Passive devices for terahertz frequencies

Terahertz technology is a relatively new field of electromagnetic study and interest is rapidly growing in the wake of dramatic imaging demonstrations. Other applications are expected to follow, and they will need passive devices with functionality already found in more familiar microwave and visible regions of the electromagnetic spectrum, but presently missing in the terahertz region. Two fundamental devices in particular are variable polarisation compensators, and tunable frequency-selective filters. This work represents the first demonstration of a variable polarisation compensator using subwavelength patterned features (artificial dielectrics). Following on from the original proposal, this work contains a complete and thorough investigation including the development of a bulk silicon micromachining fabrication process, full characterisation of the device performance in the W-band (70 – 110GHz) and comprehensive simulations of the device, including detailed simulation of three distinct new designs with improved performance (continuously-variable retardance with maximum in excess of quarter- and half wave). The third of the three designs is capable of extremely low insertion loss (<0.6 dB) and overcomes a difficulty of the original design that prevented zero retardance in a practical device. Secondly, a new tunable photonic crystal filter is proposed and demonstrated. Easily accessible external control surfaces integrated into the interlocking plates of a layer-by-layer photonic crystal allow unprecedented contol over the number and type of defects within the structure, all of which may be tuned "on-the-fly". Devices are initially investigated with a full-vector electromagnetic finite-difference time-domain technique, to reveal the influence of the design dimensions on the band gap as well as the effect of the defects. A two-plate metal device having four layers of rods is constructed and measured in the W-band. In good agreement with the simulations, it is experimentally determined that a moveable passband is centered at 81 GHz, with a quality factor of 11, and a tuning shift of 1.7 GHz for a plate movement of 450 µm