Multidiffractive Broadband Plasmonic Absorber

A novel plasmonic structure for absorbing light over broad range of wavelengths and angles of incidence is reported. It is based on a corrugated metallic grating with multiple superimposed periodical modulations for diffraction coupling of light to surface plasmons. This approach combines the advantages of regular periodic and random structures as it allows for strong as well as broadband coupling of light to surface plasmon waves. In order to demonstrate this concept, a multidiffractive structure is sequentially recorded by laser interference lithography into a photoresist, copied into an UV cross-linkable polymer, and subsequently coated by aluminum and a thin standard polymer-based photovoltaic active material layer (poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C-61 (PCBM)). Experiments and numerical simulations are carried out in order to design a three-diffraction crossed grating corrugation for plasmonic extending of the absorption band of the P3HT:PCBM into near-infrared part of spectrum. The work demonstrates that this structure enhances the absorption in the P3HT:PCBM by a factor of 2.9 in the spectral window 600-750 nm where it is inherently weakly absorbing. Over the whole visible/NIR part of spectrum 400-750 nm the number absorbed photons in P3HT:PCBM is increased by 28%.