# Surface Plasmon Nanolaser: Principle, Structure, Characteristics and Applications
## 3 Structures and Characteristics of Plasmonic Nanolasers
### Plasma Nanolaser Based on a Single Semiconductor Nanowire
The plasmonic nanolaser uses a continuous laser of 405 nm wavelength as the pump source, and the stimulated radiation spectrum has two lasing peaks at 510 nm and 520 nm. Its lasing threshold is only 2.1 kW/cm\({}^{2}\) and 3.7 kW/cm\({}^{2}\) at 8 K and 78 K temperatures. The internal ohmic loss of metal will increase sharply when the optical wavelength is close to the energy range of the inter-band/in-band conversion of metal materials, thus reducing the propagation length of surface plasmons. Therefore, the realization of plasmonic nanolasers in the ultraviolet band is still a difficult task. In 2014, [PERSON] et al. reported a surface plasmon laser with a lasing wavelength of 370 nm at room temperature, which is pumped by a laser with wavelength of 355 nm, a pulse width 10 ns and a repetition rate of 100 kHz [56]. The threshold is 35 mJcm\({}^{-2}\)/3.5 MWcm\({}^{-2}\). The low-threshold characteristics of this laser are primarily due to high-quality single crystal nanowires and good semiconductor-medium-metal contact. As shown in Figure 5, the device forms a semiconductor-dielectric-metal structure by placing GaN nanowires on Al and separating Al from GaN by SiO\({}_{2}\). Al is used here because it has less loss in the ultraviolet band than silver. High quality GaN nanowires are the precondition of realizing low-threshold lasing. The nanowires are prepared by metal-organic chemical vapor deposition (MOCVD). The morphological characterization results show that the nanowires have a triangular cross section and smooth surface. The direct surface contact  
Figure 4: Plasmonic nanolaser using epitaxially grown silver film. (**a**) Schematic of the device, (**b**) calculated mode distribution, (**c**) lasing spectra, (**d**) L-L plots [55]. between the nanowire and the substrate reduces the scattering loss, which is more advantageous for the exciton to transfer energy to the surface plasmon, meaning that the gain generated by the GaN nanowire is fully utilized. In 2016, [PERSON] et al. obtained a low-threshold SPP laser by directly placing ZnO nanowires on the Al surface and optimizing the dielectric constant combination of the semiconductor and metal materials [57]. The device is shown in Figure 6. In their paper, a high-quality Al thin film with a single crystal structure were prepared by molecular beam epitaxy (MBE), which effectively reduced the additional scattering loss caused by polycrystalline Al thin films. The device is pumped by a laser source with wavelength of 355 nm, pulse width of 0.5 ns and a repetition rate of 1 kHz. At 77 K and 300 K temperatures, the threshold values of the laser are 16 MWcm\({}^{-2}\) and 110 MWcm\({}^{-2}\), respectively. The nanolaser can achieve 386 nm wavelength lasing at 353 K, which benefits from the single crystal Al film used in the device, the larger exciton oscillation intensity in ZnO and the simple laser structure. It can be seen from many studies that this kind of plasmonic nanolaser generally has a metal-dielectric-semiconductor structure. In order to reduce the loss caused by metal and the threshold of this nanolaser, three schemes have been proposed. First, in order to obtain high-quality gain dielectric nanowires with excellent morphology, the structural composition (such as the core-shell structure) and the preparation process of semiconductor nanowires should be improved. Second, the preparation process of the metal film should be improved; thereby, a high-quality metal film whose smooth surface is not easily oxidized can be obtained. Third, a suitable dielectric layer may be added between the  
Figure 5: Room temperature ultraviolet plasmon laser. (**a**) Schematic of the device, (**b**) power-dependent spectra [56]. Figure 6: ZnO SPP nanolaser. (**a**) Schematic of the SPP nanolaser based on ZnO nanowires, (**b**) lasing spectra at different temperatures [57]. metal layer and the nanowire. The dielectric layer can store energy in the structure, thus reducing the energy loss between the interspace.