What is the difference between linear and non linear optics?

Answer 1

In the case of linear optical transitions, an electron absorbs a photon from the

incoming light and makes a transition to the next higher unoccupied allowed state. When

this electron relaxes it emits a photon of frequency less than or equal to the frequency of

the incident light (Figure 1.3a). SHG on the other hand is a two-photon process where this

excited electron absorbs another photon of same frequency and makes a transition to

reach another allowed state at higher energy. This electron when falling back to its original

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state emits a photon of a frequency which is two times that of the incident light (Figure

1.3b). This results in the frequency doubling in the output.