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New nonlinear optical crystals: Strontium and lead tetraborates
October 1995
?TI;AL ELSEVIER
Optical Materials 4 ( 1995 ) 669~574
New nonlinear optical crystals: strontium and lead tetraborates Yu. S. Oseledchik a, A.L. Prosvimin a, A.I. Pisarevskiy a, V.V. Starshenko a, V.V. Osadchuk a, S.P. Belokrys a, N.V. Svitanko a, A.S. Korol a, S.A. Krikunov b, A.F. Selevich c ~'Laboratory_ of Nonlinear Optical Frequency Converters, Industrial Institute, Prosp. Lenina 226, 330006 Zaporozhye, Ukraine General Physics Institute RAS, 117942 Moscow, Russia c Institute of Physico-Chemical Problems, Belarussian State Universi~, 220080 Minsk, Belarus
Received 20 June 1994; revised 11 May 1995; accepted 15 June 1995
Abstract The nonlinear-optical and electro-optical properties of the crystals of strontium (SrB40 7) and lead (PbB40 7) tetraborate are investigated. The transparency range of this crystals is 130-3200 nm for SBO and 235-4000 nm for PBO. The effective nonlinear coefficient and surface optical damage threshold of the SBO is higher than for any borate crystals. The SHG phase-maching conditions in these crystals are absent. The possible application of these crystals is discussed.
1. Introduction The low temperature modification of the barium borate f l - B a B 2 0 4 ( B B O ) and lithium triborate LiB305 (LBO) are well-known nonlinear optical crystals of borate series that are widely used for harmonic generation of laser radiation, sum and difference frequency mixing in visible and UV spectral range [ 1-4]. The search of a new crystals of the borate series seems to be very promising for nonlinear optics. Recently a crystal of cesium triborate C s B 3 0 5 (CBO) was described in Ref. [ 5 ]. All of these crystals have high values of the effective nonlinear coefficients and good damage resistance. The phase-matched frequency conversion could be performed in these crystals in a spectral band from 170 to 2600 nm. The S r B 4 0 7 and P b B 4 0 7 single crystals were grown at first and electro-optic properties were investigated by Bohaty et al. [6,7]. The results of the investigation of the optical and nonlinear optical properties of the new borate crystals of strontium SrB407 (SBO) and lead PbB407 (PBO) 0925-3467/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10925-3467(95) 00027-5
tetraborates are reported in this paper. These crystals were grown in our laboratory by the Czochralski method [8] and differ considerably from BBO, PBO and CBO. Thus, the short wave transparency border for SBO shifts to the VUV range. The effective nonlinear coefficient and surface optical damage threshold of this crystal are essentially higher as well. The possible application of these crystals in nonlinear optics is discussed in this paper.
2. Grown and crystal structure The needle-shaped single crystals of strontium and lead tetraborates were grown from stoichiometric melt. Crystal structure of the SBO and PBO were investigated in Ref. [9]. We have grown big single crystals of SBO and PBO from the melt by the Czochralski method. X-ray investigations of the SBO and PBO powder were carried out on a H-4A diffractometer
Yu.S. Os.eledchik et al. /Optical Materials 4 (1995) 669-674
670 Table 1 X-ray diffraction data of SBO dexp, A
1/lo dcan¢,A
hkl dexp, A
I/1o
dcam¢,/k hkl
5.35211 4.42066 4.09774 3.93633 3.06103 2.94078 2.77771 2.73068 2.67718 2.65742 2.29069 2.21247 2.11725 2.04525 2.01586 1.96860 1.96055 1.92908 1.91072 1.88096
28 5 35 30 32 12 41 50 15 100 18 22 30 12 68 16 12 18 8 25
020 100 110 011 101 111 130 031 040 121 140 200 002 220 141 022 201 150 051 230
5 5 10 8 5 5 10 7 6 5 6 8 3 7 5 5 7 5 12
1.84146 1.78478 1.71878 1.68391 1.66058 1.55473 1.54163 1.52969 1.47084 1.44586 1.42539 1.40602 1.39304 1.38140 1.36333 1.34816 1.32817 1.31249 1.30398
5.35435 4.42549 4.09000 3.93747 3.05937 2.94168 2.77841 2.72912 2.67671 2.65633 2.29065 2.21274 2.11703 2.04500 2.01471 1.96873 1.96109 1.92784 1.91115 1.88071
1.84271 1.78535 1.71885 1.68462 1.66127 1.5543 1.54293 1.52906 1.47070 1.44615 1.42586 1.40621 1.39379 1.38163 1.36344 1.34802 1.32812 1.31288 1.30425
221 060 231 132 042 142 161 201 222 170 152 232 301 311 330 321 242 033 162
Table 2 X-ray powder diffraction data of PBO
dexp, ,~k
1/1o dca~c,,~
hkl dexv, ,~
I/1o dcanc,A
hkl
5.41600 4.44700 4.12000 3.94800 3.07263 2.95200 2.80730 2.75010 2.70800 2.67270 2.33630 2.31320 2.22540 2.18160 2.12040 2.05970 2.03120 1.97540 1.97130 !.94930 1.93870
70 12 73 81 55 21 52 66 20 100 5 15 23 10 18 15 52 26 20 16 18
020 100 I10 011 101 111 130 031 040 121 131 140 200 210 002 220 141 022 201 150 211
