Non-diffracting flat-top terahertz sheet beam via cascaded refractive optical system

Terahertz (THz) transmission imaging suffers from low throughput due to point-by-point scanning and lacks beams with simultaneous transverse uniformity and longitudinal non-diffracting property. We propose a new structured beam—the non-diffracting flat-top THz sheet beam, exhibiting a flat-top profile transversely and quasi-non-diffracting propagation longitudinally—generated by a cascaded refractive optical system comprising an aspheric cylindrical lens (ACL) and a double-ridge ridge prism (DRRP). The DRRP embeds a negative ridge structure into a conventional ridge prism (RP), enabling tuning of the projection distance ( L PD ), non-diffracting region length ( L NDR ), and sidelobe-suppression ratio (SSR) beyond the single-base-angle limitation of conventional RPs. Finite-difference time-domain simulations demonstrate that the DRRP achieves an L PD 12.4 times and an SSR 22.46 times that of a conventional RP. The cascaded system generates beams with uniformity greater than 0.96 within the non-diffracting region, an average compression ratio of 8.06, and a maximum mean intensity in the flat-top region 3.91 times that of a single ACL shaped beam. Combined with a linear array detector, this beam enables single-line scanning, reducing scanning time and improving imaging quality, thereby providing a new technical pathway for high-speed THz imaging, non-destructive testing, and microscopy applications.