Topology-Optimized Metal Structure for Broadband Wavefront Engineering of Guided Terahertz Waves

We introduce a topology-optimized metal structure that attains broadband wavefront engineering of guided terahertz waves. While the recent demonstrations of dielectric-free terahertz lenses composed of curved metal surfaces facilitate low-loss and point source-coupled beamforming for the fundamental transverse-electric guided terahertz waves, such designs are significantly constrained in bandwidth due to the frequency dispersion of the mode. To address this challenge, this study employs topology optimization to tailor the effective refractive index profile via the surface structure, achieving broadband manipulation of guided terahertz waves. Experiments around 300 GHz demonstrate the capability to produce a directional beam with an about 15 mm waist from a point source over a 50 GHz bandwidth. Our approach presents a versatile design framework towards integrated terahertz systems.