Laser amplification in excited dielectrics

Today, laser processing of dielectric materials with ultrashort, femtosecond laser pulses finds a great variety of applications. While ablation, cutting or welding of different types of glasses are widespread and popular examples, nanostructuring of transparent crystals or optoporating cells is part of more recent scientific studies. Although the material systems seem to be very different, their interaction with visible and infrared wavelengths is very similar: the light is simply transmitted. This changes drastically when they are exposed to ultrashort and highly intense laser pulses. Different interaction mechanisms lead to the appearance of a transient metal-like state. Thus, pump-probe experiments are the method of choice to study the optical excitation of dielectric materials. In strong contrast to the metallic characteristics of excited dielectrics, this study provides the first observation of the amplification of a probe laser in optically excited sapphire and fused silica glasses. Not only is the observed amplification of coherent nature, it is also dependent on the incident probe pulse intensity and thus of a nonlinear order, a curiosity in itself. From the experimentally obtained energetic and temporal dependencies, a consistent three-step model was developed and is supported by simulations. Besides, the influence of temporal pulse shaping on spatial properties of the laser excitation in water is demonstrated experimentally and via simulations. It is shown that temporally asymmetric shaped laser pulses lead to a spatial distribution of laser excitation that is reduced in area but highly extended in depth. The results are put into the context of post-mortem experiments.