Two-dimensional quasi-optical Josephson junction arrays for THz oscillators

A Josephson junction is a natural choice for a THz oscillator because of its typical voltage scale of mV and an oscillation frequency of f{dollar}sb{lcub}rm J{rcub}{dollar} = 484 GHz per mV of dc bias. The disadvantages of a single Josephson junction oscillator are its low power, low impedance, and broad linewidth. Arrays of Josephson junctions have been sought to solve these problems. This thesis describes a new scheme of Josephson junction arrays for THz oscillators. It is a two-dimensional (2-D) Josephson junction array integrated with a planar antenna array on quartz or silicon substrates. We call the device the Quasi-optical Josephson Oscillator (QJO). The power is coupled out quasi-optically from broadside of the arrays. We will use a large number of Josephson junctions up to 1,000,000. Computer simulations showed that the QJO will be capable of radiating 25 {dollar}mu{dollar}W at 1 THz from an array of 4,000 junctions. An array of 1,000,000 junctions would radiate 6.4 mW at this frequency: the power rises linearly with the number of junctions used. For this device to work, in-phase-locking among the junctions in the array has to be achieved. We have studied the phase-locking problem extensively using simplified circuit models and computer modeling. A QJO of 638 junctions in a 11 x 58 array format designed according to our phase-locking analysis has been fabricated using Nb tunnel junctions. Radiated power of 11 nW has been detected from this device off the chip using a liquid helium cooled bolometer. The power is lower than anticipated, but we know what the problems are. New chips with correct designs are under fabrication now. The QJO design issues and possible problems will be discussed in this thesis.