Plasmonic nanoresonators for colour filtering

Figure 1a presents the schematic diagrams of the proposed nanoresonators. For easy fabrication, the device is designed as a subwavelength periodic MIM stack array on a magnesium fluoride (MgF2) transparent film with period P. For each stack, a 100 nm-thick zinc selenide (ZnSe) layer is sandwiched by two 40 nm-thick aluminium (Al) layers, wherein the thickness of the dielectric core is determined on the basis of the spatial extension of SP waves inside the ZnSe layer at the visible frequencies8. The 100 nm-thick ZnSe layer ensures the efficient coupling of SP modes at the top and bottom edges of the stack, whereas the 40 nm-thick Al layer prohibits the direct transmission of the incident light. The duty cycle of the stack array is about 0.7. The bottom Al grating is used to couple selectively the incident light into plasmon waveguide modes by diffraction, whereas the top Al grating efficiently reconverts the confined plasmons to propagating waves by scattering and transmits the light to the far field in the forward direction.
Figure 1: Plasmonic nanoresonators formed by MIM stack arrays.
Plasmonic nanoresonators formed by MIM stack arrays.

(a) Schematic diagram of the proposed plasmonic nanoresonators. The white arrow represents the incident white light and the red, yellow, green and blue arrows represent the transmitted coloured light from the different stack arrays. Grey, pink and blue in the structure indicate the material of aluminium, zinc selenide and magnesium fluoride respectively. Inset is the scanning electron microscopy image of the fabricated device. Scale bar, 1 μm. (b) Plasmon dispersions in MIM stack array. Red, green and blue dots correspond to the case of filtering primary RGB colours. Red and blue curves correspond to antisymmetric and symmetric modes respectively. The shaded region indicates the visible range. (c) Simulated transmission spectra for the RGB filters. The solid and dash curves correspond to TM and TE illuminations respectively. The stack period for RGB filters is 360, 270 and 230 nm. (d) Cross-section of the time-average magnetic field intensity and electric displacement distribution (red arrow) inside the MIM stack at a wavelength of 650 nm with 360 nm stack period. The colours on the right side represent the constitutive materials, defined as in a.