Crystal Growth

Optical methods have been employed in crystal growth research for around half a century. The exploitation of lasers as light sources and computers for data acquisition and processing is very recent. Laser-interferometry is evolving as a powerful technique in studying online and in situ crystal growth phenomena. This application is of considerable importance because the grown crystals can be used in high-technology applications, i.e., solid-state lasers. The crystal growth technology being referred to here is the growth of crystals from their supersaturated aqueous solutions. Once the growth process is initiated, temperature, and concentration gradients are setup in the liquid phase around the crystal. These in turn can lead to buoyancy-driven currents and influence the crystal quality. The objectives behind measurement are then to monitor the flow, thermal and concentration fields in the solutions, and also the surface topography of the grown crystal. The thermal and concentration fields can be mapped using refractive index-based techniques; it is interesting to note that the crystal surfaces can be mapped using differences in the geometric path length, as in a Michelson interferometer. A brief survey of laser-interferometry applied to research on fluid mechanics and transport phenomena during crystal growth is presented below.
The observation of growth spirals using phase contrast microscopy (PCM) was first reported by Verma [45]. Similar microscopy studies were carried out by Sunagawa [46] on mineral crystals and summarized later by the author [47]. Extensive work on the surface microtopographic investigations of solution grown crystals using PCM and DICM (differential interference contrast microscopy) has been caried out by Bennema and coworkers [48]. They observed the spirals on crystals growing from the liquid state. Tsukamoto et al [49, 50]. demonstrated that by combining optical phase contrast microscopy and differential interference contrast microscopy with a convectional TV system, mono-molecular spiral growth steps on crystals can be observed during the growth in aqueous solution. Onuma et al. [51-53] have carried out a study of crystal growth at the microscopic level on barium nitrate and K-alum using Schlieren and Mach-Zehnder interferometry. They studied the effect of buoyancy driven convection and forced flow rate on the microtopography of the crystal growing form solution. Maiwa et al. [54] studied the growth kinetics of faces of barium nitrate crystals using micro-Michelson interferometry in conjunction with the differential interference contrast microscopy. Onuma et al. [55-56] developed a real-time phase shift interferomeer, an improvement over the DICM, and applied it to the measurement of the concentration field through a micro-Mach-Zehnder interferometer. Simultaneously they used a micro-Michelson interferometer to study the growth kinetics.  Later Sunagawa [57] reviewed the research carried out by his coworkers on interferometric analysis of crystal grown from the solution.
In addition to the Japanese researchers listed above, the work carried out by Chernov,  Rashkovich, and Vekilov and their coworkers in Russia has helped in understanding the mechanics of solution-grown crystals. Rashkovich et al. [58-64] developed a Michelson interferometer for studying the growth rate, slope of growth hillock, step veloity, and the hydrodynamics of the solution around crystals growing form it. Their experimental setup can be used for studying crystals as small as a few millimeters to as large as several centimeters. Vekilov [65-67] applied Michelson interferometry to the study of KDP and ADP crystal growth kinetics, as well as to understand protein crystal growth mechanism. Sherwood and his coworkers at Glasgow have also used interferometry for studying crystal growth rates [68-69]. They have used synchrotron X-radiation for assessment of the strain in crystals and its relation to growth rate dispersion.