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Infrared properties of CsGe(Br
x Cl1−x )3, nonlinear optical rhombohedral semiconductor
View the table of contents for this issue, or go to the journal homepage for more 2007 J. Phys.: Condens. Matter 19 476209
IOP PUBLISHING JOURNAL OFPHYSICS:CONDENSEDMATTER
J. Phys.: Condens. Matter 19 (2007) 476209 (10pp) doi:10.1088/0953-8984/19/47/476209
Infrared properties of CsGe
(Br
x
Cl
1
−x
)3
, nonlinear
optical rhombohedral semiconductor
Zhi-Guang Lin1, Li-Chuan Tang2and Chang-Pin Chou1,3
1Department of Mechanical Engineering, National Chiao Tung University, Hsinchu 305, Taiwan,
Republic of China
2Department of Electrical Engineering, Chung-Cheng Institute of Technology, National Defense
University, Taoyuan 353, Taiwan, Republic of China E-mail:zglin.me91g@nctu.edu.tw
Received 16 June 2007, in final form 9 October 2007 Published 31 October 2007
Online atstacks.iop.org/JPhysCM/19/476209
Abstract
Innovative infrared nonlinear optical crystals CsGe(BrxCl1−x)3 were
synthe-sized. From their powder x-ray diffraction patterns, these crystals were char-acterized as rhombohedral structure with (R3m, No 160) space group sym-metry. The energy gap decreased from about 3.43 to 2.38 eV as the substi-tutional ratio, x , changed from zero to unity. Moreover, the powder second-harmonic generation (PSHG) measurement of CsGeBr3showed that its
nonlin-ear optical efficiency is 9.64 times larger than that of rhombohedral CsGeCl3
and 28.29 times larger than that of KH2PO4(KDP), and most important of all,
that CsGe(BrxCl1−x)3 is phase matchable. So the optical nonlinearity is
ap-proximately inversely proportional to the cube of the energy gap. The infrared transparent spectrum of rhombohedral CsGe(BrxCl1−x)3was extended to more
than 30μm, which shows the potential in the realm of nonlinear optics and can be applied to the infrared region.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
Second-order nonlinear optical (NLO) materials have played a key role in such optical fields as laser frequency conversion and optical parametric oscillation/amplification (OPO/OPA) [1,2]. For inorganic second-order NLO materials, several crystals used in ultraviolet (UV) and visible regions have been proposed in the past two decades, such as KH2PO4(KDP), KTiOPO4(KTP),
β-BaB2O4 (BBO) and LiB3O5 (LBO). But in the infrared (IR) region the current materials,
such as AgGaSe2, ZnGeP2, are not good enough for applications mainly due to their low laser
damage threshold, as their band-gaps were smaller than 1.5 eV. So the search for new NLO
3 Author to whom any correspondence should be addressed.
crystals with excellent properties, especially a high damage threshold, has become one of the key research areas in NLO material science and laser technology [3].
Crystals with a pyramidal basis are known to exhibit a fairly large optical nonlinearity. A pyramidal basis in a unit cell contains one tetrahedron with one cation and three anions located at the vertices, such as the pyramidal basis−GeCl3[4,5] in the CsGeCl3(CGC) crystal.
Ternary halides are found to be potential materials for use in nonlinear optical applications [6] and are expected to be transparent in the mid-infrared region (with the exception of the fluorides) [7]. Furthermore, CGC’s damage threshold reaches 200 MW cm−2[8]. The optical damage threshold and the transparent range of materials are related to the magnitude of the energy gap, while the optical nonlinearity is inversely proportional to the cubic power of the energy gap [9]. To meet the demand from specific applications, the linear and NLO properties of CsGe(BrxCl1−x)3 can be adjusted by varying the alloy composition. In this paper, the
synthetic method of crystals and measurements of the optical properties in each composition are reported. Nonlinear coefficients of CsGe(BrxCl1−x)3, x = 0, 1/4, 2/4, 3/4, 1, are also
determined to reveal the potential of these crystals in NLO applications.
