Intense-field interaction regime with weak laser pulses and localized plasmonic enhancement: Reference-free demonstration by 3rd- and 5th-order infrared spectroscopies.

In bulk materials, intense field interaction is accompanied by undesired nonresonant processes. Plasmonic nanostructures localize enhanced fields exclusively in their vicinity. We report a 4-fold vibrational population inversion between all the excited and the ground states in the molecular monolayer on the surface of gold nanoantennas. Excited population assessment relies on a novel reference-sample-free evaluation of the field enhancement with 5th- and 3rd-order nonlinear infrared spectroscopies and on quantitative modeling of coherent excitation dynamics. This study opens opportunities for precise population control utilizing population inversion for vibrational transitions using weak fields.