Modelling and Design of Efficient Photomixer Based Terahertz Antennas

The lack of unoccupied and unregulated bandwidth for wireless communication vanished at lower frequency spectrum and the increasing demand of high data transmission rate leads to an intensive interest in the research of THz technologies at 0.3THz to 30THz spectrum. However, the limitation of the low output power and low efficiency of current THz devices obstacles the utilization of THz technologies. Also, compared with microwave antenna, the signal generation and excitation of THz antenna require new simulation approach. Therefore, the motivation of this thesis is theoretically analyse the reason that cause the inefficiency of THz antenna, from which, the performance of such antennas is improved from the aspects of THz source with low efficiency, THz antenna with low match efficiency and THz antenna with low gain. These investigations are necessary for the development of the THz photomixer antenna in various applications .
First of all, an new equation of the generated THz power from photomixer is developed from the equivalent circuit of photomixer fed antenna. Through this equation, various factors that affect the behaviour of photomixer is examined. Furthermore, a computational simulation process that solving both optoelectronic and electromagnetic problem in a full wave electromagnetic solver. This is a prerequisite for the analysis of improving the optical to THz conversion efficiency of photomixer. After that, the optical to THz conversion efficiency of the photomixer has been gradually improved through three different aspects, by optimizing photomixer electrodes, by utilizing reflectors underneath photomixer and by implementing superstrate. As a result, the highest enhancement factor of optical to THz conversion efficiency achieved is 494. Moreover, instead of exciting planar antenna with photomixer, the concept of truncating the photoconductive substrate of photomixer to form a dielectric resonator antenna is proposed. Such design eliminated the substrate effect to improve the radiation efficiency and to avoid using bulky lens. In addition, choke filter network and dielectric superstrate are used to improve the matching and radiation of these DRAs. The proposed DRA improved the matching efficiency and antenna gain by 10 times and 3dBi, respectively. Finally, a realization design that provide physically support to the dielectric superstrate and replace central feeding slot with coplanar waveguide is presented.