POTASSIUM TITANYL PHOSPHATE (KTP) – AN OXIDE OF
CHOICE FOR NONLINEAR OPTICAL AND ELECTRO-OPTIC
DEVICES
M. Roth1, N. Angert2 and M. Tseitlin3
1
School of Applied Science, The Hebrew University, Jerusalem 91904, Israel
2
Raicol Crystals Ltd., Industrial Zone, Yehud 56712, Israel
The Research Institute, College of Judea and Samaria, Ariel 44837, Israel
3
1. Introduction
Potassium Titanyl Phosphate (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 [1]. Its
large optical nonlinearity, relatively high damage threshold and excellent
thermal stability has made it an important building block in laser
systems utilizing frequency conversion, such as the second harmonic
generation (SHG) and optical parametric oscillations (OPO), as well as
electrooptic amplitude modulation and Q-switching. A great deal of
effort has been introduced lately into developing the periodically poled
KTP (PPKTP) devices based on quasi-phase-matched (QPM) frequency
conversion. However, both birefringent and QPM properties of KTP
crystals depend on their structural characteristics, such as morphology,
chemical composition and defect distribution, which are closely related
to the specific crystal growth parameters.
Hereby, we review the correlation between the optical
performance of nonlinear optical and electrooptic KTP (and RTP)
elements and the structural and compositional variation of the crystals’
properties in course of their growth from self-fluxes. The latter are, by
definition, potassium or rubidium phosphates with the atomic ratios R =
[K,Rb]/[P] varying from 1 to 2. In the case of KTP, the most commonly
used self-fluxes are K6  K6P4O13, K4  K4P2O7, K8  K8P6O19 or
K15  K15P13O40. The solubility of KTP in these self-fluxes varies
greatly, as well as the resulting crystal morphology [2] to be discussed
first. In the following, the link between the morphology and device
performance will be demonstrated. The influence of structural defects
will be considered as well. Finally, the as-grown and artificial
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ferroelectric domain structures will be discussed, and the utilization of
the latter in novel devices will be reviewed.
2. KTP Crystal Morphology
The typical morphology of an immersion-seeded KTP crystal
grown from the K6 flux is given in Fig.1. 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. Cutting optical elements
across the growth sectors results in their optical nonuniformity, in terms
of the distribution of refractive indices, since each type of planes is
characterized by its particular growth kinetics and different impurity
incorporation mechanisms. Indeed, such optical inhomogeneity of fluxgrown KTP crystals has been reported more than a decade ago [3].
Fig. 1. Typical habit of an immersion-seeded KTP crystal grown from the
K6P4O13 flux.
We have suggested earlier [4] that top-seeded solution growth
(TSSG) on [100]-oriented seeds may yield large single-sector KTP
crystals. The complementary benefit of a planar growth interface is to
assure a maximum transverse optical uniformity for elements cut in the
X direction, e.g. noncritically phasematched OPO elements and
electrooptic Q-switches. Fig. 2 shows the results of an eye-safe OPO
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experiment (using three 20 mm long KTP elements in a ring resonator
design) with an unprecedently high conversion efficiency of 43%. It is
noteworthy that the OPO conversion efficiency in commercially
available KTP crystals rarely exceeds 30%. The performance of Xoriented pairs of KTP or RTP Q-switching elements may benefit from
the improved transverse uniformity of the crystals even more. This fact
is so important in terms of high-frequency pulse laser applications that
the Q-switching performance will be considered in great detail below.
Pump Power Density, MW/cm^
Fig. 2. OPO (1.06 & 1.57 ) efficiency of 3(4420 mm3) KTP elements.
3. Variation of Chemical Composition
Longitudinal variation in refractive indices values may be
associated with the crystal’s gradual compositional changes during
growth from self-fluxes as the flux becomes increasingly richer in
potassium with the growth temperature decrease. Fig. 3 shows the Curie
point (Tc) variation of KTP crystals as a function of growth temperature
for four different self-fluxes. The data have been derived from the
experimentally determined Tc dependencies on the KTP concentration in
the various self-fluxes [5] and the corresponding experimental solubility
curves. The remarkable Tc increase with growth temperature lowering,
especially for fluxes with smaller values of R, indicates that the
potassium stoichiometry of KTP crystals is very sensitive to the flux
composition. Similar results are obtained with RTP and RTA crystals
with respect to rubidium stoichiometry.. We have investigated the KTP
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stoichiometry using both electron microprobe analysis and Tc
measurements on samples synthesized by solid-state reactions from
powders of variable initial compositions (since KTP decomposes
incongruently below the melting point). The results show that KTP
solidifies in a relatively broad “homogeneity” range, which does not
comprise the stoichiometric composition. Its general chemical formula
can be given by K1+xTi1-yPO5+0.5x-2y, where 0 x,y 0.02, and extra
potassium ions may fill in the titanium vacancies or occupy interstitial
positions. If the potassium and titanium nonstoichiometry may vary
within a few atomic percents, a corresponding optical inhomogeneity
may be induced in the crystal. Indeed, a gradual variation in the
refractive indices from the seed area to the periphery has been observed
during growth of large KTP crystals [6]. We have recalculated this
variation using the data of [5] together with the known mass balance of
the material and found that nz increases at a rate of 210-6 °C-1, while nx,y
decrease at a rate of 10-7 °C-1.
Fig.3. Curie point versus growth temperature dependencies for K4 (R=2), K6
(R=1.5), K8 (R=1.33) and K15 (R=1.15) fluxes. The dashed straight line
