# Principle and Applications of the Coupling of Surface Plasmons and Excitons
## 3 Recent Progress in Coupling between Surface Plasmons and Excitons
### Strong Coupling between Plasmons and Excitons on Conductive Surfaces
[57] at the Universite Louis Pasteur studied the interaction of a J-aggregate and the SPP modes of a subwavelength hole array in detail. By measuring the effects of the hole array period, the angular dispersion and the J-aggregate concentration on the transmission of the array, the existence of a strong coupling regime was demonstrated with Rabi splitting energy of 250 meV. Additionally, the splitting energy was expected to vary with the square root of the absorbance. The results, which are plotted in Figure 4b, showed very good agreement with the previously predicted square root dependence. In 2006, in a similar manner to the work of [PERSON] et al., [PERSON] et al. [58] described the properties of organic semiconductor excitons during strong interactions with surface plasmons, and the dependence of the Rabi splitting on the square root of the dye layer absorption and the width inversion of the reflectometry lines at resonance were both observed. The Rabi splitting is shown as a function of the calculated absorption in Figure 5b and shows a linear dependence in the same way as the Rabi splitting when plotted as a function of the experimental dependence, but with a higher rate. Comparable reports about strong coupling between metal films and excitons can be found in the related literature [59]. Figure 4: (**a**) Scanning electron image of typical hole array milled into a silver film. (**b**) Variation of Rabi splitting as a function of the square root of the absorbance of the J-aggregate/polyvinyl alcohol (PVA) film [57]. Figure 3: (**a**) Layout of reflectometry experiment showing the sample and the BK7 hemispherical prism. (**b**) Reflectometry spectra for various angles as a function of the incident light energy [56]. In addition to the J-aggregates-based composite system, the surface plasmon effect has also been explored in graphene composite systems [60], mainly for application to the fields of biological detection and analysis. It was not until 2010 that a graphene covering over a gold sensor membrane was first reported to enhance the surface plasmon resonance (SPR) detection signals. [PERSON] et al. [61], at the Institute of High Performance Computing in Singapore, presented an SPR biosensor based on a graphene-on-gold structure; Figure 6 shows their structure, where the gold surface was covered with an SF10 glass prism. With the aim of achieving greater SPR signal amplification, increasing numbers of researchers have combined various types of nanomaterials, including graphene oxide decorated with gold nanorod-antibody conjugates [62], graphene oxide coupled with gold nanoparticles [63], multilayer graphene structures, and silicon coatings on gold films [64]. Figure 5: (**a**) Reflectometry spectra of PVA (polyvinylalcohol)-TDBC (the 5,5\({}^{\prime}\),6,6\({}^{\prime}\)-tetrachloro-1,1\({}^{\prime}\)-diethyl-3,3\({}^{\prime}\)–di(4-sulfobuthyl)-benzimidazolocarbocyanine) layer on silver at various detection angles as a function of the incident light energy. (**b**) Filled squares represent the experimental Rabi splitting values as a function of the square root of the experimental absorption of the PVA-TDBC layer, while the open circles represent the Rabi splitting as a function of the square root of the calculated absorption [58]. Figure 6: N-layer model for surface plasmon resonance (SPR) biosensor: prism \(\mid\) Au (50 nm) \(\mid\) graphene (L \(\times\) 0.34 nm) \(\mid\) sensing medium, where L is the number of graphene layers, and z0 = 100 nm is the biomolecule layer thickness [61]. [PERSON] et al. [65] showed that a composite material that consisted of a graphene sheet on an Au (111) surface exhibited both excellent conductivity and the ability to adsorb biomolecules stably. Remarkably, the signal-to-noise ratio of this structure was so high that the resonant currents passing through the different nucleobases were clearly distinguishable. [PERSON] et al.