Abstract

A common method of producing extreme-ultraviolet (XUV) radiation relies on nonlinear processes in gas atoms. Focusing intense femtosecond laser pulses on a gas target leads to the generation of high-order harmonics (HHG) of the fundamental radiation, reaching into the XUV or even X-ray range. External enhancement cavities constitute a highly efficient approach to HHG [1–3]. A MHz repetition rate laser system feeds its pulses into a passive resonator containing the focus for the gas target. After each pass through the focus, the non-converted portion of the light pulse is coherently overlapped with the successive pulse from the laser. This way, a power enhancement of several hundreds and intensities of 10¹³ W/cm² or more, which are necessary for HHG, can be reached in the cavity focus. As opposed to traditional HHG schemes based on low repetition rate amplifiers, in the case of MHz passive cavities the frequency comb structure of the seeding laser is expected to be transferred to and usable in the XUV range. So far, XUV generation enhancement cavities based on Ti:Sapphire [1,2] and Yb [3] lasers have been demonstrated.

© 2009 IEEE