Interrelation between 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 crystalline nature and the optical properties of lab-grown nonlinear optical crystals have made tremendous strides, establishing that these properties are directly proportional. In optical spectroscopic point of view, most of the researchers justify that the higher degree of crystalline nature of the crystals can be attained 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 paper, we propose an alternative and reliable interpretation to justify the degree of crystalline nature by adopting an optical spectroscopic perspective through the band energy theory of solids. To justify the claim, 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. Given this standpoint, 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 experienced for crystals of a high degree of crystalline nature, and a red shift is noted for crystals of lower crystalline nature. Among the two parameters, the absorption band width results have higher sensitivity with respect to the degree of crystalline nature compared to the absorption band position. Based on the presented results, the 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. These results are highly valid irrespective of the growth techniques, and such interpretation may lead to a new pathway whereby understanding the interrelation between optical absorption edge shape and degree of crystalline perfection of the nonlinear optical crystals can be feasible.