crystals
Article
Growth of High Quality Al-Doped CsLiB6O10
Crystals Using Cs2O-Li2O-MoO3 Fluxes
Xianchao Zhu 1,2 , Heng Tu 1 , Ying Zhao 1 and Zhanggui Hu 1, *
1
2
*
Key Lab of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing 100190, China; [email protected] (X.Z.);
[email protected] (H.T.); [email protected] (Y.Z.)
University of Chinese Academy of Sciences, Beijing 100049, China
Correspondence: [email protected]
Academic Editor: Helmut Cölfen
Received: 31 January 2017; Accepted: 9 March 2017; Published: 13 March 2017
Abstract: High quality and large size Al-doped CsLiB6 O10 (CLBO) single crystals have been
successfully grown by top-seeded solution growth (TSSG) technique using Cs2 O–Li2 O–MoO3 fluxes.
The advantages of this newly developed flux system were investigated by viscosity measurements
and growth experiments. Al-doped CLBO presents a very high transmittance in the visible region and
the weak absorption values at 1064 nm along a and c axes are only 140 and 50 ppm/cm, respectively.
The measured LIDT of Al-doped CLBO at λ = 1064 nm and τ = 5.0 ns is 5.10 GW/cm2 . Moreover,
Al-doped CLBO exhibits an apparent enhancement of the hygroscopic nature in contrast with the
undoped crystal as determined by the humidity experiments. Finally, a high fourth harmonic
generation (FHG) conversion efficiency of 63% utilizing Al-doped CLBO has been achieved by a
picosecond mode-locked Nd:YAG laser, the results also reveal that Al doping has no obvious impact
on the FHG conversion efficiency.
Keywords: nonlinear optical crystals; top-seeded solution growth; CLBO; Al doping
1. Introduction
Nonlinear optical (NLO) crystal is one type of functional crystal that has been widely used in
optical communication systems, industrial lasers, and microelectronic devices, owing to its various
properties including electro-optical, nonlinear optical, photorefractive effects, etc. [1–3]. Recently,
considerable research has been conducted on borate series nonlinear optical crystals for harmonic
generation in high-power solid-state ultraviolet (UV) lasers due to their excellent properties such
as relatively high tolerance to laser-induced damage, large nonlinear optical coefficients, moderate
birefringence, and wide transparency range in the UV region. For instance, several borate crystals,
such as β-BaB2 O4 (BBO) [4], LiB3 O5 (LBO) [5], CsB3 O5 (CBO) [6], CsLiB6 O10 (CLBO) [7,8], KBe2 BO3 F2
(KBBF) [9], YCa4 O(BO3 )3 (YCOB) [10], and YA13 (BO)4 (YAB) [11] have attracted a great deal of
attention. Among these crystals, CLBO is a newly developed borate crystal and has been successfully
applied for the fourth and fifth-harmonic generation of a Nd:YAG laser [12,13]. After firstly reported by
researchers in 1995 [7,8,14–16], CLBO is considered as a highly promising NLO crystal due to its small
walk-off angle and relatively large phase-matching angular, spectral, and temperature acceptance
bandwidths compared with other borate NLO crystals [12,17]. Additionally, in comparison with other
commercially available NLO crystals such as BBO and LBO, CLBO possesses incomparable advantages,
since it can be easily grown from congruent or near-congruent melts at extremely fast growth rate by
top-seeded solution growth (TSSG) method [16,18].
Since CLBO melts congruently at 848 ◦ C, many growth techniques could be utilized to grow
CLBO single crystals. The TSSG method has been commonly used to grow CLBO crystals. Currently,
Crystals 2017, 7, 83; doi:10.3390/cryst7030083
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Crystals 2017, 7, 83
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a self-flux poor in B2 O3 is extensively employed to grow CLBO crystal with the purpose of reducing
the viscosity of stoichiometric melt [14,19]. Meanwhile, increasing attention has been paid to obtaining
uniform supersaturation during crystal growth by means of solution stirring or crucible rotation
technique [19–21]. Kyropoulos and Czochralski methods have also been applied to grow large size
CLBO by many researchers [18,22,23].
Despite having favorable growth habit, the major challenge to the practical application of CLBO
is its highly hygroscopic behavior, which can result in the deterioration of the crystal quality and
ultimately lead to cracking [24,25]. To date, many authors have devoted their efforts to investigating
the hygroscopic nature and cracking mechanism of CLBO crystal [18,26–29], as well as the influence
of hygroscopy on the nonlinear optical properties [25]. On one hand, previous researchers stated
that water molecules attacked CLBO crystal along the a axis much more easily than along the c axis
because of relatively large channels that exist parallel to the a axis [26], and it was also observed that
water molecules were incorporated into the Cs defect [23]. Additionally, researchers reported that
surface hydration rate mainly depends on the humidity of the environment, and it was concluded that
hydration occurred at a relatively faster rate at higher humidity (>45%), and eventually resulted in
crystal cracking and the formation of Cs2 B10 O16 ·8H2 O [18,26,29]. On the other hand, many researchers
pointed out that surface hydration had an adverse effect on the optical properties, such as refractive
index distortions and decline in bulk LIDT and UV transmittance [30–32].
Thus, efforts in developing effective techniques to solve the hydration problem are very important.
During the last few years there has been considerable progress in the practical application of CLBO.
According to earlier studies, water impurity can be effectively eliminated by long time heat treatment of
CLBO crystal at 150–160 ◦ C [25,31,32], whereas it is inconvenient in practical application. Recent studies
have revealed that surface degradation of CLBO can be significantly suppressed by Al doping [33].
