Mid-infrared metasurfaces regulated by dispersive intrinsic polarization

As a dense area of “molecular fingerprint”, medium-wave infrared (MWIR) not only contains many important absorption peaks of molecular vibration signatures, but also is one of the three atmospheric transmission windows in the infrared band, and plays an irreplaceable role in gas detection, aerospace remote sensing, space communication, infrared countermeasures and other applications.

The analysis of the wavelength and polarization of mid-wave infrared photons is an important means for target detection and recognition. Traditional methods usually need to cascade multiple optical components, through time or space methods to complete, these schemes have large volume, mass and power consumption, tuning response is slow, and the presence of a large number of moving parts leads to high requirements for temperature, humidity, vibration and other environmental stability, which is not conducive to system integration and use under special working conditions, limiting its application in such performance-sensitive fields.

The emergence of metasurfaces provides a powerful and flexible device platform for independently manipulating the multi-dimensional properties of photons at sub-wavelength scales. Compared with traditional devices, more compact metasurface devices have proven their great potential in wavelength and polarization dimension regulation. However, existing methods usually use crossfinger/tile methods for multi-polarization and multi-wavelength detection, resulting in crosstalk and energy loss between different data channels. There is still no solution to how to achieve lossless arbitrary polarization manipulation at different wavelengths to simultaneously detect wavelength and fully polarization status information.

Recently, the team of researchers Guanhai Li, Xiaoshuang Chen and Wei Lu of the Shanghai Institute of Technical Physics, Chinese Academy of Sciences, together with the team of Professor Chengwei Qiu of the National University of Singapore, proposed a coherent metasurface element of the composite unit that can be reused on demand in different spectral dimensions. The superposition of the dispersive Jones matrix is used to construct wavelength decoupling to break the intrinsic polarization constraint, and any orthogonal polarization base is independently selected at multiple discrete wavelengths and multiplexed to reduce crosstalk and improve efficiency.

The results were published in Light: Science & Applications under the title “Polychromatic full-polarization control in mid-infrared light.” Dr. Chen Jin and Dr. Yu Feizhuo are the co-first authors of this paper, and Professor Li Guanhai and Professor Qiu Chengwei are the corresponding authors of this paper. The Hangzhou Advanced Research Institute of the National University of Science and Technology, the Shanghai Quantum Science Research Center, and the University of Science and Technology of China also provided assistance for this work. This work has also been funded by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Shanghai Municipal Science and Technology Commission, the Chinese Academy of Sciences and other projects.

As shown in Figure 1, in a low-illumination background, complex typical targets with different spectral and polarization information need to be distinguished by loading. However, for traditional metasurface element structures, the regulatory path is limited by the direction of geometric symmetry. In order to break the inherent intrinsic polarization constraints of different wavelength-polarization regulation, the research team proposed a dispersive Jones matrix method, which constructs a coherent structure decoupled by wavelengths with 4 all-silicon cells. In order to ensure the high performance of the superunit in all 3 wavelengths and 6 polarization channels, the periodic geometric parameters of the unit are optimized by particle swarm-genetic algorithm, and finally arbitrary polarization control in multiple wavelength dimensions is realized.

Figure 1. Multi-polarization regulation of wavelength decoupling coherent elements in the spectral dimension. (a) Schematic diagram of spectral and polarization recognition of complex targets in a low-light background; (b) the relationship between the rotation angle θ of metasurface elements and intrinsic polarization in traditional geometric phase manipulation; (c) Schematic diagram of a coherent superunit of wavelength decoupling; (d) Functional diagram of a multicolor fully polarized superlens.

The research team prepared a representative multicolor fully polarization control device, which was experimentally proved to generate three pairs of achromatic focal points on arbitrarily selected orthogonal polarization on three wavelength spatial separation channels. It simulates the functions of traditional cascaded filters, polarizers and waveplates. The three pairs of orthogonal polarization states are 3.0 μm-linear polarization, 3.6 μm-elliptically polarized (ellipticity 30°), and 4.5 μm-circularly polarized. It is worth noting that although only four coherent pixels are used in this scheme, it is still suitable for multiple wavelengths, and the polarization state is arbitrary. As shown in Figure 2, the research team further expanded and realized a ten-channel metasurface device with five sets of different orthogonal polarization states displayed on the Poincaré sphere in line with the design.

Figure 2. Preparation and regulation of intrinsic polarization-regulated metasurfaces. (a) Sample processing SEM diagram; (b) optimization results of hyperlenses based on hybrid evolutionary algorithms; (c) the representation of the five-wavelength, ten-polarization channel on a Poincaré sphere; (d) The distribution of focusing points of different polarizations in the focal plane at the selected five wavelengths, the inset shows the corresponding polarization states.

Through theoretical analysis, simulation calculation and experimental tests, the research team confirmed that the synchronous manipulation of any orthogonal polarization state at multi-discrete wavelengths without crosstalk and high efficiency based on the combination of four elements is realized by dispersive Jones matrix superposition, which provides a beneficial exploration for metasurface devices to break through the multi-dimensional and multi-degree-of-freedom regulation restrictions of spectrum-polarization from the physical level, which is helpful to meet the needs of space-based remote sensing and homeland security. (Source: China Optics WeChat public account)

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