Shaping of single-cycle sub-50-attosecond pulses with multilayer mirrors

We report on the study of multilayer mirrors that were designed to compress extreme-ultraviolet pulses below 50 as. The mirrors were optimized in the time domain to obtain the desired pulse shape and duration when combined with a broadband hybrid filter. Moreover, they appeared to be very robust to environmental parameters, ensuring a pulse compression in various conditions. We manufactured the mirrors and characterized them on a synchrotron facility both in amplitude and in phase. A measurement of the latter was carried out using the photocurrent technique, which is the only method that allows one to access the mirrors' absolute spectral phase. This led to a measurement of the offset induced by the mirrors on the carrier-envelope phase of attosecond pulses.