Applications Of HTS YBCO Bi-crystal Junction In Terahertz Technology

Terahertz(THz)wave is the last band in electromagnetic spectrum that needs to be fully explored.Because of its unique features,terahertz wave,whose spectrum is between microwave and infrared,has been widely used in some important areas,such as radio astronomy,nondestructive testing and broadband communications.THz function devices,THz emitters and THz detectors have been the main research work in terahertz technology.High sensitive terahertz detectors have become a top priority for researchers on account of low terahertz power from terahertz emitters,which restricts the development of terahertz.In this paper,the characterization and application of high temperature superconducting YBCO bi-crystal Josephson junction detectors in terahertz band have been studied.On the other hand,the terahertz signal cannot be directly measured by a general spectrum analyzer due to its high frequency by itself,and the special terahertz testing instrument is too expensive,so we used the YBCO bicrystal Josephson junction as a terahertz mixer to detect the terahertz signal by mixing the signal of lower frequency.The key three work has been done in my research work.Firstly,high temperature superconducting YBCO bi-crystal terahertz detectors have been fabricated successfully.Secondly,a home-made measurement system has been established to measure the terahertz characteristics of detectors.Lastly,high temperature superconducting YBCO bi-crystal terahertz mixers have been studied.The details are as follows:1.To improve the voltage response,we have designed a new planar antenna to enhance the coupling efficiency between the terahertz signal and the junction according to the impedance match theory.This antenna has a 20-ohm input impedance,which is lower than the wideband log periodic antenna in selfcomplementary structure.The preparation process of the terahertz detectors integrated with different antennas has been studied,and the high performance terahertz detectors with the microbridge size of 2um*6um are successfully prepared.2.A home-made measurement system has been established to measure the terahertz characteristics of detectors.The voltage responsivity and the firstorder Shapiro step height of the detector have been measured to characterize the coupling efficiency between the antenna and the detector.In the measurement,we find that when a detector is integrated with the new antenna,the peak of the voltage responsivity is at 219 GHz.Furthermore,the first order Shapiro step has the highest height when the radiation signal is at 219 GHz with the coupling efficiency attaining to-48.6 dB.Thus,we conclude that the new designed antenna has a close impedance to the detector at 219 GHz,resulting from the best impedance match at 219 GHz in simulation.3.The high temperature superconducting YBCO bi-crystal terahertz mixers integrated with a new antenna and a log periodic antenna are studied,respectively.The mixer integrated with the new antenna has a maximal harmonic number of 114 with the signal to noise ratio of 8 dB under the radiation of 207 GHz and the local signal power of 2dBm.Compared to the mixer integrated with a log periodic antenna,the mixer integrated with the new antenna has a better performance,which further indicates that the new antenna has a good impedance match,resulting in a higher coupling efficiency with the junction.