Study of Third-Order Optical Nonlinearities of Se-Sn (Bi, Te) Quaternary Chalcogenide Thin Films Using Ti: Sapphire Laser in Femtosecond Regime

The objective of the present research work is to study the nonlinear optical properties of quaternary Se-Sn (Bi, Te) chalcogenide thin films. A Z-scan technique utilizing 800 nm femtosecond laser source has been used for the determination of the nonlinear refractive index (n(2)), two-photon absorption coefficient (beta(2)) and third-order susceptibility (chi((3))). In the measurement of n(2), an aperture is placed in the far field before the detector (closed aperture), while for the measurement of beta(2), entire transmitted light is collected by the detector without an aperture (open aperture). Self-focusing has been observed in closed aperture transmission spectra. The appearance of the peak after the valley in this spectrum reflects the positive nonlinear refractive index. The calculated value of n(2) of the studied thin films varies from 1.06 x 10(-12) cm(2)/W to 0.88 x 10(-12) cm(2)/W. The compound-dependent behavior of n(2) is explained in this paper. We have also compared the experimental values of n(2) with the theoretically determined values, other compounds of chalcogenide glass and pure silica. The n(2) of the investigated thin films is found to be 3200 times higher than pure silica. The results of the open aperture Z-scan revealed that the value of beta(2) of the studied compound is in the order of 10(-8) cm/W. The behavior of two-photon absorption is described by means of the optical band gap (E-g) of the studied compound. The variation in the figure-of-merit from 0.32 to 1.4 with varying Sn content is also reported in this paper. The higher value of nonlinearity makes this material advantageous for optical fibers, waveguides and optical limiting devices.