Fabrication and spectral characterization of the porous dielectric THz waveguides using microstructured molding technique

We report two novel fabrication techniques, as well as spectral transmission and propagation loss measurements of the subwavelength plastic wires with highly porous (up to 86%) and non-porous transverse geometries. The two fabrication techniques we describe are based on the microstructured molding approach. In one technique the mold is made completely from silica by stacking and fusing silica capillaries to the bottom of a silica ampoule. The melted material is then poured into the silica mold to cast the microstructured preform. Another approach uses microstructured mold made of plastic which is co-drawn with a cast preform. Material of the mold is then dissolved after fiber drawing. We also describe a novel THz-TDS setup with an easily adjustable optical path length, designed to perform cutback measurements using THz fibers of up to 50 cm in length. We find that while both porous and non-porous subwavelength fibers of the same outside diameter have low propagation losses (alpha leg 0.02cm-1), however, the porous fibers exhibit a much wider spectral transmission window and enable transmission at higher frequencies compared to the non-porous fibers. We then show that the typical bell-shaped transmission spectra of the subwavelengths fibers can be very well explained by the onset of material absorption loss at higher frequencies due to strong confinement of the modal fields in the material region of the fiber, as well as strong coupling loss at lower frequencies due to mismatch of the modal field diameter and a size of the gaussian-like beam of a THz source.