20 12 33 10 15 17 16 10 5 14 7 11 13 18 6 11 10 8 8 10 15
051 230 112 221 060 231 132 042 241 161 212 222 170 152 232 260 311 330 321 123 340
5.41935 4.45466 4.12024 3.95163 3.07263 2.95614 2.80603 2.75098 2.70968 2.67291 2.34063 2.31503 2.22733 2.18174 2.12187 2.06012 2.03230 1.97582 1.97219 1.94920 1.94033
1.92970 1.89560 1.88630 1.85380 1.80570 1.73080 1.69230 1.66950 1.59380 1.55700 1.52100 1.47700 1.46170 1.43450 1.41330 1.40200 1.39000 1.37300 1.35640 1.30760 1.30260
1.93047 1.89598 1.88641 1.85329 1.80645 1.73107 1.69246 1.67061 1.59456 1.55726 1.52111 1.47807 1.46255 1.43546 1.41380 1.40301 1.38999 1.37341 1.35692 1.30835 1.30218
values of the crystallographic parameters are given in Table 3. Seed crystals were obtained by spontaneous crystallization. The habit of the SBO and PBO crystals equilibrium form grown during spontaneous crystallization from the melt is represented at Fig. 1. The appearance of an additional facet (130) of the crystal morphology was observed for PBO. Small rate of the crystal growth along Y axis is a distinctive feature of PBO. The PBO crystal, which was grown in spontaneously conditions, had the form of a plate or rod. This rod was widened in the X-Z plane and was lengthened along the Z axis. The SBO crystal had the form of rods only. The SBO and PBO big crystals were grown from a melt by the Czochralski method due to the congruent character of SrB407 melting at a temperature of 970°C and P b B 4 0 7 melting at a temperature of 740°C. The melts were prepared from H3BO3 ( > 9 9 . 9 % ) and SrCO3 ( > 9 9 . 5 % ) or PbO ( > 9 9 . 5 % ) . A crucible of 50 mm in diameter and 60 mm in height containing the melt was mounted into a resistance furnace, so that the temperature gradient in the near surface layer was 80100°C/cm. The mixture was heated to a temperature Table 3 Crystallographic parameters of SBO and PBO
Point symmetry group Space symmetry group Unit cell parameters,/~
Angle between optical axes Conformity between crystallographic and main optical axes
SBO
PBO
ram2 Pnm2~
mm2 Pnm2L
a =4.4255 (7) b = 10.709 (2) c=4.2341 (9) V= 200.7/~3 74 °
a =4.4547 (7) b = 10.839 (2) c=4.2437 (8) V= 204.9/~3 86 °
X - ny
X - nz
Y - n~ Z - n:
Y - nx Z - ny
to2
.fill
¢ under Cu-K,~ radiation. X-ray diffraction data of crystals powder are listed in Table 1 and Table 2. The
Fig. 1. Growth habit of the SBO and PBO crystals.
Yu.S. Oseledchik et al. / Optical Materials 4 (1995) 6 6 9 ~ 7 4 1.0
,
£n ~n
I--
/ /
/f
0.8
~LBo 1
~=~'~"-~--~
-
2 -
had achieved a length of 20 mm, it was lifted slightly above the melt and cooled at a rate of 10-20°C/h. The SBO crystals was grown in the direction of the Z axis and PBO crystals was grown in the direction of the Z or Y axis. The SBO crystal was partially faceted by the ( 011 ) and (020) planes. The PBO crystal was partially faceted by the (100), (011) and (120) planes. The facet ( 101 ) is absent in both crystals.
SBO
P80
0.6
0.4
0.2 1
!
1
2
3000
4000
',i\
0.0 O0
200
300
2000
Wovelength,
nm
3. Optical transparency and refractive indices
Fig. 2. The t r a n s p a r e n c y of the borate crystals. Table 4 S B O and P B O refractive indices
A. ~rn
SBO nr
n,
nz
PBO nx
ny
n:
0.2129 0.2661 0.3548 0.4254 0.4358 0.4916 0.5322 0.5461 0.5770 0.6708 0.6925 1.0644
1.8364 1.7883 1.7579 1.7464 1.7454 1.7365 1.7347 1.7300 1.7208
1.8384 1.7909 1.7597 1.7479 1.7467 1.7377 1.7360 1.7307 1.7218
1.8407 1.7936 1.7622 1.7506 1.7490 1.7400 1.7386 1.7333 1.7229
2.1105 1.9948 1.9620 1.9590 1.9453 1.9385 1.9361 1.9320 1.9230 1.9211 1.9055
2.1270 2.0023 1.9677 1.9647 1.9505 1.9432 1.9409 1.9367 1.9275 1.9256 1.9104
2.1273 2.0037 1.9696 1.9663 1.9522 1.9450 1.9427 1.9385 1.9291 1.9274 1.9119
Table 5 Sellmeier coefficients of SBO, P B O
SBO
PBO
n~ ny n: n, nv n:
671
Transparency measurements of the SBO and PBO samples were performed on a vacuum UV spectrometer and on a "Specord" spectrometer for the visible and IR regions. The LBO and BBO samples were investigated at the same time. White light is used for samples with path length of 2 mm, which was oriented in the Z direction. The results of these measurements are shown in Fig. 2. Two prisms with an angle of about 30 ° were fabricated from SBO and PBO crystals in order to measure the refractive indices. Every prism was oriented so that in the position of minimal deviation the beam propagated along one of the crystal axes. The measurements of the minimal deviation angle were carried out on a goniometer, the tolerance of the minimal deviation angle was less then one minute of arc. This ensured the determination of the refractive indices with an accuracy of 10 -4 .
