Focus on Superconductor Terahertz Science and Applications

A terahertz (THz) wave emitter using the stack of intrinsic Josephson junctions present in the high-T_c superconductor Bi₂Sr₂CaCu₂O_8+δ (Bi2212) has been developed. By applying a dc voltage V across the stack, the ac-Josephson effect converts this to an ac-current that emits photons at the Josephson frequency proportional to V. The Bi2212 device also behaves as and electromagnetic (EM) cavity, so depending upon the shape of the Bi2212 crystal, when the Josephson frequency matches that of a cavity resonance, the emission power is enhanced. However, the EM radiation characteristics also strongly depend upon the effects of Joule self heating of the device. In order to alleviate this Joule heating problem, we fabricated three distinct stand-alone Bi2212 sandwich device shapes, each crystal being first covered with Au on its top and bottom, and then sandwiched between sapphire plates. From our comparative studies of the three devices, we obtained important clues that could help to increase the emission power up to ∼mW and the frequency range up to several THz, as necessary for many applications such as security screening, high speed communications, medical and biological sensing, and astronomical detection, etc.

In this study, we present a new, unique fourcross shaped metamaterial terahertz (THz) filter fabricated from both gold thin films and YBa₂Cu₃O_7−d high T_c superconducting thin films. A commercial electromagnetic simulation software, CST Microwave Studio, is used to design and optimize the metamaterial filter structures. The proposed fourcross shaped rectangular filter structure consists of periodic metallic rings where strip lines are located at the sides of the ring. Fourcross metamaterial filters are fabricated by using e-beam lithography and ion beam etching techniques. Terahertz time-domain spectroscopy measurements validated the design predictions for both the center frequencies and bandwidths of the resonances due to the fourcross structures. The resonance switching of the transmission spectra was investigated by lowering the temperature below the critical transition temperature. This resonance switching effect is not observed in filters made up of metals. This novel fourcross rectangular resonator with a temperature-dependent resonance behavior holds great potential for active, tunable and low loss THz devices for imaging, sensing, and detection applications.

We present an effective engineering technique for compactly packaging high-T_c superconducting continuous-wave terahertz source modules. A terahertz-emitting device, which consists of stacks of intrinsic Josephson junctions in single crystalline Bi₂Sr₂CaCu₂O${}_{8+\delta }$, bias electrodes, a collimating lens, and other components, is packaged into a single finger-sized assembly. The rigid and stable structure used for the packaging guarantees physical and chemical stability with good thermal contact, and provides reproducible characteristics with a high yield rate. The coherent terahertz waves can be emitted from the back side of the base crystal without significant screening. The intuitive results obtained from the numerical simulation are consistent with the observed thermal properties. The modules are easy to use, and thus intended for all users unfamiliar with superconducting electronic devices.

Terahertz (THz) direct detectors based on superconducting niobium nitride (NbN) hot electron bolometers (HEBs) and biased by a simple microwave (MW) source have been studied. The frequency and power of the MW are selected by measuring the MW responses of the current–voltage (I–V) curves and resistance–temperature (R–T) curves of the NbN HEBs. The non-uniform absorption theory is used to explain the current jumps in the I–V curves and the resistance jumps in the R–T curves. Compared to the thermal biasing, the MW biasing method can improve the sensitivity, make the readout system much easier and consumes less liquid helium, which is important for long lasting experiments. The noise equivalent power (NEP) of 1.6 pW Hz^−1/2 and the response time of 86 ps are obtained for the detectors working at 4.2 K and 0.65 THz.

Principles of Hilbert-transform spectral analysis (HTSA) are presented and advantages of the technique in the terahertz (THz) frequency range are discussed. THz HTSA requires Josephson junctions with high values of characteristic voltages I_cR_n and dynamics described by a simple resistively shunted junction (RSJ) model. To meet these requirements, [001]- and [100]-tilt YBa₂Cu₃O_7−x bicrystal junctions with deviations from the RSJ model less than 1% have been developed. Demonstrators of Hilbert-transform spectrum analyzers with various cryogenic environments, including integration into Stirling coolers, are described. Spectrum analyzers have been characterized in the spectral range from 50 GHz to 3 THz. Inside a power dynamic range of five orders, an instrumental function of the analyzers has been found to have a Lorentz form around a single frequency of 1.48 THz with a spectral resolution as low as 0.9 GHz. Spectra of THz radiation from optically pumped gas lasers and semiconductor frequency multipliers have been studied with these spectrum analyzers and the regimes of these radiation sources were optimized for a single-frequency operation. Future applications of HTSA will be related with quick and precise spectral characterization of new radiation sources and identification of substances in the THz frequency range.

We report on doping and undoping experiments of terahertz (THz) emitting intrinsic Josephson junction stacks, where the change in charge carrier concentration is achieved by heavy current injection. The experiments were performed on stand-alone structures fabricated from a Bi₂Sr₂CaCu₂O${}_{8+\delta }$ single crystal near optimal doping. The stacks contained about 930 intrinsic Josephson junctions. On purpose, the doping and undoping experiments were performed over only a modest range of charge carrier concentrations, changing the critical temperature of the stack by less than 1 K. We show that both undoping and doping is feasible also for the large intrinsic Josephson junction stacks used for THz generation. Even moderate changes in doping introduce large changes in the THz emission properties of the stacks. The highest emission power was achieved after doping a pristine sample.

A high-temperature superconducting (HTS) terahertz (THz) frequency down-converter or mixer based on a thin-film ring-slot antenna coupled YBa₂Cu₃O_7–x (YBCO)/MgO step-edge Josephson junction is reported. The frequency down-conversion was achieved using higher order harmonics of an applied lower frequency (19–40 GHz) local oscillator signal in the Josephson junction mixing with a THz signal of over 600 GHz, producing a 1–3 GHz intermediate frequency signal. Up to 31st order of harmonic mixing was obtained and the mixer operated stably at temperatures up to 77 K. The design details of the antenna, HTS Josephson junction mixer, the matching and isolation circuits, and the DC and RF performance evaluation are described in this paper.

We developed a low-noise, compact, balanced superconductor–insulator–superconductor (SIS) mixer, operating in the 787–950 GHz radio frequency range. A waveguide mixer block was designed to integrate all the key components, such as a radio frequency (RF) 90° hybrid coupler, two identical SIS mixer chips, bias-tees, and an intermediate frequency power-combiner. The RF waveguide 90° hybrid coupler consists of branch lines with wide slots optimized by numerical simulation, for ease of fabrication. The balanced mixer was installed into a cartridge type receiver, originally developed for the Atacama Large Millimeter/submillimeter Array Band 10 (787–950 GHz). The receiver demonstrated double sideband noise temperatures of approximately 200 K for most of the band, without any correction for loss in front of the receiver. The local oscillator noise rejection ratio was estimated to be more than 15 dB within the measured frequency range.

IV-characteristics of stacks of two inductively interacting long Josephson junctions with the homogeneous and inhomogeneous distributions of critical currents were investigated numerically. It was assumed that the inhomogeneous linear distribution of critical currents along the junction was created by heating of one end of the stack. Even zero-field steps were found in the IV-curve of the stack with the homogeneous distribution of critical currents, whereas odd zero-field steps appeared in the IV-curve of the stack with the heated end. Due to the inductive interaction between junctions in a stack of two junctions, each of the zero-field steps splits into two steps which correspond to frequencies of collective excitations in the system. Strong coherent emission was found at the step which corresponds to the frequency of in-phase oscillations.