Geoscience Reference
In-Depth Information
A successful large-scale hydrothermal growth process affects the size, cost, yield,
and quality of KTP crystals. The cost of an autoclave is governed by its size,
the type of alloys used, and availability of heat-treated billets from which the auto-
claves are fabricated. Thus, lower temperature experiments can use autoclaves
made from more-common steel alloys, in turn lowering the cost.
The darkening of KTP crystals caused by the lack of oxygen in the growth
process can be overcome by introducing an oxidizing agent, e.g., 1
5 wt% of
2
H
2
O
2
, into the mineralizer solution.
5.5.3 Morphology
The morphology of KTP crystals is quite interesting and it does not vary much
from flux grown to hydrothermally grown. KTP usually grows as multifaceted
crystal with 14 faces typical for the KTP structure. The commonly seen crystal
faces are {100}, {011}, {201}, and {110}. These faces possess different growth
rates and their relative sequence remains constant, even though the absolute growth
rate of each face may vary with growth temperature gradient, solvent concentration,
and so on. The average growth rates for various faces of hydrothermal KTP are
given in
Table 5.14
.
The {100} face has the largest area and, thus, the smallest growth rate among
the faces. The width and height of the steps in KTP are inversely proportional to
the growth rate or to the
T in different runs. Since KTP grows as multifaceted
crystal, cutting and polishing of the crystal are quite cumbersome. Therefore, the
growth of KTP with increased volume at lower cost is very important.
As given in
Table 5.13
, the properties of KTP crystals vary with the growth
method. Hydrothermally grown KTP crystals generally show an absorption band
between 3550 and 3600 cm
2
1
due to the presence of (OH)
2
. Generally, if a crystal
used for a nonlinear interaction has even a small amount of absorption, the effi-
ciency of the nonlinear interaction will be decreased. Absorption of the laser radia-
tion occurs through the volume of the crystals illuminated by the laser beam. Thus,
heating of the crystal occurs through this volume also. Cooling, on the other hand,
occurs by conduction. Consequently, heating the surface of the crystal establishes
thermal gradients within the crystal. As the refractive index depends on the temper-
ature through the thermo-optic effect, gradient variations in the refractive index
Δ
Table 5.14 Average Growth Rate for Various Faces of
Hydrothermal KTP
Growth Rate (10
2
3
mm/day)
{100}
{201}
{011}
{110}
90
97
123
156
65
72
88
110
40
45
53
71
