Novel ultrafast laser sources: Applications in nonlinear optical microscopy

Biophotonics involves a wide class of interactions between optical radiation and biological objects. At an early stage of its development the main techniques to probe the biological samples were of linear optics. They used scattering, absorption and refraction of light for studying different processes in biology and chemistry. With the invention of lasers capable of generating coherent radiation anywhere in the optical spectrum a new era of biophotonics has begun. Lasers are now routinely capable of producing both the peak intensities and average powers that were thousands of orders of magnitude higher than obtainable with the older classical sources of radiation. The unique properties of laser radiation gave an opportunity to observe the phenomena, which were previously difficult to access and greatly expanded the types of studies performed and the amount of information obtained. One of the fields, which began to develop rapidly, was nonlinear optics. Nonlinear optics deals with interactions between optical radiation and matter in which the response of the medium, as manifested by the polarization induced by the incident fields, depends on the second- and higher order powers of the optical fields. The study of nonlinear optical effects has provided us not only with a lot of information about the interaction of the strong electromagnetic fields with matter, but also with new technologies that can be used to accomplish tasks that are more difficult or impossible with linear optics alone. One of such technologies is third-harmonic generation microscopy, based on the sensitivity of the third-harmonic generation to the interfaces in the objects. An important issue in the application of nonlinear phenomena such as third harmonic generation to microscopy is the complexity of the laser source. At present, almost all applications in ultrafast microscopy are performed with Ti:sapphire laser systems. Although the stability and in general overall performance of these systems has improved dramatically over the past several years, these systems have some drawbacks, which force to seek novel laser sources applicable for THG microscopy.