Strong-field-ionization suppression by light-field control

In recent attempts to control strong-field phenomena such as molecular dissociation, undesired ionization sometimes seriously limited the outcome. In this work we examine the capability of quantum optimal control theory to suppress the ionization by rational pulse shaping. Using a simple model system and the ground-state occupation as the target functional, we show that optimal control generally leads to a significant suppression of the ionization, although the fluence and the pulse length are kept fixed. In the low-frequency regime the ionization is reduced mainly by avoiding high peaks in the intensity and thus preventing tunneling. In contrast, at high frequencies in the extreme ultraviolet regime the optimized pulses strongly couple with the (de)-excitations of the system, which leads to different pulse characteristics. Finally, we show that the applied target functional works, to some extent, for the enhancement of the high-order-harmonic generation, although further developments in optimal control theory to find proper target functionals are required.

Figure 1

Ionization yield as a function of the pulse electric-field amplitude and frequency in a one-dimensional model atom. The pulse length is fixed to $T=826.8$ (20 fs). The results from OCT are shown as solid circles. The OCT pulses have the same fluence and length as the nonoptimized ones, but they contain several frequencies up to ${\omega }_{\max }=0.0654$ a.u. (700 nm).

Figure 2

(a) Initial laser pulse with ${\omega }_0=0.0459$ a.u. (dashed line) and the optimized pulse (solid line) with a fixed fluence $F_0=2.5$. The resulting ionization yields are about $60\%$ and $6\%$, respectively. (b) Fourier spectra of the pulses. (c) Occupation of the ground state during the process.

Figure 3

(a) Initial laser pulse with ${\omega }_0=0.4$ a.u. (dashed line) and the optimized pulse (solid line) with a fixed fluence $F_0=1.6$. (b) GS and total bound-state occupations during the initial (ini) and optimized (OCT) pulses. The resulting ionization yields are about $40\%$ and $9\%$, respectively.

Figure 4

(a) Spectrum of the optimized pulse (inset) to maximize the ground-state occupation. (b) HHG spectrum obtained with the optimized pulse in (a) compared with the results from three single-frequency pulses having the dominant frequencies of the optimized pulse [dashed lines in (a)]. The theoretical HHG cutoffs for pulses with ${\omega }_0=0.035$ and $0.05$ are marked by dashed lines in (b).