Coherent Generation and Detection of THz Radiation via Josephson Junction Arrays

by Roger Cattaneo (Stockholm University)

Friday 15 Nov 2024, 13:00 → 16:00 Europe/Stockholm

Albanovägen 18, Lärosal 19, (Albano Building 2)

Abstract
The terahertz (THz) range of the electromagnetic spectrum, informally called the "THz gap", is one of the last parts that are
still challenging for both detection and generation. THz frequencies are too high for conventional electronics, and the energy
of THz photons is too low for photonics. Numerous prototypes of THz sources already exist, based on several different
technologies: quantum cascade lasers, modulated electron beams, etc. In recent years, however, superconductive sources
consisting of specifically designed arrays of Josephson junctions (JJs) have been demonstrated to be valid candidates to
fill this gap. They are capable of monochromatic, continuous wave emission, with high efficiency. These properties derive
from the phase-locking of many JJs and a good impedance matching with free space. In this thesis, I present a detailed
analysis of THz sources based on stacks of JJs etched on Bi2Sr2CaCu2O8+δ crystals, and linear arrays based on Nb/NbxSi1−x/
Nb JJs. In the first case, an efficiency of 12% was measured via a novel technique based on radiation cooling. In the second
one, I focused on the mechanism that synchronizes the JJs, indicated by the formation of steps on the IV characteristic. Their
appearance has been correlated to an enhancement of the emission and shows that the synchronization is not direct, but
instead caused by cavity modes developing inside the electrodes. Concerning THz detection several technologies nowadays
work in this range: Golay cells, bolometers, semiconductor-based ones, etc., but still not yet with high sensitivity. Single
JJ detectors area good option, but in this thesis, I show that the synchronization of stacks or linear arrays of hysteretic
JJs can also be exploited to create a cascade switching current detector (CSCD), boosting the sensitivity by several orders
of magnitude up to 1013 V/W. The results presented in this thesis show that devices based on linear arrays and stacks of
JJs are a valid candidate to create detectors and sources operating in the THz range. Many different fields would benefit
from a reliable, efficient, tunable THz source as well as an ultra-sensitive THz detector: high-speed telecommunications,
spectroscopy, non-ionizing medical imaging, security screening, astronomical observation, and so on.