# Surface Plasmon Nanolaser: Principle, Structure, Characteristics and Applications
## 3 Structures and Characteristics of Plasmonic Nanolasers
### LSP-Based Nanolaser  
LSPs are non-propagating surface plasmon excitations generated by conducting electrons in metal nanostructures coupled with electromagnetic fields. The LSP-based nanolaser is composed by replacing the medium around the metal particles with material with gain characteristics. The metal particles themselves are the resonator of the nanolaser. Compared with SPPS-based nanolasers, LSP-based nanolasers have received less attention.  
In 2009, [PERSON] of Norfolk State University first successfully demonstrated LSP-based nano-lasers [62]. As shown in Figure 9, the nanolaser consists of an Au core, providing the plasmon mode, and silicon dioxide doped with OG-488 dye molecule providing gain. The diameter of the Au core is 14 nm, the thickness of the silicon dioxide layer is 15 nm, and the diameter of the whole device is only 44 nm. Because this single nanolaser has a small scale relative to the lasing wavelength, the radiation loss is very small. The loss of LSPs depends almost entirely on the absorption effect. It is pointed out that the laser characteristics are observed under the condition of high pump power. The lasing spectral bands of the plasma laser are 480 \(\pm\) 5 nm and 520 \(\pm\) 20 nm, respectively, at the pump power density of 0.98 MWcm\({}^{-2}\) and 0.042 MWcm\({}^{-2}\).  
In 2013, [PERSON] et al. proposed a tunable wavelength surface plasmon nanolaser based on LSPs, in which an organic dye molecule is used as a gain medium [63]. Its structure is shown in Figure 10. The fabrication process of the device is as follows: firstly, Au/SiO\({}_{2}\) nanorods are synthesized, then the monolayer nanorods are dispersed on the surface of SiO\({}_{2}\) substrate, and the device is finally coated  
Figure 8: (**a**) Schematic of the novel regularly arrayed plasmonic nanolasers, (**b**) a SEM image of a regular hexagonal array of nanoholes on the substrate, (**c**) side view SEM image of an n-type vertically aligned ZnO nanowire (NW) (the length is about 5.1 μm) array, (**d**) optically pumped lasing spectra of the device at room temperature, and (**e–f**) beam spot mapping images at pump power densities of 136.9 and 205.4 mW/cm\({}^{2}\), respectively [61].  
with polyvinyl alcohol doped with R6G dye molecules. The Au nanorods are prepared by a crystal seed process. Sodium hydroxide solution and tetraethyl orthosilicate dissolved in methanol are poured into an aqueous solution containing Au nanorods. Then, an SiO\({}_{2}\) shell encapsulating Au is formed by the reaction. Unlike the spherical Au nanoparticles used by [PERSON] et al., the nanorods used in this work can support both horizontal and longitudinal surface plasmon resonance modes. The simulation results show that the longitudinal mode can support better local field enhancement and thus provide stronger optical feedback. The device is pumped by a frequency-doubled laser with a center wavelength of 532 nm, a repetition rate of 1 Hz, and a pulse width of 25 ps. Under this pumping condition, the threshold of the laser reaches the micro-joule level. By adjusting the doping concentration of the dye, the tuning range of the laser wavelength from 562 nm to 627 nm is achieved.