Highly Localized Defect Mode in Polymer-Based THz Photonic Crystals Fabricated Using Stereolithography

A polymer-based one-dimensional photonic crystal with a defect mode was demonstrated for the terahertz frequency range. The photonic crystal was designed to achieve a photonic bandgap centered around 100 GHz with a narrow defect mode at the center frequency. The defect mode allowed a narrow band transmission at the center of photonic bandgap, while the transmitted signal was suppressed in the rest of the bandgap. The photonic crystal incorporated two identical sets of alternating compact and low-density layer pairs symmetrically enclosing a defect layer. The compact layers consisted entirely of polymethacrylate with no intentional internal structures, while the low-density layers were composed of sub-wavelength-sized columns. The columnar structures had a volumetric fraction selected to provide a desired index contrast between the adjacent layers. The photonic crystal samples were fabricated in a single-step stereolithography using a commercial system. THz transmission spectroscopy measurements were carried out to determine the optical response of the sample in a range from 82 to 125 GHz. Stratified optical layer model calculations were used to evaluate the transmission data. A distinct photonic bandgap with a defect mode centered at 99 GHz was observed in the experimental transmission spectra. A good agreement between the relevant model parameters and the corresponding design parameters was found.