A
B
C
D
2.9771 2.9776 2.9883 3.6289 3.6415 3.6492
0.01307 0.01392 0.01383 0.02416 0.03524 0.03514
0.01234 0.01077 0.01085 0.02950 0.03095 0.03077
0.02451 0.02266 0.02840 0.02594 0.02153 0.02324
of 100°C above the melting temperature. Rotation of a platinum stirrer for 6-10 h provided complete homogenization. The melt was then cooled to the temperature of crystallization, and seed crystal was brought into contact with the melt surface. The rotation rate was ~ 20 rpm. The crystal was grown to a diameter of 20 mm with a cooling rate of 5°C/day. Subsequently, the growth process was performed under constant temperature with a pulling rate of 2 mm/day. When the crystal
The measured refractive indices of SBO and PBO are listed in Table 4. The fit of the coefficients A, B, C and D of the Sellmeier equation ( 1 ) n2(A) =A + B/(A 2 - C) - D A 2,
( 1)
where A is vacuum wavelength measured in txm, was performed by the least-squares routine. The results are given in Table 5. It should be noted that the main optical axes are marked so that nx < n y < nz and do not coincide with the crystallographic axes. The correspondence between the main optical and crystallographic axes is given in Table 3. However, the difference between the values of the main refractive indices is too small in the SBO and PBO crystals, so that the phase-matched processes of the second harmonic generation and sum-frequency generation in these crystals are impossible.
672
Yu.S. Oseledchik et al. / Optical Materials 4 (1995) 669-674
~
~- KTP •
.
LBO SBO PBO
ii o
o
I
20
40
60
8'0
100
120
d, #m Fig. 3. SHG efficiencyin a powderedmaterialsat A= 1.064 I~m( in auxiliary units) as a function of particle size for KTP, LBO, SBO, and PBO. 5O
PBO crystals and~ to compare with, for KTP and LBO crystals, which have the same point symmetry group mm2. The decrease of the SHG intensity with the increase of the particle size confirms the absence of the phase-matching for SHG. The tangent of the slope angle of the curve at small values of the particle size is directly proportional to the averaged value of the effective nonlinear coefficient. Thus, it follows from the comparison of the curves l(d) shown in Fig. 3 that the averaged value of the effective nonlinear coefficient of SBO is approximately the same as of KTP: deff(SBO ) = deff(KTP )
significantly exceeding the value of the LBO nonlinear coefficient: deff(SBO) > deff(LBO).
E =~ 40
The effective nonlinear coefficient of PBO is considerably smaller than that of KTP:
50
d e f f ( V B O ) = 0.2 deff(KTP).
~ 20 ~ 10 0 0.'2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Incident wavelength, /zm
Fig. 4. The coherence length as a function of the incident wavelength for non-coherent SHG in SBO and PBO in the direction of the optical axis Y. /
Z
3
4
.J"
Fig. 5. The experimental scheme for observation of non-coherent harmonic generation in SBO and PBO crystals. 1, Nd:YAG laser; 2, KTP crystal; 3, SBO (PBO) crystal; 4, filter; 5, prism; 6, detector.
4. Nonlinear optical properties The SHG of Nd:YAG laser radiation (h = 1064 nm, 7= 15 ns) was studied for SBO and PBO powder with different average particles dimensions and was compared with SHG on the powder of wellknown KTP and LBO crystals, with used routine method [10]. The estimated error of deft measurements are not larger than 20%. Fig. 3 represents the SHG efficiency (in auxiliary units) as a function of the particle size for SBO and
In spite of the absence of phase matching, the noncoherent nonlinear optical processes could be performed with good efficiency, particularly in SBO crystal, as it has high values of the nonlinear optical coefficient and of the coherence length
it=
A 2An '
where An is the difference of the refractive indices of the waves with the frequencies 2oJ and to. The maximum value of the coherence length is achieved when the laser beam propagates along the y-axis, and the incident radiation is polarized in the YZ plane, then An = nx(2o~) --nz(~o). Fig. 4 shows the change of the coherence length of SBO and PBO for the process of the second harmonic generation as a function of the incident wavelength. To investigate non-coherent SHG: plates of SBO and PBO oriented perpendicularly to the optical y-axis were cut. The experimental setup is shown in Fig. 5. The system is based on a mode-locked Nd:YAG laser which generates pulses with the energy of 50 mJ and the duration of 15 ns. Pulse-to-pulse energy fluctuation did not exceed 5%. The incident radiation was partially converted to the second harmonic with efficiency 40%, so that the investigated crystal was radiated by the wavelengths of both 1.064 and 0.532 Izm. We have
Yu. S. Oseledchik eta!. / Optical Materials 4 (1995) 669-674
observed the third and fourth harmonics generation in SBO and PBO. The third harmonic was obtained by summing of the first and second harmonics and the fourth harmonic was obtained by doubling of the second harmonic. The conversion efficiency of the FHG did not exceed the level of 1% of the SHG. It should be noted that we have observed the generation of the third and fourth harmonics in one crystal at the same time which could not be performed elsewhere. This setup may be used to observe the higher harmonics from fifth to eighth, although with smaller efficiency.
5. Optical and mechanical properties The surface optical damage threshold was determined for SBO and PBO crystals cut along the main optical plane X Z and, to compare, for LBO and BBO crystals cut under the phase-matching angle for SHG. A high-power Nd:YAP laser ( A = 1.079 p~m) with a Gaussian beam profile and a pulse duration of 15 ns was used. The surface damage was determined by the observable burn of the surface. The results of these measurements are listed in Table 6. The measured values of LBO and BBO damage threshold differ from that published in Refs. [ 11,12]. This discrepancy arises, probably, from the difference of the experimental conditions such as the method of indication of laser beam power density, the quality of polishing of the crystal surfaces, etc. However, the ratio of the values of optical damage threshold of different crystals do not depend on this conditions. Thus, it follows from our results that the optical damage threshold of LBO is approximately two times larger than that of BBO: Id (BBO) = 0.43 Id (LBO), in good agreement with Refs. [ 11,12]. So it seems to be more correct to Table 6 Optical damage threshold of the borate crystals
1d, G W / c m 2
Ij/Id(LBO)
* Ref. [13].
