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Lecture Hall ICM - Internationales Congress Center München SPIE Optical Metrology > Optics for Arts, Architecture, and Archaeology VII > Light-Matter Interaction and Nonlinear Optics
11:50-12:10 h | ICM - Internationales Congress Center München ICM Room 12a
Subjects: Optical Measurement Systems / Optical Metrology
Optical pump-probe microscopy can provide contrast in highly heterogeneous media and a wide range of applications has emerged, primarily in biology, medicine, and more recently in cultural heritage. The localized nature of nonlinear interactions leads to high spatial resolution, optical sectioning, and large possible imaging depth in scattering media. Using pump-probe microscopy, we have studied paint samples and historic artworks in order to provide high-resolution, depth-resolved images of pigments and their degradation products. Here we demonstrate the use of this imaging method for high-resolution studies of Vermilion degradation. Vermilion (mostly cinnabar (alpha-HgS)), an important red pigment used in historical paintings, blackens over time, and metallic Hg and beta-HgS (metacinnabar) have been implicated as possible degradation products. Conventional elemental analysis techniques have trouble differentiating alpha- and beta-HgS; X-ray spectroscopic techniques can differentiate between the two phases, but obtaining sufficiently high three-dimensional spatial resolution is challenging. Pump-probe microscopy can differentiate metallic mercury, alpha-HgS and beta-HgS and map each distribution on the microscopic scale. We studied artificial degradation of alpha-HgS; femtosecond pulsed laser irradiation induces an irreversible phase shift of alpha- to beta-HgS, in which the initial presence of beta-HgS grains can increase the rate of conversion in their vicinity. Continuous ultraviolet exposure instead generates both liquid Hg and beta-HgS, with a conversion rate that increases with elevated temperatures. Finally, we reveal the presence of beta-HgS as a natural degradation product in a cross-section of discolored Vermilion in a 14th century Italian painting.