Closed-loop control of intense-laser fragmentation ofS8

Abstract

A liquid-crystal-based, laser-pulse shaper has been used in combination with an adaptive genetic feedback algorithm to investigate closed-loop control of intense laser fragmentation of ${\mathrm{S}}_{8}$ molecules. We observe that the yield ratios $\mathrm{S}_{N}{}^{+}:\mathrm{S}_{M}{}^{+}$, for the production of specific charged fragments $\mathrm{S}_{N}{}^{+}$ and $\mathrm{S}_{M}{}^{+}$, can be enhanced by $>300%$ relative to those observed using transform-limited $150\text{\ensuremath{-}}\mathrm{fs}$ laser pulses. We have explored the effectiveness of time- and frequency-domain pulse parametrizations while shaping either (i) only the spectral-phase distribution or (ii) the spectral-phase and amplitude distributions of the light. We find that pulse complexity, requiring control beyond simple manipulation of the peak pulse intensity and duration, is critical for optimizing the yield ratios for most species. The ``optimum'' pulse shapes obtained using different pulse parametrizations show significant differences while yielding similar signal enhancements. In some cases, comparison of the different optimum pulse shapes appears to be a useful method for identifying pulse traits that are, or are not, important for manipulating a particular yield ratio. The importance of specific traits in the optimum pulse shapes is also explored numerically using principal control analysis. We conclude that closed-loop control can be effective for optimizing highly nonlinear strong-field processes. However, in general, intensity variations in a focused laser beam severely limit one's ability to associate the optimization results with specific dynamical mechanisms that bear primary responsibility for the control.