In recent years, self-flux has been widely utilized to grow Al-doped CLBO crystals. However, it was
very difficult to grow high-quality Al-doped CLBO crystals because of the relatively high viscosity and
instability of self-flux system, which can lead to the generation of crystal defects such as inclusions and
decrease laser-induced damage threshold and transmittance. As was previously mentioned, MoO3
flux exhibits outstanding properties and has been successfully applied to grow many borate crystals,
such as LBO [34], CBO [35], and YAB [36], etc. It is generally considered that MoO3 can effectively
cause structural changes in the boron-oxygen network, which leads to a weaker connectivity in the
boron-oxygen network and thus drastically reducing the viscosity of the growth system [37]. In 1999,
Pylneva [38] et al. firstly reported the crystallization region in Cs2 O–Li2 O–B2 O3 –MoO3 system and
obtained an undoped CLBO crystal with size of 60 × 40 × 20 mm3 , but the viscosity of this system has
not been measured. Moreover, they also briefly described the growth of Al-doped CLBO crystal, but
there was no detailed crystal growth investigation or relevant properties characterization. Up to now
in the literature, there are no other papers related to the growth of CLBO using MoO3 flux. Therefore,
this paper is aimed at attempting to grow large size and high quality Al-doped CLBO crystals using
Cs2 O–Li2 O–MoO3 fluxes.
In the present study, high quality undoped and Al-doped CLBO crystals were successfully
grown from optimized Cs2 O–Li2 O–MoO3 flux system by TSSG method. We carried out a detailed
investigation of crystal growth as well as the viscosity measurement of this flux system. The as-grown
crystals were characterized by powder X-ray diffraction, transmittance spectra, optical weak absorption,
and LIDT. Furthermore, the effect of Al doping on the hygroscopic behavior and fourth harmonic
generation (FHG) conversion efficiency of CLBO crystal were investigated by humidity experiments
and laser output performance, respectively.
Crystals 2017, 7, 83
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2. Experimental
2.1. Crystal Growth
The starting material was synthesized from high purity reagents Cs2 CO3 , Li2 CO3 , H3 BO3 , and
MoO3 at an appropriate ratio with the addition of a certain amount of Al2 O3 . After being accurately
weighed and mixed homogeneously, it was then melted in a Ø 90 × 80 mm3 platinum crucible at
900 ◦ C in several batches. The crucible was placed in a three-zone resistance furnace controlled
by a programmable Eurotherm (model 818) temperature controller. Then, the mixture was heated
to approximately 50 ◦ C above the melting point, followed by stirring with a platinum stirrer for
48 h to enable the solution to melt completely and mix uniformly. The saturation temperature was
precisely determined by seeding measurements method, and the suitable vertical temperature gradient
was about 1 ◦ C/6 cm. A high quality [001] oriented seed crystal was slowly introduced into the
solution surface at a temperature of 1 ◦ C higher than the saturation temperature and maintained at
this temperature for half an hour. The growth rate was carried out at a very slow cooling rate in the
range of 0.05–0.1 ◦ C/day in the initial stage. The growing crystal was rotated at a rate of 20–50 rpm
with the rotation direction inverted every 2 min. After the growth finished, the crystal was carefully
drawn out of the solution surface and cooled to room temperature at a rate of 5–10 ◦ C/h.
2.2. Characterization
The viscosity of growth solution was measured by a DVII + Pro viscometer (Brookfield
Co., Middleboro, MA, USA). Powder X-ray diffraction (XRD) measurement was verified by a
computer-automated Bruker D8 Focus diffractometer equipped with Cu Kα radiation (λ = 1.54056 Å).
The as-grown Al-doped crystals were analyzed by inductively coupled plasma mass spectrometry
(ICP-MS) for the determination of Al3+ concentrations. The transmission spectrum was recorded using
a Perkin-Elmer Lambda 900 UV-Vis-near-infrared (NIR) spectrophotometer in the wavelength range
of 185–1800 nm at room temperature. Optical weak absorption testing at 1064 nm was performed by
the photo-thermal common path interferometer (PCI) method. The laser-induced damage threshold
of the crystal was determined by a Continuum Surelite II pulsed Nd:YAG laser system. The surface
etching morphology of CLBO crystal induced by humidity was observed on the (001) planes using an
optical microscope (Nikon SMZ 745T, Nikon Co., Kanagawa, Japan). In the laser output experiment,
a 532 nm Nd:YAG laser was utilized as the fundamental source, and then doubled to 266 nm for the
measurement of conversion efficiency.
3. Results and Discussion
3.1. Crystal Growth and Viscosity Measurement
As previously reported [38], the molar ratio of Cs2 O:Li2 O:B2 O3 :MoO3 for CLBO crystal growth
was in the range of 1:(1.0–1.5):(3.0–6.0):(0.5–1.0). According to our growth results of various solution
compositions, we determined that the appropriate molar ratio was 1:1:3:1. After the appropriate molar
ratio of this flux system was identified, undoped and 2.5%–10% Al-doped CLBO were successfully
grown with growth duration of 30–40 days. The as-grown undoped and Al-doped crystals were free
from visible cracks and inclusions, as shown in Figure 1. The detailed growth conditions and results
are summarized in Table 1. It is clear that the cooling range varies from 3.2 to 4.1 ◦ C in this flux system,
apparently wider than that of self-flux system (1–1.5 ◦ C) [14,19], indicating that this flux system is
conducive to the growth of large size crystals.
The volatility and the viscosity—two crucial parameters during crystal growth—both depend on
solution temperature and composition. Low volatility and viscosity are beneficial to mass and heat
transfer as well as to maintain the stability of solution components. Hence, a flux system with low
volatility and viscosity is vitally important to grow high quality crystal.
crystals.
