Study of femtosecond nonlinear optical coefficients for Bi doped Se_85-xTe₁₅Bi_x chalcogenide thin films

The present work reports the influence of selenium replacement by bismuth on the nonlinear optical parameters of ternary Se85-xTe15Bix (x=0, 1, 2, 3, 4, 5 atomic %) chalcogenide thin films. Calculation of nonlinear refractive index (n(2)), two-photon absorption coefficient (beta(2)) and third-order susceptibility (chi((3))) by well known Z-scan technique with femtosecond laser pulses were done. The Z-scan spectra for Se85-xTe15Bix upto Bi= 4 atomic % results in self-focusing behavior of n(2) is positive while for Bi=5 atomic % n(2) is negative. The behavior of n(2) by using different physical parameters are exlpained. The comparison of experimental and theoretical values of n(2) with pure silica are also studied. The presence of the valley at focus in open aperture Z-scan graph demonstrates strong reverse saturable absorption. The figure of merit (FOM) for the Se-Te-Bi chalcogenide thin films is found to be less than 1 and are beneficial for all-optical switch devices.