Characterization of laser pulses with a time-dependent ellipticity for the generation of attosecond x-ray pulses

Shambhu Ghimire, Bing Shan, Zenghu Chang
J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS 66506, USA

According to the semiclassical model, high order harmonic radiation is generated during the re-collision process, leading to a harmonic pulse duration that is less than the linearly polarized portion of a laser pulse. We reported earlier the generation of a single attosecond pulse at the plateau of the harmonics by using laser pulses whose linear portion was about a half of a laser cycle. We report here the characterization of such laser pulses with the frequency-resolved optical gating (FROG) technique.

The laser pulses with a time-dependent ellipticity were obtained with birefringence optics. A linearly polarized sub-10 fs pulses was split into both an o-ray and an e-ray in a 0.5 mm quartz plate. A 15 fs delay between them was introduced.Then, the two rays were recombined with a quarter wave plate to generate a pulse whose polarization changed from circular to linear and then back to circular again in time. The duration of the linear portion was estimated to be less than one optical cycle.

The second harmonic FROG used a thin BBO crystal with type I phase-matching (oo-e), i.e. only the ordinary wave in the crystal was measured. The laser pulses with a time-dependent ellipticity have two orthogonal polarization components. First, we oriented the x polarization of the laser pulse as the ordinary wave in the BBO to obtain the intensity and phase of this polarization component. Then, the y polarization was measured by orienting it as the ordinary wave. The x and y results were combined to yield the time-dependent ellipticity.

This work was supported by the Chemical Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.

Submitted to Ultrafast X-Ray Science, April 2004 in San Diegeo, CA.

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