2. Synthesis and measurement 2.1. Synthesis
The procedure of synthesis is illustrated in figure1, which was modified from the work done by Gu et al [10,8,11]. Christensen and Tananaev et al [4,12] used different synthesis methods, but their methods seemed complex and the productivity was poor. In this study, H3PO2(50%) was
loaded with HBr (48%), HCl (37%), and GeO2(99.999%) into a 250 ml beaker, and then heated
to 95◦C. The solution was vigorously mixed for 5 h and then cooled to room temperature. After removing the precipitate, CsBr (99.9%) was added and the temperature raised to boiling, then the mixture was naturally cooled to room temperature again. A light yellow precipitate was formed. The reaction equations were listed as follows:
H3PO2+ 6xHBr + 6(1 − x)HCl + 2GeO2= H3PO4+ 2HGe(BrxCl1−x)3+ 2H2O (1)
then
HGe(BrxCl1−x)3+ CsBr = CsGe(BrxCl1−x)3↓ + HBr. (2)
Recrystallization was done by mixing the precipitate with 1:1 concentrated HX and alcohol solution to give the yellow crystals CsGe(BrxCl1−x)3. To avoid residue of the precursor, we
repeat this procedure seven times. Then, the crystals were dried at 85◦C for 48 h under vacuum to prevent the influence of deliquescence. The color of precipitated product varied from yellow to white as soon as the substitutional ratio, x , changed from unity to zero.
2.2. Physical measurements
The CsGe(BrxCl1−x)3 crystals were synthesized and sieved into different particle sizes in
order to measure and analyze their structural and optical properties. The crystal structures were observed using an x-ray diffractometer. The composition of all samples was measured by electron-probe x-ray microanalysis (EPMA). The optical transmission spectra in the infrared region was determined by a Fourier-transform infrared spectrometer (FTIR) while the absorption edge was measured by a UV–vis spectrometer. Linear optical properties were measured by an ellipsometer. Nonlinear optical properties were determined by powder second-harmonic generation measurements.
J. Phys.: Condens. Matter 19 (2007) 476209 Z-G Lin et al
Figure 1. The synthesis procedure of rhombohedral nonlinear optical crystals CsGe(BrxCl1−x)3.
3. Results and discussion
3.1. Composition and structural properties
The results of EPMA measurement (table 1) reveal that those samples possess a Cs to Ge ratio of almost 1:1 and confirm qualitatively that chlorine atoms were successfully doped in the CsGeBr3 crystal. Though there are still some impurities, they are all smaller than 1%
(Omax 0.47%, Pmax 0.58%).
XRD measurement, which was obtained at room temperature by means of Cu Kα radiation with Siemens D5000 equipment, was employed to determine the structural parameters of all the CsGe(BrxCl1−x)3 crystals. The results are shown in figure 2: the substitution-related
diffraction peaks shifted gradually with substitute composition. Moreover, the measured pattern was indexed and analyzed by a non-profit program PowderCell [13], which was developed by Kraus and Nolze. The structural parameters of CsGe(BrxCl1−x)3 were compared with
both CsGeCl3and CsGeBr3, which were reported in JCPDS [14–17,7]. There were certain
stronger diffraction peaks observed at 2θ = 31.76◦, 27.66◦, 26.86◦, 22.60◦, 22.10◦, and 15.76◦ in CsGeBr3. These diffraction patterns were compared with JCPDS and were indexed with
(200),(1¯11), (111), (1¯10), (110), and (100) planes, respectively. The result also confirmed that
Figure 2. The x-ray powder diffraction results for nonlinear optical crystals CsGe(BrxCl1−x)3.
Figure 3. The structural parameters for nonlinear optical crystals CsGe(BrxCl1−x)3.
Table 1. The composition of the rhombohedral NLO crystals CsGe(BrxCl1−x)3 (x =
0, 1/4, 2/4, 3/4, 1) from EPMA measurements.
x 0/4 1/4 2/4 3/4 4/4 Cs 20.56 20.16 20.57 20.20 20.32 Ge 20.66 20.70 20.61 20.26 20.51 Br 0 15.26 30.15 44.89 58.17 Cl 58.04 43.15 27.74 14.19 0 O 0.45 0.47 0.35 0.34 0.46 P 0.29 0.26 0.58 0.12 0.54
CsGe(BrxCl1−x)3 crystallized in the non-centrosymmetric rhombohedral space group R3m.