crosses the curves where the Curie point equals the crystallization temperature.
Fig. 3 shows that KTP crystals grown at lower temperatures,
especially from self-fluxes with higher [K]/[P] ratios, exhibit higher Tc
values. This implies that such crystals have a higher content of
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potassium and, therefore, contain less potassium vacancies. We recall
that these vacancies are essential in formation of point defects associated
with the detrimental gray track damage in frequency doubled (into
green) KTP crystals [7]. Such point defects comprise Ti 3+ ions with
adjacent [K+] vacancies. A comparative study of the kinetics of gray
track formation in high- and low-Tc KTP crystals has been conducted
using the newly developed (at Stanford University) green radiation
induced infrared absorption (GRIIRA) method. In this experiment, a 10
kW/cm2 CW green (532 or 514 nm) excitation beam was used to induce
the gray track damage, while the resulting IR (1064 nm) absorption was
monitored. The initial stage of track formation, from several seconds to
several minutes, could be clearly observed as an increase in the IR
absorption as shown in Fig. 4. At this stage the gray track is located
within the green beam region and disappears fast when the green light is
turned off. Yet, the first minute test is sufficient to determine the quality
of the sample. The results of Fig. 4 show clearly that KTP crystals
having a higher potassium content reveal a higher resistance to graytrack formation. Such crystals may compete successfully with
hydrothermal KTP in high peak power frequency doubling.
Absorption coefficient x 106 cm-1
500
High Tc
400
Low Tc
300
200
100
0
0
20
40
60
80
Time (s)
Fig. 4. Initial stage of gray-track formation in K-deficient (low-Tc) and K-rich
(high-Tc) KTP crystals.
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4. Ferroelectric Domain Structure
High-temperature solution growth of KTP around the Curie
point inevitably brings about the formation of ferroelectric domains
during the crystal cool-down [8]. We have studied the domain formation
mechanisms within different growth sectors and found that they depend
both on the crystallization kinetics and compositional gradients (both
stoichiometric and due to impurities). Such gradients, followed by
species diffusion and charge separation, create built-in electric fields that
may be similar or much larger than the coercive fields. In the latter case,
the built-in fields define the direction of the ferroelectric domain. The
results obtained have allowed us to suggest a number of ways for
producing single-domain KTP crystals. Seeded growth in either <001>
(Z), <010> (Y), <201> or <011> can be chosen depending on the crystal
application in mind. In parallel, it has been shown that potassium-rich
crystals have much smaller built-in fields, which in similarity with
stoichiometric LiNbO3 and LiTaO3 allows using much smaller external
fields for electrical poling of KTP crystals. As a result, plates in excess
of 1 mm thickness can be easily poled increasing greatly the useful
apertures of PPKTP devices. An example of such device used for SHG
(frequency doubling) is shown in Fig. 5. The period is defined
photolithographically in this case, and its maximum width should not
exceed the coherence length (where the second-harmonic power reaches
a maximum value) for a specific interaction (wavelength).
Fig. 5. Periodically poled KTP structure for SHG.
However, high-power applications may require large apertures
(up to 1 cm) for the pump beam, which cannot be achieved by electrical
poling. We have found an excellent alternative in developing a method
of as-grown production of periodic ferroelectric domain structures in the
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entire bulk of the crystal. This has been achieved by introducing an
oscillatory undercooling regime by means of multi-step programming of
the employed temperature controller. The details of the domain
production mechanisms and growth parameters are reported elsewhere
[9]. Large KTP crystals with a well-defined periodic domain structure of
at least 10 mm length have been obtained. A fragment of such structure
for TSSG KTP pulled in Z-direction is presented in Fig. 6. Two (201)type and one (011)-type plane is seen around the seed.
Fig. 6. As-grown KTP crystal periodic domain structure (enlarged x160,  = 25 ).
Structures with periods  varying from 25 to 40  have been
easily obtained. They can be effectively used for OPO applications.
Frequency doubling requires smaller periods, such as 9  for SHG of the
1.06  radiation of the Nd:YAG laser. There is no principal limitation in
creating the narrower domain as-grown structures, and the
corresponding experiments are currently underway.
In the case of periodic domain structures, there is no need of
orienting the crystal in the appropriate phase-matching direction, and a
direction corresponding to the maximal nonlinear optical coefficient can
be chosen. Such are the conditions of the so-called quasi-phasematching (QPM). The QPM effective coefficient is given by
deff = (2)d33sin(D), where d33 is the maximum coefficient for both
KTP and RTP in either X or Y crystallographic directions and D is the
229
duty cycle (ratio between the size of the inverted domain and the QPM
period). The quality of the PDS is thus determined by the value of the
duty cycle, D = a/, where a is the actual domain width. The optimal
effective NLO coefficient and, thus, frequency conversion efficiency is
obtained at D = 0.5 [10]. Our experimental values of D were in the 0.4 –
0.6 range, which are still very useful for practical implementation of the
as-grown PDS.
The X-direction is usually chosen for light propagation, since
ferroelectric domains are formed anisotropically under the electric field
poling with a preference in the Y-direction. The value of the effective
coefficient is very important, since the conversion efficiency is
proportional to (deff)2 in the nondepletion approximation:
SHG
2 2
P2  2 d eff k P 

