We describe an infrared interferometric technique based on a two-dimensional spatial fringe analysis Fourier method for investigating the characteristic ring diffraction pattern generated by the self-phase-modulation effect induced in nematic liquid crystals (NLCs) by an infrared laser beam and for measuring the nonlinear refractive index of the NLCs. The experimental setup employs a Mach-Zehnder interferometer with a cw CO<sub>2</sub> laser emitting at 10.6 μm and a pyroelectric optoelectronic sensor matrix to detect the modulated ring-pattern intensity distribution formed in the far field by a nematic E7 sample. A Fourier-transform-based analysis of the interference fringe pattern allows comparison of the measurements with the theoretical ring-pattern intensity distribution. We show that accurate determination of the nonlinear refractive index can be obtained by analyzing the two-dimensional phase distribution of the modulated ring pattern.
© 2003 Optical Society of America
(070.2590) Fourier optics and signal processing : ABCD transforms
(070.5010) Fourier optics and signal processing : Pattern recognition
(100.2650) Image processing : Fringe analysis
(110.3080) Imaging systems : Infrared imaging
(160.3710) Materials : Liquid crystals
Enrico Allaria, Stefano Brugioni, Sergio De Nicola, Pietro Ferraro, Simonetta Grilli, and Riccardo Meucci, "Interferometric Analysis of Reorientational Nonlinear Phenomena at 10.6 µm in a Nematic Liquid Crystal," Appl. Opt. 42, 4827-4834 (2003)