Download plumbum thiogallate optical properties

2007 г
№ 12
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PLUMBUM THIOGALLATE OPTICAL PROPERTIES
V. Badikov, D. Badikov, M. Doroshenko1, V. Panyutin, V. Chizhikov*, G. Shevyrdyaeva
Kuban State University, 149 Stavropolskaya Street, 350040 Krasnodar, Russia
1Laser
Materials and Technologies Center, General Physics Institute RAS,
38 Vavilov Street, 119991 Moscow, Russia
We studied conditions of growing PbGaS plumbum thiogallate and present results of measuring dispersion of main values of refractive indices of this crystal in the 0.510 мm range. Polarization dependence of main values of absorption indices for Dy,Na:PbGaS crystal was determined near 1.314 мm. Laser properties of this compound are demonstrated.
1.
Introduction
The problem of creation of effective active media for solid state lasers of the mid-IR range on the basis
of Ln doped sulfide crystals attracts attention during last years of a number of research groups [1, 2]. Applica-loss transmission winrefer to a quality change of character of temporal relief of "laser" levels of Ln ions in sulfide crystalline hosts
relating to Ln-doped oxide crystals. The effect is stipulated by the shift into the long wave spectrum region of
the short wave edge of phonon absorption. At present laser generation near 4.3–4.4 мm is obtained on 6H11/2
→ 6H13/2 transitions of Dy3+ ion in crystals of Dy:Са PbGa2S4 calcium thiogallate [1] and Dy: PbGa2S4
plumbum thiogallate [2].
Only several compounds in the wide class of sulfide crystals with typical tetrahedral coordination allow
introduction of Ln dopants into cationic sublattice in typical "laser" concentrations 10 20 cm-3 without changing optical properties of the crystal itself. Such crystalline hosts must contain at least cations with coordination number 6 or 8 and ionic radii close to the radius of the doping ion. Sulfide crystals with mentioned structural properties are known and are well studied thank to works concerning studies of phosphors based on Lndoped thiogallates (See
binary systems MS–Ga2S3, M – Са
shows crystallographic characteristics and melting temperatures of a number of isomorphic thiogallates which
are potentially promising [5,9] for Ln ions doping.
Let's note some structural features of the compounds listed in table 1. Ga 3+ cations are in tetrahedral anionic environment and differ greatly from Ln3+ ions in anionic radius. Characteristic value of R(Ln3+ ) ≥ 1 Е.
Incorporation of Ln ions because of the specified coordination numbers and ionic radii is possible (most likely) only in the M2+ cation position. Allowing for the charge compensation, chemical formula of a doped
compound has the M1–3xLn2xGa2S4 form, x < 1/3 and x ~ 0.005 for laser crystals. Paper [5] notes that cationic
lattice of M ions consists of two sublattices. Half of the cations occupies positions with point group of symmetry D2, while the other half occupies positions with lower symmetry (either C2, or Ci). The problem of
which positions are occupied by Ln ions in MGa2S4 during doping is not solved yet.
Besides, in principle, a scenario is possible of MGa2S4 doping with Ln ions into a position of vacancy,
because there are 3 cations in these compounds for 4 anions and, in fact, there may be a vacancy in cation
sublattice. It is known [11, 12] that binary, ternary and more complex diamond-like compounds form two
types of structures: normal with mutual tetrahedral coordination of cations and anions, and defect which are
characterized by presence of ordered vacancy in cation sublattice, which upsets tetrahedral coordination of
anions by cations.
*
Corresponding author. Tel.: +7-861-2199501x261; fax: +7-861-2199517.
