Knowledge Bank

Monomolecular layers can be assembled by a number of methods and also occur naturally. The optical properties of these systems resemble neither those of the constituent molecules nor those of the bulk crystal obtained by stacking a large number of the layers. We have investigated the excited state of a model monomolecular layer consisting of a periodic lattice of classical oscillators characterized by a one-transition polarizability. Maxwell’s equations and the mechanical equations of motion for the oscillators are solved self-consistently for the excited state energies as a function of wavevector. We find for transitions polarized perpendicular to the exciton wavevector there are two polariton branches, one is superradiant with radiative lifetimes ($\lambda /a)2\sim 106$ times shorter than the free molecule, and the other does not radiate at all. Oblique incidence reflection spectra have been calculated and dramatic changes in peak frequency and bandwidth are predicted as the angle of incidence is varied. The dispersion of the subradiant branch can be probed in an ATR experiment. We have worked out the effect of the substrate necessary in such an experiment on the excited state properties of the monolayer.