Molecular engineering for optical limiting in the visible

Summary form only given. Efficient protection of opto-electronic devices and vision sensors against damage caused by high power lasers can be performed by optical limiting materials. Two-photon absorption (TPA) is an attractive mechanism for optical limiting in the visible range since the material has an instantaneous response and presents no saturation effect. Furthermore, TPA can be obtained with transparent materials which are required when the eye must be protected. For nanosecond pulses, the nonlinear absorption corresponds in fact to the three-photon process, i.e. a two-photon initiating step followed by a transient absorption from the resultant excited state (ESA). The nonlinear absorption resulting from such a two-step process is governed by a three-photon absorption coefficient /spl alpha//sub 3/ which can be written as follows in the case of a singlet S/sub 1/-S/sub n/ transition and of an instantaneous response: /spl alpha//sub 3=/N/V f/sup 6/ /spl sigma//sub TPA//spl sigma//sub ln//spl tau//sub l//-2(/spl planck//spl omega/)/sup 2/ where N/V is the concentration, /spl sigma//sub TPA/ and /spl sigma//sub ln/ are respectively the two-photon and the S/sub 1/ - S/sub n/ absorption cross-section; /spl tau//sub 1/ is the lifetime of the excited state S/sub 1/ and f the local field factor. The optimization of /spl alpha//sub 3/ requires the simultaneous optimization of all the parameters involved. We focused our attention on the diaminobiphenyl which present in highly concentrated solutions promising properties for optical limiting in the visible range for nanosecond pulses.