Microwave and modulated optical reflectance studies of YBCO thin films

Planar HTS microwave devices require high quality, homogeneous samples. In this paper, sensitive measurements of the microwave surface impedance of YBCO thin films using coplanar resonators are collated with modulated optical reflectance (MOR) measurements. MOR provides a powerful noncontact, nondestructive and high resolution means of probing local variations in the quality of thin films at room temperature, and consequently has great potential for diagnostic testing of HTS films prior to microwave device patterning. Microwave and MOR inter-comparisons of four YBCO films patterned into 5.2 and 8 GHz coplanar resonators are presented. Superior global microwave response in the superconducting state, such as low surface impedance and low levels of nonlinearity at enhanced powers, correlate with the magnitude and spatial homogeneity of the room temperature MOR signals. The presence of defects in films is investigated using both techniques. Both large scale single defects and film inhomogeneity can be detected using MOR; however, the spatial resolution of the technique is not sufficient to detect single weak links on a sub-micron scale, whose presence can result in severely degraded microwave resonator performance.