Near-resonant four-wave mixing of attosecond extreme-ultraviolet pulses with near-infrared pulses in neon: Detection of electronic coherences

Abstract

Coherent narrow-band extreme-ultraviolet (EUV) light is generated by a near-resonant four-wave mixing (FWM) process between attosecond pulse trains and near-infrared pulses in neon gas. The near-resonant FWM process involves one vacuum-ultraviolet (VUV) photon and two near-infrared (NIR) photons and produces new higher-energy frequency components corresponding to the $ns/nd$ to ground-state $(2{s}^{2}2{p}^{6})$ transitions in the neon atom. The EUV emission exhibits small angular divergence (2 mrad) and monotonically increasing intensity over a pressure range of 0.5--16 Torr, suggesting phase matching in the production of the narrow-bandwidth coherent EUV light. In addition, time-resolved scans of the NIR nonlinear mixing process reveal the detection of a persistent, ultrafast bound electronic wave packet based on a coherent superposition initiated by the VUV pulse in the neon atoms. This FWM process using attosecond pulses offers a means for both efficient narrow-band EUV source generation and time-resolved investigations of ultrafast dynamics.