Gunter Luepke

Femtosecond optical studies of cuprates

Femtosecond optical reflectivity measurements of La2-xSrxCuO4, La2CuO4+y, Bi2Sr2CuO6+z and Bi2Sr2CaCu2O8+δ thin films and single crystal samples indicate qualitative changes with fluence. At the lowest fluencies, there is a power law divergence in the relaxation time. The divergence has an onset temperature of 55±15K, independent of whether the sample is in the superconducting or normal states. At slightly higher fluencies, still perturbative, the additional response does not exhibit this power law divergence. At quite high fluencies- no longer perturbative- the metallic samples exhibit oscillations in the reflectivity amplitude. The period of these oscillations varies with the probe wavelength but not with the pump wavelength. The oscillations exhibit a decay time as long as 10 nsec.

Beam shaping of ultrashort laser pulses using diffractive optics: a theoretical study

We extended the study of a diffractive laser beam shaping system which is originally designed for continuous waves (SPIE vol. 2863, pp 237-45, 1996). The beam profile of an ultra-short laser pulse passing through this optical system is calculated. Two different numerical methods, Monte Carlo and Gaussian method, are applied to this problem. We find that the Gaussian method yields better results because the step size algorithms used in this method are well suited for this specific problem. The Gaussian numerical simulation shows that the fluence still yields a top-hat radial distribution. Beam shaping of ultra-short laser pulses using this optical scheme is feasible.

AC Current Driven Flux Dynamics in YBCO Coated Conductors

We present a time-resolved magneto-optical imaging study of YBa 2 Cu 3 O 7-δ coated conductors in alternating current regime. The evolution of the magnetic flux density distribution in the superconducting samples is imaged at small steps of the phase of the applied AC current. The flux penetration during the phase evolution is inhomogeneous due to the grain structure of the coated conductor. A quantitative analysis of the images show how grain boundary network affects the overall behavior of the flux and current density evolution.

Time Resolved Magneto-Optical Imaging in High Frequency AC Currents of YBa2Cu3O7-Delta Thin Films (Postprint)

Abstract : We present a time-resolved magneto-optical (MO) imaging study of high-temperature superconductor (HTS) in high-frequency alternating current (AC) regime. The evolution of the magnetic flux density distribution in YBa2Cu3O7-delta (YBCO) thin film samples is studied in small steps of the phase of the applied AC current. The flux distribution at 10 K exhibits instabilities including flux jumps and flux creep. A quantitative analysis of the data allows us to obtain the current density evolution. The current profile changes considerably with the phase differently from the prediction of the critical state model. These observations can be explained by the higher self-field at the sample edge and the effects of flux creep.

Large Nonlinear Kerr Angle in non-Centrosymmetric Fe/AlGaAs (001) Heterostructure

A large nonlinear magneto-optical effect is observed in a non-centrosymmetric Fe/AlGaAs (001) heterostructure. This effect is a direct consequence of interference between second-harmonic optical waves of magnetic and crystallographic origin, generated at ferromagnetic Fe interface and bulk AlGaAs, respectively. The longitudinal nonlinear Kerr rotation is measured to be 1.6 degrees along the [1-10] hard axis, about two orders of magnitude stronger than the linear equivalent. The rotational second-harmonic signal shows large magnetic contrast along all the in-plane directions, demonstrating a high sensitivity to the magnetization of an anisotropic interface in the longitudinal geometry.