Asymmetric propagation of spoof surface plasmons along doubly corrugated metal surfaces

Abstract

Spoof surface plasmons (SSPs) on the doubly corrugated metal surfaces can find a variety of applications, such as waveguides, filters, sensors, communications, and other high-performance active devices in terahertz (THz) and microwave bands. However, these studies so far are mostly on the perfect symmetric structure. In this paper, the asymmetric doubly corrugated metal surfaces are proposed to support and propagate SSP modes, which is inherently free from the conventional structure. The analytical dispersion theory of SSP modes on the structure is presented by a simplified field expansion method, which is also verified by a finite integration method. Based on the given modal analysis, the dispersion relations, propagation losses, and field profiles of the SSP modes with various geometric parameters for both 2D and 3D structures are investigated and analyzed in THz frequencies. By introducing an asymmetry of different upper and lower groove depths, the asymptotical frequency of the symmetric SSP mode at the Brillouin boundary can be largely tuned compared with the conventional symmetric structure. However, the asymptotical frequency of anti-symmetric SSPs on the structure is almost unmovable for the given structural parameters. The symmetric SSP mode demonstrates a larger propagation loss on the structure with the increased degree of asymmetry, while the anti-symmetric mode is inversely lower. By increasing the gap size between these two asymmetric corrugated metal surfaces, the propagation losses decrease for both symmetric and anti-symmetric SSP modes. SSP modes experience a larger distance on the asymmetric structure with an increased unit period and, thus, the damping losses are also enlarged. The effect of 3D structure parameters on the propagation characteristics of SSP modes with a closed sidewall is also considered for practical applications. The presented studies on the SSP modes of the asymmetric doubly corrugated metal surfaces provide new avenues to develop plenty of devices such as low-loss waveguides and filters and many other compact active devices at THz frequencies.