Polarization is a fundamental property of light that can carry and probe information with a wide range of applications including imaging,1
Advanced Photonics Nexus, Volume. 3, Issue 6, 066002(2024)
Uniformly polarized multi-output illumination by metasurfaces performing near-complete conversion of unpolarized light
Many technologies, including dot projectors and lidar systems, benefit greatly from using polarized illumination. However, conventional polarizers and polarizing beam splitters have a fundamental limit of 50% efficiency when converting unpolarized light into one specific polarization. Here, we overcome this restriction and achieve near-complete conversion of unpolarized light to a spatially uniform polarization state over several output directions with our topology-optimized metasurfaces. Our results provide a path toward greatly improving the efficiency of common unpolarized light sources, such as LEDs, for a variety of applications requiring uniformly polarized illumination. Our fabricated metasurface realizes a 70% conversion efficiency, surpassing the aforementioned limit, and achieves a polarization extinction ratio exceeding 20, when characterized with laboratory measurements. We further demonstrate that arbitrary power splitting can be achieved between three or more polarized outputs, offering flexibility in target illumination.
1 Introduction
Polarization is a fundamental property of light that can carry and probe information with a wide range of applications including imaging,1
Whereas lasers with a polarized output are available, the cheaper and more ubiquitous sources such as LEDs usually emit unpolarized or partially polarized light. Efficient extraction of fully polarized light from such sources remains a challenging problem.9
In the last decade, there have been great advances in shaping polarization states of light with optical metasurfaces, composed of a planar array of nanostructures with subwavelength thicknesses.14
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Figure 1.Output polarization states shown on a Poincaré sphere for (a) previously realized metasurfaces that split polarizations into pairwise orthogonal states between different outputs (1 and 4, 2 and 3) and (b) the proposed metasurface, which achieves spatially uniform polarization across several outputs. (c) A schematic depicting a metasurface which converts an unpolarized input into
In this work, we reveal, for the first time to our knowledge, the ultimate efficiency and flexibility in converting an unpolarized input state to a spatially uniform output polarization state. This is achieved through specially designed elements with multiple output channels, thus overcoming the efficiency limit of a single-output polarizer [Fig. 1(b)]. We implement this principle by inversely designing metasurfaces with two, three, and four outputs. Each can convert unpolarized light into a single predefined output polarization state with combined efficiency far exceeding the 50% threshold, such that the polarization state is spatially uniform across all the output directions. In experiments, we demonstrate the dual-output metasurface polarizer, with the measured efficiency reaching 70%, thus demonstrating the practical feasibility of our concept. These fundamental advances in polarization optics can improve the energy efficiency of many optical technologies employing unpolarized or partially polarized light sources.
2 Method
2.1 Deriving Fundamental Limits to Conversion Efficiency
We first formulate the general properties of any passive linear optical device with
The input state
The corresponding power transmission to each output is then
We find that full power efficiency can be achieved for
For three or more outputs, 100% total efficiency can be achieved with arbitrary output states and any power splitting portions, only subject to a condition that each output has a maximum of 50% power, as marked by the red shading in Fig. 1(d). After defining the output states and power splitting portions, analytical forms of the right singular states
2.2 Topology Optimization
We illustrate a particular case of polarization conversion by designing metagratings that split an incoming unpolarized beam into multiple diffraction orders, all having identical output-pure polarizations
We overcome this apparent roadblock by designing dielectric metasurfaces with a spatially nonlocal response, where the polarization transformations depend on the diffraction order. For this purpose, we perform inverse design with free-form topology optimization41
3 Results and Discussion
3.1 Simulation Results
In the first design, we target the splitting of an incoming unpolarized beam into two outgoing beams (
Figure 2.(a) Illustration of the dual-output polarizing metasurface. (b) Operating scheme of the metasurface in transmission, with an incident angle of
We run the topology optimization starting with a random distribution of refractive indices in the unit cell. The design is based on a silicon layer (
The modeling predicts highly efficient conversion of unpolarized light to the target diagonal state
While the resulting optimized metasurface geometry delivers the required polarization transmission performance, the underlying mechanism of its operation, and the interplay between local and nonlocal modes, may not be intuitively obvious. We employ the singular value decompositions of the scattering matrix to elucidate the mechanism with which polarizations are split and rotated for different outputs (Sec. S4 in the Supplementary Material35). Then, to obtain physical insight, we perform multipolar decomposition48 to identify the predominant local modes of the metasurface under different polarizations (Sec. S5 in the Supplementary Material35). We find that electric and magnetic dipole modes provide the strongest scattering and polarization-filtering response. At the same time, the metasurface has a nonlocal characteristic allowing for the nontrivial dependence of transmission on the diffraction orders beyond the limits of local metasurfaces, as discussed above. We provide performance comparisons to previously demonstrated metasurface polarizers and commercially available polarizers in Sec. S6 in the Supplementary Material (see also Refs. 49–51 therein). The metasurface maintains effective performance greater than the threshold 50% efficiency over the entire incident angle (
3.2 Experimental Results
We successfully fabricate the optimized design using e-beam lithography and standard silicon etching, as shown in Figs. 3(a) and 3(b). The characterization of the metasurface was then performed in free space, where we measured the
Figure 3.(a) Top-down and (b) tilted SEM images of the fabricated metasurface. (c) Simplified experimental setup. The source transmits through the metasurface, which diffracts the beam into two orders. Each arm analyses the diagonal polarization before reaching the detector. (d) Measured transmitted power, and (e) extinction ratio of the diagonal polarization for each diffraction order for an incident angle
We experimentally demonstrate the metasurface’s ability to convert unpolarized light to diagonal polarized light with an absolute efficiency close to
For another demonstration, we optimize for and experimentally characterize a two-output circular polarizer; see Sec. S10 in the Supplementary Material.35 The metasurface is able to realize conversion of unpolarized light into circularly polarized light with approximately 60% efficiency, thereby exceeding the 50% limit.
