Graphene-MoS2 spacer on metal-insulator-metal structure for enhanced surface plasmon coupled emission

In fluorescence microscopy, surface plasmon coupled emission (SPCE) has become a state-of-art technique for efficient detection of analytes at a very low concentration, where lights emitted from excited fluorophores couple to the surface plasmon mode of nearby thin metal film and result in a highly directional emission. In this work, we present the use of graphene and molybdenum disulfide (MoS2) as the spacer materials on the metal-insulator-metal structure for enhanced SPCE signal. In conventional SPCE studies a thin layer of glass acts as the spacer layer on a single silver or gold film. But the glass contributes nothing to signal enhancement. In our proposed structure, in addition to enhancing the electric field configuration on the metal surface, the spacer facilitates the participation of non-vertically oriented fluorophores in the sample in the coupling of fluorescence-plasmon, by establishing π − π interactions with the fluorescent molecules. Moreover, instead of a single metal layer, the adoption of a dielectric layer of gallium arsenide flanked by two silver films amplifies the excitation field as well as the SPCE signal. Here we report a 75-fold enhancement of signal intensity compared to isotropic fluorescence radiation at our frequency of interest, and a ∼ 2-fold amplification with respect to the peak intensity obtainable in the conventional SPCE structure. This enhancement is attributed to the very high electric field confinement of the 2D plasmonic nanostructures– graphene and MoS2.