Plasmon-coupled hybrid Fabry–Pérot cavity modes in submicron metal–dielectric–metal arrays for enhanced color filtering

Abstract

We design and experimentally demonstrate micron- and submicron-sized color filters in the visible region using metal–dielectric–metal (MDM) Fabry–Perot cavity arrays. Large-area MDM filters (150 μm × 150 μm) of varying dielectric thicknesses show polarization-independent bandpass filtering of primary and secondary colors. The lateral dimensions of each cavity and the spacing between adjacent cavities are systematically reduced to study the possibility of using these color filters as Bayer filter arrays for color cameras. Up to micron-sized dimensions with micron-sized spacing between the adjacent MDM cavities, it is observed that the dominant mechanism for color filtering is Fabry–Perot effect. However, as the size and the pitch of the arrays of MDM are reduced further to the submicron length scale, polarization-dependent transmission is observed, and the dominant effect for color filtering is observed to be the leaked radiation from the grating-coupled plasmons. By keeping the pitch and lateral dimensions of MDM arrays fixed for plasmon resonance, the dielectric spacer thickness is systematically varied. Our electromagnetic simulations and microscopy-based imaging reveal that there exists a critical dielectric thickness for a specific cavity period, where the Fabry–Perot resonance and the Plasmons couple to each other and exhibit a high transmittivity. However, at other dielectric thicknesses, two weaker transmission peaks corresponding to decoupled modes; one corresponding to Fabry–Perot effect and the other corresponding to the signature of grating-coupled plasmons are present. Such MDM filter arrays can act as polarization-insensitive Bayer filters, as well as current-drawing contacts of multispectral imaging sensors with pixel sizes and separations, each up to a micron. With the pixel sizes in submicron regime, the integrated color filters with photodiodes may be useful for polarization-sensitive color imaging and surface sensors.