The Non-Linear Interaction of Visible Light and X-Rays: The Measurement of Femtosecond Duration Soft X-Ray Pulses

Abstract

We are developing a high resolution technique for the direct measurement of pulse durations of soft x-ray pulses that arise from laser-induced plasmas driven by sub-picosecond lasers. This technique relies on the ponderomotive effect (high intensity Stark Effect) of a high intensity (~1014W/cm2), 100 fsec visible laser on the absorption properties of core transitions in noble gases at soft x-ray wavelengths. We employ cross correlation in high density krypton of a 100 fsec 616nm laser pulse with an x-ray pulse produced from a laser plasma driven by a high intensity, 300 fsec 308nm laser. The 616nm laser effectively "switches" the absorption of the x-rays in the krypton off and on with a resolution exactly equal to its pulse duration. We have demonstrated the technique by making a preliminary measurement of the pulse duration of 100 eV soft x-ray pulses derived from the interaction of a 300 fsec, 3mj 308nm laser pulse on a gold rod. We observe a cross correlation signal on the order of 1 psec. This value is in rough agreement with computer modeling codes (LASNEX) for our experimental conditions. To our knowledge this is the only demonstration of a technique for the measurement of the duration of soft x-ray pulses with a resolution of better than a few picoseconds. In addition, it is an example of a rather rare experimental occurrence: the non-linear interaction of visible light and x-rays. This measurement technology is crucial to the development of a soft x-ray "framing" camera capable of recording changes in atomic positions in solids with a resolution sufficient to resolve electron-phonon interactions.