Polarization-insensitive dual-wavelength dispersion tunable metalens achieved by global modulation method

TL;DR

This paper presents a global modulation method for designing polarization-insensitive dual-wavelength metalenses that are achromatic and super-chromatic, simplifying fabrication and enabling advanced thermal imaging applications.

Contribution

The authors introduce a novel global modulation approach that reduces the complexity of designing multiwavelength metalenses by optimizing only two nanofin parameters, improving efficiency over local modulation methods.

Findings

Successfully designed dual-wavelength metalenses at 10.6 and 12 μm

Achieved polarization insensitivity and controlled dispersion

Reduced nanostructure optimization to only two parameters

Abstract

SF$_6$ is widely used as a gas-insulator in high-voltage power electrical system. Detecting SF$_6$ leaks using unmanned aerial vehicle (UAV)-based thermal cameras allows efficient large-scale inspections during routine maintenance. The emergence of lightweight metalenses can increase the endurance of UAVs. Simultaneously controlling dispersion and polarization properties in metalens is significant for thermal camera applications. However, via a propagation phase modulation method in which the phase is tuned locally, it is difficult and time-consuming to obtain enough different nanostructures to control multiwavelength independently while maintain the polarization-insensitive property. To this end, by using a global modulation method, a polarization-insensitive dual-wavelength achromatic and super-chromatic metalens are designed respectively. The working wavelength is set at 10.6 and 12 $\rm\mu$m to match the absorption peaks of SF$_6$ and one of its decompositions (SO$_2$F$_2$), respectively. According to the operating wavelengths, only the geometric parameters of two nanofins are required to be optimized (through genetic algorithm). Then they are superimposed on each other to form cross-shaped meta-atoms. In order to control the influence between the two crossed nanofins, an additional term $\Delta f$ is introduced into the phase equation to modify the shape of the wavefront, whereby the phase dispersion can be easily engineered. Compared with local modulation, the number of unique nanostructures that need to be optimized can be reduced to two (operating at dual wavelengths) by the Pancharatnam-Berry (PB) phase based global modulation method. Therefore, the proposed design strategy is expected to circumvent difficulties in the local design approaches and can find widespread applications in multiwavelength imaging and spectroscopy.