A symmetrical wedge-to-wedge THz hybrid SPPs waveguidewith low propagation loss

Abstract

A symmetrical wedge-to-wedge THz hybrid SPPs waveguide (WWTHSW) with low propagation loss is investigated. The WWTHSW consists of two identical dielectric wedge waveguides symmetrically placed on each side of a micro wedge-patterned thin metal film. The mode characteristics of the WWTHSW, such as the propagation length (<i>L</i>_p), the normalized effective mode area (<i>A</i>) and the figure of merit (FOM) are analyzed by using the finite element method (FEM). Firstly, the influences of the height of Si micro wedge waveguide (<i>H)</i> and the gap between two wedges (<i>g</i>) on <i>L</i>_p and <i>A </i>are studied. For the same <i>g</i>, <i>A</i> first decreases and then increases with the increase of <i>H</i>. <i>A</i> achieves a minimum at an <i>H</i> of ~40 μm. However, <i>L</i>_p monotonically increases as <i>H</i> increases. The change of <i>L</i>_p slows down when <i>H</i> is greater than 40 μm. At a fixed <i>H</i>, <i>L</i>_p slightly increases with the increase of <i>g</i>. But <i>A</i> achieves a minimum when <i>g</i> is ~50 nm. Secondly, the dependencies of the mode characteristics of the WWTHSW on Si wedge tip angle (<i>α</i>) and Ag wedge tip angle (<i>θ</i>) are analyzed. At a fixed <i>α</i>, <i>θ</i> has less effect on <i>L</i>_p and <i>A</i>. As <i>α</i> increases at a fixed <i>θ</i>, <i>L</i>_p increases monotonically but <i>A</i> decreases firstly and then increases. <i>A</i> reaches a minimum when <i>α</i> increases to ~100°. Then, the change of <i>L</i>_p and <i>A</i> with the thicknesses of Ag film (<i>d</i>) and Ag wedge (<i>h</i>) are demonstrated. At a fixed <i>h</i>, both <i>L</i>_p and <i>A</i> slightly decrease as <i>d</i> increases. For the same <i>d</i>, <i>L</i>_p and <i>A</i> decrease with the increase of <i>h</i>. <i>A</i> for <i>h</i> = 0 μm is distinctly larger than those for <i>h</i> = 2 μm and <i>h</i> = 5 μm. According to the above optimizations, the parameters of the WWTHSW are chosen as <i>d</i> = 100 nm, <i>g</i> = 50 nm, <i>h</i> = 2 μm, <i>θ</i> = 80°, <i>α</i> = 100°, <i>H</i> = 40 μm. Under the optimal parameters, <i>L</i>_p of ~51 mm is obtained when <i>A</i>_mreaches ~<i>λ</i>²/10280. Compared with the previous hybrid THz plasmonic waveguide, <i>L</i>_p of the WWTHSW increases by 3 times, and <i>A</i> decreases by an order of magnitude. This result reveals that the WWTHSW enables low-loss propagation and ultra-deep-subwavelength mode confinement at THz frequencies. At last, the coupling property of the parallel WWTHSW is investigated. The coupling length of ~8958 μm is achieved without the crosstalk between two parallel waveguides. By comparison, the WWTHSW has more advantages in terms of transmission and coupling characteristics than the previous micro wedge waveguide structure and bow-tie waveguide structure. In summary, due to the excellent transmission and coupling characteristics, the WWTHSW has great potential in the fields of optical force in trapping, biomolecules transporting, and in high-density integrated circuits design.