| Thus, linear optics is the description of optical phenomena with the assumption that the induced dipole amplitudes are proportional to the electric field that induced them. If the field oscillates at an angular frequency ω, so do induce dipoles (see, Jackson). The total field in the medium - the incident field and the generated field - oscillates at frequency ω. In such a case no new frequencies can be generated and waves at different frequencies do not interact. The spectrum of incident light can of course be modified by selective absorption of a part of the spectrum of incident light but no frequency that is absent in the incident light is generated. |
| 2. |
Nonlinear Optics: |
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| Nonlinear optics deals with the study of the light- matter interaction in nonlinear regime which leads to myriads of phenomena hitherto unknown in the realm of the linear regime. |
| In the following sections, we will discuss the brief history of nonlinear optics and origin of optical nonlinearity in terms of simple classical picture. |
| 2.1 |
Brief History of Nonlinear Optics: |
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| Probably, mirage is the oldest natural phenomenon experienced by human beings that is akin to a nonlinear optical phenomenon. It is a manifestation of the light fluence dependent refractive index (thermal nonlinearity) as you will learn in these web lectures later. However, the prelude to the story of the scientific quest in nonlinear optics can be marked with the discovery of the Kerr effect [1] in 1875. Kerr in his experiments (see figure 5) with liquid and glasses demonstrated the rotation of the plane of polarization of the light propagating through a dielectric medium subjected to large electric field. This is caused by the anisotropic field dependent variation of its refractive index. In 1886, Bruhl [2] observed the Kerr effect to be stronger in conjugated organic molecule - recognized as an important class of nonlinear optical materials now. The first ever, truly nonlinear optical phenomenon of the saturation of absorption or state-filling effect was observed in 1926 by Vavilov and Levshin in Uranium glass using intense light from a spark [3]. Further growth of the exciting field of nonlinear optics was hampered due to unavailability of intense light sources and prospered only with the advent of lasers. |
