Quantum Confinement and Superradiance of Self-Trapped Excitons from 1D J-Aggregates

Abstract

In one-dimensional (1D) molecular crystals with finite length 2L ≪ λ (λ is the optical wavelength) an overwhelming part of the total oscillator strength is concentrated in the lowest excitonic state and is equal to F ₁ ≅ 0.85 f ₀(2L/a), where f ₀ is the oscillator strength of a monomer and a is the lattice constant. This leads to the superradiance from the lowest excitonic state and its domination in the absorption spectrum of the crystal. We show that self-trapping of excitons destroys this simple picture so that it takes place only for short chains with length 2L small as compared to the length 2l ₀ of self-trapping. For long enough chains the value of F ₁ does not increase with growth of L, as it occurs in linear case, but tends to the saturation limit F ₁≅5f ₀(l ₀/a). The oscillator strength of the next bright state also tends to the same limit with growth of L, but it takes place only at the length L > 9l ₀, and analogous relations are true for the following bright states. We consider also the influence of quantum confinement and self-trapping on the superradiance of 1D molecular crystals.