Focusing enhancement of terahertz surface plasmon polaritons

Subwavelength-sized metallic structures exhibit extraordinary responses to electromagnetic waves due to their geometry, generating surface plasmon polaritons (SPPs) at the metal-dielectric interface. The development and application of terahertz (THz) science, which is an emerging science, can be advanced by studying SPPs in the THz band. Because the field strength of SPPs gradually weakens in the propagation direction, a coherent phase-matched THz SPP enhancement structure was designed. Here, we demonstrated an asymmetrical circular groove structure. And using this structure, THz SPPs can be excited and can be focused and enhanced under the illumination of linearly polarized THz pulses, generating a subwavelength-sized focal spot. These functions are achieved via phase matching. Furthermore, the original structure was modified by controlling the propagation phase of the THz SPPs and a periodic enhancement structure was designed. Compared to the original structure, the periodic enhancement structure has a stronger focusing effect on THz SPPs. Moreover, we imaged the THz SPPs by using a THz real-time near-field spectral imaging system and verified the feasibility of the designed structure by comparing with the simulation results. The study results are of great significance to the application of THz SPPs and the fabrication of related devices.