Terahertz (THz) electromagnetic radiation (1012 Hz) holds immense potential for applications ranging from explosive detection to cancer diagnosis and treatment. However, the challenge of bridging the gap between microwave and infrared radiation (THz radiation) is not easily overcome. THz radiation is difficult to generate due to its high frequency, which is too elevated for semiconductor-based emitting devices but too low to be produced by solid-state laser systems. Recently, researchers from the United States, Turkey, and Japan have demonstrated that this issue can be addressed by utilizing Josephson junctions in high-temperature superconductors (Science 318, 1291).
A Josephson junction consists of two layers of superconducting materials separated by a thin insulating layer and has long been recognized as a classic example of the quantum tunneling effect. If a voltage is applied across this junction, it can generate an alternating supercurrent, leading to the emission of photons at a frequency corresponding to the energy gap of the superconductor. In other words, the Josephson junction can produce electromagnetic radiation.
Unfortunately, the energy gaps in laboratory-generated Josephson junctions based on conventional low-temperature superconductors (e.g., Niobium, Nb) are too small to produce radiation in the THz range, and the output power is also quite low. Researchers have attempted to employ techniques to create arrays of junctions to enhance output power, but synchronizing these junctions has proven difficult, making the generation of combined electromagnetic radiation even more challenging.
Figure 1. Device structure from Welp’s team (Science 318, 1291).
Ulrich Welp (from Argonne National Laboratory, USA) and his colleagues have confirmed that both issues can be resolved using high-temperature superconducting materials. Unlike low-temperature superconductors, high-temperature superconductors do not need to be engineered into the Josephson junction because they inherently contain a sufficient amount of material throughout the single-layer structures. Additionally, they possess a relatively large energy gap, allowing them to emit radiation in the THz spectrum. More importantly, Welp’s group has discovered a very straightforward method to synchronize the emissions (the phases of the waves emitted from the Josephson junctions in high-temperature superconductors), enabling output power levels in the milliwatt (mW) range. “We can envision a variety of applications such as probing, imaging… utilizing THz radiation at this power range,” Welp stated.
The research team used the high-temperature superconductor Bi2Sr2CaCu2O8, commonly known as BSCCO, with intrinsic Josephson junctions continuously formed and arranged between CuO2 superconducting layers with insulating layers of BiO and SrO. When a voltage is applied across the BSCCO sample, these layers emit electromagnetic radiation at a specific frequency but do not phase-synchronize. Similar to lasers, the trick to achieve coherent emission is to adjust the voltage until the emitted frequency matches the resonant frequency of the cavity. At this frequency, the electric fields compensate each other in phase, aiding in the synchronization of the radiation. Initially, only a few junctions were in phase, but this effect was dramatically enhanced through feedback mechanisms, resulting in the entire spectrum of emitted waves being phase-synchronized.
Figure 2. Results of the emitted radiation frequency (Science 318, 1291).
Welp’s research team fabricated BSCCO samples with a height of 300 µm, creating a system with 200,000 intrinsic Josephson junctions, emitting power levels around 0.5 µW at frequencies up to 0.85 THz.
Welp expressed his hope that with further optimization, the output power could reach 1 mW. This power level could be utilized, for example, at airports to detect traces of explosives, although he acknowledges that there are challenges in commercializing this device. “In general, the higher the power, the better the signal-to-noise ratio, allowing for faster and more accurate imaging applications” Welp added.
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