Toward intrinsic low-loss transmission in polymer-based millimeter waveguides

Polymer dielectric waveguides provide a promising platform for millimeter-wave transmission due to their low dielectric loss, flexibility, and compatibility with scalable fabrication. However, the microstructure of polymers provides multiple degrees of freedom that would affect the dielectric response in the millimeter-wave range. This will eventually determine the transmission loss but has rarely been elucidated. Here, polypropylene dielectric waveguides were fabricated by melt extrusion, and the influence of molecular orientation and crystallinity on transmission loss in the millimeter-wave range was investigated. It is noted that the shear-induced chain alignment can increase the dielectric loss of the polymer under predominantly fundamental quasi-TE-mode excitation, whereas increased crystallinity can suppress the polarization relaxation in amorphous regions. In turn, the transmission loss can be reduced to a highly competitive value of approximately 1.5 dB/m at 110 GHz by optimizing the molecular orientation and crystallinity through the extrusion process. This work provides a roadmap for the fabrication of polymer-based millimeter-wave waveguides with transmission loss approaching the intrinsic limit, and would be significant for wired millimeter-wave communications.