Near-infrared polarimetric imaging with nonlinear flat-optics

TL;DR

This work demonstrates a compact near-infrared polarimetric imaging approach based on second-harmonic generation in nonlinear all-dielectric flat optics. By engineering AlGaAs gratings with orientation-dependent SH emission, the authors enable pixel-by-pixel retrieval of the full Stokes vector using a four-unit super-pixel, eliminating moving parts. They develop a comprehensive theory and simulations showing strong SH circular and linear dichroism, and provide a robust LUT-based retrieval scheme with RMSE below a few percent across 1450–1650 nm for both linear and full-Stokes polarimetry. The method promises a scalable, passive, and cost-effective polarimetric imaging platform suitable for broadband near-IR applications, with quantified practical power requirements and detailed Supplementary analysis.

Abstract

A compact and broadband polarimetric imaging platform is presented, based on second-harmonic generation (SHG) in nonlinear flat-optics. The system employs periodic all-dielectric AlGaAs gratings to induce polarization-dependent SH emission, enabling pixel by pixel direct retrieval of the full Stokes vector from an input intensity distribution in the near-infrared range. By engineering the geometry and orientation of the polarimetric units, sensitivity to linear and circular polarization components is achieved. A superpixel design comprising four polarimetric structures allows accurate reconstruction of the polarization state without moving parts or sequential measurements. This approach offers a scalable, passive, and cost-effective solution for polarimetric imaging, particularly suited for near-infrared applications.

Figures (8)

• Figure 1: Visualization of the nonlinear polarimetric device. (a) Polarimetric super-pixel consists of four all-dielectric gratings tilted by $\pm\pi/8$$(\pm22.5^\circ)$ with respect to the crystalline lattice of the AlGaAs film. (b) Under illumination by a pump irradiation, the fundamental field generates second harmonic signal in the $\chi^{(2)}$ layer, which is most prominent for RCP or linear input parallel to the grating bars, and negligible for the LCP or LP orthogonal to the bars. (c) SH response from the polarimetric units arranged in an array of super-pixels is relayed by the telescope formed by lenses $L_1$ and $L_2$ on the CCD, allowing for the imaging polarimetry.
• Figure 2: Second harmonic response from the polarimetric units with input intensity $I^{\omega}_0\!=\!1$ MW/cm$^2$. (a) SH map at 1550 nm for the $U^y_R$ unit, rotated by the angle $\beta=\pi/8$ with respect to [010] direction ($y$-axis), as a function of the ellipticity and orientation angles of the polarization ellipse. Structure $U^y_L$ ($\beta=-\pi/8$) features inverted map, showing the opposite dependence to the handedness of the input. (b) Dependence of SH signal power on the orientation angle $\psi$ is similar to the response of a linear polarizer $I\propto\cos^2\psi$ depicted by dashed lines; solid lines correspond to slices of SH maps with $\chi=0$. (c) Dependence of the nonlinear circular dichroism on the input wavelength features a maximum near 1550 nm.
• Figure 3: Stokes parameters retrieval from the second harmonic signal intensity measurements. (a) Reference input of the tested linear SoPs displayed on the Poincaré sphere (left panel) and the error of the retrieved Stokes parameters at input wavelength of 1550 nm (right panel). (b) Dependence of the normalized RMSE on the wavelength of linearly polarized input light. (c) Polarization states produced by a quarter-wave plate rotated by an angle $\tau$ (left panel), and the corresponding retrieval error (right panel). (d) Dependence of the normalized RMSE on the wavelength of the input light.
• Figure A1: Dependencies of the normalized SH power generated by the polarimetric unit on the orientation angle for an ellipticity angle $\chi=0$. For small SH-CD values, the angular dependence becomes distorted due to the appearance of an additional harmonic. The simulated data and corresponding fits are shown by markers and lines, respectively.
• Figure S1: Transmittance of the polarimetric units at 1550 nm.