Controlling the spectral shape of coherent soft x-rays

Full text: Coherent control techniques allow to trace and control the behavior of quantum systems on their natural time scale. For molecular dynamics on the order of femtoseconds, this is achieved by applying ultrashort coherent light fields in the wavelength range from the IR to the UV, variably shaped in space and time using a laser pulse shaper consisting of some adaptive device such as a deformable mirror or a liquid-crystal display. However, to follow and to control the electronic motion in real time, light pulses with durations of attoseconds and with photon energies in the XUV to soft x-ray range are required. High-harmonic generation yields the required photon energies and pulse durations in the femtosecond range (for single harmonics) down to attoseconds (for a range of harmonics). However, the harmonic spectrum normally has a fixed characteristic shape, exhibiting a plateau of equally spaced harmonics. We demonstrate the generation of arbitrarily shaped spectra of coherent soft x-ray radiation in a gas-filled hollow fiber by adaptive control of the driving 800 nm laser pulse. Using the same setup, Bartels et al. showed the optimization and enhancement of one harmonic. However, we achieve both enhancement and suppression of high-harmonic emission in a selected wavelength region as well as the enhancement of coherent soft x-ray radiation over a selectable extended range of harmonics. To our knowledge this is the first demonstration of the suppression of single plateau harmonics. Full control over the shape of the soft-x-ray spectrum has a major impact on energy-resolved spectroscopy and on the control of electron dynamics. The suppression of single harmonics can result in a sub-half-cycle beating due to an effective bimodal spectral distribution. This consideration demonstrates the potential of our technique for extending pulse shaping into the attosecond regime. We are therefore working on the characterization of our shaped pulses and their pulse durations using XFROG. This constitutes the first steps towards building an attosecond pulse shaper in the soft-x-ray domain. Refs. 4 (author)