Three-dimensional focal spots related to two-photon excitation

We used a photopolymer consisting of urethane acrylate monomer and free-radical-type initiators as a media for recording two-photon-excited (TPE) focal spots in three dimensions. Mathematically, the initiator plays a role of kroot operator, transforming the quadratic light intensity distribution to a linear radical concentration distribution. Then, the monomer acts as a three-dimensional (3D) film, fixing the focal volume where radical concentration is above a threshold as a voxel. An ascending scan method was utilized to avoid substrate truncation and to achieve isolated complete 3D voxels, from which accurate forms of TPE focus were obtained. The 3D photographing technology led to a clear explanation of the origin of pronounced contradictions in reported spatial resolutions, and would be critical for laser precision microfabrication in various materials. (C) 2002 American Institute of Physics.