Nonlinear optical effects in natural topaz

We investigate the nonlinear optical properties of natural topaz. For the first time, the observation of nonlinear phenomena related to multiphoton absorption effects, such as the up-conversion luminescence of the material defects, is reported in natural gems. In particular, we were able to detect the presence of the AlO4-related defects which are responsible for the change of the color of topaz as a consequence of irradiation with ?-rays as well as electrons and neutrons beams. Besides, the nonlinear spectral broadening of intense laser beams permits to spot of the presence of rare earth elements, such as Ytterbium and Neodymium, which are not found by means of standard characterization techniques of natural gems, e.g., those based on X-ray fluorescence, electron paramagnetic resonance, and linear optical fluorescence and absorbance measurements. In this sense, our results demonstrate that nonlinear optical effects induced luminescence measurements are a complementary, and previously unforeseen, technique for the characterization of the optical and physicochemical properties of natural gems. Finally, using intense infrared laser pulses allowed us to trigger extreme nonlinear effects, such as multiphoton ionization and conical emission. Our results will be of interest for the investigation of novel nonlinear optical effects in materials that are transparent in the whole visible spectral range and may give insights into the development of materials for nonlinear photonics.