Coherent Control of Single Molecules via Phase-Shaped Two-Photon Excitation at Room Temperature

Coherent control has been achieved in atoms and small molecules in gas phase during the past few
decades. An intriguing demonstration of coherent control is a so-called “dark pulse” that cancels 2-photon
transition probabilities despite exposing the target to the full power spectrum of transform-limited laser
pulses. However, for larger functional molecules in condensed phase at room temperature, ensemble
measurements do typically not allow exerting full control over competing pathways due to the unavoidable
influence of the surrounding (mostly complex) environment. Here, we demonstrate room-temperature
coherent control exploiting a nonresonant 2-photon transition into a higher excited state of single
conjugated polymer chains embedded in a disordered matrix, including proof-of-principle experiments
on bulk films. To manipulate the 2-photon transition probability, we exploit complex pulse sequences,
created by a systematically varied cosinusoidal spectral phase applied to the excitation laser spectrum.
For single molecules, the phase-dependent response varies from molecule to molecule, which reflects
the spectral heterogeneity (position, linewidth) of their 2-photon transitions. These data indicate that
coherent control of single molecules requires optimization of parameters for each individual molecule.
The experimental data are reproduced by a simple model that allows to directly retrieve the 2-photon
absorption spectrum of each single molecule. Our coherent-control approach is a powerful and robust
way to obtain spectral characteristics of higher excited states of single molecules and to manipulate the
excited-state dynamics in condensed phase at room temperature. It holds the potential to be useful for
the characterization of complex organic functional materials