distance itself. Because our hologram area is small enough (150 µm × 150 µm), an image plane located just few centimeters away from the hologram can be regarded as infinitely distant. Phase-only Fourier hologram can be obtained
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by employing typical Gerchberg-Saxton algorithm to a certain image. The algorithm includes many times of iterative Fourier transformation to reduce the difference between the original image and generated holographic image. Since the generated hologram contains nearly continuous phase information, we should convert it into discrete phase steps. We simply round off each angular phase divided by π/3 in order to generate 6 phase steps. After that, every pixel has been expanded to 2 × 2 arrays to exploit Dammann grating method; hence, total pattern size becomes 4 times larger than before. Final metasurface design is obtained by mapping structure parameters according to the generated phase-only hologram which contains discrete phase information. However, the generated holographic image is distorted due to the large diffraction angle of the transmitted light; this problem occurs mainly because paraxial approximation fails. Distortion in the hologram can be corrected in the encoding step by using Rayleigh-Sommerfeld (RS) diffraction theory.30 This method can achieve an undistorted image because RS diffraction theory does not assume the paraxial condition. On the other hand, there is a heuristic way to deal with the distortion more conveniently. The idea starts from the fact that uncompensated image shows pincushion distortion. Therefore, if we encode a barrel-distorted image into hologram, the distortion can be roughly compensated. Any image editing software can be exploited to make the barrel-distorted image, but distortion level should be optimized by trial and error. The holographic image generated by our metasurface starts to be distinguishable from just few centimeters behind the hologram, and due to the characteristic of the Fourier holography, the image increases in size with increase in the distance from the hologram. Therefore, we can clearly observe the high-contrast holographic image without using any optical components such as lenses or CCDs (Figure 3). In the optical setup for our metasurface, optical components including neutral density filter, iris and lens are used to shape the incident light into the ideally circular beam (Figure 4). A linear polarizer and a half-wave plate can rotate the polarization direction to prove the polarization-independent property of our metasurface. To calculate the diffraction efficiency of the metasurface, we assume our hologram is composed of almost same number of each unit structure, so theoretical transmission power ratio RTH,TRANS ~31 % is derived from simple arithmetic mean of transmission power ratio of
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each unit structure (Figure 2). Since the measured total transmission power ITRANS reaches ~84 % of total incident power ITOT, we can conclude that around ITRANS/ITOT – RTH,TRANS ≈ 53 % of incident light pass through the metasurface without interacting with the structure. Each structure generates different transmission amplitude, so some amounts of transmitted light do not contribute to image generation, i.e. they make zeroth order spot in the center of the image plane. We have calculated the theoretical power portion RTH,IMG of the transmitted light which forms the desired image versus total transmission. We found ~55 % of transmitted light generates the desired image and the rest of ~45 % forms the zeroth order spot due to non-uniform transmitted power by each unit structure. As a result, we can roughly expect that theoretical imaging efficiency which is normalized to total incident power ηTH,DIFF equals to RTH,TRANS × RTH,IMG ≈ 17 %. In the measurement setup, we block the zeroth order diffraction to measure the optical power IIMG of the image. Measured diffraction efficiency ηEX,DIFF = IIMG/ITOT is ~6 %, and does not change with the polarization of the incident light. Despite of the discrepancy between theoretical and experimental efficiency, our hologram still shows the highest experimental efficiency compared to other reported metasurface holograms without complicated fabrication processes (Table 1). By optimizing the fabrication processes such as poly-Si growth and etching, the experimental efficiency our metasurface can be further improved up to the theoretical value of 17 %. In summary, we demonstrate a polarization-independent high-contrast metasurface hologram that operates at λ = 532 nm, and balance the trade-off between diffraction efficiency and fabrication compatibility. To improve image fidelity, we use computer-generated Fourier holography as well as Dammann grating; the result is a clearly-discriminated holographic image. We also use poly-Si as the structuring material to increase the fabrication compatibility of the metasurface because poly-Si is compatible with the processes used to fabricate CMOS devices. Although lower than those of c-Si or TiO2, the diffraction efficiency of poly-Si is sufficient for this work. Our method to design the hologram, and to optimize the trade-off between efficiency of diffraction and the compatibility of fabrication can provide a way to extend further practical applications of metasurfaces; examples include CMOS-compatible flat optical components, full-color display techniques, and security devices.
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Methods Fabrication of the metasurface A 270-nm-thick poly-Si film is deposited on a fused silica substrate by low-pressure chemical vapor deposition (Eugene Technology, BJM-100) at 700 °C. Then conventional EBL (Elionix, ELS-7800) and lift-off process are used to pattern a Cr hard mask of the metasurface. Inductively-coupled plasma-based reactive ion etching (ULVAC, NE-7800) is used to etch the poly-Si layer along the patterned Cr mask, then the mask is removed using Cr etchant (KMG, CR-7) for 2 min.
Author Information Corresponding Author *E-mail:
[email protected] Notes The authors have no competing financial interest.
Acknowledgements This work is financially supported by the LGD-SNU Incubation program funded by LG Display and the National Research Foundation grants (NRF-2015R1A5A1037668) funded by the Ministry of Science, ICT and Future Planning (MSIP) of the Korean government.
References
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Figure 1. (a) Schematic of our metasurface hologram operating at the wavelength of 532 nm. (b) Structural parameters of poly-Si circular posts. Calculated (c) transmission amplitude and (d) phase modulation according to structural parameters. Vertical axis: pitch among unit structures; horizontal axis: ratio of post diameter to pitch. Figure 2. Transmission amplitude and phase modulation vs. diameter of posts at pitch = 190 nm. Post diameters of 66, 83, 88, 93, 98 and 111 nm, are selected to achieve 6 step constant phase gap between each pair of structures, and each structure generates phase delaying of 11π/6, 9π/6, 7π/6, 5π/6, 3π/6, π/6 rad, respectively. Figure 3. (a) Encoded original binary image (400 × 200 pixels). To maximize contrast, only black and white are used. (b) Simulated holographic image with the calculated data of transmission amplitude and phase in Figure 2. Although transmission amplitude by each unit structure is not uniform, the contrast of simulated image is still high; i.e. uniform amplitude is not a necessary condition for highcontrast hologram. (c) Experimental image that appears on the image plane behind the hologram. A 532-nm laser is focused on the hologram through a conventional convex lens. The image can be easily distinguished from background noise without use of optical devices such as objective lenses or CCDs. (d) Normalized intensity profile of a cross-section of the letter ‘T’. (inset) The white line represents the position of the cross-section in the letter. Figure 4. (a) Optical setup for operation of our metasurface hologram. The device does not need polarizers or wave plates to generate a holographic image. A series of optical components between a light source and the metasurface is used to make a circular beam. (b), (c) SEM images of the fabricated metasurface. The device has no conductive materials, so a few nanometers of platinum (Pt) are coated on the sample to prevent charge accumulation during SEM observation. During the coating process, the sample is contaminated by impurities in the vacuum chamber of the Pt coater.
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Year
References
Material
Wavelength
Efficiency
Polarization
Fabrication
2017
Ours
poly-Si
532 nm
6%
Independent
Easy
2015
Kun Huang et al.31
Cr
532 nm
2%
Independent
Moderate
2016
11
Kun Huang et al.
a-Si
532 nm
