Hybrid plasmonic leaky-mode lasing on subwavelength scale

Due to the existence of diffraction limit as the basic characteristic of light, the lasing on subwavelength scale cannot be achieved by traditional methods. In order to break this diffraction limit, a stacked structure composed of metal, dielectric layer and semiconductor was designed in this paper to achieve lasing on the deep subwavelength scale and its influence on the propagation mode was discussed. In terms of structural design, we used silver, a metal with low dielectric constant, as the substrate, a 10 nm-thick LiF layer as the dielectric layer, and a ZnO semiconductor nanowire with hexagonal section as the high-dielectric-constant layer. We adopted the finite-difference eigen mode and Finite-Difference Time-Domain (FDTD) method to perform optical simulation of the designed structure. First, by changing the nanowire diameter and using the finite eigen mode, the optical modes in the dielectric layer were analyzed to obtain four mode distributions. Then the effective refractive indexes and losses of the four optical modes at different nanowire diameters were used to calculate the corresponding waveguide propagation distances and lasing threshold gains. Finally, the three-dimensional FDTD method was introduced to simulate the electric field distribution of the four modes during the steady-state laser emissionin of the nanowire. The results showed that there were hybrid plasmonic mode and hybrid electric mode in the dielectric layer between the nanowire and the metal substrate. When the diameter of ZnO nanowire was smaller than 75 nm, there was no effective physical optical mode, that is, both the hybrid plasmonic mode and the hybrid electric mode were cut off. When the nanowire diameter was larger than 75 nm, the hybrid plasmonic mode could effectively exist. The hybrid electric mode did not appear until the nanowire diameter reached 120 nm. Although the hybrid plasmonic mode could be better confined to the dielectric layer, its loss was too large and its propagation distance was relatively small. In addition, the hybrid electric mode traveled a longer distance than hybrid plasmonic mode. At the given diameter of the micron wire (D = 240 μm), the hybrid electric mode propagated for over 50 μm. In conclusion, the hybrid leaky mode on the deep subwavelength scale can break the optical diffraction limit and realize lasing.