GROWTH OPTIMIZATION OF KTiOPO4 LARGE SINGLE CRYSTALS
FOR APPLICATION IN NONLINEAR OPTICAL DEVICES
M.Tseitlin*, N.Angert**, G.Shwarzman**, M.Roth***
* - College of Judea and Samaria, Ariel, Israel
** - Raicol Crystal Ltd, Yehud, Israel
*** - The Hebrew University of Jerusalem, Jerusalem, Israel
1.Introduction
Potassium titanyl phosphate ( KTiOPO4, or KTP) belongs to the family
of isomorphic compounds with the general composition of MTiOXO4, where
X = {P or As } and M = {K, Rb, Tl or Cs (for X=As only)}, exhibiting the
mm2 point group symmetry at room temperature with unit cell constants
of a = 12.822 Å, b = 6.4054 Å and c = 10.589 Å (polar axis) [1]. KTP has
several unique properties, such as large non-linear optical (NLO) coefficients,
wide acceptance angles, phase matching properties and high optical damage
threshold, making this crystals attractive for frequency doubling and
parametric devices [2-3]. It has a high conversion efficiency for second
harmonic generation (SHG). Its large electro-optic coefficients and low
dielectric constant make it very useful for various electro-optic applications,
such as Q-switches and modulators [2]. The figure of merit of a KTP electrooptic waveguide modulator is reported to be twice that of any other inorganic
material. This indicates that KTP is a promising material for integrated optics
applications [4]. The crystals are transparent over a wide wavelength range,
chemically and thermally stable and non-hygroscopic. However, many high
power industrial and medical applications require further improvement of
growth technology of large KTP single crystals and their quality in terms of
chemical homogeneity, defect structure and optical uniformity.
2.Experimental
KTP large single crystals were grown by the modified top-seeded
solution growth (TSSG) method with pulling on crystallographically oriented
seeds.
This method implies growth from concentrated solutions solidifying at higher
temperatures than usually accepted in order to increase the growth rate due to
lower viscosities of high-temperature melts, in similarity with our previous
report [5]. The basic experimental setup was slightly modified with respect to
the one used previously. The main part of the growth system was a vertically
oriented resistance heated tubular furnace (ID 160 mm) accommodating
platinum crucibles of desirable dimensions. A cylindrical steel frame provided
support for both the furnace and the crystal puller unit mounted above the
furnace. A platinium coated pulling rod was attached to the puller unit
equipped with stepping motors allowing for computer controlled translational
and rotational motion. The growth system was equipped with a balance that
allowed controlling the crystal weight during the process. The electrical power
was supplied to the resistance heaters through a temperature controller
(Eurotherm 818) interfaced with a desktop computer. Thus, all variable
parameters, namely the rotation and pulling rates, weight and time/
temperature profiles, were computer controlled.
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Solutions of KTP in the K6 (K6P4O13) self-flux and a K6M (K6P4O13 +
modificator) flux were prepared using Aldrich 99,99% purity TiO2, KPO3 and
K4P2O7 chemicals obtained by thermal decomposition of KH2PO4 and
K2HPO4 (Merck Suprapur) respectively. The charge was loaded into a 1200ml Pt crucible and subjected to a 48 h soaking with flux homogenization aided
by a Pt stirrer. The crucible was usually filled to 85% of its volume. Seed
rotation rates varied from 70 to 20 rpm and pulling rates, when applied, from
0.2 to 1.2 mm/day.
The temperature was controlled within 0.1oC. Ramped reduction of the
solution temperature was carried out at a rate of 0.01 to 0.15oC h-1.
Concentration KTP
( per 1 g of K6)
g
3. Crystal growth
The way to increase growth rate is to enlarge the
supersaturation temperature. The value of the latter is limited by the
onset of spontaneous nucleation in the solution. In order to overcome
this difficulty we have tried to modify the flux composition. We find
that the addition of PbO to the self-fluxes leads to sufficiently higher
growth rate. Modified flux leads to an increasing solubility of KTP,
so at the same growth temperature it is possible to achieve a
significantly higher KTP concentration {Fig.1}.
