Non Linear Spectroscopy of Single Molecules in Low Temperature Solids

Abstract

The fluorescence exciliition spectra ol small and dilute samples of solid solutions kept at helium temperatures display sharp homogeneous lines of individual solute molecules. Single molecule lines arc sensitive to external perturbations such as electric fields or pressurc. The fluctuations of the fluorescence signal show characteristic times ranging from nanoseconds to minutes. They give information about the dynamics within the molecule (intersystem crossing, Rabi nutation) as well i ~ s in the surrounding matrix (spectral diffusion of the resonance frequency). Non-linear optical experiments have been performed on single molecules. First, single pentacene molecules in p-terphenyl crystals demonstrate paramagnetic resonance in their triplet states. This non-linear efrect appears when the molecule is subjected to resonant microwaves in addition to optical excitation. Microwave pulses were used to produce transient effects, and were synchronised with the molecule itself. We performed non-linear optical experiments of the pump-probe type on the narrow excitation lines of single terrylene molecules in a pterphenyl crystal. At low saturations, the pump beam was found to shift the optical transition of the molecule as expected : proportionally to the intensity and inverse pump detuning. The variations of the fluorescence signal (proportional to the excited state population of the molecule) were also studied with pump and probe closer to resonance. Because of triplet saturation, the signal variations are rather weak and high pump powers had to be used. Upon tuning the probe beam through the resonance, W-shaped structures were obtaiped, recalling Autler-Townes splittings. The shape of the structures is correctly reproduced by numerical simulations of the optical Bloch equations in two intense laser fields and indicates photon exchange between pump and probe beams. These results show the possibility of non-linear and quantum optical experiments on single molecules in condensed matter, even though thc shifts observed are much weaker than disorder-induced inhomogeneous broadening. I 09:oo QThE32 Q U A N T U M EFFEC'TS A N D SPECTRAL DYNAMICS OF SINGLE TEIlRYLENE MOLECULES I N A /+TERPHENYL CRYSTAL.