Quantum-light probe for molecular femtosecond dephasing in dyes
We measure femtosecond (fs) molecular coherence (i.e., how long the molecular oscillation remains synchronized before becoming random) using one of the very popular experiments based on the interference of two indistinguishable photons. In our experiment, pairs of indistinguishable photons are produced by the parametric down-conversion in a BBO crystal. When a dye sample is placed in one arm, it modifies the arrival-time correlations measured in coincidence counts. From this change, we directly determine the molecular dephasing time to be approximately 40.1 ± 0.07 fs for the IR-140 dye. Because the technique operates at ultralow photon flux, it avoids optical distortions and reduces the risk of sample damage, using the same underlying spectroscopy principle without femtosecond laser pulses, enabling ultrafast measurements in sensitive chemical and biological systems.
