Proceedings Article | 25 August 2020
KEYWORDS: Long wavelength infrared, Infrared radiation, Metals, Polymers, Metamaterials, Scattering, Diffraction, Laser scattering, Reflectivity, Aluminum
Metasurfaces having deep sub-wavelength infrared periodicities have been explored in the past for perfect absorbers [1], active photonic systems, etc. Typically, these metasurfaces exhibit absorption and scattering resonances when the polarization is perpendicular to the stripes, while when the polarization is parallel to the stripes, a broadband high reflectivity is achieved. Metamaterial Longwave Infrared (LWIR) stripe arrays are challenging to fabricate in large (~1” or large) formats, needed for advanced nano-manufacturing, because their sub-wavelength periodicities are too small for standard photolithography in some cases. We employ photolithography to pattern stripe arrays with large periodicities (>7 μm), in order to experimentally verify LWIR diffraction, which has not been generally explored as much as shorter wavelengths, due to challenges in measuring LWIR diffraction in the laboratory. We employ tunable LWIR and Short-wave Infrared SWIR lasers to verify the diffraction from sparser arrays. We pattern metamaterial LWIR arrays using advanced (non-standard) next-generation lithography equipment, and present experimental reflectivity and backwards scattering from these metamaterial LWIR arrays. We simulate, using critical coupling analytical models and the Finite Difference Time Domain (FDTD) numerical algorithm, the reflectivity, scattering, absorption, and transmission of these metamaterial (Aluminum) arrays, and compare to the laser-based measurements. We also measure characteristics of single- and arrayed polymer (polyethylene) fibers, and contrast the results to those of Al stripes in the metasurface. Finally, we compare these measurements to those of a rectangular array of ~ 200 nm Al dots on glass, which showed that forward scattering cuts off for wavelength larger than periodicity, as expected from diffraction theory. Stripe-based metasurface arrays such as these may enable new active metasurfaces in the future, since electrical functionality is easily incorporated in the wire-like high-conductivity stripes extending across the metasurface. Analogous polymer fiber arrays may enable a new generation of smart textiles, if they are integrated with conductive metal fibers or themselves contain conductive additive particles (e.g., metal or carbon nanoparticles). In both cases, being LWIR metamaterials, these metal and polymer stripe and fiber arrays will allow unusual control of thermal functionality – another route, besides electrical, to ‘smart’ active metasurfaces and metamaterials.
