ANALYSIS OF EFFECTIVE COUPLING STRENGTH IN HIGH-FREQUENCY SQUIDS

Understanding the coupling mechanism between SQUID (superconducting qu antum interference device) loop and resonator in high-frequency SQUIDs is essential for the application of these devices in sensitive magnet ometers or controllable microwave devices. We have investigated, there fore, two kinds of planar microstrip tank circuit resonators: (1) S-sh aped lambda/2-resonators with direct or indirect coupling to the SQUID loop with optional flux-focussing pads and (2) conventional rf-washer -SQUID structures for which we have found resonant modes of the washer providing a coupling to the SQUID loop. A lumped element representati on of SQUID loop and resonator is presented in order to describe the e ffective SQUID-resonator coupling in case of direct or indirect coupli ng. According to this approach, the square of-the effective coupling c onstant k in both cases is equal to the ratio of the magnetic energy s tored in the SQUID loop to that stored in the resonator. For the indir ect coupled S-shaped lambda/2-resonators, k can be described in terms of a geometry-defined mutual inductance. The predictions of the lumped element description are compared with simulations using a standard fu ll-wave analysis computer program in order to quantify the effect of t he different layouts on microwave current distribution and on k. A met hod for the determination of k by simulation is described, leading to useful design rules for the optimization of high-frequency SQUIDs. The simulation results will also be compared with experimental data. (C) 1995 American Institute of Physics.