Al Doping
Concentration
undoped
2.5% Al-doped
Crystals
2017,
7, 83
5%
Al-doped
10% Al-doped
Al:Li
(Molar Ratio)
0
2.5:97.5
5:95
10:90
(a)
Saturation
Temperature (°C)
800
802
808
811
(b)
Cooling Range
(°C)
3.4
4.1
3.3
3.2
Size (mm3), Weight (g)
61 × 59 × 35, 125
68 × 59 × 47, 170
54 × 35 × 25, 64
50 × 49 × 26, 61
Inclusions
free
free
free 4 of 11
almost free
(c)
Figure 1.
of as-grown
crystals:
(a) undoped
CLBO;CLBO;
(b) 2.5%
Al-doped
CLBO; (c)CLBO;
5% Al-doped
Figure
1. Photos
Photos
of as-grown
crystals:
(a) undoped
(b)
2.5% Al-doped
(c) 5%
CLBO.
Al-doped CLBO.
Table 1.
1 shows
that the measured
saturation
was in the
range
of 800–811
which
Table
The detailed
growth conditions
andtemperature
results of undoped
and
Al-doped
CsLiB°C,
6 O10
was remarkably
lower than that of self-flux system (837–844 °C) [39]. Consequently, this flux system
(CLBO) crystals.
can reduce volatility so as to avoid component deviation caused by the relatively high evaporation
of Cs. In the case of Al-doped CLBO
grown from self-flux system, it is difficult
to accurately
Saturation
Al Doping
Al:Li
Cooling
Size (mm3 ),
Temperature
determine
saturation
temperature
due
to the relatively
great
fluctuation
of saturation Inclusions
temperature
Concentration
(Molar
Ratio)
Range
(◦ C)
Weight (g)
◦ C)
induced by volatiles. Additionally, the (seed
crystal can be easily etched by the volatiles from the
undoped
800falling. However,
3.4
61 growth
× 59 × 35,
125
free
solution,
resulting in the0 growing crystal
in our
experiments
utilizing
2.5%
Al-doped
2.5:97.5
802
4.1
68
×
59
×
47,
170
free
MoO3 flux, these disadvantages could be effectively eliminated. The viscosity of a Cs2O–Li2O–B2O3–
5% Al-doped
5:95
808
3.3
54 × 35 × 25, 64
free
MoO
3 system in the temperature range of 780–880 °C has been determined by the present authors for
10% Al-doped
10:90
811
3.2
50 × 49 × 26, 61
almost free
the first time, and the results are illustrated in Figure 2. As can be seen from the figure, the viscosity
increases gradually with the increase of Al doping concentration at the same temperature. Or rather,
Table 1 shows
thatrapidly
the measured
saturation
temperature
was in the
of concentration
800–811 ◦ C, which
the viscosity
increases
from 198.4
to 239.5
cP with increasing
Alrange
doping
only
◦ C) [39]. Consequently, this flux system
was
remarkably
lower
than
that
of
self-flux
system
(837–844
from 2.5% to 5% at 800 °C, but it changes slightly with Al doping concentration in range of 0%–2.5%
can
volatility
so as to avoid
component
deviation
by the relatively
high evaporation
andreduce
5%–10%.
The viscosities
of undoped,
2.5%,
5%, andcaused
10% Al-doped
CLBO growth
solutions of
at
Cs.
In
the
case
of
Al-doped
CLBO
grown
from
self-flux
system,
it
is
difficult
to
accurately
determine
their corresponding saturation temperature are 182.5, 198.4, 205.3, and 209.8 cP, respectively.
saturation
due to the
relatively
great fluctuation
saturation
induced
Obviously,temperature
there is a tremendous
decrease
in viscosity
comparedofwith
self-fluxtemperature
system (the viscosity
by
volatiles.
Additionally,
the
seed
crystal
can
be
easily
etched
by
the
volatiles
from
the
solution,
beyond 1000 cP) [40], which often inevitably leads to low growth repeatability and the formation of
resulting
in in
thecrystal.
growingThe
crystal
falling.
However,
in our
experimentsofutilizing
3 flux, these
inclusions
above
results
confirm
thatgrowth
the introduction
MoO3 MoO
can enormously
disadvantages
could
be
effectively
eliminated.
The
viscosity
of
a
Cs
O–Li
O–B
O
–MoO
in
2
2
2
3
3 system
reduce the viscosity of the growth solution. Thus, the Cs2O–Li2O–B2O3–MoO3 system has
a great
◦ C has been determined by the present authors for the first time, and
the
temperature
range
of
780–880
advantage over the Cs2O–Li2O–B2O3 system in that it possesses a clearly lower volatility and a
the
results arelower
illustrated
in Figure 2. As can be seen from the figure, the viscosity increases gradually
significantly
viscosity.
with the increase of Al doping concentration at the same temperature. Or rather, the viscosity increases
rapidly from 198.4 to 239.5 cP with increasing Al doping concentration only from 2.5% to 5% at 800 ◦ C,
but it changes slightly with Al doping concentration in range of 0%–2.5% and 5%–10%. The viscosities
of undoped, 2.5%, 5%, and 10% Al-doped CLBO growth solutions at their corresponding saturation
temperature are 182.5, 198.4, 205.3, and 209.8 cP, respectively. Obviously, there is a tremendous decrease
in viscosity compared with self-flux system (the viscosity beyond 1000 cP) [40], which often inevitably
leads to low growth repeatability and the formation of inclusions in crystal. The above results confirm
that the introduction of MoO3 can enormously reduce the viscosity of the growth solution. Thus,
the Cs2 O–Li2 O–B2 O3 –MoO3 system has a great advantage over the Cs2 O–Li2 O–B2 O3 system in that it
possesses a clearly lower volatility and a significantly lower viscosity.
Crystals 2017, 7, 83
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Crystals 2017, 7, 83
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Figure 2. Viscosities of CLBO growth solutions with different Al doping concentrations.
