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Mid-infrared multifunctional metasurface with polarization modulation

Updated: May 11, 2023

Mid-wave infrared (MWIR), as a dense area of "molecular fingerprints", not only contains many important molecular vibration characteristic absorption peaks, but also is one of the three atmospheric transmission windows in the infrared band. It plays an irreplaceable role in other applications. Xinxin Gem provide germanium windows for Mid-wave infrared (MWIR) applications.

The analysis of dimensions such as the wavelength and polarization of mid-wave infrared photons is an important means of detecting and identifying targets. The traditional method usually requires cascading multiple optical components, and it is completed by dividing time or space. These solutions have large volume, mass and power consumption, and the tuning response speed is slow. At the same time, the existence of a large number of moving parts leads to The device has high requirements for environmental stability such as temperature, humidity, vibration, etc., which is not conducive to system integration and use under special working conditions, which limits its application in areas sensitive to such performance.

The emergence of metasurfaces provides a powerful and flexible device platform for independently manipulating the multi-dimensional properties of photons at the subwavelength scale. Compared with traditional devices, more compact metasurface devices have proven their great potential in terms of wavelength and polarization dimension regulation. However, existing methods usually use interdigitation/blocking for multi-polarization and multi-wavelength detection, resulting in crosstalk and energy loss between different data channels. How to achieve lossless arbitrary polarization manipulation at different wavelengths to simultaneously detect wavelength and full polarization state information remains unresolved.

In this paper, the research team proposes a combined unit coherent metasurface unit that can be multiplexed on demand in different spectral dimensions. The wavelength decoupling is constructed by the superposition of the dispersion Jones matrix 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.

As shown in Figure 1, in a low-illumination background, complex typical targets with different spectral and polarization information need to be distinguished through payloads. However, for the traditional metasurface unit structure, its regulation path is limited by the geometric symmetry direction. In order to break the intrinsic polarization constraints inherent in different wavelength-polarization modulations, the research team proposed a dispersion Jones matrix method by using four all-silicon units to construct a wavelength-decoupled coherent structure. In order to ensure the high performance of the supercell in all three wavelengths and six polarization channels, the periodic geometric parameters of the cell are optimized by particle swarm-genetic algorithm, and finally realize arbitrary polarization regulation in multiple wavelength dimensions.

Figure 1. Multi-polarization modulation of wavelength-decoupled coherent units in the spectral dimension. (a) Schematic diagram of spectral and polarization recognition of complex targets in low-illumination background; (b) Relationship between rotation angle θ and intrinsic polarization of metasurface unit in traditional geometric phase manipulation; (c) Schematic diagram of wavelength-decoupled coherent superunit; (d ) Functional schematic diagram of a multicolor fully polarized metalens.

The research team prepared a representative polychromatic full polarization control device, which was experimentally proven to generate three pairs of achromatic focusing points on arbitrarily selected orthogonal polarizations on spatially separated channels of three wavelengths. It mimics the function of a traditional cascade of filters, polarizers, and waveplates. The three pairs of orthogonal polarization states are 3.0 μm-linear polarization, 3.6 μm-elliptical polarization (ellipticity 30°) and 4.5 μm-circular polarization. It is worth noting that although this scheme uses only four coherent pixels, it is still applicable to multiple wavelengths and the form of the polarization state is arbitrary. As shown in Figure 2, the research team further expanded and realized a ten-channel metasurface device, and the five sets of different orthogonal polarization states displayed on the Poincaré sphere are consistent with the design.

Figure 2. Fabrication and tuning of intrinsic polarization-tuned metasurfaces. (a) SEM image of sample processing; (b) optimization results of metalens based on hybrid evolutionary algorithm; (c) representation of five wavelengths and ten polarization channels on the Poincaré sphere; (d) selected five wavelengths , the distribution of focus points on the focal plane for different polarizations, and the inset shows the corresponding polarization states.

Through theoretical analysis, simulation calculations and experimental tests, the research team has confirmed that based on the combination of four units, the synchronous manipulation of any orthogonal polarization state at multiple discrete wavelengths without crosstalk and high efficiency can be achieved through the superposition of dispersion Jones matrices. Breaking through the limitations of spectrum-polarization multi-dimensional and multi-degree-of-freedom regulation at the level provides a useful exploration and helps meet the needs of space-based remote sensing and homeland security.

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