Modifying the Spectral Weights of Vibronic Transitions via Strong Coupling to Surface Plasmons

Strong light–matter coupling results in the formation of hybrid half-light half-matter excitations with modified energy levels. The strong coupling of excitons with photons in organic molecular systems has received much attention recently owing to the potential for engineering their photophysical properties and even the prospects for controlling chemical reactions. One means to affect chemical reactions is to control the molecular excited states within their vibronic manifolds. Here we demonstrate the modification of the spectral weight of the vibronic transitions and excimer emission in an archetype organic molecule, diindenoperylene (DIP), by strong coupling to surface-plasmon polaritons. The vertically aligned DIP molecule is grown on an ultrasmooth film of Ag with a 3 nm thick alumina spacer. Through angle-resolved reflectivity measurements we demonstrate the strong coupling between the surface-plasmon mode of the Ag and the vibronic transitions in DIP. Temperature-dependent photoluminescence measurements show the shift in the spectral weight of the emission peaks when the molecules are in the strong coupling regimechanges that are attributed to the polaritonic control of the oscillator strengths of emissive states owing to modification in the Franck–Condon factor and efficient energy transfer to lower-lying excimer states. This work demonstrates the potential to modify the material properties through a strong light–matter interaction.