3.2. XRD and ICP Analysis
The powder X-ray diffraction (XRD) measurement was performed to verify the structure of
as-grown Al-doped crystals. As displayed in Figure 3, we found that the positions and intensities of
main diffraction peaks are well in accordance with these of undoped CLBO crystal. The results
confirm that Al doping do not change the essential structure of CLBO.
Figure
different Al
Al doping
doping concentrations.
concentrations.
Figure 2.
2. Viscosities
Viscosities of
of CLBO
CLBO growth
growth solutions
solutions with
with different
In order to determine the effective Al3+ segregation coefficient, ICP analysis was carried out to
measure
the Al3+Analysis
concentration in the crystal. The effective segregation coefficient K can be
3.2.
XRD
3.2.
XRD and
and ICP
ICP Analysis
calculated by the following equation: k = Cs/C0, where Cs and C0 correspond to Al3+ concentration in
The powder
powder
X-ray
diffractionin(XRD)
(XRD)
measurement
performed
to
the
structureAlof
of3+
3+ concentration
the crystal
and AlX-ray
the growth
solution,was
respectively.
Based
on the
measured
The
diffraction
measurement
was
performed
to verify
verify
the
structure
as-grown
Al-doped
crystals.
As
displayed
Figure
we
that
the positions
positions
and intensities
intensities
of
3+ segregation
concentration
in thecrystals.
as-grown
thein
calculated
coefficients
of 2.5%,
as-grown
Al-doped
Ascrystal,
displayed
in
Figure 3,
3, effective
we found
foundAl
that
the
and
of
main
diffraction
peaks
are
well
in
accordance
with
these
of
undoped
CLBO
crystal.
The
results
5%, and
10% Al-doped
are 0.028,
0.034,
0.031,CLBO
respectively,
which
are confirm
slightly
main
diffraction
peaks areCLBO
well incrystals
accordance
with these
of and
undoped
crystal. The
results
confirm
that those
Aldo
doping
do notthe
change
the essential
higher
than
of previous
study
[39].
that
Al doping
not
change
essential
structurestructure
of CLBO.of CLBO.
In order to determine the effective Al3+ segregation coefficient, ICP analysis was carried out to
measure the Al3+ concentration in the crystal. The effective segregation coefficient K can be
calculated by the following equation: k = Cs/C0, where Cs and C0 correspond to Al3+ concentration in
the crystal and Al3+ concentration in the growth solution, respectively. Based on the measured Al3+
concentration in the as-grown crystal, the calculated effective Al3+ segregation coefficients of 2.5%,
5%, and 10% Al-doped CLBO crystals are 0.028, 0.034, and 0.031, respectively, which are slightly
higher than those of previous study [39].
Figure3.
3. XRD
XRD patterns
patterns of
of undoped
undopedand
andAl-doped
Al-dopedCLBO
CLBOcrystals.
crystals.
Figure
3.3. Transmission
Spectrum the effective Al3+ segregation coefficient, ICP analysis was carried out to
In order to determine
measure
Al3+ concentration
in theon
crystal.
The effective the
segregation
K cancut
bealong
calculated
For the
investigation
of Al doping
the transmittance,
as-growncoefficient
crystals were
(001)
3+
by
the following
equation:
k = to
Csabout
/C0 , where
to measurement.
Al concentration
the crystal
s and C0 correspond
wafer
and optically
polished
1 mmCthickness
for spectrum
The in
transmission
3+ concentration
and
Al3+ concentration
theAl-doped
growth solution,
respectively.
Based
on 185
the measured
Alare
spectrum
of undoped in
and
CLBO crystals
ranging
from
to 1800 nm
presented in
in
the as-grown
crystal, from
the calculated
effective
Al3+ segregation
coefficients
of 2.5%,
5%, and high
10%
Figure
4. As is evident
Figure 4, both
undoped
and Al-doped
crystals exhibit
extremely
Figure 3. XRD patterns of undoped and Al-doped CLBO crystals.
Al-doped
CLBO
crystals
0.028,
0.034, and
0.031,
which
areinslightly
higher
than those
transmittance
and
exceedare
92%.
Moreover,
there
wasrespectively,
no absorption
band
the range
of 200–380
nm,
of
previous
study
[39].
demonstrating
Mo ion did not crystallize into the crystal, and thus the ultraviolet cutoff
3.3.
Transmissionthat
Spectrum
wavelength could not be shifted [35]. Consequently, it can be concluded that as-grown crystals
3.3. Transmission
Spectrum
For excellent
investigation
of Al
doping
transmittance,
the as-grown
were cut along (001)
present
optical
quality
andon
Althe
doping
has no adverse
effect oncrystals
the transmittance.
waferFor
and
optically
polished
to
about
1
mm
thickness
for
spectrum
measurement.
The
transmission
investigation of Al doping on the transmittance, the as-grown crystals were
cut
along (001)
spectrum
of
undoped
and
Al-doped
CLBO
crystals
ranging
from
185
to
1800
nm
are
presented in
wafer and optically polished to about 1 mm thickness for spectrum measurement. The transmission
Figure 4. As
is evidentand
from
Figure 4,CLBO
both undoped
and Al-doped
crystals
exhibit
extremely
high
spectrum
of undoped
Al-doped
crystals ranging
from 185
to 1800
nm are
presented
in
transmittance
and
exceed
92%.
Moreover,
there
was
no
absorption
band
in
the
range
of
200–380
nm,
Figure 4. As is evident from Figure 4, both undoped and Al-doped crystals exhibit extremely high
demonstratingand
thatexceed
Mo ion
not crystallize
into
crystal, band
and in
thus
ultraviolet
cutoff
transmittance
92%.did
Moreover,
there was
nothe
absorption
thethe
range
of 200–380
nm,
wavelength
could
not
be
shifted
[35].