Moreover, the splitting differences between(1¯11) with (111) and (1¯10) with (110) get closer as the containment of Br decreases in CsGe(BrxCl1−x)3. The cell parameters, which were refined
from powder XRD in figure3, showed that the lattice constant became larger as Br increased while the cell angle became smaller as Br increased. Therefore, the structural distortion of CsGe(BrxCl1−x)3(R3m) will increase as Br increases.
J. Phys.: Condens. Matter 19 (2007) 476209 Z-G Lin et al
Figure 4. Absorption coefficient near the band edge of CsGe(BrxCl1−x)3plotted in coordinatesα2
and hν. The inset shows the Br composition dependence of the energy gap obtained.
3.2. Optical transparent properties
For the absorption edge measurements, thin plates (≈500 μm) of CsGe(BrxCl1−x)3were used.
In figure 4, the absorption spectrum measured at room temperature in the UV–visible light range is shown. The recorded curves can be approximated with straight lines in the coordinates
α2 and hν, where α is the absorption coefficient and hν is the photon energy. The straight
line approximation is applied to the rapidly increasing portions of the curves in figure4. Thus, the fundamental absorption edge is described by theα = A(hν − Eg)1/2dependence, where
A is a constant and the approximate band-gap Eg can be determined from the cross points of
the straight lines with the abscissa. This dependence corresponds to direct allowed electronic transitions [18]. In the inset of figure4, the energy gap values are plotted versus Br composition. The absorption edge is found to decrease from about 3.43 to 2.38 eV as the substitutional ratio,
x , changes from zero to unity.
Infrared spectra were recorded on the spectrometer (Bomem, DA8.3) in the range from 120 to 4000 cm−1 with the specimens pressed into thin plates (≈500 μm). From figure5, FTIR measurements showed that the long wavelength limit of the transparent range of the crystals exhibited a similar dependence on substitute composition. The crystal CsGeCl3had an
infrared cut-off wavelength at approximately 30μm, which was shorter than the cut-off value of CsGeBr3 (approximately 47μm). The infrared absorption edge of CsGe(BrxCl1−x)3 with
x = 1/4, 2/4, 3/4 lay approximately from 32 to 37 μm. This result agreed with the
effective-mass concept that the infrared transparency range of CGB is expected to be wider than that of CGC owing to the fact that the Br atom is heavier than Cl. The results of FTIR at room temperature are presented in figure6. The transmission range of the crystals extends wider as Br increases. The longest infrared transparency wavelength is usually limited by the phonon absorption of the crystal. Moreover, the absorption edge is limited by the energy band-gap of the crystal.
3.3. Second-order nonlinear optical measurements
Powder SHG measurements were performed on a modified Kurtz-NLO [19] system using 1260 nm light. Since the SHG efficiency of powders has been shown to depend strongly on
Figure 5. The full transmission range of the nonlinear optical crystals CsGe(BrxCl1−x)3(a) x= 1,
(b) x= 3/4, (c) x = 2/4, (d) x = 1/4, (e) x = 0.
Figure 6. The transmission edge and absorption edge of nonlinear optical crystals CsGe(BrxCl1−x)3.
particle size [19,20], polycrystalline CsGe(BrxCl1−x)3was ground and sieved (Newark Wire
Cloth Company) into six distinct particle-size ranges, 19–37 μm, 37–74 μm, 74–105 μm, 105–210μm, 210–420 μm and 420–840 μm (see figure7). To make relevant comparison with known SHG materials, crystalline KDP was also ground and sieved into the same particle-size ranges. All of the powders were placed in separate capillary tubes. We filled the capillary tube as full as possible with CsGe(BrxCl1−x)3powders. But it was averaged slightly loosely
in the two extremities. Though the powder was suspended in air in the two extremities, we chose the compact part when performing the nonlinear optical characterization. The SHG radiation (630 nm) was collected in transmission and detected by a photomultiplier tube (Oriel
J. Phys.: Condens. Matter 19 (2007) 476209 Z-G Lin et al
Figure 7. The powder second-harmonic generation results for rhombohedral nonlinear optical
crystals CsGeBr3.
