# Surface Plasmon Nanolaser: Principle, Structure, Characteristics and Applications
## 3 Structures and Characteristics of Plasmonic Nanolasers  
### Plasma Nanolaser Based on a Single Semiconductor Nanowire  
This kind of laser uses nanowires to achieve the integration of the gain medium and the resonant cavity [53]. The photons generated by the nanowires are coupled with the metal layer to form surface plasmons, which propagate along the direction of the nanowire, transmit and oscillate in the F-P cavity formed by reflection at both ends of the nanowire, are amplified by the gain medium and achieve lasing. The working principle is shown in Figure 2.  
Figure 1: Surface plasmon polariton (SPP) propagation along an interface between metal and dielectric material. In 2009, the [PERSON] group at the University of California, Berkeley reported a deep subwavelength hybrid plasma nanolaser based on nanowire structures [54]. Experiments have demonstrated that the laser can support a mode size as small as \(\lambda^{2}/400\) due to the lasing behavior of the mixed mode formed by surface plasmons and the optical mode. The laser can confine light to a size that is less than 100 times the diffraction limit. As shown in Figure 3, the laser is composed of metal, medium and nanowire. The nanowire is placed on the metal, and the nanowire and the metal are separated by a nanometer-scale insulating dielectric layer. By using semiconductors and dielectric layers with large differences in their refractive indices, the optical field can be confined to a dielectric layer having a thickness of only a few nanometers. The composite surface plasmon mode supported by this structure has the advantages of a wide frequency band, small mode size, and low transmission loss. In this work, the nanowires used by the researchers were CdS semiconductor materials prepared by chemical vapor deposition. The characteristic wavelength was 489 nm, the dielectric layer was MgF\({}_{2}\), and the metal layer was silver with lower loss in the desired frequency range. Although the structure has the advantage of low propagation loss, the laser emission still operates at an ultra-low temperature (T < 10 K), and the pump optical power density is very high (about 100 MW/cm\({}^{2}\)). In the surface plasmon laser, the metallic material silver is the best choice for realizing a low-threshold surface plasmon laser because it has a small surface plasmon loss in the visible to near-infrared spectral range. However, because the silver films prepared by thermal evaporation or magnetron sputtering are usually polycrystalline and generally have a surface roughness of a few nanometers root mean square (RMS), the propagating loss of SPPs on the metal surface is so severe that the lasing threshold of surface plasmon nanolasers is still too high. This shortcoming restricts its practical application. Therefore, this kind of nanolaser is difficult to achieve lasing even under  
Figure 3: (a) Principle diagram and scanning electron microscope (SEM) picture of the hybrid plasmonic nanolaser. (b) Distribution of the model between the nanowire and the metal layer [54]. Figure 2: The working principle of the nanowire cavity [53]. continuous light pumping conditions. In 2012, [PERSON] et al. adopted an optimized two-step growth method to realize the epitaxy growth of silver thin films on (111)-type single crystal silicon [55]. A single crystal silver thin film with a thickness of 80 nm was obtained. A surface plasmon laser with a low threshold was obtained by continuous laser pumping at low temperature. Its structure is shown in Figure 4. Although this method can fabricate high-quality silver thin film, it has not been reported by other research groups that surface plasmon lasers are fabricated by similar processes, due to the complicated preparation process. In this work, the surface plasmon resonator has the advantage of low loss, due to the silver layer having an atomically smooth surface, and the nanowires of the core-shell structure have good end faces and side morphology.