2
1.5
(b)
1
0.5
(a)
0
900
950
1000
1050
1100
Temperature, deg. C
Fig.1. Comparison of the solubility of KTP in two fluxes: (a) K6;
(b) K6+PbO
Large KTP crystals for OPO and electro-optic applications have been
grown from the K6M flux, usually with the initial crystallization
temperature of 1026oC corresponding to a 1.2 weight ratio (g/g) of
KTP and K6. Crystals grown from modified fluxes are colorless and
without inclusions. The maximum Pb ions incorporation into KTP
crystals grown from the flux is 7500 ppm (data of ICP analyses). At
the same time we have observed, in such type of crystals, defects
named “striations”. Striations are parallel to the growing facets for
each growth pyramid and connected with non-equilibrium solute
incorporation due to the existing non-stable conditions of crystal
growth. Pb incorporation does not lead to essential changes in most of
the physical parameters (Curie temperature, electrical conductivity,
etc.).
The solubility of KTP in different self-fluxes varies greatly, as well as
the crystal morphology [6]. A typical habit of an immersion-seeded
KTP crystal grown from a K6M solution is demonstrated in Figure 2 .
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Fig.2. Typical morphology of KTP crystals grown from a K6M
flux.
Such crystals exhibit fourteen facets belonging to four families of
crystallographic planes, namely: {100}, {110}, {011} and {201}.
Accordingly, fourteen growth sectors develop simultaneously on the
submerged seed. All crystals grown in this work were pulled on
{100} seeds. We have suggested earlier [5] that TSSG on [100]oriented seeds may yield large single-sector KTP crystals with an
additional benefit of planar growth interface, i.e. maximum transverse
optical uniformity for elements cut in the X-direction. This includes
non-critically phasematched OPO elements and electrooptic Qswitches. The latter are not very sensitive to optical uniformity along
the laser beam propagation direction [7]. Large KTP crystal with
dimensions 53(X) x 82(Y) x 65(Z) mm (weight 540g) were obtained
in course of a 52 days long growth process using the above mentioned
conditions of crystal growth {Fig.3}.
Fig.3. Large crystal of KTP grown from the flux K6M with
pulling on X-oriented seed.
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4. Performance of optical elements
If l is the typical crystal dimension, in the case of submerged
seed, or volumetric growth, dnx/dl ~ l2.In the case of one-dimensional
growth (TSSG with pulling) dnx/dl = const, or is independent of the
pulled crystal length. Naturally, the smaller are the lateral dimensions
of the TSSG-pulled crystal the smaller is the refractive index
gradient. In practical terms, large optically uniform crystals can be
pulled from very large crucibles. We have fabricated long (25-40
mm) OPO elements from such crystals and tested their nonlinear
optical performance. High conversion efficiencies of over 30% have
been obtained for the eye-safe 1.57 signal frequency excited by a
Q-switched Nd:YAG (1.06 ) laser. Use of three 20 mm long
elements in a ring resonator has allowed obtaining a 43% conversion
efficiency for 7 ns pulses (12 Hz repetition rate) at a 200 MW/cm2
power density. Apparently, small (and constant) dnx/dl values in the
x-direction cause only slight mismatch, well within the acceptance
limit of the noncritically phasematched OPO process. It is noteworthy
that no refractive index gradient exists in the Z-Y plane. This explains
the high quality of KTP switches obtained which exhibit an extinction
ratio of 300:1 and, unlike LiNbO3 [8], show no signs of
photorefractive damage even at high peak power operation.
5. Conclusion
KTP crystals with weights up to 550 g have been grown using
large volume Pt crucibles (1200 ml) and optimized flux composition
(doped with Pb ions) by puling on [100]-oriented seeds. This
technique allows to obtain KTP X-plates with cross sections of more
than 50(Z) x 60(Y) mm2 and fabricate long (25-40 mm) OPO
elements.
References
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[4] J.D.Bierlein and H.Vanherzeele, J.Opt.Soc.Amer. B6 (1989) 622.
[5] N.Angert, L.Kaplun, M.Tseitlin, E.Yashchin, M.Roth
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[6] G.M.Loiacono, T.E.McGee, G.Kostecky,
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