Detuning energy

As follows from the equation, the efficiency of 2PA depends on the values of the corresponding one-photon transition dipoles and the detuning energy... 

This expression is derived from Eq. (18) for the case when the second term dominates control of electronic coupling between two excited states. The first term vanishes for centrosymmetric chromophores in Eq. (18) because A/rol 0. Substitution of Eq. (18) into Eq. (17) results in Eq. (27) for centrosymmetric compounds. This relation shows the proportionality of 8 to the square of Mqi M i and the excitation energy of the TP excited state 02, respectively. The TP excited state is in most cases S2- A further important term in Eq. (27) is the detuning energy 01 02/2 resulting in an increase of 8 if 0i — 02/2... 

Quantum chemical calculations have ascribed this large TPA response to the role of high energies of two-photon excited states with a large detuning energy term and an unusually large transition dipole moment /a 12 (Eq. 7). 

On the contrary, D-jt-A-jt-D molecules are generally known to provide high (7TPA values from the lowest two-photon excited states with a weaker detuning energy term this has been observed recently in conjugated porphyrins [93], in extended phenylenevinylene oligomers for which ajpA values up to 5300 GM have been obtained [94], and in squaraines [95]. 

The magnitude of absorption shift depends on the detuning energy i.e. the difference between pump and one photon allowed transition energy and is proportional to the square of applied electric field. Within the three level model the frequency shift of absorption spectrum (cf. Charra and Nunzi (1989)) is given by... 

Blanchard et al. studied optical Stark effect in PTS single crystal film as a function of detuning energy AE = - w. At small values the optical Stark effect is phonon mediated because of strong exciton-phonon coupling in these materials. At large it is mainly excitonic Stark effect. 

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