4. Sequentially absorbed two-photon stereolithography

As shown in figure 2 , it is possible to carry out electronic excitation in a single step, in which both photons are used simultaneously to produce an electronic state which initiates polymerization. It is also possible to pass through an intermediate stage where a first photon excites a molecule A which, during its lifetime in the excited state, can receive a second photon which, for example, will induce polymerization. While triplet-triplet absorptions are well known in the literature (cf. for example