Plasmonic black metals by broadband light absorption in ultra-sharp convex grooves

We have recently reported broadband (450-850 nm) and efficient (96% on average) light absorption on gold surfaces with arrays of ultrasharp convex grooves via excitation and subsequent adiabatic nanofocusing and absorption of gap surface plasmon modes (Sondergaard 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 similar to 650 nm reaching finally similar to 60% at 1700 nm, but with a remarkable dip around 1150-1250 nm featuring only similar to 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.