*5.3.3 Nonlinear optical properties*

Nonlinear optics (NLO) is a wing of optics that explains the behaviour of light in nonlinear media, i.e. the media in which the polarization density, P responds nonlinearly to the electric field, E of the light. It is noted that nanocomposite materials showed large values of optical nonlinearities and fast response time. Thus, these materials can potentially be used in areas such as image processing, optical switching, optical modulation, optical information processing, and medical applications like cancer therapy [82]. Fellahi *et al.* [83] studied the nonlinear optical properties of fluorine doped and undoped In2O3 thin films using X-ray diffraction, electrical resistivity, transmission and third harmonic generation. The best value of nonlinear optical susceptibility χ(3) is obtained from the doped films with low electrical resistivity of 6 × 10−3 Ω cm. This is because free carrier concentration in fluorine-doped In2O3 samples is higher than that in undoped In2O3 [83].