~"(ns I
BBO
LBO
SBO
PBO
15.0 0.1 1.3 15.0 0.1 1.3
4.0 15.0" 10.4" 0.43 0.6* 0.55"
9.3 25.0" 18.9" -
14.7
3.6
673
Table 7 Microhardness of the borate crystals
LBO SBO PBO
Hk, k g / m m 2
Hky, k g / m m 2
Hk:,kg/mm 2
768 1750 1270
755 1460 1170
607 1350 1190
compare the optical damage thresholds of different crystals measured under the same conditions. The ratios I d ( S B O ) / I d ( L B O ) and I d ( P B O ) / l d ( L B O ) are also listed in Table 6. Note the high value of the optical damage threshold of SBO. We have also tested the possibility of the use of these crystals in electro-optics. The linear Pockels effect was used to measure the magnitude of the matrix elements rij of the electro-optical coefficients tensor. The crystals of SBO and PBO belong to the orthorombic system, point symmetry group being mm2. To determine the matrix elements rll, r22, r33,/42, ?'51, the samples were cut in the direction of the main optical planes and at 45 ° in the X Z and YZ planes. The electric field was induced as across, as along the beam propagation direction, and the light polarization plane rotation was measured. It was revealed for all possible orientation of both SBO and PBO crystals that the half-wave voltage was no less than 50 kV. It follows from this data that the values of the matrix elements ro are no more than 5 × l0 -3 m/V. This is too small value to propose the use of this crystals in electro-optical devices. The microhardness of the SBO and PBO crystals was measured in the directions of the main optical axes. The results are given in Table 7 in comparison with LBO [8]. The measurements indicate that the microhardness of the SBO is significantly greater than that of any other borate crystals. The density of SBO was measured to be 4.011 g/ cm 3 and the density of PBO is 5.852 g / c m 3. The crystals of strontium and lead tetraborate are not hygroscopic.
6. Applications 1.58
0.39
The crystal of strontium tetraborate (SBO) has an excellent mechanical and optical strength, high nonlinear coefficients and wide transparency band, including the area of vacuum ultraviolet. The properties of lead
674
Yu.S. Oseledchik et al. / Optical Materials 4 (1995) 669-674
tetraborate are more modest. But the absence of phasematching limits the application of these materials in nonlinear optics. The efficiency of non-coherent harmonic generation in SBO is quiet high, but not good if compared with coherent processes which are performed with well-known materials, such as LBO. However, some application of these materials could be proposed. SBO and PBO may be used as materials for creation of nonlinear optical waveguides. As the values of unit cell parameters of these materials are similar, one of them may be grown over another without the appearance of defects. The refractive indices of SBO and PBO differ significantly: An =0.2. Thus, the optical waveguide may be produced using SBO as the basis material, and PBO as the waveguide film material. These waveguides will be transparent for the radiation with the wavelength more than 230 nm and may be used for the nonlinear optical conversion of the radiation in the ultraviolet spectral region. The thickness of the waveguide film could be fitted to obtain the necessary values of the effective refractive indices which satisfy the phase-matching conditions. SBO crystal may also be used as a cross-correlator for the measurement of the pulse duration of the ultrashort pulses of excimer laser radiation [ 13]. Two pulses are crossed in a thin plate of SBO, and a non-coherent second harmonic generation occurs. The measurement of the conversed radiation pulse duration makes it possible to determine the duration of the incident pulses. The cross-correlator of SBO is unique in the spectral range of 130-160 nm.
Acknowledgements The authors are grateful to prof. B.S. Hudson of Oregon University for helpful discussion, to A. Mitrofanov of Lebedev Physics Institute (Moscow), and to prof. I. Klimiv and T. Dudok of the Physical Optics Institute (L' viv, Ukraine) for kind technical assistance. This work was supported, in part, by a Soros Foundation Grant awarded by the American Physical Society.
References [ 1 ] C.L. Tang, W.R. Bosenberg, T.U. Kachi, R.J. Lane and L.K. Cheng, Laser Focus World 26 (1990) 87. [2] J.Y. Huang, Y. Shen, C. Chen and B. Wu, Appl. Phys. Lett. 58 (1991) 1579. [ 3 ] A. Borsuttzky, R. Brunder, C. Huang and R. Wallenstein, Appl. Phys. B 52 (1992) 55. [4] T. Ukachi, R.J. Lane, W.R. Bosenberg and C.L. Tang, J. Opt. Soc. Am. B 9 (1992) 1128. [5] Y. Wu and T. Sasaki, in: CLEO'93, 1993, CMD5. [6] L. Bohaty, Z. Kristallogr. 164 (1983) 279. [7] L. Bohaty, J. Liebertz and S. Stahz, Z. Kristallogr. 172 (1985) 135. [8] Yu. Oseledchik, A. Prosvirnin, V. Starshenko et al., J. Cryst. Growth 135 (1994) 373. [9] A. Perloff and S. Block, Acta Cryst. 20 (1966) 274. [ 10] S.K. Kurts, IEEE J. Quantum Electron. QE-4 (1968) 578. [ 11 ] S. Zhao, C. Huang and H. Zhang, J. Cryst. Growth 99 (1990) 805. [ 12] C.T. Chen, QELS'89 (1989) MAAL. [ 13] C.K. Rhodes, Appl. Phys. B 42 (1987) 11.
