# Nanoplasmonic Sensor Based on Surface Plasmon-Coupled Emission: Review
## 5 Surface Plasmon-Coupled Emission  
The theory of SPCE is similar to SPR and can be derived from [PERSON] and [PERSON] equations [16, 25, 26, 27] discussed in the previous section. On the other hand, SPCE is the reverse process of SPR, as seen in the angle dependent absorption of thin metal film and it has numerous applications in bioscience. SPCE sensors have high sensitivity, and the mechanism related to the coupling between fluorophores and surface plasmons due to exclusive collection of emitted light occurs only near the bio-affinity surface. The surface plasmons are quantum phenomena due to collective oscillations of electron charge density at the interface between a gold or a silver layer and medium. The near field wave, which damps rapidly in the short length at the interface of metal layer and medium is the evanescent waves. If the gold or silver layer is coated on the surface of high refractive index glass, the plasmons, which are generated in the metal layer, can couple in the substrate. The strong evanescent field was induced using surface plasmon wave. It can excite a fluorophore with thickness of about 200 nm placed on top of gold or silver films in the aqueous solution. The reverse process is also possible, i.e., the fluorophores that are near the gold and silver layer can be excited with laser beam. It can couple with surface plasmons in the metal layer and penetrate to substrate, while appearing at a sharply defined angle p-polarized form. This is SPCE, and it is closely related to SPR [25; 28; 29; 30; 31]. The coupling between the field of the excited fluorophore and the thin gold or silver layers results in the creation of surface plasmons. For a thin gold or silver layer and the high-refractive index of the glass substrate, the surface plasmons can couple to photons in the glass substrate and a surface plasmon is converted to light [2]. As mentioned above, two configuration setups for the SPCE suggest an experiment in Figure 5. The first configuration setup (Figure 5a) is Kretschmann configuration and the incident light passes through the prism. When the wave number of light and plasmons matches together, the resonance occurs in the oscillation of surface plasmons near the fluorophores, thereby resulting in the excitation of the fluorophores by the evanescent wave [32; 33]. The second configuration is reverse Kretschmann (Figure 5b). The fluorophores can be excited directly by laser beam from the fluorophores side, and the near-field dipoles are coupled with surface plasmon on the metal layer, caucusing the direct emission in the prism with the wavelength near the excitation wave length, and the emission appears near the SPR angle with p-polarization. To compare the Kretschmann configuration with the reverse Kretschmann, the intensity of SPCE is very large and strong in the Kretschmann, since the excitation of fluorophores depends on the evanescent wave which is strong in the resonance. Typically, direction of emission, background, polarization of emission and distance of coupling are significant parameters for the evaluation of the SPCE signal and it depends on optical parameter of fluorophores, concentration of fluorophores and optical parameters of layers. Sometimes, the calculation of power flow needs to evaluate the response of fluorophores that must be derived from Sommerfeld identity. Hence, the power flow was achieved based on [PERSON]'s equation in the layers system, which was coated on the prism as follows [34]:  
\[\frac{P_{z}}{P_{1}}=0.75\times\text{Re}\Bigg{[}\frac{n^{*}}{n_{1}^{3}n}\int \limits_{0}^{\infty}dn_{\rho}\frac{n_{\rho}^{3}\sqrt{n^{2}-n_{\rho}^{2}}}{ \left|n_{1}^{2}-n_{\rho}^{2}\right|}(a-b)(a^{*}+b^{*})\Bigg{]} \tag{17}\]  
where \(n\), \(n_{1}\), \(n_{\rho}\) are refractive index, refractive indices of first medium, and that corresponding to wave vector component in z-direction, respectively. \(P_{z}\), \(P_{1}\), \(a\) and \(b\) are the power flow in z direction, input power, electrical field coefficient for forward and backward electromagnetic component, respectively.