Biexciton states and two-photon absorption in molecular monolayers

Exciton–exciton interactions in doubly electronically excited molecular monolayers, may result in the formation of bound two-exciton states (biexcitons), whose binding is induced by the change in the molecular permanent dipole moments upon electronic excitation. We solve for the biexciton energies in the dipole approximation, and explore the effects of biexcitons on the monolayer nonlinear susceptibility. Calculations are made for monolayers with molecular dipole moments oriented either parallel or perpendicular to the monolayer surface.