Hybrid Color Filters for Multispectral Imaging

Hybrid SWM-IR narrow bandpass filters with high optical density

Narrow bandpass filters (NBFs), which are designed to accept a narrow wavelength range and simultaneously reject a much wider range, show great potential in applications such as spectral imaging, lidar detection, fluorescence microscopy, and others. In this paper, we propose and numerically simulate NBF technology for infrared (IR) optical applications. The filter is a combination of plasmonic nanostructures and improved induced transmission layers. The operating wavelength range is from 1360 to 5000 nm [short wave mid-infrared radiation(SWM-IR)], with a FWHM of less than 10 nm and maximum optical density of around 10. Therefore, our SWM-IR hybrid filter can distinguish much smaller differences in terms of spectrum information and reduce the background noise level even if using an optical amplifier.

Achieving a wide color gamut based on polarization interference filters in a liquid crystal display

We propose two polarization interference filters (PIF₁ and PIF₂) used in the backlight unit of a liquid crystal display (LCD) to achieve a wide color gamut. Both PIF₁ and PIF₂ consist of two polarizers and two 720° super twisted nematic liquid crystal polymer (LCP) layers, where two polarizers are crossed in PIF₁, and two polarizers are parallel in PIF_2. The PIFs can eliminate unwanted cyan and yellow parts in the output spectrum, which can improve the color gamut of LCD. In our calculation, when the PIF₁ is employed in the LCD with normal color filter and QD-LED backlight, the color gamut increases from 107.3% NTSC to 124.6% NTSC, which is 13.7% NTSC larger than that of the LCD with high-performance color filter. When the PIF₂ is employed in the LCD with normal color filter and QD-LED backlight, the color gamut of LCD with a normal color filter is improved by 6.8% NTSC larger than that of LCD with high-performance color film, and the transfer efficiency is close to that of the LCD with high-performance color film. We define the color gamut enhancement ratio (CGER) to compare the influence of PIFs and the high-performance color filter on the color gamut enhancement performance of LCD. Compared with the high-performance color filter, the two kinds of PIFs have a higher CGER. The PIFs have a great potential for achieving a wide color gamut.

Wide viewing angle polarization interference filter using double liquid crystal layers with opposite twisted direction

We demonstrate a wide viewing angle polarization interference filter (PIF), which consists of two crossed polarizers and double liquid crystal layers with opposite twisted direction. The polarization interference effect of the PIF is achieved by the double liquid crystal layers, and an additional compensation layer between the double liquid crystal layers can further improve the optical performance of the PIF. By using the Jones matrix method, we derive the transmittance formula of the PIF with/without compensation layer, which can be used to design any required color filter by selecting the appropriate twist angle and thickness of the liquid crystal layer. We design blue, green and red PIFs and analyze their optical performance. The proposed PIF has a wider viewing angle (±30°), and the color saturation of the PIF is also considerably high.

Dichroic Circular Polarizers Based on Plasmonics for Polarization Imaging Applications

Dichroic circular polarizers (DCP) represent an important group of optical filters that transfer only that part of the incident light with the desired polarization state and absorb the remainder. However, DCPs are usually bulky and exhibit significant optical loss. Moreover, the integration of these kinds of DCP devices can be difficult and costly as different compositions of chemicals are needed to achieve the desired polarization status. Circular polarizers based on metasurfaces require only thin films in the order of hundreds of nanometers but are limited by their sensitivity to angle of incidence. Furthermore, few existing solutions offer broadband operation in the visible range. By using computational simulations, this paper proposes and analyses a plasmonic DCP structure operating in the visible, from 400 nm to 700 nm which overcomes these drawbacks. The resulting circular dichroism transmission (CDT) is more than 0.9, and the maximum transmission efficiency is greater than 78% at visible wavelengths. These CDT characteristics are largely independent of angle of incidence up to angles of 80 degrees.