Tight focus and field enhancement of terahertz waves using a probe based on spoof surface plasmons

In order to improve the resolution of terahertz near-field microscopic imaging technology, an ultra-thin thickness-graded silver-plated strip probe with the same duty cycle is designed to realize the excitation of spoof surface plasmons. By comparing with two other probes with different structures, it can be found that the thickness-graded silver-plated strip probe can produce a strong electric field enhancement effect. Thereafter, the influence of the polarization direction of the incident electric field and the number of periodic metal stripes on the electric field which are generated at the tip of the probe is investigated. It is found that this case is highly consistent with the electric field distribution in Richards-Wolf vector diffraction theory when the incident light is linearly polarized. The electric field intensity generated at the tip of the thickness-graded silver-plated strip probe can be flexibly and effectively manipulated by changing the polarization direction of the incident electric field. When the number of thickness-graded silver-plated strips is 12, the minimum size of the focal spot is 20 mu m, which is lambda/150. When the number of thickness-graded silver-plated strips is 4, the electric field intensity enhancement factor at the focal spot is 849. The electric field intensity enhancement factor at the focal spot increases continuously as the number of periodic metal stripes increases, and the size of focal spot decreases continuously as the number of periodic metal stripes decreases. This result shows that the tight focusing and electric field enhancement of terahertz waves can be achieved by using an ultra-thin thickness-graded silver-plated strip probe. The research results in this paper have important guiding significance for manipulating the electric field in the terahertz band.