Research On Microstructure Terahertz Photoconductive Antenna

Photoconductive antenna(PCA),which has a simple structure,and is easily to obtain higher power and broadband terahertz waves,is currently the most widely used terahertz wave radiation source.However,the terahertz antenna has rather low radiation efficiency.And there are no complete design tools available for PCA structure design which takes both electromagnetic simulation and semiconductor numerical simulation into consideration.Now the research on terahertz radiation sources integrating with the designable radiation peak frequency and adjustable the spectrum width is a new field in terahertz radiation source research,which has the practical applications in the highly sensitive molecular sensing.By introducing the metallic microstructure into the conventional PCA,the radiation peak frequency and spectrum width of PCA can be adjusted by virtue of the resonant behavior of metallic microstructure in the THz band.The radiation principle and the simulation design of the microstructure PCA,and the experimental platform of the PCA have been done in the dissertation.The detailed contents are as following:The working principle and related basic theory of PCA are introduced.Then,based on the Drude-Lorentz(DL)model,the effect of the carrier lifetime in the semiconductor materials,laser pulse parameters,light intensity of pump light and bias voltage on the PCA THz wave radiation characteristics are qualitatively simulated.It was found that the pure analytical method based on the theoretical model could not reflect the influence of the parameters of spot size,shape,and position on the terahertz wave radiation characteristics,nor could it solve the radiation effect of the PCA structure.Based on the fundamental theory of finite-difference time-domain(FDTD)method for the terahertz wave's radiation of PCA,numerical simulation platforms of PCA for half-wave and full-wave FDTD are established.Using the full-wave simulation,it was found that the radiated terahertz wave's electric field is well related to the excitation position of light.The radiated electric field near the negative electrode excitation is the largest,mainly because the position has a large photoelectron free path.Based on half-wave and full-wave FDTD,respectively,it was also found that the radiation characteristics of the H type dipole PCA will increase with increasing of the length-width ratio of the antenna.The simulation results are consistent with the experimental results of the relevant literature,which also validates the effectiveness of the half-wave/full-wave FDTD algorithm.At the same time,the time-consuming comparison of the two algorithms is given,and the scope of application of the algorithm is also verified.The terahertz time domain spectroscopy(THz-TDS)system for PCA testing was established.The effect of different apertures(including small and large)of the type II dipole PCA on the peak frequency of radiation terahertz waves was experimentally studied.Experimental results show that the more insignificant the resonance characteristics can be achieved with the larger aperture.Then,the radiation saturation characteristics of the large-aperture PCA under high light intensity conditions were observed experimentally.Due to the protection of the applied bias voltage,the saturation characteristics under high bias voltage were not observed.The resonant characteristics of a series of split rings resonator in the microwave frequency range are studied firstly.And then the resonant characteristics of the metallic microstructure in the terahertz band are studied by scaling.Furthermore,a series of new PCA structures with microstructured arrays are proposed,and the equivalent circuit of the microstructured PCA are given too,and the radiation characteristics of the microstructured PCAs are analyzed.And by adjusting the size of the microstructure,especially the arm length of electrical split ring resonators in the microstructured PCA,a relatively large peak frequency shift can be obtained.It has also been found that by changing the symmetry of the resonant ring structure,the spectral width of the resonant ring structure can be adjusted.Finally,the problem of radiation pattern caused by the different simulation results and test results of the microstructured PCAs is discussed and analyzed.