Plasmonic black gold by adiabatic nanofocusing and absorption of light in ultra-sharp convex grooves

Abstract

Excitation of localized and delocalized surface plasmon resonances can be used for turning excellent reflectors of visible light, such as gold and silver, into efficient absorbers, whose wavelength, polarization or angular bandwidths are however necessarily limited owing to the resonant nature of surface plasmon excitations involved. Nonresonant absorption has so far been achieved by using combined nano- and micro-structural surface modifications and with composite materials involving metal nanoparticles embedded in dielectric layers. Here we realize nonresonant light absorption in a well-defined geometry by using ultra-sharp convex metal grooves via adiabatic nanofocusing of gap surface plasmon modes excited by scattering off subwavelength-sized wedges. We demonstrate experimentally that two-dimensional arrays of sharp convex grooves in gold ensure efficient (>87%) broadband (450-850 nm) absorption of unpolarized light, reaching an average level of 96%. Efficient absorption of visible light by nanostructured metal surfaces open new exciting perspectives within plasmonics, especially for thermophotovoltaics.

Extremely confined gap surface-plasmon modes excited by electrons

“…The results obtained in the course of our EELS study allow us to add an important element to the interpretation of very efficient and broadband light absorption by ultra-sharp convex grooves in gold 25 . The incident light, which propagates downwards (that is, in the xy plane) for this purpose, couples through scattering off the groove wedges to the MIM sGSP mode (not probed by our EELS setup), which is adiabatically focused and, consequently, absorbed as it propagates into the depth of the groove.…”

“…In fact, it was already shown in the Supplementary Information of ref. 25 that a small inclined angle (B20°) of the incident light moderately improves the overall absorption, which we can now ascribe to excitation of the MIM aGSP mode. It should be stressed that the introduction of a small asymmetry results in a weak excitation of the MIM aGSP mode, so that the absorption is still dominated by the MIM sGSP mode.…”

“…However, as we will discuss later, the slight geometric asymmetry of the groove is crucial in understanding the plasmonic black gold effect studied in ref. 25. Finally, Fig.…”

“…The presence of the MIM aGSP mode is confirmed at extremely narrow gaps of only 5 nm. Furthermore, we argue why the excitation of this mode, featuring very strong absorption, has a crucial role in the experimental realization of non-resonant light absorption by ultra-sharp convex grooves with fabrication-induced asymmetry 25 .…”

“…However, GSP modes have, to our knowledge, never been experimentally examined with EELS. The GSP modes occur in a variety of geometries, from the simplest one-dimensional (1D) metal-insulator-metal (MIM) waveguide 23,24 and metal nanowire-on-film geometries 22 to advanced structures such as the convex groove, V-groove and trench and stripe waveguides 25,26 . Furthermore, GSP modes offer enhanced properties compared with the usual propagating SP mode, such as extreme light confinement with improved propagation distances 27,28 , negative refraction 29,30 , highly efficient light absorption 25 and electrically driven circuitry 31 , to name a few important and distinct features of GSP modes in plasmonics.…”