?TI;AL ELSEVIER
Optical Materials 4 ( 1995 ) 669~574
New nonlinear optical crystals: strontium and lead tetraborates Yu. S. Oseledchik a, A.L. Prosvimin a, A.I. Pisarevskiy a, V.V. Starshenko a, V.V. Osadchuk a, S.P. Belokrys a, N.V. Svitanko a, A.S. Korol a, S.A. Krikunov b, A.F. Selevich c ~'Laboratory_ of Nonlinear Optical Frequency Converters, Industrial Institute, Prosp. Lenina 226, 330006 Zaporozhye, Ukraine General Physics Institute RAS, 117942 Moscow, Russia c Institute of Physico-Chemical Problems, Belarussian State Universi~, 220080 Minsk, Belarus
Received 20 June 1994; revised 11 May 1995; accepted 15 June 1995
Abstract The nonlinear-optical and electro-optical properties of the crystals of strontium (SrB40 7) and lead (PbB40 7) tetraborate are investigated. The transparency range of this crystals is 130-3200 nm for SBO and 235-4000 nm for PBO. The effective nonlinear coefficient and surface optical damage threshold of the SBO is higher than for any borate crystals. The SHG phase-maching conditions in these crystals are absent. The possible application of these crystals is discussed.
1. Introduction The low temperature modification of the barium borate f l - B a B 2 0 4 ( B B O ) and lithium triborate LiB305 (LBO) are well-known nonlinear optical crystals of borate series that are widely used for harmonic generation of laser radiation, sum and difference frequency mixing in visible and UV spectral range [ 1-4]. The search of a new crystals of the borate series seems to be very promising for nonlinear optics. Recently a crystal of cesium triborate C s B 3 0 5 (CBO) was described in Ref. [ 5 ]. All of these crystals have high values of the effective nonlinear coefficients and good damage resistance. The phase-matched frequency conversion could be performed in these crystals in a spectral band from 170 to 2600 nm. The S r B 4 0 7 and P b B 4 0 7 single crystals were grown at first and electro-optic properties were investigated by Bohaty et al. [6,7]. The results of the investigation of the optical and nonlinear optical properties of the new borate crystals of strontium SrB407 (SBO) and lead PbB407 (PBO) 0925-3467/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10925-3467(95) 00027-5
tetraborates are reported in this paper. These crystals were grown in our laboratory by the Czochralski method [8] and differ considerably from BBO, PBO and CBO. Thus, the short wave transparency border for SBO shifts to the VUV range. The effective nonlinear coefficient and surface optical damage threshold of this crystal are essentially higher as well. The possible application of these crystals in nonlinear optics is discussed in this paper.
2. Grown and crystal structure The needle-shaped single crystals of strontium and lead tetraborates were grown from stoichiometric melt. Crystal structure of the SBO and PBO were investigated in Ref. [9]. We have grown big single crystals of SBO and PBO from the melt by the Czochralski method. X-ray investigations of the SBO and PBO powder were carried out on a H-4A diffractometer
Yu.S. Os.eledchik et al. /Optical Materials 4 (1995) 669-674
670 Table 1 X-ray diffraction data of SBO dexp, A
1/lo dcan¢,A
hkl dexp, A
I/1o
dcam¢,/k hkl
5.35211 4.42066 4.09774 3.93633 3.06103 2.94078 2.77771 2.73068 2.67718 2.65742 2.29069 2.21247 2.11725 2.04525 2.01586 1.96860 1.96055 1.92908 1.91072 1.88096
28 5 35 30 32 12 41 50 15 100 18 22 30 12 68 16 12 18 8 25
020 100 110 011 101 111 130 031 040 121 140 200 002 220 141 022 201 150 051 230
5 5 10 8 5 5 10 7 6 5 6 8 3 7 5 5 7 5 12
1.84146 1.78478 1.71878 1.68391 1.66058 1.55473 1.54163 1.52969 1.47084 1.44586 1.42539 1.40602 1.39304 1.38140 1.36333 1.34816 1.32817 1.31249 1.30398
5.35435 4.42549 4.09000 3.93747 3.05937 2.94168 2.77841 2.72912 2.67671 2.65633 2.29065 2.21274 2.11703 2.04500 2.01471 1.96873 1.96109 1.92784 1.91115 1.88071
1.84271 1.78535 1.71885 1.68462 1.66127 1.5543 1.54293 1.52906 1.47070 1.44615 1.42586 1.40621 1.39379 1.38163 1.36344 1.34802 1.32812 1.31288 1.30425
221 060 231 132 042 142 161 201 222 170 152 232 301 311 330 321 242 033 162
Table 2 X-ray powder diffraction data of PBO
dexp, ,~k
1/1o dca~c,,~
hkl dexv, ,~
I/1o dcanc,A
hkl
5.41600 4.44700 4.12000 3.94800 3.07263 2.95200 2.80730 2.75010 2.70800 2.67270 2.33630 2.31320 2.22540 2.18160 2.12040 2.05970 2.03120 1.97540 1.97130 !.94930 1.93870
70 12 73 81 55 21 52 66 20 100 5 15 23 10 18 15 52 26 20 16 18
020 100 I10 011 101 111 130 031 040 121 131 140 200 210 002 220 141 022 201 150 211
20 12 33 10 15 17 16 10 5 14 7 11 13 18 6 11 10 8 8 10 15
