Nanostructured plasmonic metapixels

Abstract

State-of-the-art pixels for high-resolution microdisplays utilize reflective surfaces on top of electrical backplanes. Each pixel is a single fixed color and will usually only modulate the amplitude of light. With the rise of nanophotonics, a pixel's relatively large surface area (~10 μm²), is in effect underutilized. Considering the unique optical phenomena associated with plasmonic nanostructures, the scope for use in reflective pixel technology for increased functionality is vast. Yet in general, low reflectance due to plasmonic losses, and sub-optimal design schemes, have limited the real-world application. Here we demonstrate the plasmonic metapixel; which permits high reflection capability whilst providing vivid, polarization switchable, wide color gamut filtering. Ultra-thin nanostructured metal-insulator-metal geometries result in the excitation of hybridized absorption modes across the visible spectrum. These modes include surface plasmons and quasi-guided modes, and by tailoring the absorption modes to exist either side of target wavelengths, we achieve pixels with polarization dependent multicolor reflection on mirror-like surfaces. Because the target wavelength is not part of a plasmonic process, subtractive color filtering and mirror-like reflection occurs. We demonstrate wide color-range pixels, RGB pixel designs, and in-plane Gaussian profile pixels that have the potential to enable new functionality beyond that of a conventional 'square' pixel.

Figure 1

Plasmonic MIM pixel (metapixel) concept. (a) Reflective LCoS microdisplay: schematic example of typical RGB-filter pixel array reflectors. Relatively large, pigment-based filters control color and hence each pixel can only be one color. From the conventional pixel to (b) the nanostructured plasmonic metapixel concept; demonstrating modulation of color. The designs are based on nanostructuring the available reflector area whereby for each target wavelength, a minimum of two absorption modes are tailored to sit either side of this wavelength thus eliminating the requirement for RGB filters in (a). The proposed pixel designs (i) and (ii), represent both 1D (sub-pixel 1D gratings) and 2D nanostructuring (sub-pixel isolated nanostructures) respectively, for achieving the different polarization dependent optical properties shown in the simulated reflection spectra above the schematics. For the 1D case (i), color modulation or near-perfect reflection can be controlled. For the 2D case (ii), multi-color modulation can be achieved. Hence now, pixels can include multiple state and multiple color functionality, yet still offer high reflection (due to MIM geometry).

Figure 2

1D grating plasmonic-MIM pixel: (a) Schematic of 1D MIM grating with common grating parameters defined: grating widths, w _g (nm), grating period, Λ (nm), duty cycle, Γ = w _g/Λ. (b) Shows the simulated reflection response of a typical 1D plasmonic MIM pixel with two resonant absorption modes, with the associated field profiles shown in (c). A more detailed set of simulations is found in the Supplementary Material. (d) Optical characterization results of a range of pixels exhibiting vivid colors under varying polarization conditions and (e) the SEM images of the min and max gratings for several rows. In (d), the three insets are zoomed in versions of selected RGB colors in the larger array. (f) Experimental reflection measurements, λ vs. Λ (which varies in the x-axis in (d)), of different pixel rows (1–20), showing multiple distinct resonant mode (absorption) profiles, which increase in wavelength with increasing grating period. Between the two modes there is the region of high reflection, due to Al back reflector, which is spectrally dependent on grating width and grating period.

Figure 3

RGB plasmonic pixel design: (a,b) Gradient spacing spirals under SEM and optical microscope under two orthogonal polarizations conditions for the analyzer (unpolarized incident). (c) SEM images of RGB pixel design, schematically shown in (e), at varying magnifications. (d) Optical characterization of an array of plasmonic pixels exhibiting RGB colors under varying polarization conditions (normalized to Al mirror) and (f) the associated reflection measurements of the main unit cell with orthogonal polarization conditions. This displays either thin-film silver reflection or dual-mode resonant absorption (color filtering) depending on analyzer condition.

Figure 4

2D plasmonic-MIM pixel: (a) Schematic of nanostructured MIM plasmonic structures, with the nanostructure dimension in the x-dimension, L_x, and y-dimension L_y, with symmetric grating period, Λ_xy. (b,c) Selected FDTD simulations (reflection response and associated field profiles) of a typical 2D plasmonic MIM pixel with two resonant absorption modes - taken from larger set of simulations shown in Supplementary Material. α is the x-z plane intersection, β the x-y plane and γ the z-y plane. (d,e) Experimental optical characterization results of a range of pixels exhibiting vivid colors under varying polarization conditions, with selected pixels exhibiting RGB behavior shown in (g). (f) SEM images showing the extremes of the range of different arrays; from smallest-to-largest grating period (A–E) and smallest-to-largest L_x parameter.

Figure 5

Gaussian plasmonic MIM pixels: (a,b) SEM and optical images (microscope) of 2D Gaussian nanostructured pixels, with the overlaid Gaussian functions in x/y. The optical images are with two orthogonal analyzer conditions and a crossed-polarization state, showing gradient color functions associated with each pixel. Λ_x and Λ_y is the grating period in x and y respectively. ΔL _x and ΔL _y are increase in the geometry of the nanostructures dictated by the Gaussian functions, where the total length of each structure is L ₀ + ΔL _x and L ₀ + ΔL _y for x-and-y dimensions respectively and L ₀ is the initial length, 60 nm (same for both axes). (c,d) As previous, but with a 1D Gaussian profile. The inset in (d), (i) is of the RGB channels of one of the Gaussian rows exhibiting how each pixel encodes varying responses depending on the wavelength. (e,f) Show the encoding of a 2D Gaussian with first-order partial derivative and second-order derivatives (Laplacian operator) for orthogonal polarization states.