Influence of microscopic defects in type-II superconducting thin films on the magnetic flux penetration

The magnetic flux penetration into thin type-II superconducting films with circular defects is investigated. The artificial circular defects (diameter = 40 μm) in an YBa2Cu3O72d thin film (thickness = 300 μm) were prepared by pulse-laser irradiation. The flux penetration into the zero-field-cooled superconducting film was visualized by means of the magneto-optic method. A stepwise increase of the external magnetic field allowed a detailed investigation of the influence of local defects on the flux penetration. For a magnetic field parallel to a long sample (longitudinal geometry) with a long cylindrical defect a single parabolic discontinuity line appears. Also in the case of a thin superconducting film exposed to a transverse magnetic field (transverse geometry), a single parabolic discontinuity line has been supposed in the vicinity of a local defect. On the contrary, our investigations show that the flux and current distribution around a single defect in a superconducting thin film can be determined not by a single, but by two discontinuity parabolas. In thin superconducting films in transverse geometry screening currents in the Meissner region (j front

Integrated optical cross strip interferometer

A thick, bimodal segment of specific length and height between two single mode sections of a planar waveguide can serve as an integrated optical interferometer. It is realized by etching a wide strip form a guiding film. A vertically guided, laterally unguided beam of light is then made to traverse the strip perpendicularly. For a wide range of materials the structure can be dimensioned such that it shows the proper behavior of an interferometer: depending on the phase gain of the two modes in the thick region, the guided light interferes either almost completely destructively at the transition to the output segment, i.e. the power is radiated away into the substrate and cover regions, or constructively, i.e. most of the power passes the device. We believe that for certain applications structures of this kind can be a simple substitute for instruments like Mach-Zehnder interferometers or directional couplers. This is illustrated by two numerically simulated examples: A polarizer constructed from silicon based waveguides, which offers 30 dB polarization discrimination and 0.1 dB insertion loss with a total length of only 10 micrometers, and a proposal for an integrated magneto optic isolator experiment, where the freedom in the lateral direction can be exploited for a proper tuning of the device

Structures in superconducting YBa2Cu3O7-delta thin films investigated by magneto-optic technique

Using a reflection magneto-optic technique we have investigated natural inhomogeneities and artificial struetures in YBCO thin films exposed to an external magnetic field. The artificial structures were mechanically scratched by scanning a diamond tip with different loading over the film, surface. Alternatively planar structures with reduced oxygen content could be patterned by heating the YBCO film, with a focused laser beam in nitrogen atmosphere. Depending on the laser annealing parameters different screening properties concerning the applied magnetic field could be achieved. As a magneto-optically active layer we used EuS films evaporated on glass as well as bismuth- and gallium-doped lutetium-iron-garnet films grown onto (111) oriented gadolinium-gallium-garnet substrates by liquid phase epitaxy. In contrast to measurements with EuS films that show only weak faraday rotation for temperatures higher than 20 K the magneto-optic studies have been expanded to about 60 K by using the garnet films

Zerstörungsfreie magnetooptische Charakterisierung von natürlichen und künstlichen Defekten an 3-Zoll-HTSL-Wafern bei 77K

Doppelsei tige 3-Zoll-HTSL-Wafer wurden magnetooptisch charakterisiert. Der vorgestellte Aufbau erlaubt einen schnellen und zerstorungsfreien Nachweis von lokalen und ausgedehnten Inhornogenitiiten in der kritischen Stromdichte rnit hoher Ortsauflosung im Mikrometerbereich. Zusatzliche Goldschichten auf den HTSL-Wafem, wie sie manchmal in der Bauteilherstellung eingesetzt werden, beeinflussen nicht das Ergebnis der Messungen. Die hohe rnagnetische Empfindlichkeit erlaubt sogar den Nachweis lokaler Defekte bei 77 K, wo der Kontrast in den kntischen Stromen schwiicher ist und die Charakterisierung von HTSLFilmen vie1 schwieriger ist als bei tieferen Temperaturen. Der Aufbau kann daher selbst unter Bedjngungen eingesetzt werden, bei denen Kiihlung rnittels Flussig-Helium oder Kleinkuhler nicht zur Verfugung steht. Die Empfindljchkeit wurde an natiirlichen und lainstlichen Defekten iiberpriift, wobei letztere durch fokussierte Laserstrahlung hergestellt wurden

Nondestructive magneto-optical characterization of natural and artificial fefects on 3" HTSC Wafers at liquid nitrogen temperature

Double-sided 3" HTSC Wafers were characterized by the magneto-optic technique. The presented apparatus allows a nondestructive and fast detection of local and extended inhomogeneities in the critical current density with high lateral resolution in the micrometer range. Additional gold-layers on the HTSC wafers, as they are sometmes used for the device production, do not influence the characterization result. The high sensitivity of the presented apparatus allows even the detection of local defects at higher temperature (77 K) where contrasts in the critical current are weaker and the magneto-optical characterization of HTSC thin Alms is milch more difficult than at lower temperatures. So the apparatus can be used even under conditions where cooling with liquid helium or closed-cycle refrigerators is not available. The sensitivity was tested on natural and artificial defects, the latter being prepared by means of a focused laser beam