Frequency Conversion of Light

Coherence and monochromaticity are two important characteristics of the laser light. Laser light radiation of nonlinear optical materials triggers the emission of light, either of different wavelength or of different coherence.4,51 Let us consider the mechanism of light emission in the case of nonlinear polarization. 

The electric field of the light, E, emitted in a situation of nonlinear polarization is expressed by the following nonlinear Maxwell equation  

assuming that the amplitude remains constant during a propagation distance equal to the wavelength (see Eq. (5.18)), we get Eq. (5.19). 

I i Intensity of incident light, 1 sample path length, 1 c coherent length, n, refractive index If phase matching condition A k =0 does not hold, I decreases, i.e., 1 m depends on the incident angle 0. m = 2 second harmonic generation (SHG) m=3 third harmonic generation (THG) 

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In the introduction to this chapter we gave an intuitive explanation of the origin of nonlinear optical effects and stressed the key role played by high power lasers and coherent light beams. These two concepts are defined here. We will describe one specific characteristic of laser light, namely the absorption saturation, and finally we will discuss susceptibility and frequency conversion of light. 

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