Interrelation between the exponential absorption-optical band width and degree of crystalline perfection in nonlinear optical bulk single-crystals

Abstract

Over the years, investigations into the correlation between the degree of crystallinity and the optical properties of lab-grown nonlinear optical crystals have made tremendous strides, and it has been established that these properties are highly sensitive to the degree of crystalline nature. In optical spectroscopic point of view, most of the researchers justify that the degree of crystalline nature of the crystals can be evaluated only by the enhancement of optical transmittance whereby most of the typical and related statements in the crystal growth and characterization-related research articles are as follows: “enhancement of optical transmittance of the grown crystal is due to the enhancement of the crystalline perfection”. In this study, we propose an alternative and reliable method to assess the degree of crystallinity by adopting an optical spectroscopic perspective based on the band energy theory of solids. We consider the UV-vis-NIR spectroscopic results of a few prominent nonlinear optical crystals, which have been previously published in esteemed journals by crystal growth experts. Based on the observed results, the high degree of crystallinity concerning the crystals' absorption band width is found to be lower compared to the lower degree of crystallinity of the crystals. In addition, a blue shift is exhibited by crystals with a high degree of crystallinity, and a red shift is noted for crystals with a lower degree of crystallinity. Among these two parameters, the absorption band width results have higher sensitivity with respect to the degree of crystalline nature compared to the absorption band position. However, based on the presented results, the exponential absorption band width and absorption band position results can be strongly considered to justify the degree of crystalline nature, followed by the optical transmittance of the nonlinear optical crystals. The results of the interrelation between the degree of crystalline nature and exponential absorption band width are found to be highly valid with irrespective of the growth techniques, and such interpretation may lead to a new pathway whereby understanding the interrelation between optical properties and degree of crystalline perfection of the nonlinear optical crystals can be feasible.