Threshold enhancement in two-photon photpolymerization

It is demonstrated that threshold reduction of two-photon polymerization is achievable by means traditionally employed for sensitivity enhancement for single photon photoinitiation, such as heavy atom enhancement or intersystem crossing, electron donor agent, concentration increase of initiator. It is shown that measured threshold is in reverse proportion to square root of initiator concentration, whereas observed length of induction period exhibits reverse proportionality to the square of light intensity. Overall, experimentally observed threshold values of two-photon induced photopolymerization are effected by all intermediate stages of energy transformation in the photochemical sequences leading to photoinitiation, in particular inter-system crossing of excited initiating molecules as well as by monomer reactivity.