Department of Physics, Stockholm University
Thursday 08 December
13:00 - 16:00
The THz range is one of the few portions of the electromagnetic spectrum where detection and, in particular, generation are still challenging. Many different fields would benefit from a reliable, efficient, tunable source: high-speed telecommunications, spectroscopy, non-ionizing medical imaging, security screening, etc. As of now, this result has been achieved only in part, still with considerable limitations, of which one of the hardest to overcome is their poor efficiency. Numerous prototypes of such sources have been developed and are based on several different technologies: semiconductors, as quantum cascade lasers; modulated electron beams as Backward Wave Oscillators; or down-conversion effects from non-linear optical processes. However, in recent years superconductive sources, which consist of specifically designed arrays of Josephson junctions, have been proven to be a valid candidate to fill this gap.
In this thesis, I present a detailed analysis of superconductive devices we fabricated for such purpose based on thin Bi2Sr2CaCu2O8+δ crystals. We etched stacks of Josephson junctions on them as they intrinsically contain alternating superconductive and non-superconductive layers in their atomic structure. The key to achieving a highly efficient and coherent THz emission from such arrays lies in two aspects: the synchronization of a large number of junctions, possible thanks to direct magnetic interactions, and a good impedance matching between the device and free space as the electrodes act as high-quality antennas. These properties allowed our devices to reach an electromagnetic power emission on the order of 100 µW, as well as a record-high efficiency of 12 %. This value is more than one order of magnitude larger than those reported previously for other THz sources, not too far from the theoretical limit of 50 %.
Further improvements, led by the results from our experiments and simulations, will be used in the future to broaden their tunability and achieve higher values both in power and efficiency. The latter, as well as having an on-chip device, is particularly relevant for building cheaper and portable THz sources.