Spatially resolved microwave field distribution in YBaCuO disk resonators visualized by laser scanning

Abstract

The applicability of superconducting films in high-performance microwave devices suffers from inhomogeneous growth and local defects. Therefore we have installed a laser scanning system to visualize with spatial resolution the distribution of the microwave fields in 2-in.-diam YBaCuO disk resonators. The TM310 mode was imaged at a loaded quality factor of 8.3×104 via changes of the resonant frequency at 2.21 GHz and 40 K. The field distribution agreed with a magnetic-wall model, indicating at the contour of the disk fringe fields, but without strong edge enhancement. The data also indicated an inhomogeneous effective microwave penetration depth. The spatial resolution is presently limited to about 2 mm by heat diffusion.

Imaging local sources of intermodulation in superconducting microwave devices

“…A synthesized signal generator (transmitted microwave power to 0 dBm) excited a resonant mode of the CPW resonator. The local heating of the sample by the "hot-spot" of a focused laser beam shifted both and of the device due to changes in the local magnetic penetration depth and the stored energy in the resonator [12], [13], [17]. This caused a change in the transmission curve that is proportional to the local rf current density squared [ ] and leads to the change in transmitted power given by [12], [18] (1)…”

“…Among these efforts, the LSM observations [12], [13] suffered from modest spatial resolution and failed to see an expected re-distribution of current in an HTS resonator as the rf power increased [12]. Room temperature modulated optical reflectance measures the local carrier density, and can be used to find defects that affect the superconducting microwave performance.…”

Microwave Current Imaging in Passive HTS Components by Low-Temperature Laser Scanning Microscopy (LTLSM)

“…A synthesized signal generator (transmitted microwave power ≈ -15 to 0 dBm) excited a resonant mode of the CPW resonator. The absorbed laser power produces a thermal spot on the HTS film surface at any location x, y of the probe, oscillating in time with the frequency of laser beam modulation f m. The thermal interaction of the probe with the sample shifted both f0 and Q of the device due to changes in the local magnetic penetration depth and the stored energy in the resonator [32,34,39]. This caused a change in the S12(f) transmission curve that is proportional to the local rf current density squared JRF 2 (x,y) at the location of the probe, and leads to a change in transmitted power P [see Fig.…”

“…The second (rf current imaging) mode produces a map of local rf current density [32,34,38]. A synthesized signal generator (transmitted microwave power ≈ -15 to 0 dBm) excited a resonant mode of the CPW resonator.…”

“…It is possible to manifest local resistive characterization of the SC transition [28][29][30], the spatial variations of magnetic and electromagnetic fields [31][32][33][34], and to carry out 2-D defectoscopy of structural and technological faults in operating SC devices and circuits [35][36][37]. In this work, we apply the LTLSM technique to analyze the redistribution of rf current flow in the vicinity of typical microdefects in YBCO films leading to anomalously high rf photoresponse, and possibly to nonlinear microwave behavior of SC devices.…”