Detection range is an important factor affecting the transmission characteristics of polarized light through fog. We first selected certain spectral bands from visible to IR wavelengths that exhibit lower path loss. For both radiation fog and advection fog, these optimized wavelength ranges include 0.4-1.1 μm, 1.48-1.56 μm, 1.63-1.86 μm, 2.03-2.18 μm, and 2.39-2.45 μm, and radiation fog in particular contains 3.5-4.3 μm. The long-wave IR wavelengths were excluded due to higher absorption losses. We further investigated the transmission performance of circular and linear polarization in variable foggy environments, exploring the impact of the detection range in particular. Using polarization-tracking Monte Carlo simulations for varying particle size, wavelength, refractive index, and detection range, we show that circular polarization outperforms linear polarization when transmitting in both radiation and advection fog. For radiation fog, circular polarization persists longer than linear polarization for 5 μm and 9 μm particles over the entire optimized wavelength range from the visible to mid-wave IR (MWIR). However, linear polarization outperforms circular polarization for 1 μm particles over the entire MWIR and a part of the short-wave IR (SWIR). For advection fog, circular polarization persists longer than linear polarization for all three particle sizes (10, 20, and 40 μm) over the entire optimized wavelength range from the visible to SWIR. We show that circular polarization retains a higher degree of polarization and has better enhancement in some detection ranges.
We fabricated a 4-in large-area flexible infrared nanowire grid polarizer using a nanoimprint and metal thermal evaporation process. To protect the Si master template, as well as to prolong the service life of it, we first fabricated a nickel template as an alternative by an electroforming process. Then, the nanowire grid structure was transferred from this template to IPS substrate by a thermal nanoimprint process. Finally, Al was deposited on the IPS nanowire structure by vertical thermal evaporation technology. The results of the infrared optical test reveal that the TM transmittance of the polarizer is greater than 60% in the 4-5.71 μm and 5.73-6.7 μm wavelength ranges, and, especially, it is greater than 70% in the wavelength ranges of 4.70-5.69 μm and 5.75-6.59 μm. The extinction ratio is more than 20 dB in the wavelength range of 3.6-6.7 μm, proving that the polarizer has good polarization characteristics. The flexible infrared nanowire grid polarizer has potential applications in the fields of curved surface monitoring equipment and polarized imaging equipment.
Using a reversal nanoimprint and metal evaporation process, we fabricated a micropolarizer array for the 2.5-7 μm wavelength region. The micropolarizer array has a unique unit, which is composed of 2×3 arrays on an intrinsic silicon substrate. Each array consists of a 200 nm period bilayer Al grating in a 1.3 mm×1.3 mm aperture. The transmittance of transverse magnetic polarization of each array is greater than 65% in the 2.5-7 μm wavelength range, and the extinction ratio is over 35 dB in the 3-4 μm and 6-7 μm wavelength range. This fabricated micropolarizer array has lower costs and better compatibility with microfabrication processes.
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