Generation of femtosecond to sub-femtosecond x-ray pulses in free-electron lasers

Generation of high power, femtosecond to sub-femtosecond x-ray pulses is attracting much attention within the x-ray free-electron laser (FEL) user community. At the existing FEL facilities, such as the Linac Coherent Light Source at SLAC, several methods have been developed to produce such short x-rays. Low-charge operation mode and emittance-spoiling scheme have successfully delivered short pulses for user experiments with duration less than 10 fs. A nonlinear compression mode has been recently developed and the pulse duration could be about 200 as. We will review the recent experimental progress at the LCLS for achieving few-femtosecond x-rays, and also discuss other short pulse schemes for reaching sub-femtosecond regime.