Multi-Focal Geometric Phase Metalens for Full-Stokes Polarization Detection

Polarization, as a degree of freedom of light, can provide important information beyond intensity and wavelength. Infrared polarization detection technology has been a hotspot in recent years, utilizing the polarization characteristics of light for target identification, remote sensing, biomedicine, and quantum communication. With the development of metasurface technology, the miniaturization of polarization detection has attracted attention. In this paper, a multi-focal metalens for polarization detection in the C-band is proposed based on the geometric phase, which achieves the minimum Stokes parameter error with the least number of foci. A metalens with three foci is designed to measure the phase difference and intensity of incident light by utilizing orthogonal circular polarization components with opposite topological charges. The full-Stokes parameters of 54 sets of incident polarized light are calculated, with an average error of only 3.32% compared to theoretical values. In addition, an artificial neural network-based backward optimized input (ANN-BOI) model is developed, which utilizes the target transmittances to obtain the desired metasurface structural parameters. The results reveal an average error of approximately 0.0256 between the transmittances obtained from simulations based on the predicted metasurface structural parameters and the set target transmittances. With the aid of this established model, a polarizer-functional metasurface is designed, and a dual-layer metasurface is implemented in combination with the polarization detection metasurface, thus optimizing the measurement devices. The proposed design scheme enables compact and fast polarization detection with excellent performance, contributing to further exploring the practical applications of ultra-compact polarization detector devices.