Nonlinear microscopy of historic art

Fine paintings are more than skin deep: Human vision can penetrate tens to hundreds of microns beneath the surface, which can include dozens of layers of paint. But until now, the only way to quantitatively analyze a painting’s layering structure has been to slice away slivers of paint with a scalpel. Now Duke University’s Warren S. Warren and collaborators have shown that femtosecond pump–probe microscopy of paintings can nondestructively create three-dimensional images that reveal layers of paint up to hundreds of microns deep and distinguish different pigments and materials. Developed by Warren and others for biomedical imaging applications, the technique makes use of sample-mediated interactions between two pulsed laser beams focused together in an object. The first beam, or pump, induces molecular excitations, which affect how the second beam, the probe, interacts with the sample. Some molecular processes, such as excited-state photoabsorption, attenuate the probe; others, such as stimulated emission, enhance its intensity. Whatever the process, the magnitude of the pump–probe interaction depends on the product of the beam intensities. Measuring the scattered probe light, therefore, gives information about what molecules are present where the beams are most tightly focused. In the 14th-century painting shown in the figure (Puccio Capanna’s The Crucifixion, already extensively analyzed by the scalpel method), Warren and colleagues showed that pump–probe microscopy could penetrate the full thickness of the paint in the Virgin Mary’s blue robe. The robe was painted with an unusually thick layer of the pigment lapis lazuli, which at the time was more costly than gold. (T. E. Villafana et al., Proc. Natl. Acad. Sci. USA 111, 1708, 2014 .)—Johanna L. Miller