Studies on nonlinear optical ammonium pentaborate crystals

In this paper, the properties of a nonlinear optical crystal, ammonium pentaborate were discussed. Ammonium pentaborate (AB5) was synthesized by reacting ammonium carbonate and boric acid in 1:10 molar ratio. AB5 single crystals were grown by slow evaporation solution growth method at 32 °C temperature using water as solvent. The lattice parameters of grown crystal were determined using single crystal X-ray diffraction analysis. The functional groups and vibrational behaviour of chemical bonds present in the crystal were enumerated by FT-IR and FT-Raman studies. Hardness number of the material was measured for various loads using Vickers microhardness tester. Thermal stability and decomposition behaviour were studied upto 1000 °C by TG/DT and DSC analysis. UV–Vis-NIR analysis revealed that grown crystals have optical transmission from 230 to 900 nm and lower cut-off wavelength is around 230 nm. Optical band gap of the material was found to be 5.20 eV. Second harmonic generation was confirmed by Kurtz–Perry technique using Nd:YAG wavelength of 1064 nm. Z-scan measurement using Gaussian beam of He–Ne laser of wavelength at 632.8 nm reveals the self-focusing, saturable absorption behaviour of the crystal and third order nonlinear optical parameters, refractive index (n₂), nonlinear absorption coefficient (β) and susceptibility (χ⁽³⁾) were also calculated. Laser damage threshold value was calculated as 10.29 GW/cm² using 1064 nm pulsed Nd-YAG laser having pulse width of 10 ns.