 Lh( B,  ) ,


P  n2 n2 0 c 3 
(1)
where L is the crystal length and h(B,) is the Boyd-Kleinman
factor (h(B,) = 1 in our case [9]).
Since deff = 3.05 pm/V in the regular, phase-matched,
interaction, and deff ≈ 9.5 pm/V (d33 = 14.9 pm/V) for D = 0.5, QPM
gives an order of magnitude gain in the SHG conversion efficiency at
lower pumping powers. An internal doubling efficiency of 64% at 2
MW/cm2 power has been reported for a single pass (through a 1 cm long
PPKTP crystal) frequency doubling of the 1.06  Nd:YAG radiation into
the green (532 nm) [11]. The even more exciting advantage of QPM is
that any SHG, OPO or other nonlinear interaction can occur in the entire
transparency range of the crystal rather than very narrow spectral
intervals defined by the birefringent phase-matching. In particular, the
possibility of frequency doubling of the IR diode lasers in to the blue
becomes feasible. Compact blue lasers are subject to intensive
development at the present time.
5. KTP and RTP Q-switching elements
Availability of a variety of electro-optic materials has triggered
the extensive use of Q-switching for generation of high peak-power
pulses in solid-state lasers based on the electro-optic effect. Initially,
230
water-soluble crystals of potassium dihydrogen phosphate (KDP) and its
isomorphs [12] were widely used in the longitudinal mode, i.e. when the
electric field is applied along the direction of the optical beam. They are
hygroscopic and require hermetic housings with protective windows, but
have high optical uniformity and are useful for large aperture
applications. Higher transmission, or lower insertion losses, and high
contrast ratio at average powers in the kW range [13] can be achieved
with lithium niobate (LN, LiNbO3) operating in a transverse electrode
orientation with the light propagating along the optical axis. However,
LN also has several limitations: low damage threshold (~ 10 MW/cm2),
piezoelectric ringing [14] and pyroelectric depolarization [15]. Lithium
tantalate (LT, LiTaO3), isomorphic with LN, exhibits no piezoelectric
ringing, but its twice as large damage threshold is still not sufficient for
use with high peak-power high repetition rate diode pumped solid state
lasers (DPSSL). Therefore, high damage threshold BBO (BaB2O4)
crystals are employed lately for small aperture DPSSL in spite of their
low electro-optic coefficients, short length (high half-wave voltage, V)
and high cost. Very good Q-switching properties have been
demonstrated with KTP (KTiOPO4) crystals. When operated in a
thermally compensated double-crystal mode the KTP device shows
excellent resistance to thermal depolarization at high average power
densities [16]. The basic setup is shown in Fig. 7.
Fig. 7. Thermally compensated design of the double-crystal KTP or RTP Q-switch.
We will explain briefly the principal of operation of the device.
Phase retardation () in electrooptic crystals with compensated two231
element configuration can be expressed, for light propagating in the Xdirection and polarized at 45° in the ZY plane, by the following
expressions:
2L

