Research On High Temperature Superconducting Terahertz Sources

Terahertz (THz) waves are electromagnetic waves of which the spectral roughly spans from1011Hz to1013Hz. Comparing with waves in other frequency ranges, THz wave has its unique characteristics, which leads astonishing applications in various areas, such as astronomy, biology, computers and communications, and attracts tremendous researchers’attention. Due to a lack of effective generation and detection techniques, especially solid state sources that emit coherent, narrow-band, high-powered continuous wave in the THz frequency range, the research in THz technology is limited. This dissertation contributes to the exploration on intrinsic Josephson junctions as emitters in the terahertz range.Intrinsic Josephson junctions (IJJs) in high transition temperature (Tc) superconductor Bi2Sr2CaCu2O8(BSCCO) work as solid state emitter, the merits of which are following.1). The emitters work in the middle and low THz range, which cover the frequencies gap other THz sources do not.2). Josephson junction can convert direct current (dc) voltage to high-frequency oscillation. When Josephson junctions operated in the resistive state,1mV voltage drop generates a frequency of about484GHz.3). Because BSCCO has the superconducting-insulating periodic layered structure and the properties of each unit are nearly equal, high quality single crystal BSCCO is an ideal material for the fabrication of THz emitter.4). The energy gap of BSCCO is about several tens of mV, which in principle large enough to allow for frequencies up to10THz, while the low Tc superconductor, such as Nb, is limited up to about700GHz.The dissertation is focused on the properties and applications of BSCCO THz emitters. The main contents are classified as follows:1. The main properties of BSCCO intrinsic Josephson junction stacks as THz emitter have been studied, including,1) fabrication techniques both for mesa-structured THz emitter and double-sided ones;2) measurement system;3) the electronic transport properties and terahertz emission power measured in low temperature environment. There are two regions where THz emission occurs, at moderate input power region and high input power region.2. The properties mesa-structured THz emitters have been studied, including the denpendence of THz emission power and frequencies both on the bath temperature and the number of active junctions.3. The possibility of the enhancement of THz emission power by two mesas in series has been studied. We compared the frequencies and powers between the two cases when only one mesa biased and two in series. Simulations based on the finite element method have been performed.4. In order to determine the linewidth of the radiation with high spectral resolution, measurements with a superconducting integrated receiver of high sensitivity were performed. We compared the linewidth of low bias region and high bias region, and made an analysis of the effect of the "hot spot" on the linewidth. Moreover, we present possible mechanisms on the unusual dependence of the linewidth on the base temperature.5. We have explored several applications of BSCCO THz emitters, such as THz imaging technologies and THz gas sensor.