Table 2. The ellipsometry measurements of the rhombohedral NLO crystals CsGe(BrxCl1−x)3
(x= 0, 1/4, 2/4, 3/4, 1). x CsGe(BrxCl1−x)3 0.00 0.27 0.52 0.78 1.00 α630 nm(1/mm) 1.49 3.80 5.26 4.45 8.88 n630 nm 1.71 1.89 1.78 1.58 1.63 n1260 nm 1.67 1.86 1.78 1.64 1.68
Instruments). The SHG signal was collected by a data-acquisition (DAQ) interface and was monitored by a personal computer with the analysis program.
If the SHG process was phase matchable and satisfied the type-I phase-matching conditions, the intensity of the SHG response could be written as [21]
I2ω(¯r, θ) = 128π5I2 ω n2 ωn2ωλ22ωc L¯rdeff2 sin2π2 ¯l¯r pm(θ − θpm) π 2¯lpm¯r (θ − θpm) , (3)
where ¯lpm = λ/[4| nB,2ω| sin 2θpm], and θpm is the phase-matching angle. Here nB,2ω =
nE,2ω− nO,2ωdenotes the birefringence of the material at the second-harmonic wavelength. In
the event that¯r ¯lpmor¯r ¯lpm, equation (3) could be simplified to
I2ω→ [(256π4I2 ω)/(n2ωn2ωλ22ωc)]L ¯lpmdeff2 , ←− ¯r ¯lpm [(128π5I2 ω)/(n2ωn2ωλ22ωc)]L ¯rd 2 eff, ←− ¯r ¯lpm . (4)
The SHG signals became saturated when the average particle sizes were larger than ¯lpm
and independent of the particle size.
Chen et al [22] derived a useful empirical formula, which possessed the correct asymptotic forms in equation (4), to depict the overall variation in the second-harmonic intensity with a particle size¯r I2ω= 256π4Iω2 n2 ωn2ωλ22ωc L ¯lpmdeff2 1− exp[−(¯r/A)2] (5) with A≈ 9¯lpm.
Because the absorption coefficient of CsGeBr3 at 630 nm was too large, the saturated
PSHG intensity decayed. To modify such a situation, the absorption coefficients (from table2)
Figure 8. The comparison of integrated powder second-harmonic generation intensity of nonlinear
optical crystals KDP and CsGe(BrxCl1−x)3.
Figure 9. The effective powder second-harmonic generation coefficients of nonlinear optical crystals CsGe(BrxCl1−x)3and their energy gaps.
were adopted to calculate the real saturated PSHG intensity using I2ω = I2totalω exp[−(α)z].
The square of the effective nonlinearity, d2
eff, averaged over the orientation distribution of
crystalline powders of CsGe(BrxCl1−x)3, was determined by equation (6) using a reference
NLO crystal, e.g. KDP.
d2 effCGB= deff2 KDP Itotal 2ω,CGBn 2 ω,CGBn2ω,CGB Itotal 2ω,KDPn 2 ω,KDPn2ω,KDP ≈ d2 effKDP Itotal 2ω,CGBn 3 CGB I2totalω,KDPn 3 KDP (6) when n≈ nω≈ n2ω.
3.4. Nonlinear optical properties
Figure8revealed that the SHG efficiencies of CsGe(BrxCl1−x)3were higher than that of KDP.
