Second Harmonic Generation

p>The objective of this experiment is to observe non-linear effect, second harmonic generation in 2nd order non linear crystals. Second harmonic generation (also called frequency doubling or abbreviated SHG) is a nonlinear optical process, in which photons with the same frequency interacting with a nonlinear material are effectively "combined" to generate new photons with twice the energy, and therefore twice the frequency and half the wavelength of the initial photons.
Due to nonlinear atomic polarization, crystals can emit a different frequency of light other than absorbed. In order to see the second order effect of atomic polarization, which causes doubling in frequency of the incident light, the incident light must be intense and coherent source, like a laser. Potassium Dihydrogen Phosphate (KDP), Urea etc. are the crystals that exhibit nonlinear polarization properties. When light hits a crystal, it excites the atomic dipoles. In classical optics, it is assumed that this polarization of atoms is linear to the magnitude of electrical field of the incident light. In reality, this polarization is a nonlinear phenomenon, but the nonlinear components are evident only for high intensity.

Experiment:

Potassium Dihydrogen Phosphate (KDP) crystal growth

Peak wavelength λp :

The two measurable quantities connected with the z - scan are non linear absorption and non linear refraction. These parameters are associated with the imaginary and real part of the third order non linear susceptibility and provide important information about the properties of the material.

Second harmonic generation of light - frequency doubling in Urea and KDP crystals