Chalcogenide Letters Vol. 20, No. 5, May 2023, p. 353 - 365 Study of femtosecond nonlinear optical coefficients for Bi doped Se 85-x Te 15 Bi x chalcogenide thin films P. Yadav a , C. Tyagi b* , A. Devi c , N. Gahlot d a,d St. Andrews Institute of Technology & Management, Gurugram, Haryana, India b Department of Applied Sciences, The NorthCap University Gurugram, Haryana, India c Amity University Haryana, Amity Education Valley, Pachgaon, Manesar, Gurugram, Haryana 122413 The present work reports the influence of selenium replacement by bismuth on the nonlinear optical parameters of ternary Se 85-x Te 15 Bi x (x=0, 1, 2, 3, 4, 5 atomic %) chalcogenide thin films. Calculation of nonlinear refractive index (n 2 ), two-photon absorption coefficient (β 2 ) and third-order susceptibility (χ (3) ) by well known Z-scan technique with femtosecond laser pulses were done. The Z-scan spectra for Se 85-x Te 15 Bi x 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. (Received February 18, 2023; Accepted May 15, 2023) Keywords: Chalcogenide glasses, Nonlinear refractive index (n 2 ), Two-photon absorption coefficient (β 2 ), Third-order susceptibility (χ (3) ), Z-scan technique 1. Introduction The present curiosity in optical limiting devices for defense application is motivating passionate research to characterize new nonlinear materials. A large number of materials are available in literature for optical limiters. Among these, chalcogenide glasses are of significant interest because of non-resonant type third order nonlinearity with response time of < 1 picosecond and highest nonlinear refractive index [ ~ 1000 times higher than silica] [1-3]. In recent research scenario, researchers showed great interest towards active plasmonics device made by chalcogenide glasses. This interest is because of their higher bandwidth, fast response time and high nonlinear enhancement factor [4]. Chalcogenide glasses can be used as highly sensitive sensors [5], changed environmental conditions [6]. Due to high optical losses, these materials can be also used in photonic applications [7]. Moreover, due to their linear and nonlinear optical properties, chalcogenide glasses are advantageous materials for optical switches, optical imaging, Raman amplification [8], optical regenerators, parametric amplifiers [9], electro-optic modulators[10], supercontinuum generation, and so on [11-12]. In this regard, the Se-Te combination is regarded as the prototypical semiconducting chalcogenide, playing a significant role in applications including nonlinear optics, higher photosensitivity, higher hardness, higher crystallisation temperature, and reduced ageing impact [13-14]. * Corresponding authors: ctyagi05@gmail.com https://doi.org/10.15251/CL.2023.205.353