SPASER Properties Of Active Silver Nanotubes And Associated Dimers

The surface plasmons of metal nanoparticles and nanostructures can induce large enhanced near-fields and strong scatterings and break through the diffraction limit, which lead to many applications in biological and chemical sensing, surface enhanced Ranman scattering, biomedicine, and photoelectronic device. By combining metal nanostructures with dielectric medium incorporating gain materials, the gain materials will transfer energy to the surface plasmons, and then lead to the enormous amplification of the desired surface plasmon response. Surface plasmon amplification by stimulated emission of radiation(SPASER) could generate enormous light emission and extremely large near fields, which have been well used in nanolaser, surface enhanced spectroscopy, metamaterials, and new photonic device. In this paper, we investigate the SPASER properties of elliptical silver nanotubes, silver nanotubes and associated dimers by using Mie scattering theory and Finite element method(FEM). The influences of the geometry and polarization of incident light on the SPASER properties of above nanostructures have been discussed in detail. The corresponding physical mechanisms also have been carefully clarified. The main research contents and results are as follows:(1) The SPASER properties of active silver elliptical nanotubes have been investigated by means of FEM. It is found that, as the gain coefficient increase to the critical value, super-resonance of the active silver elliptical nanotube will occur under parallel polarization or vertical polarization. Under the parallel polarization, the gain threshold value for the super-resonance of the active silver elliptical nanotube is larger than that under the vertical polarization. We further investigate the influences of the shell thickness on the gain threshold and resonance wavelength of the active silver elliptical nanotube. For two polarizations, the resonance wavelengths of the active silver elliptical nanotube both exhibit red shift with decreasing the shell thickness. It is found with the decrease in the shell thickness that gain threshold for super-resonance decreases under parallel polarization while that for vertical polarization increases. In addition, as the incident angle is 45, the two super-resonances can be observed in the active elliptical silver nanotube with increasing the gain coefficient, leading to the dual-frequency SPASER.(2) We investigate the SPASER properties of the active silver nanotubes and associated dimers by using Mie scattering theory and FEM. In the active Ag nanotube, as the gain coefficient of the core increases to a critical value, a super-resonance occurs. The SPASER phenomenon also can be found in the active Ag nanotube dimer. The strong couplings between two nanotubes lead to larger gain threshold for the active Ag nanotube dimer compared with the active Ag nanotube. At the super-resonance, the maximal surface enhanced Raman scattering factor at the “hot spot” in the active Ag nanotube dimer can achieve about 8×1018, which is large enough for single molecule detection. Furthermore, with increasing the separation between two Ag nanotubes, the gain threshold value for the super-resonance of the active Ag nanotube dimer decreases while the corresponding super-resonance wavelength increases first and then decreases. This is because the competition between the decrease of the coupling strength and the increased dielectric constant of the core.