Full-Color Imaging with Broadband Achromatic Metalens in the Visible
A description of the fabrication and use of a broadband achromatic metalens with high focusing efficiency used in transmission for full-color imaging applications in the visible range of the spectrum.
Metasurfaces are capable of tailoring light properties at subwavelength resolution thereby making them useful in many applications including polarization manipulation [1, 2], holographic imaging [3, 4], and tunable optical components . By producing a hyperbolical phase profile, metasurfaces can work as optical lenses (so-called metalenses) - able to converge incident light beams with considerable efficiency [6, 7]. Compared to conventional bulky lenses, which rely on specifically polished surface profiles on transparent optical materials to attain the required gradual phase change, metalenses are capable of focusing incident light using a more compact form factor.
Metalenses consist of arrays of optical nanoantennas arranged on a surface enabling manipulation of the properties of incoming light. Chromatic aberration, an intrinsic effect originating from the specific resonance and limited working bandwidth of each nanoantenna, is one of the most difficult problems in developing full-color optical systems. Despite their advantages, metalenses can still suffer from strong chromatic aberration, especially in the visible spectrum .
In this work, we designed and fabricated a series of GaN-based, integrated-resonant unit elements (IRUEs) to achieve an achromatic metalens operating across the entire visible spectrum in transmission mode . The experimental results show the focal length remained nearly unchanged as the incident wavelength varied from 400 nm to 660 nm, demonstrating elimination of chromatic aberration for a bandwidth around 49% of the central working wavelength. As a proof-of-concept demonstration, full-color imaging was realized using our broadband achromatic metalens imaging across the visible spectrum.
In principle, full-color images without aberration can only be reconstructed through a perfect achromatic lens that is able to converge light at any wavelengths within the working range into the same focal plane. Figure 1(a) shows a schematic diagram for an achromatic metalens working in a transmission scheme, where a clear colorful image can be obtained on the imaging plane. The building blocks of our achromatic metalens are composed of both GaN nanopillar and inverse structures. Scanning electron microscope (SEM) images of these features are shown in Figs. 1(b) and 1(c).
Figure 1. (a) Schematic for the achromatic metalens at visible light. An undistorted image can be formed by using the broadband achromatic metalens at the designed focal plane. Scanning electron microscope (SEM) images at (b) the boundary of GaN nanopillars and inverse structures (top view) and (c) the region of GaN nanopillars (tilted view). The height of GaN nanostructures is fixed at 800 nm. Scale bars: 500 nm.
Figure 2(a) shows the measured cross-sectional intensity profiles for the achromatic metalens with a designed focal length f = 235 μm and numerical aperture (NA) = 0.106. For the wavelength ranging from 400 nm to 660 nm, the achromatic focusing can be observed with the focal points all close to the designed position (white dashed line). To further verify the optical performance of achromatic metalenses fabricated from GaN-based structures using the design principle of IRUEs, three lenses with different NA values were fabricated and studied experimentally. We found that the focal lengths remain unchanged across the entire range of the visible spectrum for all of the designed achromatic metalenses, as shown in Fig. 2(b). Figure 2(c) shows the measured focusing efficiency for the three achromatic metalenses. The efficiency is defined as the ratio of the optical power of the focused circularly polarized beam to the optical power of the incident beam with opposite helicity . The highest efficiency can be up to 67%, while the average efficiency is about 40% over the full working bandwidth.
Figure 2. (a) Experimental light intensity profiles for the achromatic metalens with NA = 0.106 at various incident wavelengths. The white dashed line indicates the position of designed focal plane. Measured (b) focal length and (c) operation efficiency as a function of incident wavelength obtained from three achromatic metalenses with different NA values. Error bars: Standard deviation of measured efficiencies from four different samples.
For an achromatic metalens working in transmission mode, any colorful target can be imaged at the same imaging plane.. Figure 3 shows full-color images formed through our achromatic metalens further presenting its effective imaging performance. After color correction, according to the effecting variety of our achromatic metalens over the visible region, we can obtain almost the same color images as the original pictures as shown in Figs. 3(d), 3(e), and 3(f). All the images shown in Fig. 3 demonstrate the effectiveness of eliminating the chromatic effect by incorporating the IRUEs with a Pancharatnam-Berry phase (PBP) over the entire visible spectrum.
Figure 3. Full-color (a, d) Alcedinidae, (b, e) Erithacus rubecula and (c, f) Eurasian eagle owl images formed by the achromatic metalens. Captured images from achromatic metalens (a-c) before and (d-f) after color correction.
In summary, we have shown a broadband achromatic metalens utilizing a series of GaN-based resonant elements working in the visible region. To our knowledge, this work shows the broadest working bandwidth of achromatic metalenses operating in the visible spectrum in transmission mode. Our future research plan is to develop more practical applications using achromatic metalenses, such as a light field camera and developing achromatic metalenses with a working wavelength covering the visible to the near-infrared. 
Pin Chieh Wu, Tzu-Ting Huang, Din Ping Tsai
Research Center for Applied Sciences
Shuming Wang, Zhenlin Wang, Tao Li, Shining Zhu
National Laboratory of Solid State Microstructures
College of Engineering and Applied Sciences
School of Physics
Department of Electrical Engineering
National United University
Yi-Chieh Lai, Mu-Ku Chen, Hsin Yu Kuo, Bo Han Chen, Yu Han Chen
Department of Physics
National Taiwan University
Jung-Hsi Wang, Chieh-Hsiung Kuan
Department of Electrical Engineering and Graduate Institute of Electronics Engineering
National Taiwan University
Department of Electronic Engineering
Chang Gung University
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