Nature Communications | Article
Resonant plasmonic and metamaterial structures allow for control of fundamental optical processes such as absorption, emission and refraction at the nanoscale. Considerable recent research has focused on energy absorption processes, and plasmonic nanostructures have been shown to enhance the performance of photovoltaic and thermophotovoltaic cells. Although reducing metallic losses is a widely sought goal in nanophotonics, the design of nanostructured 'black' super absorbers from materials comprising only lossless dielectric materials and highly reflective noble metals represents a new research direction. Here we demonstrate an ultrathin (260 nm) plasmonic super absorber consisting of a metal–insulator–metal stack with a nanostructured top silver film composed of crossed trapezoidal arrays. Our super absorber yields broadband and polarization-independent resonant light absorption over the entire visible spectrum (400–700 nm) with an average measured absorption of 0.71 and simulated absorption of 0.85. Proposed nanostructured absorbers open a path to realize ultrathin black metamaterials based on resonant absorption.
(a) Schematic representation of a three-layer MIM system with the top layer patterned as metallic gratings of width w. (b) Schematic representation of trapezoid array. (c) SEM images of metallic gratings with w=60 and 120 nm, and trapezoid arrays. Scale bar is 500 nm. (d) Schematic representation of the unit cell of a single trapezoid, width varying from 40 nm to 120 nm over 300 nm. (e) Measured extinction spectrum for the 60 nm and 120 nm wire gratings and trapezoid arrays for TM polarization (inset). (f) Measured extinction spectrum for TE polarization (inset). (g,h) Simulated extinction spectrum for TM and TE polarizations.
(a) Three-dimensional schematic view of 120-nm metallic grating showing the cross-section along x–z plane (red plane). (b) Calculated magnetic field intensity for 120 nm stripe at 555 nm on the x–z plane as shown in Figure 2a. (c) Three-dimensional schematic view of single trapezoid unit cell showing the cross-section along x–y plane at the top Ag/bottom SiO2 interface (green plane). (d) Calculated magnetic field profiles in the x-y plane for single trapezoid unit cell as shown in Figure 2c at λ=515 nm and (e) at λ=555 nm.
(a) SEM images of two different crossed metallic gratings with w=60 and 120 nm. Scale bars are 500 nm (b) Measured and (c) Simulated extinction spectra for two different crossed metallic gratings. (d) Extinction spectra for 120-nm-wide crossed-grating structure as a function polarization angle and wavelength. (e) SEM image of the fabricated crossed trapezoid arrays (left) and a single unit cell of crossed trapezoid (right). Scale bars are 500 and 100 nm, respectively. (f) Measured, and (g) Simulated extinction spectra using digitized SEM images (inset) for different incident electric field polarization angles (inset). (h) Extinction spectra for crossed trapezoid array as a function polarization angle and wavelength.
(a) Schematic drawing of a periodic crossed trapezoid arrays with the angle of incidence shown as θ. (b) Extinction plotted as a function of wavelength and angle of incidence. Broadband extinction is preserved even at higher angles of incidence.
Orange line corresponds to the absorption calculated by the A=1-T-R formula (same with the 0° data in Fig. 3g) and grey line plots the total absorption within the metallic portion of the three-layer MIM stack calculated by using the formula Pabs = 1/2 ω ɛ″|E|2.