Nonlinear Optical Responses I

Abstract

A laser is an intense, monochromatic, and coherent light source. By making the best use of these characteristics, we have become able to easily observe the nonlinear optical responses of atoms, molecules, and solids such as higherharmonic generation, four-wave mixing, parametric oscillation, multiphoton absorption, induced-Raman scattering, etc. In general, the interaction between the radiation field and matter is relatively weak so that the effects of this interaction can be described in terms of perturbational methods. These nonlinear optical responses are discussed in this chapter. However, we have such strong power in ultrashort laser pulses, as discussed in Chap. 5, that the perturbative treatment is not justified in describing the nonlinear optical phenomena under these laser fields. These phenomena will be discussed in Chap. 7. On the other hand, nonlinear optical responses which are discussed in this chapter can be classified according to how many times this interaction works in each nonlinear optical response. The lowest-order nonlinear optical responses are sum-frequency and second-harmonic generation (SHG). These will be discussed in Sect. 6.1. This sum-frequency and higher harmonic generation in solids is useful to understand the electronic structure as well as the microscopic optical processes of solids but also is important from an engineering point of view. First of all, laser light generation with shorter wavelengths is the more difficult, as already discussed in Sect. 4.3.4. Because of this, we are now producing coherent ultraviolet light as higher harmonics in crystals by using Nd:glass lasers or YAG lasers as a strong fundamental source. Secondly, we obtain the light source in the visible region by converting the infrared (≈ 1μm) laser light of cheap and stable semiconductor lasers into the second harmonics.