Second-harmonic generation

The goal of this experiment in nonlinear optics is to study the conversion of a (very small) fraction of laser light from its original wavelength of 980 nm (i.e. invisible to the naked eye) to its second-harmonic at a wavelength of 490 nm. This conversion takes place by mild focussing of an intense laser beam in a nonlinear birefringent crystal of BBO (= β BaBO4). You will study the physical processes that determine the nonlinear conversion and hopefully appreciate the central role of phase matching. These phase matching conditions ensure that the generated second harmonic light interferes constructively in the forward propagating direction. Angle tuning of the birefringent crystal allows you to change the phase matching conditions through changing the refractive indices. Playing with the geometry, the various polarizations, angles, and crystals you will hopefully be rewarded with a very weak blue beam at the nanoWatt level, starting with a dangerously strong infrared beam of tens of milliWatts. Please be careful, read the required safety procedures carefully, and follow them strictly; we don’t want you to hurt yourself or somebody else!