Performance Improvements And Applications On High-Temperature Superconducting Terahertz Emitters

For the past decade,the high-Tc superconducting Bi2Sr2CaCu2O8(BSCCO)terahertz source,which is a new type of terahertz solid-state sources,has received wide attentions.It has advantages like relatively large radiation power,wide range of adjustable frequency,small volume,low power consumption,and so on.Therefore,it is also considered one of the ideal terahertz sources to solve the "terahertz gap" in sub-millimeter wave range.This dissertation mainly focuses on the innovation of the performance improvements and applications in high-Tc superconducting terahertz sources.The main results are as follows:1.BSCCO terahertz sources of in-line structure.We studied the relationships between the performance of BSCCO emitter between the number of junctions,the size and self-heating in the BSCCO stack.For improving the performance of BSCCO emitter,We proposed the fabricating method of in-line structure,which can realize gold-BSCCO-gold samples with only one-time photolithography,resulting in high fabricating efficiency and sample production yield.The BSCCO terahertz sources with the in-line structure have many advantages like large radiation power,good thermal conductivity,wide adjustable frequency range and so on.2.The expansion of emission frequency range of the BSCCO emitters.The lowest frequency can be down to 0.108 THz,and the highest frequency up to 2.09 THz.For the lower emission frequency,we studied the relationship between the size of resonant cavity and the radiation frequency,and then designed square BSCCO terahertz stacks with large side length.After the experimental test,we detected the lowest-frequency terahertz waves of 0.108 THz,which is also the lowest emission frequency in BSCCO emitters.For the higher emission frequency,we studied how the self-heating and Jc affect the emission frequency.We proposed the idear that we fabricated an in-line structure mesa using slightly overdoped BSCCO crystal,and then embedded it in a new designed sandwich structure for efficient heat exhaust.We realized the BSCCO emitter with an operating temperature up to 86 K and an highest emission frequency above 2THz.It is worth mentioning that a record high frequency of 0.577 THz was observed at 80 K,which is important for potential applications using liquid nitrogen as coolant.3.The improvement of emission power for BSCCO emitters.The increase of the side width and the number of Josephson junction can improve the emission power.Using the fabricating method of in-line structure,we fabricated square BSCCO stacks with optimizing size and thickness,and achieved a maximum emission power up to 94 ?W.In addition,in order to improve the radiation power of the in-line structure BSCCO stacks,a layer of gold film was evaporated on the top of the above IJJ stacks to form the reflector,which can reflect the terahertz wave to the direction of the light path.With the contrast experiment,we observed the obvious enhancement of the emitting power for the two samples,with up to 135%improvement in some bias points and the maximum emission power was improved up to 123 ?W.4.We successfully realized terahertz spectroscopy of dilute gases using BSCCO terahertz emitter as a frequency tunable terahertz sources.We build a set of simple and practical gas-detection system based on BSCCO terahertz emitter.Using Josephson relation,by simply sweeping the bias voltage across the stack,we observed clear absorption dips at different pressures of ammonia and water vapors,with a frequency resolution of about 1 GHz.This is the first time to realize terahertz spectrocscopy of dilute gases using BSCCO terahertz emitter.For comparision,with a more stable system and superconducting integrated receiver(SIR),we observed a smallest absorption linewidth of about 5 MHz at pressures well below 0.1mbar,reaching the level of trace gas detection,indicating that BSCCO emitters have great potentials for applications in environmental monitoring.