Abstract
We have recently reported broadband (450–850 nm) and efficient (96% on average) light absorption on gold surfaces with arrays of ultra-sharp convex grooves via excitation and subsequent adiabatic nanofocusing and absorption of gap surface plasmon modes (Søndergaard et al 2012 Nature Commun. 3 969). Here, we significantly extend our spectroscopy investigations of one- and two-dimensional (1D and 2D) groove arrays in gold covering the wavelength range of 500–1700 nm and report first results on broadband light absorption by 1D groove arrays in nickel. For 1D groove arrays (periods 250 and 350 nm, groove depth 450 nm) in gold, the experimental characterization as well as numerical simulations based on the surface integral equation method reveal gradually increasing reflectivity for wavelengths above ∼650 nm reaching finally ∼60% at 1700 nm, but with a remarkable dip around 1150–1250 nm featuring only ∼10% reflectivity. Results indicate that the dip position can be adjusted with the precise groove geometry, a feature that could prove particularly useful for selective thermal emitters in thermophotovoltaics. Furthermore, investigations of field enhancement at the groove bottoms of 1D groove arrays in gold, mapped via diffraction-limited two-photon photoluminescence (TPL) scanning microscopy, reveal very selective polarization properties of excitation and TPL emission from the groove bottoms. 1D groove arrays in nickel were fabricated by making parallel 300 nm periodic adiabatic grooves of depths 100, 200, 300, 400 or 500 nm in a 600 nm thick nickel film. Their experimental characterization verifies that the structures are indeed very dark, exhibiting only 5–8% reflectivity over an entire wavelength range 400–1700 nm for the deepest grooves, which is in good correspondence with simulations.
Highlights
Extraordinary optical transmission [14] due to nanofocusing by the tapering [15] and the FE accessible at the slit bottom has been characterized [16]
We characterize 1D- and 2D plasmonic black gold arrays of high quality, fabricated using focused ion beam (FIB) milling in a 900 nm thick gold film, and we significantly extend our spectroscopy investigations to the relatively wide wavelength range 400–1700 nm assessed experimentally using halogen lamp as well as supercontinuum (SuperK) laser illumination, and theoretically via simulations based on the Green function surface integral equation method for periodic structures [9]
We employ diffraction-limited TPL scanning microscopy investigations mapping the FE at the groove bottoms of 1D plasmonic black gold and revealing very selective polarization properties of excitation and TPL emission at the narrow groove bottom, similar to those observed in detail with V-grooves [17]
Summary
During our initial attempts to fabricate black gold, we decreased the groove period until there were no longer any flat sections between the grooves. The FIB milling was terminated once the top level of 2D arrays started to decrease below the flat gold surface, limiting thereby the groove depths. This procedure was introduced after repeated testing in order to obtain the deepest and most narrow grooves at relatively short periods (compared to the incident wavelengths) and with the best similarity and symmetry between the in-plane x- and y-oriented milled grooves, which could be an issue due to slight material re-deposition on the initially milled grooves. We use the FIB to mill 1D ‘plasmonic black nickel’ arrays of parallel 300 nm periodic adiabatic grooves of depths d ∼ 100, 200, 300, 400 or 500 nm in a 600 nm thick nickel film deposited by e-beam evaporation on silicon substrates
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