National Institute of Laser NILES, Cairo University, Cairo, Egypt.Nonlinear optics is the branch in physics in which the electric polarization density of the medium is considered as a nonlinear relation with the electromagnetic field of the light. As a widespread field of applications in electromagnetic wave propagation, light nonlinear-interaction with matter leads to a wide range of phenomena, like phase conjugation, Raman scattering and optical frequency conversion. The nonlinearity is usually obtained only at very high light intensities values of the electric field analogous to interatomic electric fields, approximately around 108 V/m, which is over than million suns, so that such intensities can be delivered only by lasers. Moreover, a variety of the analytical tools considered in nonlinear optics are of wide-ranging character, such as the symmetry considerations and perturbative techniques, and can similarly well be considered in other branches in nonlinear dynamics. Consequently, numerous developments in nonlinear optics have been made, with a lot of efforts with fields of, for example, laser and maser in 1960s and 1970s. Nonlinear optics clarifies nonlinear response of properties such as polarization, frequency, phase or path of incident light. These nonlinear interactions induce optical nonlinear phenomena: second harmonic generation SHG, third harmonic generation THG and Sum-frequency generation SFG. These phenomena due to nonlinear propagation of high intensity laser light in some crystals will be discussed and its applications will be considered.
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