Integrated metasurface chip for versatile polarization generation

Polarization is one of the key properties for information processing and signal delivery. It plays an important role in our daily life, from basic display products to imaging devices. Manipulating the polarization state therefore becomes an important research theme of optics. Conventional approaches to manipulate polarizations utilize birefringent crystals, in which the phase retardation is gradually accumulated during light propagation; thus the significant thickness and single functionality are the two most obvious weaknesses. Recently, metasurfaces are sub-wavelength artificial structures^1-3, enabling control of the optical properties at a subwavelength spatial resolution⁴. They provide a great flexibility in generating versatile polarization state of light⁵ in a single metasurface chip.

In recent work, we present a metasurface polarization generator (MPG) capable of producing light beams with arbitrary polarizations in a reflection scheme⁶. All produced beams with desired polarizations are from a linearly polarized incidence with a single optically thin chip. Using aluminum (Al) nanoantennas as the building block allows for our proposed MPG working in a wide range of wavelength at visible light⁷.

Figure 1a shows the schematic of our MPG under an x-polarized illumination, which enables generating six polarization states in reflection. The unit elements on the metasurface are Al nanoantennas of equal dimensions (170-nm-length, 50-nm-width and 50-nm-thick with the period of 230 nm × 230 nm) with varying orientations. We fabricated a MPG device consisting of six areas, each made of one type of supercells responsible for generating a light beam with a particular polarizing state into a pre-determined scattering angle. Figure 1b shows the scanning electron microscope (SEM) image of small regions from each area.



Figure 1. (a) Schematic for versatile polarization generation using integrated plasmonic metasurfaces with fixed incident polarization. (b) SEM images from the fabricated metasurface sample. The Al nanoantenna arrays are defined on a 50-nm-thick SiO2 spacer deposited on an Al mirror.

An array of anisotropic elements is used to realize Pancharatnam-Berry (P-B) phase-based metasurfaces in response to circularly polarized (CP) light then produce the anomalous beam with reflection angle θr which is determined by the generalized Snell's law as θr = sin-1(λ /Lx) under a normal illumination⁸. Considering that a left-hand circularly polarized (LCP) light incidence will deflect an anomalous reflected beam with right-hand circularly polarized (RCP) by +θr, while a RCP incidence will lead to a LCP output along -θr direction. By reversing the orientation of the nanoantennas, we will have the inverse response that interferes with the original array. Therefore, an incidence of linear polarization (LP), which can be viewed as a combination of LCP and RCP, will create two circularly polarized reflection beams with opposite handedness in two different directions, ±θr simultaneously, as shown in Fig. 2a. Once LCP and RCP are produced, we can use them to generate LP states by simply superpositioning the LCP and RCP beams through combining two supercells with opposite nanoantenna orientations (see Fig. 2b). The MPG consists of two supercells with a position offset d, providing a phase difference between two generated CPs that is necessary for producing LP scattered wave along a particular direction. For the proof of principle, we merge two supercells with different position offsets to generate six kinds of polarizations all from a single MPG device, including LCP, RCP, LP along horizontal (LP-H) corresponding to x polarization, LP along vertical (LP-V) corresponding to y polarization, LP along +45° (LP+45˚), and LP along -45° (LP-45˚).



Figure 2. Schematic for (a) circular polarization generation via intrinsic property of P-B phase-based metasurfaces and (b) linear polarization generation via superposition of circular polarizations under an x-polarized light illumination. The angle of reflection is predicted as θr = sin-1(λ /Lx), where λ is the incident wavelength and Lx is the length of supercell.

Using the Stokes parameters as the sphere's coordinates, any polarizations can be uniquely described as a point on or within a unit sphere. It is a convenient way to make polarization transformation clear to view. Figure 3 shows the comparison of experimental results (color dots) and theoretical predictions (black dots) at six incident wavelengths. The measured results show a good agreement with the theoretical prediction, while slight inaccuracy comes from defects of the fabricated nanostructures and from the non-ideal components used in the optical setup. These results validate that the proposed MPG is indeed capable of generating multiple polarization waves over the entire visible spectrum.



Figure 3. Poincaré sphere experimental results (color dots) compared with theoretical predictions (black dots) for different incident wavelengths.

In summary, we have proposed and experimentally demonstrated an integrated metasurface chip, which is able to generate versatile polarizations in reflection at visible light. Our future research plan is to develop an integrated polarization generator combining with active materials that is capable of dynamically switching the polarization state in demand. The MPG with versatility and compactness works over a broad visible range paves a way in achieving a complete set of flat optics for far-reaching applications such as integrated optoelectronic circuits, quantum communications and light beam shaping, just name a few.

Pin Chieh Wu, Yao-Wei Huang, Cheng Hung Chu, Din Ping Tsai
Research Center for Applied Sciences
Academia Sinica
Taipei, Taiwan

Wei-Yi Tsai, Wei Ting Chen, Ting-Yu Chen, Jia-Wern Chen, Chun Yen Liao
Department of Physics
National Taiwan University
Taipei, Taiwan

Greg Sun
Department of Engineering
University of Massachusetts Boston
Boston, MA

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