# Nanoplasmonic Sensor Based on Surface Plasmon-Coupled Emission: Review
## 2 Photoluminescence  
Photoluminescence is the molecular emission in higher wavelength or lower energy than absorption of light. This phenomenon is called fluorescence. The molecules are usually excited before emission, and there is a molecular transfer to the excited state [4]. The occurrence of photoluminescence, or phosphorescence, is determined by the situation of excited state and time delay. If there is rapid emission of low photon energy by the molecule, the fluorescence will occur with a radiative life time of about \(10^{-8}\) s, whereas phosphorescence photon will be emitted later than \(10^{-3}\) s [5]. In the fluorescence process, a molecule absorbs high-energy photons and some molecules exit from the ground state to the vibrational or rotational excited state, depending on the energy level structure of the molecule. Sometimes, the electrons of ground state can absorb the energy of photons and the molecular changes to the electronic excited state. Figure 1 shows the energy level of a molecule. The emission in the range of UV to visible light corresponds to transition from the electronic excited state to the ground state, and the emission wavelength in the infrared range is due to transition of molecule between vibrational and rotational excited state to ground state [5]. Figure 1 shows the Jablonski diagram that depicts the absorption and emission of molecules. In the photoluminescence case, the molecules absorb the high-energy photon and remain excited from ground state (S\({}_{0}\)) to excited state (S\({}_{1}\), S\({}_{2}\)) during \(10^{-15}\) s (1 fs). The molecular transition occurs rapidly to lower excited state (S\({}_{1}\)) during \(10^{-12}\) s (1 ps). The procedure of transition to ground state contains two processes as follows [6]: The first process is fluorescence transition, where the electrons change from excited state to ground state with a constant electron spin. This means that the electrons transfer between two singlet states during \(10^{-8}\) s (0.1 ns) with the radiation process defined by radiative decay (\(k_{r}\)) parameter (Figure 1). This process can be gone along by a nonradiative transition defined by nonradiative decay parameter (\(k_{nr}\)) that can be explained using internal or external quenching mechanism. The second process is related to the production of the phosphorescence photons that can be explained by intersystem transition from singlet state (S\({}_{1}\)) to triplet state (T\({}_{1}\)). The intersystem transition causes the variation of the total spin of molecule that the triplet state has lower energy than singlet states. The triplet state contains sublevels, and the spin of electron o molecule is constant during the transition with the emission of photon or nonradiative decay. Moreover, the transition from triplet state to ground state occurs by emission of photon and change in the electron spin. In the processes of fluorescence and phosphorescence, the energy of emitted photon is lower than the energy of excited photon. Therefore, the wavelength of emitted photon is longer than excited photon, which is the Stokes shift. Figure 1: Jablonski diagram showing the processes of fluorescence and phosphorescence. Quantum yield and lifetime are the significant parameters for evaluation of fluorescence that can be derived from Jablonski diagram, and they are based on radiative and nonradiative decays. Fluorescence quantum yield (Q) can be described by emitted photons and exited photons as follows [6]:  
\[\textit{Fluorescence Quantum yield}=\frac{\textit{Number of emitted Photon}}{\textit{Number of excited Photon}}\]  
\[Q=\frac{k_{r}}{k_{r}+k_{nr}} \tag{1}\]  
Typically, the fluorescence quantum yield is between 0 and 1. Because the fluorescence process is the Stokes transition process, the fluorescence quantum yield is always lower than unity.