Tightly focusing terahertz wave using gradient-type slotted grating based on spoof surface plasmons.

The d ₁ -d ₂ -d ₃ -d ₄ -d ₅ gradient-type spoof surface plasmons (SSP) grating was designed and found to exert an obvious effect on electric field localization. Two gradient-shaped planar ports were added to the bottom of this grating to form a gradient-type slotted SSP grating and achieve tight focusing and local electric field enhancement for a terahertz wave. The size of the focal spot was optimized to 0.01λ. The single-gradient-type slotted SSP grating was considered as a unit and arranged in one and two dimensions to generate a longitudinal focal line and square focal spots array. This did not only improve the resolution of terahertz imaging, but also simultaneously scan multiple focal spots to increase the speed of terahertz imaging. This work makes the manipulation of terahertz wave more flexible and efficient which has great potential in terahertz high-resolution near-field scanning imaging.