Research On High Temperature Superconducting Terahertz Harmonic Mixer

Terahertz (THz) radiation with the wavelength of 0.03 mm to 3 mm is one of the electromagnetic waves which cover frequency range of 0.1 THz to 10 THz. THz wave has many special advantages that there is a very important scientific significance and broad application prospects. But it is still an electromagnetic frequency window to comprehensive research and development.With considerably nonlinear, high temperature superconductor (HTS) Josephson junctions have good response to THz waves. It also has the advantage of high sensitivity, low noise and high frequency conversion efficiency as a THz mixing detector. This paper is purposed to research HTS YBCO grain boundary Josephson junction terahertz harmonic mixing detectors. The HTS bi-crystal junction with good stability and excellent performance was prepared. And the THz harmonic mixing system was built. With this system, we have achieved the detection of the weak THz signal and we have measured and analyzed the frequency conversion efficiency and noise temperature in THz frequency band of the HTS harmonic mixing detector. The research work and the results are as follows:1. The fabrication techniques of the YBCO grain boundary Josephson junctionhave been studied. The Josephson junctions with characteristic voltage of 2 mVhave been fabricated which have very good terahertz response at 650 GHz and have only 1 micro wide micro bridge.2. The research about high temperature superconductor YBCO grain boundary junction as a THz harmonic mixer has been done. Two THz harmonic mixer systems have been developed that both tested by VDI THz source with the power of 20 μW at 623 GHz and the 200th harmonic mixing are both observed successfully on two systems that means these systems have good noise and sensitive performance.3. The frequency conversion of the mixing system has been measured by the harmonic mixing experiments. Its relationships with the harmonic number, bias current, the LO power and the characteristic parameter of the junction are analyzed which provide a theoretical and practical guidance for select the working point of the HTS Josephson grain boundary harmonic mixer.4. Using the system we developed, the noise temperature of the YBCO grain boundary Josephson junction has been tested. At 220 GHz, a good result is observed that the minimum noise temperature is about 1207 K.