Image formation, filtering and applications to third order nonlinear measurement techniques

Date du colloque&nbsp;: 10/2011</p

Kerr-induced nonlinear focal shift measurements

International audienc

Linear optical characterization of transparent thin films by the Z-scan technique

We report experimental characterization of a very small rectangular phase shift (&lt;0.3 rad) obtained from the far-field diffraction patterns using a closed aperture Z-scan technique. The numerical simulations as well as the experimental results reveal a peak-valley configuration in the far-field normalized transmittance, allowing us to determine the sign of the dephasing. The conditions necessary to obtain useful Z-scan traces are discussed. We provide simple linear expressions relating the measured signal to the phase shift. A very good agreement between calculated and experimental Z-scan profiles validates our approach. We show that a very well known nonlinear characterization technique can be extended for linear optical parameter estimation (as refractive index or thickness)

Nonlinear Characterization Techniques inside a 4f System

International audienc

Kerr-induced nonlinear focal shift in presence of nonlinear absorption

We report a theoretical formulation for the nonlinear Kerr-induced focal shift of converging lenses in a high intensity regime. A numerical relation expressing the on-axis intensity of a focused Gaussian beam is derived in the case of a nonlinearly absorbing and diffracting lens induced in a medium. The concept of an effective Fresnel number is used to provide a simple linear relationship between the focal displacement and the nonlinear phase distortions. The influence of nonlinear absorption on the sensitivity of the focal shift measurements is also discussed

Optimization of Z-scan technique inside a 4f system

International audienc

Absolute measurement of the nonlinear refractive indices of reference materials

We report absolute measurements of the nonlinear refractive index on carbon disulfide ( CS 2 ) and fused silica. These materials are commonly used as standard references in nonlinear optical experiments. To obtain more accurate values than those usually used, we have combined the z-scan method inside a 4-f imaging system (in order to analyze the spatial distortion of the diffracted pump beam) with the “Kerr shutter” experiment (to evaluate the temporal pulse width durations for three different wavelengths such as 1064, 532, and 355 nm). We obtained surprisingly n 2 values one order of magnitude less than the one usually taken into account in the picosecond regime and a more significant dispersion of the nonlinear refraction index. Experimental and simulated Z-scan transmittance profiles as well as acquired autocorrelation functions in the Kerr-gating experiments are presented here in order to validate our measurements

Determination of photo-induced changes in linear optical coefficients by the Z-scan technique

We introduce a Z-scan technique as a tool to characterize small phase shift (&lt;1 rad) and photodarkening, both effects induced inside photosensitive materials by light illumination. Theoretical analysis supported by experiments is presented for permanent refraction and absorption Gaussian profiles. Simple relations are derived in order to estimate the changes in the linear coefficients. Particularly, we investigate quantitatively the photo-induced modifications in the linear optical constants of As2S3 caused by subbandgap irradiation (17 ps, 1064 nm)

Fifth-order nonlinear susceptibility: Effect of third-order resonances in a classical theory

We compute the fifth-order nonlinear susceptibility in the frame of a classical model based on an anharmonic oscillator, taking into account the local field corrections. A third-harmonic resonance is evidenced, which explains the strong enhancement of some measured values of the corresponding nonlinear index and its sign changes with the wavelength. The ratio between the fifth-order nonlinear index and the fifth-order nonlinear absorption is computed and is in good agreement with experimental data measured in carbon disulfide CS2