Coherent x-rays driven by ultrashort-pulse lasers: generation, application, and prospects

Ultrashort laser pulses represent an ideal starting point for frequency conversion of light to almost any wavelength from the THz to x-rays. High-harmonic upconversion (HHG) is a unique process enabled by the combined strong field laser field and the few-cycle pulse duration of a femtosecond laser pulse. HHG makes it possible to generate coherent light in the spectral region from the vacuum-UV into the x-ray region at sub-nm wavelengths. HHG sources are now finding increasingly diverse application for both science and technology, in topics ranging from basic studies of atomic processes, to materials dynamics revealed through time and angle-resolved photoemission. Furthermore, the coherent nature of the HHG process makes possible unprecedented control over light in a new region of the spectrum, making it possible to, for example, control the polarization state and spectral bandwidth, creating the most complex time-domain waveforms ever measured and characterized. Here we review recent work, as well as efforts at commercial implementation of HHG sources.