Studies on nonlinear optical ammonium pentaborate crystals

Abstract

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.