Although the two-photon absorption (TPA) was theoretically predicted over the 70 years ago and observed in the 60'ies with the advent of lasers, it is only during the last decade that one has realized the tremendous technical potential of TPA. This potential can be traced to the fact that the TPA cross section displays a squared dependence on the intensity of the incident radiation and that transitions to electronically excited states are achieved with photons of half the one-photon excitation energy. This makes TPA absorption both confocal and transparent. Proposals of a wide range of TPA applications has followed in the wake of this realization; here to mention such apparently different applications as up-converted lasing, 3-dimensional optical data storage, new confocal microscopy, medical photodynamic therapy, and Optical Power Limiting. We use response theory to evaluate multi-photon absorption cross sections. The response technology offers a decisive advantage in that the cross sections for a given order of the multi-photon processes can be obtained from the same order of the polarizability.

We have showed that the TPA of conjugated polymers is dominated by one single state, which has a common localization and correlation length. The TPA cross section of the dominating state fulfills a power law dependence on the conjugation length.

We have introduced a concept refering to dimensionality in the design of TPA materials. The TPA cross section can be drastically enhanced by creating a two-dimensional charge-transfer conjugated network, which is due to strong electronic coupling within the systems.

We have examined a series of one-dimensional organic molecules that exhibit large two-photon absorption cross sections in the visible region and that have been synthesized in different laboratories. It is found that the most crucial role for large two-photon absorption is played by the pi-center. For molecules with a given pi-center a symmetrical structure with strong donor groups can result in a maximum two-photon absorption cross section.

The effects of solvent polarity on the TPA cross sections of push-pull organic molecules have been examined at an ab initio level with reaction field models.

We have discovered that for multi-branched structures connected by an amino group, the electronic couplings among different branches are very weak, while the vibronic coupling can give a significant contributions to the TPA cross section. A general approach based on the linear coupling model has been proposed to simulate the vibronic profile of a TPA spectrum.

If you are interested in projects in these areas contact Hans Ågren or Yi Luo .