We performed a series of successful experiments for the optimization of the population transfer from the ground to the first excited state in a complex solvated molecule (rhodamine 101 in methanol) using shaped excitation pulses at very low intensities (1 absorbed photon per 100–500 molecules per pulse). We found that the population transfer can be controlled and significantly enhanced by applying excitation laser pulses with crafted pulse shapes. The optimal shape was found in feedback-controlled experiments using a genetic search algorithm. The temporal profile of the optimal excitation pulse corresponds to a comb of subpulses regularly spaced by , whereas its spectrum consists of a series of well-resolved peaks spaced apart by approximately 6.5 nm corresponding to a frequency of . This frequency matches very well with the frequency modulation of the population kinetics (period of ), observed by excitation with a short transform-limited laser pulse directly after excitation. In addition, an antioptimization experiment was performed under the same conditions. The difference in the population of the excited state for the optimal and antioptimal pulses reaches even at very weak excitation. The results of optimization are reproducible and have clear physical meaning.
The saturation energy density corresponds to the magnitude of the excitation which reduces the absorption coefficient of a thin optical layer by times (see, e.g., Ref. 22)
Note that in this work Ref. 31 only the spectral position, amplitude, spectral width, magnitude, and sign of the linear and quadratic chirp in the excitation pulse were controlled.
The actinic (absorbed) energy of 0.5 nJ (see inset in Fig. 9) corresponds to the photon density of for our experimental conditions. The concentration of molecules in the solvent can be calculated using the extinction coefficient for R101 (Ref. 25) to be . For a path length of 0.02 cm a ratio of excited to nonexcited molecules is .