5-Meter-Long Reliable PP/Ag-Coated Hollow Waveguide Fabricated with UV Activation for Flexible Low-Loss Transmission of THz Waves

The robust low-loss polymer/metal-coated terahertz (THz) hollow waveguide (HWG) developed to date generally has a length less than 1.2 m. This limits its applications in 6G THz wired communication, sensing and imaging where several-meter-long waveguides are demanded. In this work, a 5-meter-long polypropylene (PP)/silver (Ag)-coated THz HWG with a 4 mm bore size were prepared using a dual-wavelength ultraviolet (UV) activation method based on optimal design of waveguide structure dimensions. XPS, wetting angle and SEM analyses indicated that UV activation could generate oxygen-containing groups and increase the hydrophilicity of the PP tubes without increasing surface roughness. This contributed to forming a dense and homogeneous Ag reflective film and enhancing the PP/Ag interfacial bonding strength from level 5 to level 2. The PP/Ag-coated HWG has a 0.1∼1.0 THz wide transmission window and low dispersion (|β₂|<1.3 ps/THz/cm) within 0.1∼2.5 THz range. At 0.1 THz, the HWG sample exhibited a straight loss of 1.1 dB/m and a bending loss of 1.4 dB/m when rolling it up in multiple coils with a radius of 15 cm. It could stably transmit THz wave at -78.5 and 60 °C (output power fluctuation<1%). The loss growth was less than 3% after 120 min of anti-vibration test (10∼500 Hz), 200 h of hydrothermal aging (60 °C, 85RH%), and 20 high/low temperature cycling tests (-40∼60 °C). In contrast, the HWG sample without UV activation was completely damaged after 10 h hydrothermal aging. This study presents a viable methodology for fabricating flexible, low-loss and reliable long THz HWG for commercial applications mentioned above.