Theoretical aspects of femtosecond double-pump single-molecule spectroscopy. II. Strong-field regime.

We investigate femtosecond double-pump single-molecule signals in the strong-field regime, which is characterized by nonlinear scaling of the signal with the intensity of the pump pulses. The signals can be decomposed into population and coherence contributions. In contrast to the weak-field regime (in which only the coherence contribution is important) both contributions are relevant in the strong-field regime and reveal the vibrational dynamics of the chromophore. Other than in the weak-field regime, the detection of vibrational beatings is not limited by the electronic dephasing time of the chromophore. Moreover, the signals in the strong-field regime are more robust with respect to the environment-induced modulation of the chromophore parameters. It is shown that excited-state absorption in chromophores with three electronic states is reflected in the phase dependence of single-molecule signals. The simulations reveal that the information content of femtosecond double-pump single-molecule signals is enhanced in the strong-coupling regime.