E-mail address: [email protected] (V. Chizhikov)
© V. Badikov, D. Badikov, M. Doroshenko, V. Panyutin, V. Chizhikov, G. Shevyrdyaeva
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V. Badikov, D. Badikov, M. Doroshenko, V. Panyutin, V. Chizhikov, G. Shevyrdyaeva
Table 1. Isomorphic thiogallates potentially promising for Ln ions doping
Chemical formula of the compound MGa2S4
Space group of symmetry [5]
Periods of identity of the structure, Е,
Z =32 [5]
V0 = abc/Z, Е3
Coordination numbers
K(M ) / K(Ga3+) [5]
Ionic radii
R(M ) / R(Ga ), Е [10]
Bulk concentration
of positions M2+ , 1022 cm–3
Melting temperature, °C
CaGa2S4
SrGa2S4
PbGa2S4
EuGa2S4
Na0.5La0.5Ga2S4
24
D2h
24
D2h
24
D2h
24
D2h
24
D2h
(Fddd)
a = 20.087
b = 20.087
c = 12.112
152.72
8/4
(Fddd)
a =20.840
b =20.495
c =12.212
163.00
8/4
(Fddd)
a= 20.706
b =20.380
c = 12.156
160.30
8/4
(Fddd)
a =20.716
b =20.404
c =12.200
161.15
8/4
(Fddd)
a =20.384
b =20.384
c =12.075
156.79
8/4
1.12 / 0.47
1.25 / 0.47
1.29 / 0.47
0.65
0.61
0.62
1.25 / 0.47 Na (1.16) / 0.47
La (1.18) / 0.47
0.62
0.64
1132 [7]
1230 [6]
890
> 1200
> 1200
Examples of such normal ternary compounds with point group of D2d symmetry and defect ones with
point group of S4 symmetry are given in table 2. Formation of a structure with ordered vacancy is regarded as
a result of splitting Ag lattice with point group of D2d symmetry, containing two non-equivalent positions in
a primitive cell into two sublattices with lower S4 symmetry and subsequent filling of one of the sublattices
with Cd2+ (or Hg2+
al values of the periods of the structure identity
for crystals in table 2 are imported from [11, 13]; volume of the V0 primitive cell makes up half of the volume
of the elementary one and corresponds to the volume accounting for 4 anions and 3 cations (faulted compounds) or 4 anions and 4 cations (normal defectless compounds).
Table 2. Normal and defect thiogallates
Compound
Point group of symmetry
a, Е
c, Е
V0 = a2c/2, Е3
AgGaS2
D2d
5.757
10.304
170.75
CdGa2S4
S4
5.576
10.08
156.70
HgGa2S4
S4
5.507
10.23
155.12
AgGaSe2
D2d
5.992
10.886
195.43
CdGa2Se4
S4
5.742
10.73
176.89
HgGa2Se4
S4
5.741
10.78
177.65
Comparing the data for V0 value in tables 1 and 2 one can make a conclusion that in MGa2S4 compound
there is a vacancy in cationic sublattice. Point group of symmetry of sublattice of vacancy in this case may be
equivalent to the point group of symmetry of M sublattice, the effective "ionic" radius of the vacancy must be
comparable to the ionic radius of M cation and there will be eight-fold coordination of the vacancy by anions.
Thus, MGa2S4 doping with Ln ions one cannot exclude a possibility of filling either sublattice of vacancy or
both sublattices, M and vacancy, by dopant.
2.
Crystal growth
At initial stage of preparation for PbGa2S4 crystals growth we synthesized charge mixture of plumbum
2S3. We used pure (99.999) Pb, Ga and S elements. PbS
melting point, according to reference data [14], is 1113 °C and that of Ga 2S3 1120 °C. Synthesis of plumbum sulfide and gallium sulfide was performed in fused silica ampoules evacuated till residual pressure
210 mm mercury. Temperature in the furnace was 1150 °C. In order to homogenize the melt, ampoules
were aged for several hours at specified temperature and after that they were cooled in the mode of the
switched off furnace conditions.
We have not found data about character and melting temperature of plumbum thiogallate in literature.
However, according to [5], MGa2S4 (M – Са, Sr, Pb, Eu) compounds form an isomorphic group and accordgruent character of melting of plumbum thiogallate, which was substantiated by the results of reconstruction
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Plumbum thiogallate optical properties
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made by us by method of differential thermal analyses (DTA) of a part of T–X projection of constitutional
diagram of pseudo-binary PbS–Ga S system.
With the purpose of making DTA experiments we prepared samples from compositions in the (1 – x)PbS
– (x)Ga S system with 2 mole % increment in the range x from 0.2 to 1.0. Each sample weighted not more
than 0.7 g. The following process was used to homogenize the composition of the charge in the samples.
From plumbum sulfide charge and gallium sulfide charge we prepared a charge of specified composition x
at this temperature and
subsequent cooling in the switched off furnace. The synthesized charge was pounded in agate mortar till fine
powder. This powder was placed into a fused silica ampoule, which was evacuated, and then the powder was
isothermally annealed at 650 єC during one month with subsequent quenching. The sample isolated from the
charge homogenized by this procedure was placed into a small fused silica ampoule, which was evacuated till
residual pressure 210 mm mercury. The samples were subject
ature range at rate 2 єC per minute in both heating and cooling modes. It must be noted that gas-melt volume
ratio in the small ampoule did not exceed 2/1.
