Unique thermal characteristics of polarization-insensitive metalenses over traditional refractive and diffractive lenses.

The thermal characteristics of lenses play an essential role in the performance of optical systems, particularly infrared detection systems. Metalenses, composed of sub-wavelength nanostructured surfaces, have recently emerged as a promising technology to realize flat, lightweight, and mass-producible lenses. However, their thermal behavior remains largely unexplored. This study examines the impact of uniform temperature variations (both theoretically and experimentally) and laser heating (theoretically) on the performance of polarization-insensitive metalens (PIM) at a representative wavelength of 10.6 µm. We compare the thermal performance of PIMs with refractive and diffractive lenses, by a comprehensive structural, thermal, and optical performance (STOP) analysis through finite-element analysis (FEA) and finite-difference-time-domain (FDTD) simulations. A comprehensive analysis of various thermal aberration indicators reveals that the PIM exhibits smaller thermal aberrations than both the aspheric lens and the harmonic diffractive lens, and its performance is closer to that of a traditional diffractive optical element (DOE). We also find that the unique nanostructures of PIM make it highly sensitive to the refractive index change induced by temperature variations, allowing the PIM to achieve unique capabilities compared to other lenses. A typical example is that the PIM consistently demonstrates an opposite shift in focal length and depth of focus, contrary to conventional cases. We also experimentally verify the near-athermal properties of 5-centimeter-aperture metalens, confirming the advantages of large-aperture PIMs over refractive lenses in athermal infrared imaging applications. Additionally, our numerical analysis demonstrates that the broadband achromatic metalens notably outperforms the DOE in simultaneously achromatic (8 to 12 µm) and athermal (-60 to 80°C) performance. It is reasonable to speculate that the thermal diffraction efficiency of the achromatic metalens is likely superior to that of the achromatic multi-level diffractive lens (MDL). Our results confirm the strong potential of metalenses in athermal imaging applications.