Consequently,
it
can
be
concluded
that
as-grown
crystals
demonstrating that Mo ion did not crystallize into the crystal, and thus the ultraviolet cutoff wavelength
present excellent optical quality and Al doping has no adverse effect on the transmittance.
Crystals 2017, 7, 83
6 of 11
could not be shifted [35]. Consequently, it can be concluded that as-grown crystals present excellent
optical
quality
Crystals
2017,and
7, 83 Al doping has no adverse effect on the transmittance.
6 of 11
Crystals 2017, 7, 83
6 of 11
Figure 4. Transmission spectrum of undoped and Al-doped CLBO crystals.
Figure 4. Transmission spectrum of undoped and Al-doped CLBO crystals.
3.4. Laser-Induced Damage Threshold
FigureThreshold
4. Transmission spectrum of undoped and Al-doped CLBO crystals.
3.4. Laser-Induced Damage
Laser-induced damage is one of the major limitations associated with the development of
Laser-induced
damage
is one
of theit major
limitations
associated
withinput
the development
high-power
solid-state
lasers,
because
can seriously
restrict
the available
power. Bulk of
3.4. Laser-Induced
Damage
Threshold
laser-induced
damage
is mainly
related
to crystal
quality,
so we
bulkpower.
LIDT ofBulk
high-power
solid-state
lasers,
because
it can
seriously
restrict
themeasured
availablethe
input
Laser-induced damage is one of the major limitations associated with the development of
as-grown damage
crystal for
the purpose
of to
evaluating
its prospect
practical applications.
TheofCLBO
laser-induced
is mainly
related
crystal quality,
so weinmeasured
the bulk LIDT
as-grown
high-power solid-state lasers, because it can seriously
restrict the available input power. Bulk
3 was employed and two (001) faces were optically
crystal
sample
with
dimensions
of
5
×
5
×
5
mm
crystallaser-induced
for the purpose
of evaluating
prospect
in practical
The CLBO
crystal
damage
is mainly its
related
to crystal
quality,applications.
so we measured
the bulk
LIDTsample
of
polished. The bulk LIDT was3 tested by means of the 1-on-1 test procedure using a Q-switched
with dimensions
of
5
×
5
×
5
mm
was
employed
and
two
(001)
faces
were
optically
polished.
The bulk
as-grown crystal for the purpose of evaluating its prospect in practical applications. The CLBO
Nd:YAG laser (1064 nm) operating in a longitudinal single mode. The pulse repetition rate and
LIDT crystal
was tested
bywith
means
of the 1-on-1
using a Q-switched
Nd:YAG
laser
(1064 nm)
sample
dimensions
of 5 × test
5 × 5procedure
mm3 was employed
and two (001)
faces were
optically
pulse duration were 1 kHz and 5 ns, respectively. During the measurement, ten sites were chosen
polished.
The bulk LIDTsingle
was tested
means
of repetition
the 1-on-1 rate
test procedure
a Q-switched
operating
in a longitudinal
mode.byThe
pulse
and pulseusing
duration
were 1 kHz
as test points at each pulse fluence, and laser-induced damage was confirmed to have occurred by
Nd:YAG
laser (1064During
nm) operating
in a longitudinal
single
mode.
The as
pulse
repetition
rate
and
and 5visual
ns, respectively.
the
measurement,
ten
sites
were
chosen
test
points
at
each
pulse
observation of a laser spark. The pulse fluence which causes the corresponding damage
pulse
duration
were
1
kHz
and
5
ns,
respectively.
During
the
measurement,
ten
sites
were
chosen
fluence,
and laser-induced
damage
confirmed
to have
occurred
by sample.
visual observation of a laser
probability
of 0% is defined
as thewas
laser-induced
damage
threshold
of the
testpulse
points at each which
pulse fluence,
and
laser-induced damage
wasprobability
confirmed to
by
spark.asThe
causesdamage
the corresponding
of have
0%
isoccurred
defined
as the
Figure 5fluence
shows the measured
probability ofdamage
undoped
and Al-doped
CLBO
crystals.
visual observation of a laser spark. The pulse fluence which causes the corresponding damage
The results
show that
the LIDT
undoped
laser-induced
damage
threshold
ofofthe
sample.and Al-doped CLBO crystals along the c axis are 33.98
probability of 0% is 2defined as the laser-induced
damage threshold of the sample.
J/cm2 (6.80
GW/cm
and 25.48 damage
J/cm2 (5.10
GW/cm2), of
respectively.
of newly-grown
Figure
5 shows
the) measured
probability
undoped The
and LIDT
Al-doped
CLBO
crystals.
Figure 5 shows the measured damage probability of undoped and Al-doped CLBO
crystals.
2 at 1064 nm
undoped
CLBO
significantly
exceed
that
of
conventional
undoped
CLBO
(5.72
GW/cm
The results
show
that
the
LIDT
of
undoped
and
Al-doped
CLBO
crystals
along
the
c
axis
The results show that the LIDT of undoped and Al-doped CLBO crystals along the c axis are 33.98 are
along 2the c axis) [15]; 2the enhancement of
LIDT may be 2attributed to high crystallinity and low
2 (5.10
2) and
2 (5.10
2), respectively.
33.98 J/cm
(6.80GW/cm
GW/cm
) and25.48
25.48J/cm
J/cm
GW/cm
), respectively.
