Femtosecond laser pulse shaping for enhanced ionization

We demonstrate how the shape of femtosecond laser pulses can be tailored in order to obtain maximal ionization of atoms or molecules. For that purpose, we have overlayed a direct- optimization scheme on top of a fully unconstrained computation of the three-dimensional time- dependent Schrodinger equation. The procedure looks for pulses that maintain the same total length and integrated intensity or fluence as a given pulse that serves as an initial guess. It allows, however, for changes in frequencies —within a certain, predefined range— and overall shape, leading to enhanced ionization. We illustrate the scheme by calculating ionization yields for the H + molecule when irradiated with short (≈ 5 fs), high-intensity laser pulses. The magnitude of the obtained enhancement, as well as the shape of the solution optimal field depend strongly on the constrains imposed on the search space. In particular, when only small frequencies are allowed, the solution merely increases the peak intensity through temporal compression, as expected from a simple tunneling picture. If larger frequencies are allowed the structure of the solution field is more complicated. Copyright c EPLA, 2009