Spectral encoding of x-ray/optical relative delay

We present a new technique for measuring the relative delay between a soft x-ray FEL pulse and an optical laser that indicates a sub 25 fs RMS measurement error. An ultra-short x-ray pulse photo-ionizes a semiconductor (Si3N4) membrane and changes the optical transmission. An optical continuum pulse with a temporally chirped bandwidth spanning 630 nm - 710 nm interacts with the membrane such that the timing of the x-ray pulse can be determined from the onset of the spectral modulation of the transmitted optical pulse. This experiment demonstrates a nearly in situ single-shot measurement of the x-ray pulse arrival time relative to the ultra-short optical pulse. (C) 2011 Optical Society of America