PINPOINT POLYMERIZATION, in which laser light is absorbed two photons at a time within tiny volumes, can be used to turn chemical reactions on and off and to fabricate microstructures from the inside out. An Arizona-Caltech collaboration (Joseph Perry, jwperry@u.arizona.edu, 520-626-9331) has developed a new highly sensitive resin which when bombarded by intense laser light is converted into polymer, but only within the tiny micron-sized laser focus. By scanning the laser, a pattern of chemical changes is imposed on the sample. Although not exactly a holographic process since interference effects are not at work, the photo-polymerization does result in permanent changes in the local environment, such as index of refraction. The implications of this are chemical (reactions can be activated in tiny zones and not in neighboring zones), optical (rows of fluorescent binary bits can be encoded), and mechanical (microstructures can be built, including waveguides, photonic crystals, and arrays of cantilevers---see the figure at Physics News Graphics). For example, the photonic crystal (honeycomb structures which exclude or trap light at select wavelengths) was built by polymerizing some sections of the solid and then washing away the unwanted parts with solvents, a process not unlike photo-lithography except that it's done in three dimensions and with two-photon excitation. (Cumpston et al., Nature, 4 March 1999.)