Optical Filters Using Metal-based Metamaterial

Since it was first discovered that enhanced optical transmission can occur through periodic arrays of holes with sub-wavelength dimensions formed in optically-thick metallic films, many potential applications have been conceived. These include, for example, wavelength filters, light extraction from light emitting diodes, and sub-wavelength photolithography. In the present study, this phenomenon was utilized to fabricate color filters consisting of periodic arrays of sub-wavelength holes and slits in aluminum films. The optical properties of the different metamaterials were experimentally evaluated, and the results were compared to those obtained from calculations using the finite difference time domain method. In the case of a hexagonal array of circular nanoholes, the transmittance showed no dependence on the polarization direction of the incident light. Thus, by suitable choice of the hole size and the lattice periodicity, these metamaterials could be made to act as polarization-independent red-green-blue color filters. In contrast, for an aluminum film containing a nanoslit pattern, the transmittance was strongly polarization dependent. When the incident light was polarized parallel to the slit length, the highest transmittance occurred at shorter wavelengths. The opposite effect was observed for light polarized perpendicular to the slit length, where high transmittance occurred at longer wavelengths, and significant absorption took place at shorter wavelengths. This is different to the behavior of wire-grid polarizers at infrared wavelengths, and is most likely the result of surface plasmon resonance and extraordinary diffraction. In addition to allowing simple device fabrication, the use of an aluminum film enables excitation of surface plasmons in the visible region due to its high plasma frequency.