Abstract

The transition from the multiphoton to the tunneling regime for ionization is characterized by the Keldysh parameter $\gamma =\sqrt{I_p/2U_p}$, where Up ∝ I ×λ². A value of γ smaller than 1 implies tunnel ionization. The scaling of the Keldysh parameter with the wavelength allows studying atomic and molecular systems deep in the tunneling regime, when mid-infrared wavelengths are applied. In the present study we used a home-built few-cycle IR light source [1] with a center wavelength at 2.1 μm that is based on three OPCPA stages pumped by an optically synchronized 50 ps, 11 mJ Nd:YLF laser. Difference frequency mixing is employed for generating the IR seed, resulting in a signal that is self CEP-stabilized without the need for an active feed-back loop. The quality of the self-stablization of the CEP was demonstrated by imaging the directional control of the electron emission in the photoionization of xenon [2].

© 2011 Optical Society of America