On the basis of thermograms as well as on the basis of additional growth processes in the vicinity of
compositions x 0.50 and 0.33 we determined the compounds existing in the (1 – x)PbS – (x)Ga S pseudobinary system, character and melting temperature and eutectic points. PbGa S (x 0.50) compound melts
congruently with melting temperature (890±5) °C. T–X projection of the constitutional diagram of the investigated system is characterized by three eutectic points with the coordinates: x 0.60, T (840±5) °C; x
0.38, T (750±5) °C and x 0.25, T (760±5) °C. Beside PbGa S there exists one more compound
Pb Ga S (x 0.33), which melts congruently with melting temperature (790±5) °C. According to [15], this
15
compound crystallizes into a structure with space group of D 2h (Pcab) symmetry and lattice parameters a
12.38 Е, b 11.90 Е, c 11.03 Е and Z 8.
PbGa S crystals were grown by Bridgemen-Stockbarger method from a charge of stoichiometric composition, consisting of plumbum sulfide and gallium sulfide in fused silica ampoules evacuated to residual
pressure 210 mm mercury having diameter 24 mm and length 200 mm. Weight of the charge in an amtemperature of the charge was brought to
hours. After this the ampoule was lowered to crystallization zone. In order to determine optimal parameters of
the growing process we conducted several preliminary growing processes. Optical quality of grown crystal
was chosen as a criterion of optimal parameters of growth. Optimal parameters of the growing process according to this criterion were as follows: speed of passing crystallization zone – (6 ± 2) mm per day, temperature gradient in the
20 days.
Plumbum thiogallate crystals doped with dysprosium were grown according to a method analogous for
undoped crystals using the same parameters of the growing process from the charge having Pb1–3x–y
Dy xNa yGa S composition.
It must be noted that we used Dy (99.99) to synthesize dysprosium sulfide charge – Dy S and the procedure of synthesis had certain characteristic properties, which are related to high melting temperature of
Dy S (from 1470 to 1730 єC according to different reference data) and intense interaction of metallic dysprosium with the walls of the fused silica ampoule. The synthesis was conducted in a two-zone furnace into
which the fused silica ampoule with dysprosium and sulfur evacuated till residual pressure 210 mm mercury was placed. Dysprosium in glasstwo hours temperature at the end with dysprosium was smoothly increased till 900 єC and till 300 єC at the
minutes with heat evolution. Then temperature at the "cold" end was increased during 2 hours till 900 єC,
aged for 12 hours with the purpose of completion of the reaction of synthesis. After that the ampoule was
cooled in the mode of the switched off furnace conditions.
Because of high hygroscopic property of sodium sulfide – Na S, we did not synthesize the charge of this
compound. Metallic Na (99.999) was added directly into the ampoule with the charge of the crystal being
grown, then the ampoule was evacuated and placed into a growing furnace.
At first stage we grew Dy:PbGa S crystals from the charge without Na with parameter x
ufied charge composition. Atomic percentage of dysprosium in the charge was two times smaller than concen-
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V. Badikov, D. Badikov, M. Doroshenko, V. Panyutin, V. Chizhikov, G. Shevyrdyaeva
tration, which was used by the authors of paper [1] when doping calcium thiogallate with dysprosium. We
concentrated our further efforts on growing crystals with higher optical qualities containing more Dy . We
grew a crystal with charge composition x y 0.005 and x y 0.007. In our opinion, optical homogeneity of the crystalline host improves when co-doped with sodium. It is evident that optimal, even from the point
of view of optical homogeneity, ratio x/y in the charge is not equal a unity because Dy and Na ionic radii
in positions with coordination number 8 are different (R(Dy )/R(Na ) 0.89) and therefore coefficients
of insertion of these ions into plumbum thiogallate will be different. Besides, it is known (see, e.g. [16] and
references therein) that sodium co-doping of Ln doped halides changes their spectroscopic characteristics. In
our opinion, the problem of sodium influences upon spectroscopic and generation properties of
Dy,Na:PbGa S crystals requires a special consideration.
3.
Optical properties
Optical property of the grown crystals was considered satisfactory when in polished crystalline wa
mm thick and aperture not less then 10 mm there were no visible optical heterogeneities or inclusions in the
form of black dots. These heterogeneities and inclusions one might observe by shadow method or by distortion of the contrast fringe pattern obtained by placing a tested parallel-plane wafer into Haidinger laser interferometer [17].
Additionally optical heterogeneities in wafers were registered by method of comparison. PM15 wafer
made according to USSR GOST 1121-75 from K8 glass of high optical quality was used as an etalon. Optical
mW) passed through an aperture with diameter 1.7 mm and thickness 1.5 mm. On the screen placed at distance 4 m after the aperture there formed a diffraction image of the light source. Parameters of the installation
n the screen with a contrast sufficient for observation.
