Liquid-phase adaptive femtosecond quantum control: Removing intrinsic intensity dependencies

Femtosecond adaptive pulse shaping of 800-nm laser pulses is applied to control the multiphoton molecular excitation of the charge-transfer coordination complex $[\mathrm{Ru}(\mathrm{dpb}{)}_3\mathrm{](}{\mathrm{PF}}_6{)}_2$ (where $\mathrm{dpb}{\text{=4,4}}^{\prime }-\mathrm{diphenyl}{\text{-2,2}}^{\prime }-\mathrm{bipyridine})$ dissolved in methanol. A phase-only femtosecond pulse shaper provides a mechanism for multiparameter (128) variation of the incident field, and a closed-loop evolutionary algorithm optimizes pulse shapes within the vast search space. Molecular emission at 620 nm is used as experimental feedback which is proportional to the excited-state population in the long-lived ${}^{\mathrm{3}}\mathrm{MLCT}$ (metal-to-ligand charge-transfer) state. The dominant intensity dependence of the multiphoton excitation process is removed by using second-harmonic generation (SHG) in a thin optical crystal as a general “reference” signal. Successful control of the emission/SHG ratio demands that the field adapt to the electronic structure or dynamic needs of the molecule in solution. This suggests that adaptive femtosecond pulse shaping can provide a general means of finding field shapes capable of selectively exciting molecules based on their unique optical properties.