LIDT
of newly-grown
J/cm2 (6.80
GW/cm
The The
LIDT
of newly-grown
dislocation density of crystal [19,20,41]. Hence, the results further prove the high quality of 2
2 atGW/cm
undoped
CLBO
significantly
thatof of
conventional
undoped
CLBO
(5.72
undoped
CLBO
significantly exceed
exceed that
conventional
undoped
CLBO (5.72
GW/cm
1064 nm at
as-grown crystals and their promising applications in high-power solid-state lasers.
along
the cthe
axis)c [15];
enhancement
of LIDT of
may
be attributed
to high crystallinity
and low
1064 nm
along
axis)the
[15];
the enhancement
LIDT
may be attributed
to high crystallinity
dislocation
density
of
crystal
[19,20,41].
Hence,
the
results
further
prove
the
high
quality
of of
and low dislocation density of crystal [19,20,41]. Hence, the results further prove the high quality
as-grown
crystals
and
their
promising
applications
in
high-power
solid-state
lasers.
as-grown crystals and their promising applications in high-power solid-state lasers.
Figure 5. Bulk LIDT of undoped and Al-doped CLBO crystals.
3.5. Optical Weak Absorption
Figure
5. Bulk
LIDT
undopedand
and Al-doped
Al-doped CLBO
Figure
5. Bulk
LIDT
ofofundoped
CLBOcrystals.
crystals.
The PCI system is a precise weak absorption measurement apparatus based on surface thermal
lensing
technique.
The probe beam crosses the pump beam at a certain angle. As the crystal sample
3.5. Optical
Weak Absorption
The PCI system is a precise weak absorption measurement apparatus based on surface thermal
lensing technique. The probe beam crosses the pump beam at a certain angle. As the crystal sample
Crystals 2017, 7, 83
7 of 11
3.5. Optical Weak Absorption
Crystals 2017, 7, 83
Crystals 2017, 7, 83
7 of 11
7 of 11
The PCI system is a precise weak absorption measurement apparatus based on surface thermal
lensing
technique. aThe
probe beam crosses
the pump beam change
at a certain angle.
As thethe
crystal sample
is
is
is irradiated
irradiated by
by a pump
pump beam,
beam, the
the refractive
refractive index
index change can
can occur
occur in
in the heated
heated area.
area.
irradiated by the
a pump beam,
the refractive
index changeincan
occur in
the heatedcommon-path
area. Accordingly,
Accordingly,
Accordingly, the probe
probe beam
beam suffered
suffered phase
phase distortion
distortion in this
this area
area induced
induced by
by common-path point
point
the
probe
beam
suffered
phase
distortion
in
this
area
induced
by
common-path
point
diffraction
diffraction
interference.
The
perceived
phase
distortion
signals
of
the
probe
beam
are
collected
diffraction interference. The perceived phase distortion signals of the probe beam are collected by
by aa
interference. The
perceived
phase distortion
signals
of the probe
beam areby
collected
by a amplifier
photodetector
photodetector
behind
an
aperture,
and
then
are
accurately
processed
the
lock-in
photodetector behind an aperture, and then are accurately processed by the lock-in amplifier and
and
3 were
behind an aperture,
and then are accurately
processed
amplifier and transmitted
tothis
the
transmitted
of
55by
×× 55the
×× 5lock-in
mm
polished
for
3 were optically
transmitted to
to the
the computer.
computer. Samples
Samples with
with size
size
of
5
mm
optically
polished
for
this
3 were optically polished for this measurement.
computer.
Samples
with
size
of
5
×
5
×
5
mm
measurement.
measurement.
The optical
weak absorption
curves of
of undoped and
Al-doped CLBO
crystals at
at the critical
The
The optical
optical weak
weak absorption
absorption curves
curves of undoped
undoped and
and Al-doped
Al-doped CLBO
CLBO crystals
crystals at the
the critical
critical
wavelength
of
1064
nm
were
obtained,
as presented
in Figures
6 and67,and
respectively.
The twoThe
highest
wavelength
of
1064
nm
were
obtained,
as
presented
in
Figures
7,
respectively.
wavelength of 1064 nm were obtained, as presented in Figures 6 and 7, respectively. The two
two
peaks
are regarded
as the weakthe
absorption
of two end surfaces
of the crystal,
while the stable
area
highest
highest peaks
peaks are
are regarded
regarded as
as the weak
weak absorption
absorption of
of two
two end
end surfaces
surfaces of
of the
the crystal,
crystal, while
while the
the
between
the two peaks
represents
the measured
bulk weak bulk
absorptionabsorption
of the crystal.the
As seen from
stable
stable area
area between
between the
the two
two peaks
peaks represents
represents the
the measured
measured bulk weak
weak absorption of
of the crystal.
crystal. As
As
the two
figures,
the figures,
weak absorption
values
of undoped
CLBO
crystal along
acrystal
and c directions
werec
seen
from
the
two
the
weak
absorption
values
of
undoped
CLBO
along
a
and
seen from the two figures, the weak absorption values of undoped CLBO crystal along a and c
only 80 andwere
50 ppm/cm,
respectively,
while the weak absorption
values
of Al-doped CLBO
crystal
directions
directions were only
only 80
80 and
and 50
50 ppm/cm,
ppm/cm, respectively,
respectively, while
while the
the weak
weak absorption
absorption values
values of
of
along a andCLBO
c directions were
approximately
140 and 50 ppm/cm,
respectively. The
measured
weak
Al-doped
Al-doped CLBO crystal
crystal along
along aa and
and cc directions
directions were
were approximately
approximately 140
140 and
and 50
50 ppm/cm,
ppm/cm,
absorption values
are dramaticallyabsorption
lower compared to
previous investigation
(600 ppm/cm
for a
respectively.
respectively. The
The measured
measured weak
weak absorption values
values are
are dramatically
dramatically lower
lower compared
compared to
to previous
previous
direction
and
150
ppm/cm
for
c
direction)
[42],
as
listed
in
Table
2.
