Photonic-Crystal-Based Polarization Converter for Terahertz Integrated Circuit

In this paper, the fabrication and characterization of newly developed photonic crystal (PC) polarization-controlling devices on a silicon-on-insulator wafer for integrated terahertz applications are presented. The polarization converter is composed of periodic asymmetric loaded PC slab waveguide. Square-and circular-hole PC slab waveguides were studied using a 3-D finite-difference time-domain method. For a square-hole PC-based polarization rotator, polarization rotation efficiency higher than 90% was achieved within the normalized frequency band of a/lambda = 0.258 - 0.267. In circular-hole PC polarization converter, the polarization conversion efficiency dropped to 70% for the afore-mentioned frequency band. Low polarization conversion efficiency of the circular-hole PC-based device is attributed to scattering loss at the top loaded layers. Thus, the square-hole PC structure is a better candidate for integrated terahertz polarization-controlling devices. Planar terahertz integrated circuit technology was developed to implement the proposed device. Characterization setup was designed using rigorous numerical methods to use the newly introduced Agilent Millimeter-wave PNA-X network analyzer (up to 500 GHz) as a source. Scattering parameter characterizations provide a good measure of polarization extinction ratio. For the devices designed for the central frequency of f = 200 GHz, it was observed that, within the frequency band of 198-208 GHz (a/lambda = 0.26-0.272), the ratio of S(21) to S(11) was higher than 15 dB. The bandwidth is in good agreement with our preliminary design presented before.