Superconductors could plug the terahertz 'gap'
The THz frequency range of the electromagnetic spectrum remains poorly utilised. Semiconductor devices cannot produce such high frequencies and this range is too low for solid-state lasers. However, there are many potential applications for devices that operate between 0.3 and 30 THz in fields as wide-ranging as astronomy, biology, chemistry and physics. This multitude of applications attracted the attention of EU-funded researchers. Within the project THZ RADIATION (Controlling terahertz radiation using layered superconductors), they showed that Josephson vortices, moving through periodically modulated layered superconductors, can generate THz radiation. Josephson junctions, made by sandwiching a thin layer of insulating material between superconductors, display unusual quantum effects. In particular, voltage that is applied across a junction sets up an oscillating current, causing the junction to emit photons at a frequency matching the superconductors' energy gap. In other words, Josephson junctions produce electromagnetic radiation. It is difficult to synchronise junctions and make a coherent beam of radiation where all wavelengths are in-phase. Using advanced computer simulations, researchers investigated the effects of ordered out-of-plane magnetic dots on the radiation emitted. They found that applying both alternating current (AC) and direct current (DC) could solve this problem. Like a laser, the trick in making the emission in-phase is to vary the voltage applied until the emitted frequency matches the characteristic frequency of the system. At this frequency, oscillations induced by the vortex lattice in the different layers are in phase. Specifically, the junctions are 'encouraged' to synchronise when the moving vortices form a rectangular lattice. Phase synchronisation cannot be observed even if there are small deviations of the sidewalls of the system from their vertical position. Researchers searched for other boundary and initial conditions that affect the collective behaviour of arrays of Josephson junctions. Their goal was to identify factors contributing to less ordered dynamics of the square Josephson vortex lattice. The THZ RADIATION project findings will open the way for the development of devices emitting THz radiation whose frequency and power can be tuned over a wide range. The higher available power would enhance the signal-to-noise ratio leading to faster and more reliable sensing and imaging.
Keywords
Superconductors, electromagnetic radiation, THz frequency range, Josephson junctions, THZ RADIATION, sensing, imaging
