Conventionally, pigment or dye based color filters have been used for commercialized display panels and image sensors. However, due to their chemical / ultraviolet / thermal instabilities, plasmonic color filters, which have control of the color by adjustment of the shape, size, and lattice configurations of their sub-wavelength unit structures without alteration of the intrinsic properties of constituent materials, have been suggested as an alternative. Compared to the conventional counterpart, plasmonic color filters have orders of thinner magnitude profiles and excellent stability, making them a good potential candidate for application on displays and image sensors. However, the plasmonic color filters proposed thus far have low transmittance and narrow color gamut, mainly due to the Ohmic loss of metal.
Here, we presented the bright and saturated transmissive plasmonic RGB color filters using the orthogonality control of multiple resonance modes. We started by calculating ideal spectra for color filters showing the highest brightness, while satisfying a given standard chromaticity such as that of sRGB, DCI-P3, or BT-2020. While it would be practically impossible to realize these ideal spectra either by plasmonic color filters or conventional dye filters, they served as the ultimate reference against which actual color filters can be measured. We considered new plasmonic color filter designs that utilized the Ohmic loss of metal as an advantage by introducing two resonance modes to block unwanted wavelengths. Based on coupled mode theory (CMT), it is shown that two resonance modes with proper orthogonality can closely approximate the ideal spectral response. It was derived that the red and the blue filters needed orthogonal modes and the green filter needed non-orthogonal modes, according to CMT. Metal-Insulator-Metal (MIM) nanodisk arrays for excitation of orthogonal modes and metal di-atomic arrays of square-cross structures for excitation of non-orthogonal modes were adopted. Specifically, it was identified that the artificial magnetic resonance in MIM nanodisk arrays played a significant role in producing excellent transmittance and color purity. The structural parameters were optimized to obtain bright and saturated optical properties using particle swarm optimization with the Finite-Difference Time-Domain method.
After being optimized, RGB color filters were successfully designed. These color filters had transmittances of over 71% for the green and over 80% for the other two, which were higher than that of any plasmonic transmissive RGB color filters proposed thus far. The 62.2% sRGB color space area coverage by these filters also exceeded that of their proposed predecessors. Furthermore, the optical properties of the color filters were insensitive to the incident angle of the light. In summary, we proposed and demonstrated the brightest and the most saturated plasmonic transmissive RGB color filters with good angular performance.