Two-photon absorption:: from optical power limiting to 3D microfabrication

We report here recent results obtained in the field of molecular engineering for two-photon absorption (TPA); this paper deals with two specific applications: optical limiting in the visible range and microfabrication. The nonlinear absorption and optical power limiting properties of a conjugated system (fluorene) and polyfluorenes induced by TPA, were performed in the visible range for nanosecond time duration pulses. The TPA cross-section of small oligomers increases with the square of their lengths: this trend is rationalized in terms of excitonic coupling between monomers within the range of the effective length of the oligomer. We also report on the characterization of efficient photoinitiators for radicalar polymerization at 532 and 1064 nm by TPA. Symmetric molecules optimized for TPA and for optical limiting properties with a D-pi-D structure (hearing tertiary amines as a donor group D and a biphenyl or a fluorene for the transmitting pi electron group) were proposed for the visible. In the infrared, the selected photoinitiators present the general structure D-pi-A-pi-D, in which A is an acceptor group. The initiation efficiencies of these systems were evaluated by the determination of the threshold intensities for a given exposure duration. Compared to the commercial resins for UV photopolymerization and generally involved in TPA, these optimized initiators led to a significant increase of the sensitivity during fabrication. Weaker incident intensities and faster scanning speeds could be used. This approach allows the fabrication of three-dimensional micro-objects with a low cost nanosecond pulsed microlaser. (c) 2005 Academie des sciences. Published by Elsevier SAS. All rights reserved.