Imaging Microwave Currents in Planar Microwave Devices
We have developed a microwave microscope that can image the distribution of RF currents in resonant microwave devices, including planar filters. We have imaged current distributions at the edges of patterned thin films and observe large enhancements of current density at these edges. In superconducting devices these enhancements can give rise to breakdown of the superconducting state and admission of RF flux in the form of discrete vortices. This in turn leads to a strong nonlinear response and intermodulation of two or more signals. We will demonstrate the power of this microscope by examining the microscopic properties of an RF SQUID metamaterial. The SQUID metamaterials have a collective resonant response between 10 and 20GHz, tuned by very small DC magnetic fields. Through RF current imaging we find that the SQUIDs do not all oscillate in phase while being tuned, but break in to domains with different resonant frequencies. These domains can be directly imaged with the microwave microscope through their RF current magnitude distribution. This imaging gives us deep insights into the practical applications of RF SQUID metamaterials, for example as agile tunable notch filters for RF interference mitigation.