Geoscience Reference
In-Depth Information
clearly. Thus, the solution for this problem was to use a second crystal which effi-
ciently converts the near-IR to the visible—a harmonic generator crystal
[135,136]
.
The requirement was for a material with high conversion efficiency and appropriate
index of refraction characteristics to allow phase matching. The AT&T Bell Labs
group
[135]
tried with LiNbO
3
.
Subsequently, the DuPont group discovered KTP to be phase matchable most
efficiently at 1.064
m. It has very high
laser damage resistance, coupled with low optical losses and it is transparent over a
wide range of wavelengths with favorable angular, spectral and temperature band-
widths and has high conversion efficiencies
[134,137,138]
. Also, it was found that
KTP's conversion efficiency was twice that of LiNbO
3
and hence it is the most
commonly used 0.53
μ
m to an easily visible green light at 0.53
μ
m harmonic generator. The additional advantage is that the
preparation of LiNbO
3
, which could be grown from the melt, was very expensive.
Also, the starting materials for the growth of LiNbO
3
were equally expensive. In
contrast to this, the starting materials for the growth of KTP are rather cheaper and
the growth techniques are relatively inexpensive. The lower cost preparative meth-
ods could have an impact on its use, and indeed, more generally on the commer-
cialization of nonlinear optical materials and electro-optic devices and systems on
the whole. KTP crystals are mechanically, chemically, and thermally stable and
nonhygroscopic. It is remarkable for both high nonlinear optical coefficients and
high conversion efficiency. The material has excellent physical properties and can
be used in a type II phase-matching configuration. In the recent years, KTP has
been developed for quasi-phase-matched guided-wave devices to access the visible
spectrum.
KTP belongs to the family of compounds that have the formula unit MTiOXO
4
,
where M
μ
P or As. All members belong to
the orthorhombic space group, P
na21
, point group, mm2, with the following lattice
cell parameters: a
K, Rb, Tl, NH
4
or Cs (partial) and X
5
5
371.115
˚
3
. An important
crystallographic property with reference to the optical, chemical, and solid-state
behavior of KTP is that there are two formula units in the asymmetric unit so that
the proper solid-state formula is K
2
(TiOPO
4
)
2
. The structure of KTP is composed of
helices of TiO
6
octahedra linked via phosphate bridges. This leads to an open frame-
work structure in which the charge-balancing cations are incorporated within the
channels (
Figure 5.36
)
[139]
. The nature of the KTiOPO
4
host framework suggested
the possibility that modification of the internal structural characteristics, and hence
the electro-optic properties, might be achieved by gas-phase absorption and descrip-
tion. The K-ion exists in a high-coordination number site and is weakly bonded to
both the Ti octahedral and P-tetrahedral channels existing along the Z-axis [001]
direction, where K can diffuse, with a diffusion coefficient, several orders of magni-
tude greater than in the X
10.616
˚
, V
12.814, b
6.404, c
5
5
5
5
Y plane.
In spite of the above-mentioned unique properties of KTP, its application has
been limited by a shortage of crystals of sufficient size and quality. In particular,
the crystal growth processes have tended to be plagued by spurious nucleation pro-
blems and the inclusions of the solvents. The first attempt to prepare KTP crystals
was by Ouvard during 1890
[140]
, who melted TiO
2
,K
4
P
2
O
7
, and K
3
PO
4
. Masse
