As a passive and non-contact detection technology, the infrared thermal imaging system operating in the longwave infrared (LWIR, 9∼12 μm) region is extensively applied in environmental monitoring and electric equipment maintenances [1], [2]. Sulfur hexafluoride (SF6), exhibiting an absorption peak at 10.6 μm, is commonly used as an electrical gas-insulator in high-voltage and extra-high-voltage systems owing to its outstanding dielectric properties [3], [4]. Discovering and then preventing its leakage can protect electrical equipment and environment simultaneously [3]. For electrical equipment inspections, integrating a thermal imaging system into an unmanned aerial vehicle (UAV) can not only save human resources but also inspect those hard-to-reach areas by human beings [2]. However, conventional optical lenses in the thermal imaging systems are bulky, cumbersome and single-functioned [5], significantly increasing the payload of UAVs.

Metasurface is a two-dimensional (2D) planar structure composed of meta-atoms or meta-molecules with sub-wavelength size, which can control the optical properties of electromagnetic waves such as amplitude, phase and polarization [6], [7], [8]. Different from traditional optical devices which realize electromagnetic wavefront manipulation based on phase accumulation, the phase in metasurfaces can be accurately controlled by sub-wavelength spaced structures with thicknesses at the wavelength scale or below and thus has the advantages of being ultra-thin, ultra-light and easy integration [9], [10]. Metalenses [11], [12], [13], [14], optical devices based on metasurfaces, have attracted considerable interest because of their potential applications in compact imaging systems for both consumer and industry products. These innovative metalenses hold promise in addressing the challenges posed by traditional optical lenses, as mentioned above. Notably, both polarization and dispersion are key parameters in the design of metalenses [11].

Compared to polarization-dependent metalenses, polarization- insensitive metalenses are more suitable for realistic applications. Their capability to function effectively with light of any polarization allows for greater versatility and practicality. A variety of methods have been proposed to achieve polarization-insensitive functionality. A straightforward way is to select C4-symmetric nanoantennas such as square, or isotropic circle to be meta-atoms [15], [16], [17], [18], [19], [20], [21]. For the anisotropic meta-atoms, polarization-insensitivity can be achieved by an area-division method [22], [23], which is based on the fact that light can be decomposed into two orthogonally-polarized components. The metalens can thus be divided into two areas: one area controls the focus of the right-circularly polarized (RCP) light while the other controls the left-circularly polarized (LCP) one. In addition, some researchers have obtained polarization-insensitivity by an amplitude-division method, in which a meta-atom controls the focus of both RCP and LCP at the same time [24], [25]. Through this method, the ratio of the two orthogonally-polarized components can be tuned flexibly.

On the other hand, controlling the dispersion in metalens is equally significant particularly for multiwavelength applications. For highly symmetric (e.g., square or circle) meta-atoms, multiwavelength achromatic focusing have been realized by numerous means, including cascading multiple metalens [15], area-division modulation based on wavelengths [16], [17], multiwavelength amplitude modulation [18], [19], dispersion modulation on the basis of propagation phase [20], [21] etc. However, all these methods have their own disadvantageous, such as bulky (cascading multiple metalens) [15], limited efficiency (area-division and amplitude modulation methods) [16], [17], [18], [19], and poor designing flexibility (dispersion engineering via propagation phase) [20], [21]. For the anisotropic meta-atoms (such as cuboids), it is also possible to realize polarization-insensitivity and dispersion controlling at multiple wavelengths concurrently. Polarization-insensitivity is achieved by rotating the cuboids by 45° to induce a same phase response to the x− and y−component of the electric field [26]. In this case, dispersion engineering is realized via the propagation phase, through which the phase is tuned locally by varying the geometric parameters of the meta-atoms. However, this approach introduces numerous parameters, resulting in poor designing flexibility.

Normally, the propagation phase can be considered as a local phase modulation, while the Pancharatnam–Berry (PB) phase can be viewed as a global effect [27]. Here, based on the PB phase modulation, we propose a global design strategy to circumvent the design difficulties of poor designing flexibility in dispersion engineering mentioned above. We achieve a polarization-insensitive metalens with dispersion controllability at two wavelengths of 10.6μm (corresponding to the absorption peak of the SF6 gas) and 12μm. The numerical aperture (NA) value is selected as 0.4 to meet the requirement of the thermal imaging system incorporated in the UAV-based thermal cameras. The method of amplitude-division is applied through which each meta-atom can focus both RCP and LCP simultaneously. To engineer dispersion between two design wavelengths, only the dimension of two nanofins are demanded to be optimized independently (via genetic algorithm (GA)) by utilizing our method. The two optimized nanofins are superimposed to form cross-shaped meta-atoms. The dispersion property of the resultant metalens is tuned by incorporating a wavefront shape adjustment term, Δf, into the phase equation, which can manipulate the interaction between the two nanofins. The dispersion can either be eliminated or enhanced, which leads to an achromatic metalens or a super-chromatic metalens. Compared to the local designing method, the global designing strategy proposed here is simple, timesaving and effective. It should be known that the design method in this paper can be extended to other bands, such as visible, terahertz and others. In addition, the polarizations (RCP and LCP) and wavelengths can be controlled independently. The multi-dimensional controllable function opens up a new door in the design of multifunctional and multiplexed metasurfaces.