Etalon wa
duced the num
ere
placed. The wafers were oriented so that they did not change the condition of polarization of incident radiation. Typical number of ob
ctory optical quality). A decrease in the number of observed rings is connected with phase distortions introduced by the wafer into the stopped beam of radiation of He–Ne laser.
Phase distortions are stipulated by fluctuations of the refractive index дn of the wafer. In order to quantitavely
determine the value of fluctuations дn on the basis of phase distortions in the fringe pattern on the screen it is
necessary to model representations of дn distribution. For example, in case of total deterioration of the fringe
pattern, the upper evaluation of дn ~ л/2L, where L
kness of the wafer.
Grown PbGa S crystals have light-yellow coloring and marked cleavage plane parallel to crystallographic plane bc. Further on we use representation of periods of structure identity a, b, c (see table 1) proposed in [5]. Transition to generally accepted at present arrangement of crystallographic and crystallophysical
systems of coordinates for crystals with structure of orthorhombic syngony do not preset a problem. When
making wafers with working plane orientation not parallel to bc plane one should use a special technique.
On the basis of the results of measurements of transmission spectra on crystalline wafers with different
orientation we determined that the edge of the band of fundamental absorption makes up (470±10) nm and the
band of two-phonon absorption typical of sulfides begins near 9500 10000 nm.
Preliminary orientation of the grown crystals was made on asterism figures. After this, displaying of directions and planes of crystallographic system of coordinates abc were performed with the help of X-ray diffractometer DRON 2.0. The wafers of a, b, c
for finding positions of optical axes in the crystallographic system of coordinates on the basis of interference
figures in monochromatic light (633 nm).
Optical axes lie in the bc plane, axis b is bisector of the acute angle between the optical axes. Angle between optical axes 2V makes up 25є (633 nm). According to our data, there takes place a significant dispersion of directions of optical axes in plumbum thiogallates in visible and near IR ranges. Thus, 2V near 690 nm
tends to zero while in the near IR range optical axes lie in ab plane. In more detail you can trace the character
of dispersion of optical axes directions on the basis of dispersion of refractive indices given in the paper.
мm range were performed according to auto collimation method on three oriented prisms with refracting anab, bc и ca
planes were reflecting ones, thus radiation propagated along c, a and b, correspondingly. Use of the three
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Plumbum thiogallate optical properties
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prisms allows evaluating correctly errors in measurements of refractive indices caused by errors in prism orientation during manufacturing. This has a particular importance for optically biaxial crystals. In our case the
difference in measured values of identical refractive values on two differently oriented prisms did not exceed
0.003 near 1
icated instrumental error in this spectral range did not exceed 0.001. The results
of measurements of dispersion of main values of refractive indices are given in table 3 and results of approximation of dispersion of refractive indices are given by function of the type:
n2 A1 A3/(A2 2
A5/(A4 2),
where 
S is an optically positive crystal with optical axis directed along b. In this approximation na
nc
no and nb
ne. Therefore,
plumbum thiogallate optical properties must be similar in directions a and c.
Transmission spectra of Dy,Na:PbGa S crystals (x y
by SF-20 spectrophotometer at room temperature with the help of linearly polarized radiation. Measurements
a, b, c cutoffs. Inasmuch as, in general case, coefficients of
insertion of dysprosium and sodium into plumbum thiogallate may depend upon the direction of growth, wafers of the specified cutoffs were made only from crystals grown along the direction close to c. Every wafer
was measured twice. One measurement was made when wafer was placed (e.g., a cutoff) relative to polarization of incident radiation into position E//b, the other - into position E//c.
On the basis of measured transmission spectra peak absorption coefficients near 1.3 мm, stipulated by
the transition of the trivalent ion of dysprosium from ground state 6H15/2 into an excited state 6H9/2 6F11/2 [1]
were calculated. Absorption band near 1.3 мm with full width about 100 nm is characterized by presence of
its spectral position, this peak is mostly acceptable for exciting dysprosium ions in Dy,Na: PbGa S by
Nd:YAG laser radiation at 1.318 мm wavelengths. Polarization dependence of main values of the absorption
coefficient at 1.314 мm is the following: бa (2.2 ± 0.2) cm 1, бb (2.0 ± 0.2) cm 1 and бc (1.0 ± 0.1)
cm 1. When radiation propagates in one of the main planes of the crystal (e.g. in ac plane) absorption coefficient for ordinary wave бo бb, and for extraordinary wave бe(и) is found from the equation:
ne(и) бe(и) nc бcsin (и) na бacos (и),
where angle и is counted from crystallographic axis c. Analogous in structure expressions take place for ab
and bc planes. The equation is valid in approximation of a small coefficient of extinction k ( k/n 10 or, in
terms of absorption coefficient, б 200 cm–
, which is always held true for laser crystals. Problems of polarization of eigen waves in absorbing crystals of medium syngonies and rhombic syngony are given in [19].
