Research On Generation And Application Of Continuous-Wave THz Radiation By Photomixing

Continuous-wave (CW) terahertz (THz) radiation source by photomixing has been an active area of research at home and abroad in recent years, because it has the advantages of narrow line-width, wide tunable range, simple structure, working at room temperature, and low price, and can be widely used in several applications, including terahertz spectroscopy, imaging and sensing, etc. However, the effects of energy conversion efficiency of photomixing, radiation efficiency of photoconductive antenna etc. limit its broad use in reality.Some researches on the generation and application of CW THz radiation by photomixing have been done in this dissertation, including ensemble Monte Carlo simulation of CW THz radiation from low-temperature grown GaAs (LT-GaAs) photoconductor, radiation impedance characteristics of photoconductive antennas, design and optimization of dual-band dipole antenna in the terahertz frequency range and CW THz images enhancement, etc. The main works of the dissertation are listed as follows:1) The methods of generating and detecting THz wave and its radiation properties are introduced, and the typical applications of THz radiation are listed. The recent advances in CW THz radiation by photomixing are reviewed, and the problems existed in this source and the ways to improve its radiation power are analyzed. The photomixing theory is then described, and the basic characteristics of photoconductive antenna and the finite integration technology (FIT) involved in antenna design are also introduced.2) Ensemble Monte Carlo method is used to simulate the procedure of radiating CW THz from LT-GaAs by photomixing, and the main factors affecting the intensity of CW THz radiation mostly, including bias electric field and carrier lifetime are studied from the microscopic view. The material properties and band structure of LT-GaAs are firstly presented, and then the elements involved in ensemble Monte Carlo simulation are briefly introduced. The simulation flowchart of radiating CW THz by photomixing is given. The simulation results show that, due to the effects of space charge screening and scattering, the intensity of CW THz radiation firstly increases as the bias electric field increases, and then saturates finally after reaching peak value; Although the possibilities of scattering drop as temperature reduces, the initial energy of photoexcited carrier decreases, so the power of CW THz radiation is lower at low temperature than at normal temperature when the bias electric field is weak, and becomes higher when the bias electric field is above a certain threshold. The simulation results also demonstrate that optimizing carrier lifetime can improve the intensity of CW THz radiation.3) The performance of the radiation impedance for three kinds of photoconductive antennas, including bow-tie antenna, spiral antenna and dipole antenna, are firstly studied. The computation results show that the radiation impedance of the dipole antenna at its resonant frequency is larger than those of the other two types of broadband antennas; although the interdigitated electrode photomixer can enhance the power of irradiated light that the photoconductive antenna can endure, it has inhibiting effect to the radiation impedance at high radiation frequency. A dual-band dipole antenna structure working within terahertz frequency range is then proposed considering its future use in material recognition and THz communication etc. The performance of radiation impedance and electrical characteristics are selected as objectives, and the main structure parameters of the photoconductive antenna are optimized. The simulation results show that the designed antenna has the property of dual-band radiation in the THz frequency range.4) The produced photoconductive antennas are tested by pulsed THz system and the performance is compared with that of the company abroad. The experimental platform of CW THz radiation by photomixing is built using photoconductive antennas as THz radiation source and detector. The experimental results are analyzed.5) A multi-scale image enhancement algorithm combining wavelet denoising is proposed. It performs multi-scale decomposition by building laplacian pyramids first, and then enhances detailed images by exponential transform in spatial domain. To remove the influence of enlarged image noises, the wavelet soft threshold method is used to denoise the approximation image when rebuilding the pyramids, and furthermore nonlinear transform is also adopted for image enhancement in wavelet domain. Several experimental results demonstrate that, the proposed algorithm can remove noise efficiently, and the enhanced THz images have clear details and sharped edges, which is useful for further image processing and recognition.