L
( n z  n y )  n z3 rc 2V ,


d
First element:

Second element:
* 
(2)
 
3
2L* *

L*
,
( n y  n*z )  n*z rc*2


d
3
where
 ny 
rc 2  r33    r23 ,
 nz 
L , L* - element length;
n x , n y , n z , n *x , n *z - refractive indices;
d - distance between electrodes;
V - voltage between electrodes.
Static birefringence ( n z  n y  f (T ) ) is a function of
temperature.
However, if n y  n *y , n z  n *z , L  L* , the birefringence for
two elements is expressed by
    * 
2 3
L
n z rc 2V

d
(4)
and is, practically, independent of temperature. With this setup, i.e. light
propagating along X- and electric field applied along the Z-direction, the
half-wave voltage is
(5)
V  (  / rc 2 n z3 )( d / L ) .
Comparison of KTP-family crystals at hand indicates that RTP
has a several orders of magnitude higher electrical resistivity and,
therefore, no signs of electrochromism as compared to KTP. Therefore,
RTP crystals have been used throughout this study. We have determined
experimentally the electro-optic coefficients of RTP, and the results are
given in Table 1.
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Table 1
:Electro-optic coefficients (pm/V) and half-wave voltages (V) of RTP for d = L
at  = 633 nm.
Property
Literature data
Our
measurements
r13
10.9
12.5
r23
15.0
17.1
r33
33.0
39.6
rc1
23.6
30.2
rc2
20.3
23.6
V (X)
4560
3920
V (Y)
3900
3080
Although V (Y) > V (X) we have used the setup of Figure 6 in
view of the better optical uniformity of the crystal in the YZ plane as has
been discussed above. Small aperture devices may be fabricated for light
propagation in the Y direction.
The insensitivity of optical transmission of a compensated RTP
Q-switch to average temperature changes has been evaluated by placing
it in a thermally regulated mount between crossed polarizers (45° to the
Z-axis). The results are shown in Fig. 7.
Fig. 12. Transmittance versus temperature of an RTP Pockels cell between
crossed polarizers (straight line, two 11 mm long crystals) and of a single
crystal of equal length (sine, one 22 mm long crystal).
No transmission changes have been observed in the 25 – 125 °C
temperature range with two crossed 11 mm long RTP crystals, whereas a
single 22 mm long crystal (equivalent length) shows a well defined
periodic retardation with a period of about 2.5 °C. Identical results have
233
been obtained with the electrical field on and off, namely the half-wave
voltage of the compensated Q-switch does not vary in the temperature
interval studied as well. These outstanding results have been already
utilized and implemented practically. We have reported recently [16] on
a novel large aperture RTP-based double-crystal electro-optic Q-switch
exhibiting high damage threshold, low insertion losses, excellent thermal
stability and negligible piezoelectric ringing at repetition rates up to 75
kHz. 4.6 W average output power has been achieved at 75 kHz for a
diode end-pumped Nd:YVO4 laser with 31% light to light and 40%
slope efficiencies. No other crystal can operate as an electro-optic Qswitch in excess of 30 kHz without piezoelectric ringing.
6. Conclusion
In summary, adequate choice of the flux-growth technique (in
terms of seeding and pulling direction) as well as thorough control of the
crystal chemical composition and stoichiometry can yield optically
uniform KTP and RTP materials with a greatly reduced concentration of
defects. The improved performance of these KTP-family materials in a
variety of linear and nonlinear optical applications (both birefringent and
QPM) has been demonstrated.
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