The aspect ratio of potassium dideuterium phosphate (DKDP) crystals

Large crystals of potassium dideuterium phosphate (DKDP) are used for third harmonic generation in powerful lasers. To allow the optimal production of optical elements with a shape required by these applications, the aspect ratio, i.e., the ratio of a crystal's dimensions along the crystallographic axes, needs to be controlled. To highlight the factors that determine the aspect ratio, we have monitored the normal growth rate, local slope, and step velocity for both the prismatic {100} and the pyramidal {101} faces of DKDP crystals growing in solutions with saturation temperatures ranging from 52 to 70 degreesC over a range of supersaturation from 0 to 0.06 in the presence and absence of 1 ppm aluminum ions. We employ optical two-beam interferometry with the growing crystal face as one of the mirrors to determine the above three kinetic variables with a frequency of 1 s(-1). We find that in the probed temperature range, Al3+ ions are not effective in retarding the growth of the {100} faces to bias growth in the Z direction as is required for growth of crystals of sufficient size for use as optical elements at high power laser systems. The step kinetic coefficient does not change with the addition of Al3+ and, for the shallow vicinal on the (101) face, is in the range of 0.09-0.17 cm s(-1).