Theoretical study on the closed-aperture Z-scan curves in the materials with nonlinear refraction and strong nonlinear absorption

The features of the closed-aperture (CA) Z-scan transmittance curves have b een studied in detail when the third-order nonlinear refraction and absorpt ion, both of which can be independently positive or negative, are simultane ously present. We found that whether the peak-valley or valley-peak structu re exists or not, depends on the ratio rho of the imaginary part chi ((3))( 1) to the real part chi ((3))(R) of the third-order nonlinear susceptibilit y chi ((3)) which are caused by nonlinear refraction and absorption respect ively. This communication also gives the rho dependences of the distance be tween the peak position and valley position, the difference between peak tr ansmittance and the valley transmittance, the ratio of peak height to the v alley depth for CA Z-scan, and especially the exact critical value rho (c) which determines whether the peak-valley or valley-peak structure exists or not, with Gaussian beams for the first time to our knowledge. (C) 2001 Els evier Science B.V. All rights reserved.