Table 3. Refractive indices of PbGa2S4 crystal
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л, мm
nb
0.546
0.577
0.700
0.800
0.900
1.000
1.100
1.200
1.300
1.400
1.500
1.600
1.700
1.800
1.900
2.000
3.000
4.000
5.000
2.985
2.943
2.831
2.781
2.752
2.730
2.716
2.703
2.694
2.688
2.682
2.679
2.674
2.671
2.667
2.664
2.649
2.642
2.631
na
2.853
2.809
2.708
2.663
2.636
2.617
2.603
2.593
2.586
2.579
2.575
2.570
2.567
2.563
2.562
2.557
2.548
2.541
2.535
nc
2.830
2.795
2.706
2.671
2.647
2.631
2.619
2.610
2.603
2.598
2.593
2.589
2.586
2.583
2.581
2.577
2.568
2.561
2.552
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V. Badikov, D. Badikov, M. Doroshenko, V. Panyutin, V. Chizhikov, G. Shevyrdyaeva
6.000
7.000
8.000
9.000
10.00
2.621
2.609
2.594
2.580
2.562
2.526
2.516
2.506
2.493
2.477
2.542
2.535
2.522
2.509
2.493
Table 4. Parameters of the function approximating dispersion of refractive indices
PbGa2S4
nc
A1
8.425629
A2
0.0825464
na
7.732889
0.0944660
A3
A4
587.07
A5
0.306557
464.10
0.338149
nb
10.472910
0.0835000
859.61
0.408855
The following conclusions may be made on the basis of data on anisotropy of the absorption coefficient.
For maximum utilization of pumping radiation active elements must have a special orientation and pumping
radiation polarization match this orientation. A more correct characterization of spectroscopic parameters
must contain data of anisotropy of cross-section of stimulated radiation.
To illustrate the effects connected with anisotropy of absorption coefficient, we conducted generation
tests on an active element from Dy,Na:PbGa S (x y 0.007) crystal with special orientation. Normal to
working surfaces of the element was in ac plane in direction 24є to c axis. Absorption coefficients in this direction at 1.314 мm for both eigen waves coincide and equal 2.0 cm . The element with thickness 6.14 mm
was not AR coated and was mounted upon a copper clamp in a resonator 30 mm long. The resonator was
formed by a flat mirror, through which linearly polarized pumping radiation of a Nd:YAG laser at 1.318 мm
wavelength was introduced (free running mode, 150 мs, 200 mJ) and a spherical output coupler (radius of
curvature 200 mm, 96% [email protected] мm). Relative to polarization of pumping radiation, active element in one
experiment was installed in position E // b and in position E parallel to ac plane in the other. Generation
thresholds for both incident radiation and absorbed radiation did not depend upon orientation of the element
relative to vector E. Details of measuring procedure and description of measuring equipment are given in [2].
Dependence of output pulse energy at 4.33 мm (pulse spectral width ~ 40 nm, pulse duration ~ 200 мs) upon
absorbed pump energy is presented in picture. In both cases generation threshold was ~30 mJ, reached output
pulse energy was ~ 0.8 mJ at absorbed pump energy ~ 80 mJ.
Conclusion
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Plumbum thiogallate optical properties
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The growth conditions of a new promising sulfide crystal PbGa 2S4, and Dy:Na:PbGa2S4 are studied. We
provided detailed measurements of the refractive indices dispersion plumbum thiogallate and anisotropy of
absorption coefficients Dy,Na:PbGa S at 1.314 мm. We conducted generation tests on an active element
from Dy,Na:PbGa S and output pulse energy 0.8 mJ at absorbed pump energy 80 mJ was obtained.
The work is supported by Russian Foundation of basic research trough the Department for Education
and Science of the Krasnodar region Administration grant 06-02-96633.
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Оптические свойства тиогаллата свинца
В. Бадиков, Д. Бадиков, М. Дорошенко, В. Панутин, В.Чижиков, Г. Шевердяева
Изучены условия роста кристаллов PbGa2S4. Представлены результаты измерений дисперсии
основных значений показателя преломления в этих кристаллах от 0,5 до 10 мм. Определена
поляризационная зависимость основных значений индекса поглощения для кристалла Dy,Na:
PbGa2S4 приблизительно 1,314 мm. Определены лазерные свойства этой смеси.
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