Crystal growth and nonlinear optical studies of m-dinitrobenzene doped urea

Urea is an attractive material for frequency conversion of high power lasers to UV but the frequent use of urea (U) crystal at normal atmosphere is troublesome due to its hygroscopic nature. The hygroscopic character of urea has been modified by making the solid solution with m-dinitrobenzene (DNB). The phase diagram of DNB and U shows the formation of a monotectic and a eutectic. There is also a large region of immiscibility, where two immiscible liquid phases are in equilibrium with a liquid of single phase. The solubility of U, DNB and UDNB and the stability of solutions have been studied in methanol at different particular temperatures. van't Hoff equation was used to determine the enthalpy and entropy for the different saturation temperatures for each material. Transparent single crystals of UDNB have been grown from a saturated solution in methanol and the optimum conditions to grow the good quality crystals have been determined. Optical properties, e.g., second harmonic generation (SHG), refractive index and band gap of the UDNB crystal were measured and their values compared with parent compounds. Besides modification in hygroscopic nature, UDNB also shows the higher SHG signal than the parent components.