Fabbricazione e caratterizzazione di dispositivi superconduttivi basati sulle giunzioni di Josephson intrinseche del BSCCO

Superconducting THz devices are becoming more and more important since this range of the radiation spectrum is still difficult to be detected and emitted with other kinds of technologies, consequently the future repercussions of this technology may be of high relevance. Indeed, thanks to its high frequency (around 1000 times higher than radio waves), its easy and fast tunability and the fact that the emitted radiation can be almost monochromatic, this technology can be used to analyze phenomena whose energies are in the order of 10 μeV and also for non-invasive imaging and for high-speed telecommunications. A very important property of superconducting THz devices is that a stack of them connected in series is able to produce coherent radiation. Thus a single Josephson junction can emit THz radiation with a total power of the order of pW, while hundreds of them can emit a total power of the order of mW due to their coherence. However, to achieve this result it is necessary to use highly pure materials in the form monocrystals, which represent the starting blocks for producing these stacks of Josephson junctions (also known as mesa) with areas of the order of a few square μm. The ultimate goal of my thesis project was to make a device consisting of a stack of several coherent Josephson junctions that emitted and to detected THz radiation. To this purpose, during this work I produced and characterized a number of devices based on the BSCCO, which is a high-temperature superconductor with a layered crystal structure that can be viewed as a series of Josephson junctions along the direction perpendicular to the superconducting layers. My thesis project consists of two parts: the first one took place at the University of Torino and the second one at the Stockholms Universitet. In Torino I synthesized BSCCO whiskers, which are submillimetre high-purity monocrystals. I characterized them by measuring their variation of resistance in function of the temperature, their superconducting critical temperatures and their composition. Indeed, BSCCO exists in several crystal phases with different stoichiometry and during my work two phases have been observed: the Bi2Sr2CaCu2O8+δ (also known as 2212) phase and the Bi2Sr2Ca2Cu3O10+δ (also known as 2223) phase. However, only the 2212 phase is known to be able to generate THz radiation, so that selecting single crystals with the proper stoichiometry represents an important starting point for the fabrication procedure. In Stockholm I produced the devices intended for THz measurements. I have been working in the clean room of the Albanova Research Centre and I used several fabrication techniques such as cleaving, photolithography, e-beam evaporation, FIB, SEM and plasma etching. These samples were used not only to further investigate BSCCO properties in an extended temperature range but principally for producing and detecting THz radiation which has been characterized with different techniques in different temperature conditions and in the presence of different magnetic fields. I have also fabricated some logarithmic periodic THz antennas based on a Niobium/Aluminium Oxide Josephson junction to be used as detectors for the THz radiation. The results show that a significant power has been emitted in the THz range and were confirmed also by using a bolometer. Further experiments will be performed on these samples to accurately measure the power and the frequency of the emitted frequency.