We report the formation of hybrid states of self-assembled PTCDI-C7 organic molecule excitons and surface plasmon polaritons (SPP).
Ten self-assembled monolayers with no host matrix are directly evaporated onto a gold thin film forming a ultra-dense and organized, 30nm thick layer. The π- π stacking among molecules leads to the formation of H-aggregates with alignment of molecular dipole moments along the local electric field vector. This collective excitations are known to give rise to a sharp excitonic peak in absorption with large oscillator strength, which are favorable properties for the observation of strong coupling.
Experimental wavevector-resolved reflectance spectra display an anticrossing, attesting the strong coupling regime with a Rabi splitting energy ΩR=102 meV at room temperature. By contrast, no anticrossing has been observed for PTCDI-C7 molecules evaporated in a different experimental condition and with a reduced local order. We interpret the observed strong coupling regime as resulting from the high degree of organization and the controlled molecular dipole orientation.
Under optical pumping, we observe an enhancement of the coupling efficiency between the molecular emission and the SPP mode. This observation is consistent with the small oscillator strength of the lowest Frenkel state of exciton and to the large Stokes shift of PTCDI-C7 molecules induced by H-aggregate stacking.
The use of ultra-dense layers of self-assembled molecules opens interesting perspectives for the control of the molecular dipole orientation at the nanoscale to maximize interaction with the SPP field and therefore the strong coupling strength.
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