Study of third-order nonlinear optical properties of APB inorganic single crystal with a Q-switched Nd-YAG laser under 532 nm and 1064 nm excitation wavelengths

Third-order nonlinear optical properties of an inorganic single crystal ammonium pentaborate tetrahydrate (APB) was investigated by well-established Z-scan technique with two excitation wavelengths (532 nm & 1064 nm).The measurement were performed at nine different pulse energy for both the excitation wavelength. A total energy of 0.45 mJ, 0.67 mJ, 0.90 mJ, and 0.94 mJ were used at 532 nm, which have power densities of 0.38 GW/cm², 0.57 GW/cm², 0.76 GW/cm², and 0.92 GW/cm² respectively. Energy 1.25 mJ, 1.44 mJ, 1.54 mJ, 1.66 mJ, and 2.25 mJ were used at 1064 nm, which have power densities of 0.52 GW/cm², 0.59 GW/cm², 0.64 GW/cm², 0.69 GW/cm², and 0.93 GW/cm², respectively.The third-order nonlinear susceptibility χ⁽³⁾,absorption coefficient (β), and the refractive index (n₂) of APB crystal are determined at both the wavelength (532 & 1064 nm). The values of nonlinear refractive index (n₂) at 532 nm excitation were − 1.55 × 10^–5 cm²/GW, − 1.63 × 10^–5 cm²/GW, − 1.87 × 10^–5 cm²/GW and − 2.74 × 10^–5 cm²/GW while the values of absorption coefficient (β) were 0.038 cm/GW, 0.042 cm/GW, 0.057 cm/GW, 0.093 cm/GW and 0.093 cm/GW. At 1064 nm excitation the values of refractive index (n₂) were − 1.69 × 10^–5 cm²/GW, − 1.89 × 10^–5 cm²/GW, − 1.91 × 10^–5 cm²/GW, 1.99 × 10^–5 cm²/GW and − 2.16 × 10^–5 cm²/GW and value of absorption coefficient (β) were 0.082 cm/GW, 0.086 cm/GW, 0.108 cm/GW, 0.132 cm/GW and 0.137 cm/GW, respectively. The value of χ⁽³⁾ were 1.51 × 10^–12 esu, 1.64 × 10^–12 esu, 2.07 × 10^–12 esu, 3.26 × 10^–12 esu and 3.26 × 10^–12 esu at 532 nm and 4.84 × 10^–12 esu, 5.12 × 10^–12 esu, 6.30 × 10^–12 esu, 7.73 × 10^–12 esu and 8.03 × 10^–12 esu at 1064 nm excitation. It was shown that the increase of power densities leads to increase of third-order nonlinear susceptibility χ⁽³⁾. It was also shown that the crystal possess positive absorption coefficient and negative nonlinearity, i.e. self-defocusing behavior. The analysis and comparison of experimental values of nonlinear parameters are presented.