Biosensing Using Diffractively Coupled Plasmonic Crystals: the Figure of Merit Revisited

that of an isolated single metal nanostructure. [ 19–21 ] How to characterize its 3D distribution and put it into practice for the optimal biosensing is not a trivial topic. The denominator in the FoM defi nition contains the linewidth. Due to scattering and absorption losses in metallic nanostructures, randomly distributed single nanostructures usually exhibit resonances of ≈ 100 nm wide. [ 22 ] There are multiple ways to reduce the linewidth of SPs resonances. One of the most dramatic is based on the diffractive coupling. The so-called surface lattice resonance (SLR) of the plasmonic crystals, which stems from the diffractive coupling of the LSPR of the nanostructures in periodic arrays, is an effective strategy to reduce the high metal loss, narrow the resonance linewidth and thus boost the FoM. [ 11,23–25 ] The SLR mode has also been applied in surface enhanced Raman spectroscopy for its largely amplifi ed electric fi eld. [ 26,27 ] The realization of the SLR notably depends on the match of the substrate index, on which the metal nanostructures are fabricated, to that of the medium, in which the target biomolecules are. [ 28 ] Nonetheless, widely used substrates such as quartz have a much larger index than that of the aqueous buffers ( n ≈ 1.54 vs ≈ 1.33), which greatly damps the coupling strength and broadens the resonance. In addition, it has also been demonstrated that the substrates with lower indexes could further increase the sensitivity. [ 29 ] Therefore, it is highly desirable to fabricate the metal nanostructures on a cladding with a lower refractive index for optimum biosensing applications. [ 30 ]