Abstract: An optical frequency comb setup including a semiconductor cascade laser drivable by a laser driver, emitting a laser beam through an end facet of the semiconductor cascade laser with a frequency comb with at least two given individual emission frequencies, repetition frequency, carrier envelope offset frequency shows improved comb stability and/or comb formation and/or comb bandwidth. This is achieved by an external cavity added outside of the cavity of the semiconductor cascade laser, having a reflective element with a mirror surface reflecting the at least two individual emission frequencies being arranged in a relative distance to the end facet allowing to adapt repetition frequency and/or carrier envelope offset frequency and/or the dispersion seen by the light in the optical frequency comb setup.

Abstract: A method for generating an ultrashort charged particle beam, comprising creating a high intensity longitudinal E-field by shaping and tightly focusing, in an on-axis geometry, a substantially radially polarized laser beam, and using the high intensity longitudinal E-field for interaction with a medium to accelerate charged particles.