To
our
knowledge,
the
measured
investigation
investigation (600
(600 ppm/cm
ppm/cm for
for aa direction
direction and
and 150
150 ppm/cm
ppm/cm for
for cc direction)
direction) [42],
[42], as
as listed
listed in
in Table
Table 2.
2.
weak
absorption
values
inmeasured
the present
workabsorption
are by far the
minimum
levels
reported
inare
articles.
Since
To
our
knowledge,
the
weak
values
in
the
present
work
by
far
the
To our knowledge, the measured weak absorption values in the present work are by far the
the weak absorption
values are
closely correlated
with the optical
quality ofclosely
the crystal, the newly
minimum
minimum levels
levels reported
reported in
in articles.
articles. Since
Since the
the weak
weak absorption
absorption values
values are
are closely correlated
correlated with
with
grown
crystals
possess
outstanding
optical
quality.crystals possess outstanding optical quality.
the
optical
quality
of
the
crystal,
the
newly
grown
the optical quality of the crystal, the newly grown crystals possess outstanding optical quality.
Figure
Weak
curves of undoped
Figure 6.
Weak absorption
Figure
6. Weak
absorption curves of undoped CLBO
CLBO crystal
crystal along
along (left)
(left) aa and
and (right)
(right) cc directions.
directions.
Figure
7. Weak
curves
Figure
Weak absorption
absorption
curves of
of5%
5%Al-doped
Al-dopedCLBO
CLBOcrystal
crystalalong
along(left)
(left)aaaand
and(right)
(right)cc cdirections.
directions.
Figure 7.
7. Weak
absorption curves
of
5%
Al-doped
CLBO
crystal
along
(left)
and
(right)
directions.
Table 2. Weak absorption values of CLBO crystals.
Table 2. Weak absorption values of CLBO crystals.
CLBO
CLBO Crystals
Crystals
Undoped
Undoped CLBO
CLBO
5%
Al-doped
5% Al-doped CLBO
CLBO
Conventional
Conventional CLBO
CLBO [42]
[42]
−1
Weak
Weak Absorption
Absorption Value
Value at
at 1064
1064 nm/ppm·cm
nm/ppm·cm−1
[100]
[001]
[100] Direction
Direction
[001] Direction
Direction
80
50
80
50
140
50
140
50
600
150
600
150
Crystals 2017, 7, 83
8 of 11
Table 2. Weak absorption values of CLBO crystals.
CLBO Crystals
Weak Absorption Value at 1064 nm/ppm·cm−1
[100] Direction
[001] Direction
80
140
600
50
50
150
Undoped CLBO
5% Al-doped CLBO
Conventional CLBO [42]
3.6. Humidity
Experiment
Crystals 2017,
7, 83
8 of 11
In order
to investigate the influence of Al doping on the hygroscopic behavior of CLBO crystal,
3.6. Humidity Experiment
a humidity experiment was conducted at a constant temperature in a humidity chamber. Two optically
In order to investigate the influence of Al doping on the hygroscopic behavior of CLBO crystal,
polished samples with dimensions of 4 × 4 × 3 mm3 —namely, A (undoped CLBO crystal) and B
a humidity experiment was conducted at a constant temperature in a humidity chamber. Two
(5% Al-doped
crystal)—were
kept in theofprogram-controlled
at relative
humidity of
opticallyCLBO
polished
samples with dimensions
4 × 4 × 3 mm3—namely,chamber
A (undoped
CLBO crystal)
60% andand
temperature
of 25 ◦CLBO
C for crystal)—were
different durations.
surface micrographs
of theat(001)
plane are
B (5% Al-doped
kept in The
the program-controlled
chamber
relative
60% and
of 25 °C forwith
different
durations. The surface
of theregular
given inhumidity
Figure 8of(under
antemperature
optical microscope
100 magnification).
Frommicrographs
Figure 8, some
are given
in Figure
an optical
microscope
100 magnification).
From Figure
etch pits(001)
can plane
be clearly
observed
on8 (under
the edges
of (001)
plane inwith
sample
A with an increase
in duration
8, some regular etch pits can be clearly observed on the edges of (001) plane in sample A with an
time from 0 to 1200 h. In contrast, there were no observable etch pits in sample B, even over a period of
increase in duration time from 0 to 1200 h. In contrast, there were no observable etch pits in sample
50 days.B,The
results demonstrate that Al doping could substantially improve the hygroscopic nature
even over a period of 50 days. The results demonstrate that Al doping could substantially
of CLBO.
improve the hygroscopic nature of CLBO.
(A, 0 h)
(A, 420 h)
(A, 840 h)
(A, 1200 h)
(B, 0 h)
(B, 420 h)
(B, 840 h)
(B, 1200 h)
Figure 8. Microphotographs of the (001) plane in (A) undoped and (B) Al-doped CLBO crystals for
Figure 8. Microphotographs of the (001) plane in (A) undoped and (B) Al-doped CLBO crystals for
different durations.
different durations.
3.7. 266 nm UV Laser Conversion Efficiency
3.7. 266 nm UV
Laser Conversion
Efficiency
A picosecond
mode-locked
Nd:YAG laser (pulse width: 25 ps, pulse repetition frequency: 10
Hz) was employed
as the fundamental
light
source
for the
outputrepetition
and fourth
harmonic 10 Hz)
A picosecond
mode-locked
Nd:YAG laser
(pulse
width:
25laser
ps, pulse
frequency:
generation (FHG) conversion efficiency measurements. Samples of uncoated and optically polished
was employed as the fundamental light source for the laser output and
fourth harmonic generation
undoped and Al-doped CLBO crystals with dimension of 3 × 3 × 10 mm3 were cut along the angles of
(FHG) conversion
efficiency
measurements.
