The present work focuses on the study of nonlinear optical properties of bis(4-dimethylaminodithiobenzyl)-nickel (BDN) dye encapsulated in poly-methylmethacrylate (PMMA) matrix. The Z-scan measurements are carried out using nanosecond laser pulses of Nd: YAG laser at 532nm wavelength with maximum energy of 200mJ per pulse as excitation source. The nonlinearity in terms of excited state absorption (ESA) has been discussed. A switching from saturation absorption (SA) to reverse saturation absorption (RSA) is reported from absorptive study in samples of different thickness. The nonlinear parameters viz. nonlinear absorption coefficient ( β ), nonlinear index of refraction ( n<sub>2</sub> ) and third-order nonlinear susceptibility ( χ<sup>3</sup> ) are calculated. The thermal diffusivity that provides the rate at which a temperature disturbance travels within the medium is used to estimate thermal conductivity of solid-state polymeric samples. The nonlinear refractive index gradient is determined in terms of thermo-optic coefficient ( dn/dT ).The observed nonlinearity is attributed to laser induced acoustic motion with rise time of thermal lens of nanosecond order that agrees well with literature reported earlier. The study shows that BDN dye encapsulated polymers are excellent material for stable optical limiting and pulse shaping applications.