4-Nitrobenzoic acid (4-NBA) single crystals were studied for their linear and nonlinear optical properties. The crystals were grown by slow solvent evaporation method at room temperature. The structure and functional groups wereconfirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopic studies. The 4-NBA crystal has an optical absorption at 263nm and a wide transparency window for the visible light. Theoretical predictions for themeasured optical absorption and charge transfer mechanism in 4-NBA were explained with HOMO and LUMO illustrations. The crystal is found to be birefringent and has produced an optical quality interference pattern. The optic plane of the crystal is observed along $langle$100$angle$ direction. The crystal exhibits nonlinear optical effects viz. reverse saturable absorption and self defocussing of laser beam. Considerable nonlinear refraction ($n_2$) and nonlinear absorption ($eta$) coefficients and third-order nonlinear optical susceptibility ($chi^{(3)}$) were determined using Z-scan technique.
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