Pump-Probe Experiments with a Single Molecule: ac-Stark Effect and Nonlinear Optical Response

We have measured the ac-Stark effect on single terrylene molecules in $p$-terphenyl crystals at superfluid helium temperatures. With the pump beam far enough from resonance, the molecular transition shifts as expected, i.e., proportional to the pump intensity and to the inverse pump detuning. Experiments closer to resonance with stronger beams show Autler-Townes-like structures that can be well reproduced by means of Bloch equations with two intense laser fields. These experiments are a demonstration of nonlinear optical effects with the localized electronic states of a single molecule trapped in a solid.