Abstract

Third-order nonlinear optical (NLO) properties related to excitons are expected to overcome an empirical rule which states that the figure of merit, χ⁽³⁾/τα, are constant for any materials.¹ Here χ⁽³⁾ is a third-order susceptibility, τ is α response time and a is an extinction coefficient. Molecular aggregates called J aggregates are supposed to be one dimensional structures and show sharp absorption spectra (J band) which can be well described by one dimensional Frenkel excitons with large coherence length (= N molecules). The optical nonlinearities of J aggregates are expected to show χ⁽³⁾ = N^× (x ≥ 2) enhancement.² The large enhancement is expected in J aggregates because the coherence length becomes more than hundreds in some cases. In Frenkel excitons electrons and holes created by absorption of light are situated in the same molecule and the transition dipole-dipole interactions between the neighboring molecules form excitons. The dipole-dipole interactions are effective even at high temperature, then J aggregates can exist even at room temperature. This nature is sharp contrast with Wannier excitons which are formed with electron-hole interactions and unstable at high temperature.

© 1995 IEEE