Samples
of
uncoated
and
optically
polished
undoped
(θ, φ) = (61.6°, 45°) and θBrewster = 56.3° according to type-I phase matching direction. Figure
9 displays
3
and Al-doped
CLBOefficiency
crystalsofwith
dimension
3 × 10
mm
cut along
the angles of
the conversion
the 266
nm radiationofas3a ×
function
of 532
nm were
input peak
power density.
◦ , 45◦ ) and
Remarkably,
the FHG
conversion
from
532 nmphase
to 266 matching
nm of CLBO
increasesFigure
gradually
(θ, φ) = (61.6
θBrewster
= 56.3◦efficiency
according
to type-I
direction.
9 displays
with
increasing
the
532
nm
input
peak
power
density.
The
maximum
conversion
efficiencies
the conversion efficiency of the 266 nm radiation as a function of 532 nm input peak powerofdensity.
undoped and Al-doped CLBO crystals can reach 65% and 63% at their corresponding peak power
Remarkably,
the FHG conversion efficiency from 532 nm to 266 nm of CLBO increases gradually with
density of 325 MW/cm2, respectively. Despite the FHG conversion efficiency of Al-doped CLBO
increasing
the
532 nm input peak power density. The maximum conversion efficiencies of undoped
crystal is slightly lower than that of undoped CLBO crystal, the difference between them being very
and Al-doped
CLBO
crystals
can reachefficiency
65% andmay
63%
their corresponding
peakofpower
density of 325
small. The
decrease
of conversion
beat
connected
with the reduction
the crystalline
perfection due to Al doping [43]. The results illustrate that Al doping has no obvious influence on
the FHG conversion efficiency of CLBO.
Crystals 2017, 7, 83
9 of 11
MW/cm2 , respectively. Despite the FHG conversion efficiency of Al-doped CLBO crystal is slightly
lower than that of undoped CLBO crystal, the difference between them being very small. The decrease
of conversion efficiency may be connected with the reduction of the crystalline perfection due to Al
doping [43]. The results illustrate that Al doping has no obvious influence on the FHG conversion
Crystals 2017,
83
9 of 11
efficiency
of7,CLBO.
Figure
power
density
dependence
at 532atnm532
of fourth
generation generation
(FHG) conversion
Figure 9.9.Peak
Peak
power
density
dependence
nm ofharmonic
fourth harmonic
(FHG)
efficiency
of
CLBO
crystals.
conversion efficiency of CLBO crystals.
4. Conclusions
4.
In summary,
summary,undoped
undoped
and
Al-doped
CLBO
crystals
free from
inclusions
were successfully
In
and
Al-doped
CLBO
crystals
free from
inclusions
were successfully
grown
grownthe
using
thedeveloped
newly developed
2O–Li23
O–MoO
fluxes.
2
O–Li
2
O–B
2
O
3
–MoO
3
system
using
newly
Cs2 O–Li2Cs
O–MoO
fluxes.3 The
Cs2The
O–LiCs
O–B
O
–MoO
system
could
2
2 3
3
could drastically
lower
the viscosity
of the growth
compared
that of extensively
used
drastically
lower the
viscosity
of the growth
solutionsolution
compared
to that oftoextensively
used self-flux
self-flux The
system.
The as-grown
exhibited
The measured
weak absorption
system.
as-grown
crystals crystals
exhibited
excellentexcellent
quality. quality.
The measured
weak absorption
values
values
of undoped
and Al-doped
were remarkably
smaller
previous
studies.
LIDT
of
undoped
and Al-doped
CLBO CLBO
were remarkably
smaller
than than
previous
studies.
TheThe
LIDT
of
2
2
of undoped
CLBO
came
6.80
GW/cm—19%
—19%higher
higherthan
thanthat
that of
of conventional
conventional undoped CLBO.
undoped
CLBO
came
upup
to to
6.80
GW/cm
Our results
resultsfurther
furtherillustrate
illustrate
that
doping
significantly
enhance
the resistance
of CLBO
to
Our
that
Al Al
doping
cancan
significantly
enhance
the resistance
of CLBO
to water.
water.
The
undoped
and
Al-doped
CLBO
with
FHG
conversion
efficiencies
greater
than
63%
have
The undoped and Al-doped CLBO with FHG conversion efficiencies greater than 63% have been
been realized,
and
Al doping
has a influence
weak influence
onconversion
FHG conversion
efficiency.
Therefore,
the
realized,
and Al
doping
has a weak
on FHG
efficiency.
Therefore,
the newly
newly grown
Al-doped
CLBO crystal
has
been demonstrated
to be a promising
highly promising
NLO for
crystal
grown
Al-doped
CLBO crystal
has been
demonstrated
to be a highly
NLO crystal
UV
for UV
light generation.
light
generation.
Acknowledgment: This work was supported by National Key R&D program of China (No. 2016YFB0402100).
Acknowledgments: This work was supported by National Key R&D program of China (No. 2016YFB0402100).
Author Contributions:
Contributions: Xianchao
Xianchao Zhu
Zhu finished
finished the
the experiment
experiment and
and wrote
wrote this
this manuscript;
manuscript; Heng
Tu heng
helped
Author
Tu helped
performthe
theanalysis
analysis
with
constructive
discussions;
Ying provided
materials
and tools;
analysis
tools; Hu
Hu
perform
with
constructive
discussions;
Ying Zhao provided
materials
and analysis
Zhanggui
designed
experiments
revised the
manuscript.
All authors
read and
the manuscript.
Zhangguithe
designed
the and
experiments
and
revised the
manuscript.
All approved
authors read
and approved the
manuscript.
Conflicts
of Interest: The authors declare no conflict of interest.
Conflicts of Interest: The authors declare no conflict of interest.
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