Introduction to nonlinear optics

Abstract

Nonlinear optics (NLO) is a branch of physics dealing with electromagnetic fields whose wavelength (frequency, color) is modified by the interaction with a medium. For example, in second harmonic generation an infrared laser beam is converted into a visible beam when going through a crystal. In white light generation, a pulse with narrow spectrum generates a broadband continuum of new frequencies.Due to the high energies needed to observe such phenomena, NLO started in 1961, just one year after the realization of the first laser.With NLO techniques, it is possible to generate optical pulses whose duration is in the order of the femtosecond (10-15 seconds) or even attosecond (10-18 seconds). Such extremely short pulses are routinely used for detecting and even controlling the dynamics of molecules and atoms on the same time scale.I will give a brief introduction to NLO. I will show its importance for science, describe the nonlinear effects that can be observed and some of their applications, with particular focus on those of interest for biology. I will introduce the pump-probe technique for ultrafast spectroscopy, and describe an experiment on the primary step of human vision. Finally, I will describe the applications of NLO to microscopy, showing its advantages in terms of resolution in respect to conventional techniques