Pd-free activation for low-loss silver-coated polymer waveguides in millimeter-wave transmission

• Pd-free activation strategy was developed for silver coating on polycarbonate hollow waveguides. • The silver-coated polymeric hollow waveguides was fabricated with length of up to 4 m and transmission loss as low as 0.73 dB/m at 220 GHz—a value approaching the theoretical limit. • The waveguides maintain stable performance under bending and demonstrate applicability for ranging in automotive radar systems. Waveguides are crucial components for enabling stable signal links in rapidly developing millimeter-wave and terahertz applications. Among various configurations, metal-coated polymeric hollow waveguides offer a compelling combination of low transmission loss, lightweight flexibility, and cost-effectiveness, yet their reliable fabrication remains challenging. The conventional approaches, which typically rely on palladium-based activation to achieve metal-polymer adhesion, suffer from high cost, limited scalability, and environmental concerns. This work proposes a disruptive Pd-free surface modification strategy using a silane coupling agent to address the intrinsic adhesion issue between polycarbonate substrates and silver coatings. The introduced aminosilane layer forms chemical bonds onto the inner surface of a polymer hollow tube, enabling the further growth of silver layer. Through systematic parameter optimization, we fabricated polymer hollow-core waveguide with a silver-plated inner surface, achieving lengths of up to 4 m and a transmission loss as low as 0.73 dB/m at 220 GHz—a value approaching the theoretical limit. Furthermore, we demonstrated a prototype automotive millimeter-wave radar system incorporating the fabricated waveguide for ranging applications. This work establishes a scalable, low-cost, and efficient pathway to high-performance hollow waveguides, suitable for advanced millimeter-wave and terahertz systems in communications, data center interconnects, sensing, and beyond.