051 230 112 221 060 231 132 042 241 161 212 222 170 152 232 260 311 330 321 123 340
5.41935 4.45466 4.12024 3.95163 3.07263 2.95614 2.80603 2.75098 2.70968 2.67291 2.34063 2.31503 2.22733 2.18174 2.12187 2.06012 2.03230 1.97582 1.97219 1.94920 1.94033
1.92970 1.89560 1.88630 1.85380 1.80570 1.73080 1.69230 1.66950 1.59380 1.55700 1.52100 1.47700 1.46170 1.43450 1.41330 1.40200 1.39000 1.37300 1.35640 1.30760 1.30260
1.93047 1.89598 1.88641 1.85329 1.80645 1.73107 1.69246 1.67061 1.59456 1.55726 1.52111 1.47807 1.46255 1.43546 1.41380 1.40301 1.38999 1.37341 1.35692 1.30835 1.30218
values of the crystallographic parameters are given in Table 3. Seed crystals were obtained by spontaneous crystallization. The habit of the SBO and PBO crystals equilibrium form grown during spontaneous crystallization from the melt is represented at Fig. 1. The appearance of an additional facet (130) of the crystal morphology was observed for PBO. Small rate of the crystal growth along Y axis is a distinctive feature of PBO. The PBO crystal, which was grown in spontaneously conditions, had the form of a plate or rod. This rod was widened in the X-Z plane and was lengthened along the Z axis. The SBO crystal had the form of rods only. The SBO and PBO big crystals were grown from a melt by the Czochralski method due to the congruent character of SrB407 melting at a temperature of 970°C and P b B 4 0 7 melting at a temperature of 740°C. The melts were prepared from H3BO3 ( > 9 9 . 9 % ) and SrCO3 ( > 9 9 . 5 % ) or PbO ( > 9 9 . 5 % ) . A crucible of 50 mm in diameter and 60 mm in height containing the melt was mounted into a resistance furnace, so that the temperature gradient in the near surface layer was 80100°C/cm. The mixture was heated to a temperature Table 3 Crystallographic parameters of SBO and PBO
Point symmetry group Space symmetry group Unit cell parameters,/~
Angle between optical axes Conformity between crystallographic and main optical axes
SBO
PBO
ram2 Pnm2~
mm2 Pnm2L
a =4.4255 (7) b = 10.709 (2) c=4.2341 (9) V= 200.7/~3 74 °
a =4.4547 (7) b = 10.839 (2) c=4.2437 (8) V= 204.9/~3 86 °
X - ny
X - nz
Y - n~ Z - n:
Y - nx Z - ny
to2
.fill
¢ under Cu-K,~ radiation. X-ray diffraction data of crystals powder are listed in Table 1 and Table 2. The
Fig. 1. Growth habit of the SBO and PBO crystals.
Yu.S. Oseledchik et al. / Optical Materials 4 (1995) 6 6 9 ~ 7 4 1.0
,
£n ~n
I--
/ /
/f
0.8
~LBo 1
~=~'~"-~--~
-
2 -
had achieved a length of 20 mm, it was lifted slightly above the melt and cooled at a rate of 10-20°C/h. The SBO crystals was grown in the direction of the Z axis and PBO crystals was grown in the direction of the Z or Y axis. The SBO crystal was partially faceted by the ( 011 ) and (020) planes. The PBO crystal was partially faceted by the (100), (011) and (120) planes. The facet ( 101 ) is absent in both crystals.
SBO
P80
0.6
0.4
0.2 1
!
1
2
3000
4000
',i\
0.0 O0
200
300
2000
Wovelength,
nm
3. Optical transparency and refractive indices
Fig. 2. The t r a n s p a r e n c y of the borate crystals. Table 4 S B O and P B O refractive indices
A. ~rn
SBO nr
n,
nz
PBO nx
ny
n:
0.2129 0.2661 0.3548 0.4254 0.4358 0.4916 0.5322 0.5461 0.5770 0.6708 0.6925 1.0644
1.8364 1.7883 1.7579 1.7464 1.7454 1.7365 1.7347 1.7300 1.7208
1.8384 1.7909 1.7597 1.7479 1.7467 1.7377 1.7360 1.7307 1.7218
1.8407 1.7936 1.7622 1.7506 1.7490 1.7400 1.7386 1.7333 1.7229
2.1105 1.9948 1.9620 1.9590 1.9453 1.9385 1.9361 1.9320 1.9230 1.9211 1.9055
2.1270 2.0023 1.9677 1.9647 1.9505 1.9432 1.9409 1.9367 1.9275 1.9256 1.9104
2.1273 2.0037 1.9696 1.9663 1.9522 1.9450 1.9427 1.9385 1.9291 1.9274 1.9119
Table 5 Sellmeier coefficients of SBO, P B O
SBO
PBO
n~ ny n: n, nv n:
671
Transparency measurements of the SBO and PBO samples were performed on a vacuum UV spectrometer and on a "Specord" spectrometer for the visible and IR regions. The LBO and BBO samples were investigated at the same time. White light is used for samples with path length of 2 mm, which was oriented in the Z direction. The results of these measurements are shown in Fig. 2. Two prisms with an angle of about 30 ° were fabricated from SBO and PBO crystals in order to measure the refractive indices. Every prism was oriented so that in the position of minimal deviation the beam propagated along one of the crystal axes. The measurements of the minimal deviation angle were carried out on a goniometer, the tolerance of the minimal deviation angle was less then one minute of arc. This ensured the determination of the refractive indices with an accuracy of 10 -4 .
