Design of an ultra-broadband super absorbing plasmonic metamaterials

Abstract

In recent years, the need for thin films structures that absorb electromagnetic radiation over a broadband spectrum has been shown to be paramount in solar harvesting for photovoltaic cells, thermal emitters or infrared detectors systems operating in 1–3μm and in 3–5μm atmospheric windows used for both civilian and military applications. Recently, Cui et al. (Nano Lett 12:1443–7, 2012), Lobet et al. (Opt. Expr 22:12678–12690, 2014) presented saw-toothed anisotropic metamaterials acting as thin film broadband absorbers in the infrared regime using alternating gold and dielectric layers. The full absorption normalized width at half the maximum was Δτ/τ = 86% over the 3–5:5μm wavelength range. In the present work, we design a metamaterial absorber (MMA) showing averaged absorptivity of. (Ā) = 93:3 %, over a much broader spectrum, between 200 nm and 5:2μm. This MMA is made of a periodical array of truncated pyramids with a squared base disposed in air (Fig. 55.1). The truncated pyramids deposited on a semi-infinite gold substrate consist of 20 layers a stratified medium with alternating Au (thickness tAu= 15 nm) and Ge (thickness tGe= 35 nm) slices whose lateral side linearly reduces from L D 600 nm at the basis to l D 150 nm at the top of the pyramid. A stack of twenty layers is used in the present MMA resulting to a 1μm high thin structure. The truncated pyramids are deposited on a 200 nm thick gold film. A SiO2matrix with a refractive index of nSiO2= 1:44, acts as semi-infinite substrate. The periodicity is set to 800 nm. Gold and germanium permittivities are described using tabulated data (Potter, Germanium (Ge). In: Palik (ed) Handbook of optical constants of solids. Academic, Orlando, 1985; Johnson and Christy, Phys Rev B 6:4370–4379, 1972). Numerical results were obtained with a rigorous coupled-wave analysis (RCWA) method, which is perfectly suited for periodic systems. Since the gold layer is sufficiently thick, no transmission.(T = 0) occurs, and the absorption (A) is directly deduced from reflectivity (R) calculations using A = 1 – R. We distinguish three parts in the absorption spectrum: a first part ranging from 200 nm to 2:0μm (UV, visible, NIR, MIR) with an averaged absorption of (ĀI) = 92:1 %, a second part encompassing wavelengths between 2:0 and 2:7μm (MIR) showing a weaker absorption of. (ĀII) = 80:1% and, finally, a third part from 2.7 to 5:2μm (MIR) with an absorption of (ĀIII) = 97:8% (Lobet et al., Opt. Expr 22:12678–12690, 2014). The absorption mechanism is attributed to the excitation of stationary surface plasmon (SSP) at the interface between metal and dielectric layers. Each layer of the pyramid contributes to several narrow absorption peaks and, since SSP are shape and size dependent, the associations of different sizes of the layers sum up constructively, boosting absorption in a broadband manner. Modelling of the absorption of individual layers was conducted using a Discrete Dipole Approximation DDSCAT code (Draine and Flatau, J Opt Soc Am A 11:1491, 1994) and enabled to identify modes optically active in individual systems. Those modes correspond to excitation of localized surface plasmons modes (Lobet et al., Opt. Expr 22:12678–12690, 2014). We identified plasmonic modes in absorption spectra of square parallelepipeds of thickness tAu= 15 nm and length L = 150, 300, 450, and 600 nm embedded in a Ge dielectric medium. Those peaks correspond to dipolar, sextupolar and octopolar modes, according to field map distributions. We show the mode resonance wavelength shift Δλ with respect to the resonance of the smallest system (L = 150 nm) as a function of the aspect ratio R = L/t which is consistent with the dispersion relation of surface plasmon polariton of infinite thin film of Au (Lobet et al., Opt. Expr 22:12678–12690, 2014). This proves that, when L increases, longer wavelengths are absorbed, which explains the broadband spectrum. The ultra-broadband characteristics of the MMA, extending from visible to mid-IR ranges, give huge versatility for the design of various devices.