3.3 Designs for Three and More Outputs
We then extend our method to design metasurfaces that generate three or more outputs. As discussed above and visualized in Fig. 1(d), there is freedom in the distribution of output powers for more than two diffraction orders. We present a metasurface with nonequal power splitting portions for each of the three outputs [Fig. 4(a)], with the optimized design shown in Fig. 4(b). The unit cell is chosen to be 1600 nm by 800 nm. The reason for the rectangular unit cell is to avoid diffraction in air along the
Figure 4.(a) Operating principle of a three-output metasurface polarizer. The incident beam is normal, and
It is instructive to discuss a question on whether it is possible to recombine the multiple output beams into one. Fundamentally, for two-output splitting, the beams are mutually incoherent and cannot be recombined coherently with any passive device. However, for three of more outputs, the beams can be partially spatially coherent. There is potential in future work to explore different beam recombination schemes and utilize them for further tailoring different structured light and dot projection schemes.
4 Conclusion
We anticipate that metasurfaces facilitating highly efficient shaping of unpolarized light into uniformly polarized outputs will find various applications, including polarized imaging with basic unpolarized sources from LED and multimode lasers. Our multi-output devices achieve this functionality in a single-layer metasurface, dramatically reducing the footprint required. Our results also demonstrate that nontrivial combinations of local and nonlocal resonances in topologically optimized metasurfaces can overcome limitations associated with arrays of weakly coupled resonators, thereby opening a path to broader polarization manipulation functionalities.
Neuton Li is a PhD candidate at the Australian National University (ANU) and the Centre of Excellence for Transformative Meta-Optical Systems (TMOS). He obtained his MSc and BSc degrees from University of Melbourne. Previously, he was a Fulbright Scholar at the California Institute of Technology. His main research interests lie in topology optimization and inverse design of optical metasurfaces.
Jihua Zhang is a researcher in Songshan Lake Materials Laboratory. Before joining Songshan Lake Materials Laboratory in 2024, he was a research fellow in the TMOS at Australian National University. He obtained dual PhD degrees from University Paris-Saclay and Huazhong University of Science and Technology in 2016. His research focuses on metasurfaces and integrated circuits for quantum photonics.
Shaun Lung received his doctorate degree from the ANU in 2022, with a focus on polarization manipulation and measurement using metasurfaces, working primarily in experimental and computational aspects. He is working as a postdoctoral researcher at Friedrich Schiller Universität, Germany, in 2023. His interests remain within both computational and experimental optics, with a strong inclination toward metasurface development.
Dragomir N. Neshev is a professor of physics at the ANU and director of the ARC Centre of Excellence for Transformative Meta-Optical Systems. He received his PhD in physics from Sofia University in 1999. He leads the Experimental Photonics Group at ANU and has made significant contributions to optics, including pioneering dielectric meta-optics and nonlinear metasurfaces. He is a fellow of Optica and a member of SPIE.
Andrey A. Sukhorukov is a professor at the Research School of Physics of the ANU and a member of the TMOS Centre. He leads a research group on nonlinear and quantum photonics, targeting the fundamental aspects of the miniaturization of optical elements down to micro- and nanoscale. In 2015, he was elected a fellow of Optica for contributions to nonlinear and quantum-integrated photonics, including frequency conversion and broadband light manipulation in waveguide circuits and metamaterials.
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[35] See Supplemental Material at [URL will be inserted by publisher] for further details in mathematical derivations, metasurface simulations, and experiments.
[46] J. P. Hugonin, P. Lalanne. RETICOLO software for grating analysis(2021).
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Neuton Li, Jihua Zhang, Shaun Lung, Dragomir N. Neshev, Andrey A. Sukhorukov, "Uniformly polarized multi-output illumination by metasurfaces performing near-complete conversion of unpolarized light," Adv. Photon. Nexus 3, 066002 (2024)
Category: Research Articles
Received: Jun. 13, 2024
Accepted: Aug. 27, 2024
Published Online: Sep. 24, 2024
The Author Email: Neuton Li (Neuton.Li@anu.edu.au), Andrey A. Sukhorukov (Andrey.Sukhorukov@anu.edu.au)