A
B
C
D
2.9771 2.9776 2.9883 3.6289 3.6415 3.6492
0.01307 0.01392 0.01383 0.02416 0.03524 0.03514
0.01234 0.01077 0.01085 0.02950 0.03095 0.03077
0.02451 0.02266 0.02840 0.02594 0.02153 0.02324
of 100°C above the melting temperature. Rotation of a platinum stirrer for 6-10 h provided complete homogenization. The melt was then cooled to the temperature of crystallization, and seed crystal was brought into contact with the melt surface. The rotation rate was ~ 20 rpm. The crystal was grown to a diameter of 20 mm with a cooling rate of 5°C/day. Subsequently, the growth process was performed under constant temperature with a pulling rate of 2 mm/day. When the crystal
The measured refractive indices of SBO and PBO are listed in Table 4. The fit of the coefficients A, B, C and D of the Sellmeier equation ( 1 ) n2(A) =A + B/(A 2 - C) - D A 2,
( 1)
where A is vacuum wavelength measured in txm, was performed by the least-squares routine. The results are given in Table 5. It should be noted that the main optical axes are marked so that nx < n y < nz and do not coincide with the crystallographic axes. The correspondence between the main optical and crystallographic axes is given in Table 3. However, the difference between the values of the main refractive indices is too small in the SBO and PBO crystals, so that the phase-matched processes of the second harmonic generation and sum-frequency generation in these crystals are impossible.
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Yu.S. Oseledchik et al. / Optical Materials 4 (1995) 669-674
~
~- KTP •
.
LBO SBO PBO
ii o
o
I
20
40
60
8'0
100
120
d, #m Fig. 3. SHG efficiencyin a powderedmaterialsat A= 1.064 I~m( in auxiliary units) as a function of particle size for KTP, LBO, SBO, and PBO. 5O
PBO crystals and~ to compare with, for KTP and LBO crystals, which have the same point symmetry group mm2. The decrease of the SHG intensity with the increase of the particle size confirms the absence of the phase-matching for SHG. The tangent of the slope angle of the curve at small values of the particle size is directly proportional to the averaged value of the effective nonlinear coefficient. Thus, it follows from the comparison of the curves l(d) shown in Fig. 3 that the averaged value of the effective nonlinear coefficient of SBO is approximately the same as of KTP: deff(SBO ) = deff(KTP )
significantly exceeding the value of the LBO nonlinear coefficient: deff(SBO) > deff(LBO).
E =~ 40
The effective nonlinear coefficient of PBO is considerably smaller than that of KTP:
50
d e f f ( V B O ) = 0.2 deff(KTP).
~ 20 ~ 10 0 0.'2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Incident wavelength, /zm
Fig. 4. The coherence length as a function of the incident wavelength for non-coherent SHG in SBO and PBO in the direction of the optical axis Y. /
Z
3
4
.J"
Fig. 5. The experimental scheme for observation of non-coherent harmonic generation in SBO and PBO crystals. 1, Nd:YAG laser; 2, KTP crystal; 3, SBO (PBO) crystal; 4, filter; 5, prism; 6, detector.
4. Nonlinear optical properties The SHG of Nd:YAG laser radiation (h = 1064 nm, 7= 15 ns) was studied for SBO and PBO powder with different average particles dimensions and was compared with SHG on the powder of wellknown KTP and LBO crystals, with used routine method [10]. The estimated error of deft measurements are not larger than 20%. Fig. 3 represents the SHG efficiency (in auxiliary units) as a function of the particle size for SBO and
In spite of the absence of phase matching, the noncoherent nonlinear optical processes could be performed with good efficiency, particularly in SBO crystal, as it has high values of the nonlinear optical coefficient and of the coherence length
it=
A 2An '
where An is the difference of the refractive indices of the waves with the frequencies 2oJ and to. The maximum value of the coherence length is achieved when the laser beam propagates along the y-axis, and the incident radiation is polarized in the YZ plane, then An = nx(2o~) --nz(~o). Fig. 4 shows the change of the coherence length of SBO and PBO for the process of the second harmonic generation as a function of the incident wavelength. To investigate non-coherent SHG: plates of SBO and PBO oriented perpendicularly to the optical y-axis were cut. The experimental setup is shown in Fig. 5. The system is based on a mode-locked Nd:YAG laser which generates pulses with the energy of 50 mJ and the duration of 15 ns. Pulse-to-pulse energy fluctuation did not exceed 5%. The incident radiation was partially converted to the second harmonic with efficiency 40%, so that the investigated crystal was radiated by the wavelengths of both 1.064 and 0.532 Izm. We have
Yu. S. Oseledchik eta!. / Optical Materials 4 (1995) 669-674
observed the third and fourth harmonics generation in SBO and PBO. The third harmonic was obtained by summing of the first and second harmonics and the fourth harmonic was obtained by doubling of the second harmonic. The conversion efficiency of the FHG did not exceed the level of 1% of the SHG. It should be noted that we have observed the generation of the third and fourth harmonics in one crystal at the same time which could not be performed elsewhere. This setup may be used to observe the higher harmonics from fifth to eighth, although with smaller efficiency.
5. Optical and mechanical properties The surface optical damage threshold was determined for SBO and PBO crystals cut along the main optical plane X Z and, to compare, for LBO and BBO crystals cut under the phase-matching angle for SHG. A high-power Nd:YAP laser ( A = 1.079 p~m) with a Gaussian beam profile and a pulse duration of 15 ns was used. The surface damage was determined by the observable burn of the surface. The results of these measurements are listed in Table 6. The measured values of LBO and BBO damage threshold differ from that published in Refs. [ 11,12]. This discrepancy arises, probably, from the difference of the experimental conditions such as the method of indication of laser beam power density, the quality of polishing of the crystal surfaces, etc. However, the ratio of the values of optical damage threshold of different crystals do not depend on this conditions. Thus, it follows from our results that the optical damage threshold of LBO is approximately two times larger than that of BBO: Id (BBO) = 0.43 Id (LBO), in good agreement with Refs. [ 11,12]. So it seems to be more correct to Table 6 Optical damage threshold of the borate crystals
1d, G W / c m 2
Ij/Id(LBO)
* Ref. [13].
