| We have investigated properties of laser switched niobium in superconducting circuits. The switches consist of 10-40 nm thick niobium lines which can be driven normal by laser light sent down an optical fiber. These switching events have been monitored and measured by both rf and dc SQUIDs.; We have used these switches to implement optically induced inductance modulation of a niobium coil. In a series of designs, we have increased the modulation ratio from 1.0:1.1 to nearly 1:20. These ratios are in very good agreement with a detailed three-dimensional model developed to study these inductance effects. These modulated inductors can be used as low noise, constant flux superconductive switches, and we have designed 1/f noise reduction chopping circuits using these elements.; In the course of this modulation work, we have encountered flux trapping within the superconducting line during the switching process. In a series of tests, we were able to rule out a number of possible causes of this flux trapping, and using the results from a mechanical fiber optic switch, we hypothesize that the speckle pattern of the light from the fiber is primarily responsible for trapping. This hypothesis is supported by experiments with a CCD imaging system.; Because we can reliably trap single vortices with our laser switching, we were able to modify our system to measure the temperature dependent pinning force of vortices trapped in our superconducting line. The absolute values agree well with other experimenters' estimations based on de-pinning and movement of groups of vortices. The temperature dependence also agreed qualitatively with those results, as well as some theoretical predictions.; We have attempted to adopt these techniques to YBCO superconductors; however, we have so far been unable to drive these lines normal with our existing laser systems. We have obtained a single mode laser system capable of much higher power densities which we plan to use with our existing high-Tc samples. |
