Abstract
We consider theoretically near-field absorption spectra of molecular aggregates stemming from a scattering scanning near-field optical microscopy type setup. Our focus is on the dependence on the direction and polarization of the incoming electromagnetic radiation, which induces a Hertz dipole with a specific orientation at the tip-apex. Within a simple description, which is based on the eigenstates of the aggregate, absorption spectra are calculated for the near field created by this dipole. We find that the spatial patterns of the spectra have a strong dependence on the orientation of this tip-dipole, which can be understood by considering three basic functions that only depend on the arrangement of the aggregate and the molecule tip distance, but not on the orientation of the tip-dipole. This allows direct access to spatial dependence of the aggregate eigenstates. For the important cases of one- and two-dimensional systems with parallel molecules, we discuss these spectra in detail. The simple numerically efficient approach is validated by a more detailed description where the incoming radiation and the interaction between the tip and molecules are explicitly taken into account.
Highlights
We model the electromagnetic field stemming from the tip as a Hertzian dipole located at position R⃗dip a few nanometers above the aggregate
From Eq (10), the two-fold importance of the molecular transition dipoles is visible: First, they determine the interaction with the electromagnetic radiation; second, the mutual interaction is caused by the transition dipoles
We have investigated the dependence of near-field spectra of molecular aggregates on the direction of an “excitation-dipole.”
Summary
There has been a growing interest in selfassembled molecular aggregates on dielectric surfaces.[1,2,3,4,5,6] The surface degrees of freedom barely couple to molecular excitations, and fluorescence is only weakly quenched.[1,7,8] Strong interactions between the transition dipoles of the molecules lead to delocalized excitonic eigenstates where an electronic excitation is coherently shared by many molecules.[9,10,11] These eigenstates depend sensitively on the arrangement of the molecules. In this type of setup, the molecules are irradiated by the external field.[24,35]. A simple model where only the near field from a dipole induced in the tip is considered gives in many cases good results. V by summarizing our results and giving an outlook of the usefulness of the present study in the context of wavefunction reconstruction
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