~"(ns I
BBO
LBO
SBO
PBO
15.0 0.1 1.3 15.0 0.1 1.3
4.0 15.0" 10.4" 0.43 0.6* 0.55"
9.3 25.0" 18.9" -
14.7
3.6
673
Table 7 Microhardness of the borate crystals
LBO SBO PBO
Hk, k g / m m 2
Hky, k g / m m 2
Hk:,kg/mm 2
768 1750 1270
755 1460 1170
607 1350 1190
compare the optical damage thresholds of different crystals measured under the same conditions. The ratios I d ( S B O ) / I d ( L B O ) and I d ( P B O ) / l d ( L B O ) are also listed in Table 6. Note the high value of the optical damage threshold of SBO. We have also tested the possibility of the use of these crystals in electro-optics. The linear Pockels effect was used to measure the magnitude of the matrix elements rij of the electro-optical coefficients tensor. The crystals of SBO and PBO belong to the orthorombic system, point symmetry group being mm2. To determine the matrix elements rll, r22, r33,/42, ?'51, the samples were cut in the direction of the main optical planes and at 45 ° in the X Z and YZ planes. The electric field was induced as across, as along the beam propagation direction, and the light polarization plane rotation was measured. It was revealed for all possible orientation of both SBO and PBO crystals that the half-wave voltage was no less than 50 kV. It follows from this data that the values of the matrix elements ro are no more than 5 × l0 -3 m/V. This is too small value to propose the use of this crystals in electro-optical devices. The microhardness of the SBO and PBO crystals was measured in the directions of the main optical axes. The results are given in Table 7 in comparison with LBO [8]. The measurements indicate that the microhardness of the SBO is significantly greater than that of any other borate crystals. The density of SBO was measured to be 4.011 g/ cm 3 and the density of PBO is 5.852 g / c m 3. The crystals of strontium and lead tetraborate are not hygroscopic.
6. Applications 1.58
0.39
The crystal of strontium tetraborate (SBO) has an excellent mechanical and optical strength, high nonlinear coefficients and wide transparency band, including the area of vacuum ultraviolet. The properties of lead
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Yu.S. Oseledchik et al. / Optical Materials 4 (1995) 669-674
tetraborate are more modest. But the absence of phasematching limits the application of these materials in nonlinear optics. The efficiency of non-coherent harmonic generation in SBO is quiet high, but not good if compared with coherent processes which are performed with well-known materials, such as LBO. However, some application of these materials could be proposed. SBO and PBO may be used as materials for creation of nonlinear optical waveguides. As the values of unit cell parameters of these materials are similar, one of them may be grown over another without the appearance of defects. The refractive indices of SBO and PBO differ significantly: An =0.2. Thus, the optical waveguide may be produced using SBO as the basis material, and PBO as the waveguide film material. These waveguides will be transparent for the radiation with the wavelength more than 230 nm and may be used for the nonlinear optical conversion of the radiation in the ultraviolet spectral region. The thickness of the waveguide film could be fitted to obtain the necessary values of the effective refractive indices which satisfy the phase-matching conditions. SBO crystal may also be used as a cross-correlator for the measurement of the pulse duration of the ultrashort pulses of excimer laser radiation [ 13]. Two pulses are crossed in a thin plate of SBO, and a non-coherent second harmonic generation occurs. The measurement of the conversed radiation pulse duration makes it possible to determine the duration of the incident pulses. The cross-correlator of SBO is unique in the spectral range of 130-160 nm.
Acknowledgements The authors are grateful to prof. B.S. Hudson of Oregon University for helpful discussion, to A. Mitrofanov of Lebedev Physics Institute (Moscow), and to prof. I. Klimiv and T. Dudok of the Physical Optics Institute (L' viv, Ukraine) for kind technical assistance. This work was supported, in part, by a Soros Foundation Grant awarded by the American Physical Society.
References [ 1 ] C.L. Tang, W.R. Bosenberg, T.U. Kachi, R.J. Lane and L.K. Cheng, Laser Focus World 26 (1990) 87. [2] J.Y. Huang, Y. Shen, C. Chen and B. Wu, Appl. Phys. Lett. 58 (1991) 1579. [ 3 ] A. Borsuttzky, R. Brunder, C. Huang and R. Wallenstein, Appl. Phys. B 52 (1992) 55. [4] T. Ukachi, R.J. Lane, W.R. Bosenberg and C.L. Tang, J. Opt. Soc. Am. B 9 (1992) 1128. [5] Y. Wu and T. Sasaki, in: CLEO'93, 1993, CMD5. [6] L. Bohaty, Z. Kristallogr. 164 (1983) 279. [7] L. Bohaty, J. Liebertz and S. Stahz, Z. Kristallogr. 172 (1985) 135. [8] Yu. Oseledchik, A. Prosvirnin, V. Starshenko et al., J. Cryst. Growth 135 (1994) 373. [9] A. Perloff and S. Block, Acta Cryst. 20 (1966) 274. [ 10] S.K. Kurts, IEEE J. Quantum Electron. QE-4 (1968) 578. [ 11 ] S. Zhao, C. Huang and H. Zhang, J. Cryst. Growth 99 (1990) 805. [ 12] C.T. Chen, QELS'89 (1989) MAAL. [ 13] C.K. Rhodes, Appl. Phys. B 42 (1987) 11.