Sparse-Aperture Quasi-Achromatic Spintronic Terahertz Emitter Enabled by Binary-Phase Metasurface and Multi-Spectral Intensity Optimization.

Terahertz (THz) technology exhibits enormous potential application in imaging and next-generation communications. However, the lacking of compact broadband THz sources capable with focusing and wavefront manipulation capabilities remains a significant challenge for its practical application. To address this, we propose a design paradigm that combines a sparse-aperture shared-aperture architecture with a multispectral weighted intensity optimization method, enabling the direct construction of a Pt/W binary-phase metasurface on a spintronic terahertz emitter. The device has an overall aperture of 2 cm and features three groups of centrosymmetric sector-shaped sub-apertures, each group individually optimized for specific sub-bands. Numerical results demonstrate that the device achieves broadband quasi-achromatic focusing over the continuous 1.0-1.5 THz band, characterized by overlapping depth-of-focus ranges and stabilized focal-spot intensity at a common design plane. The spectral intensity fluctuation is suppressed to below 33%. Meanwhile, at all representative frequencies, the edge preservation index remains above 0.68, verifying the robust broadband focusing performance. This work establishes a new paradigm for active terahertz metasurface design and provides a viable pathway toward compact, efficient, and quasi-achromatic integrated photonic systems for advanced imaging and communication applications.