Nonlinear refractive index measurement by Fresnel diffraction from phase object

Highlight
• A simple technique is proposed for measuring the nonlinear refractive index.
• The technique is based on the Fresnel diffraction from the phase object.
• The central fringe intensity is different for the focusing and defocusing media.
• By increasing the laser power, the number of the diffraction fringes is increased.
• The nonlinear refractive index is measured by best fitting the experimental data.

A simple and sensitive technique, based on Fresnel diffraction from phase objects, is proposed for specifying the nonlinear refractive index of a sample. In this technique, the nonlinear refractive index is obtained by measurement of the normalized intensity distribution of the Fresnel diffraction patterns. The fringe diffraction patterns are formed due to the propagating monochromatic plane wave beam through a phase object. The phase object is the result of the changes in the refractive index due to the heating of the sample by absorbed small fraction of the pump laser power. The change in the refractive index at this region imposes a nonlinear shift on the phase of the illuminating probe beam that leads to the Fresnel diffraction. Simulation and experimental studies show that the technique is reliable and quite sensitive to refractive index change.