Ammonium oxalate Single crystals were grown by the slow evaporation solution growth method using water as a solvent and subjected to optical characterization. Good optical quality of crystal was observed and the grown crystals were characterized by single crystal X-ray diffraction study which confirms that the grown crystal belongs to Pnma space group. The UV-Vis-NIR spectra reveal a low UV cut off value for Ammonium oxalate. Absence of SHG affirms the Centro-symmetric class of this crystal and hence the Z-scan technique is employed for THG. Through the Z-scan technique, the nonlinear refractive index 2 n and the absolute value of the third order non-linear optical susceptibility (3) c have been measured at the wavelength of 632.8nmand were found to be 2.3191 × 10-9cm2/W and 7.4278 × 10-6esurespectively. Also, at the same wavelength, the real (3) R c and imaginary (3) I c parts of susceptibility (3) c have been calculated as 1.4228x10-7 esu and 7.4264x10-6 esu respectively. Ammonium oxalate exhibits saturation absorption and self-defocusing performance and its non-linear absorption co-efficient b is determined as7.4126 × 10-6 cm/W.
