Signal enhancement techniques for rf SQUID based magnetic imaging systems

Abstract

We have investigated the rf SQUID (radio-frequency superconducting quantuminterference device) and its coupling to tank circuit configurations to achieve an optimalfront-end assembly for sensitive and high spatial resolution magnetic imagingsystems. The investigation of the YBCO rf SQUID coupling to the conventionalLC tank circuits revealed that coupling from the back of the SQUID substrate enhances theSQUID signal while facilitating the front-end assembly configuration. The optimalthickness of the substrate material between the SQUID and the tank circuit is 0.4 mm forLaAlO3 resulting in an increase of the SQUID flux–voltage transfer function signal,Vspp, of 1.5 times,and 0.5 mm for SrTiO3 with an increase of Vspp of 1.62 times compared to that for direct face to face couplings. For rf coupling with acoplanar resonator, it has been found that the best configuration, in which a resonator issandwiched between the SQUID substrate and the resonator substrate, provides aVspp about 3.4 times higher than that for the worse case where the resonator and the SQUID arecoupled back to back. The use of a resonator leads to a limitation of the achievable spatialresolution due to its flux focusing characteristics. This resulted in a favouring of the use of theconventional tank circuits when considering the desired high spatial resolution. The effectof the YBCO flip chip magnetic shielding of the SQUIDs in the back-coupling with theLC tank circuit configuration has also been investigated, with a view to reducing the SQUIDeffective area to increase the spatial resolution and also for studying the effect of thecoupling of various kinds of transformers to the SQUIDs. It is revealed that there is no veryconsiderable change in the flux–voltage transfer function signal level with respect tothe effective shield area, while the lowest working temperature of the SQUIDswas slightly shifted higher by a couple of degrees, depending on the shield area.