Moreover, all of them were phase matchable, as was KDP. That is, as the particle size becomes substantially larger than the coherence length of the crystal, the collected SHG intensity does not gain any more and saturates at a certain value. The saturated PSHG intensities were estimated from the transmission signals for various particle sizes, and showed that the SHG responses enhance as Br increases. From table2and dKDP(= 0.36 pm V−1) [23], deff values
were calculated and are shown in figure9. The effective PSHG coefficients increased as Br increased. The nonlinearity (see figure10) of deff2/n3 of CsGe(Br
J. Phys.: Condens. Matter 19 (2007) 476209 Z-G Lin et al
Figure 10. The nonlinearity of deff2/n3for nonlinear optical crystals CsGe(BrxCl1−x)3.
a similar dependence on substitution composition. There are some reasons for the significant SHG signals of rhombohedral CsGe(BrxCl1−x)3crystals. First of all, the SHG responses were
contributed from the structural distortion and the off-center Ge ion in the unit cell. From the results of XRD, the structural distortion increases as Br increases. Moreover, the cell angle distortion also becomes larger as Br increases. So the position of the B-site cation, Ge, is closer to the cell corner as Br increases. Second, the optical nonlinearity is approximately inversely proportional to the cube of the energy gap [9]. So the energy gap decreased and the NLO susceptibilities increased as the atomic weights of halides increased.
4. Conclusions
The structural and optical properties of rhombohedral NLO crystals, CsGe(BrxCl1−x)3 (x =
0, 1/4, 2/4, 3/4, and 1), have been investigated experimentally to reveal the anion substitution effect. Based on the results, the linearly increasing x caused an increase in lattice constant and second-order NLO susceptibility, but a decrease in energy gap. Because the optical damage threshold and the transparent range of materials are related to the magnitude of the energy gap, while the optical nonlinearity is inversely proportional to the cubic power of the energy gap [9], we could modulate the nonlinear susceptibility coefficient, energy gap, laser damage threshold and transparency range of halides at the same time by anion substitution.
Acknowledgment
The authors are indebted for the financial support of the National Science Council of the Republic of China under the grant NSC 95-2112-M-009-042.
References
[1] Burland D M 1994 Chem. Rev.94 1
[2] Chemla D S and Zyss J (ed) 1987 Nonlinear Optical Properties of Organic Molecules and Crystals (Orlando, FL: Academic)
[3] Dmitriev V G, Gurzadyan G G and Nikogosyan D N 1999 Handbook of Nonlinear Optical Crystals 3rd edn (Berlin: Springer)
[4] Christensen A N and Rasmussen S E 1965 Acta Chem. Scand. 19 421
[5] Ewbank M D, Cunningham F, Borwick R, Rosker M J and Gunter P 1997 CLEO’97 Paper vol CFA7, p 462 [6] Hagemann M and Weber H-J 1996 Appl. Phys. A 63 67
[7] Seo D-K, Gupta N, Whangbo M-H, Hillebrecht H and Thiele G 1998 Inorg. Chem.37 407
[8] Gu Q, Pan Q, Wu X, Shi W and Fang C 2000 J. Cryst. Growth212 605–7
[9] Shen Y R 2002 The Principles of Nonlinear Optics (New York: Wiley)
[10] Gu Q, Pan Q, Shi W, Sun X and Fang C 2000 Prog. Cryst. Growth Charact. Mater.40 89–95
[11] Gu Q, Fang C, Shi W, Wu X and Pan Q 2001 J. Crystal Growth225 501–504
[12] Tananaev I V, Dzhurinskii D F and Mikhailov Y N 1964 Zh. Neorg. Khim. 9 1570–7 [13] Kraus W and Nolze G 1996 J. Appl. Cryst.29 301–3
[14] Schwarz U, Hillebrecht H, Kaupp M, Syassen K, von Schnering H-G and Thiele G 1995 J. Solid State Chem.
118 20–7
[15] Schwarz U, Wagner F, Syassen K and Hillebrecht H 1996 Phys. Rev. B53 12545
[16] Thiele G, Rotter H W and Schmidt K D 1987 Z. Anorg. Allg. Chem.545 148
[17] Thiele G, Rotter H W and Schmidt K D 1988 Z. Anorg. Allg. Chem.559 7–16
[18] Pankove J I 1971 Optical Processes in Semiconductors (Englewood Cliffs, NJ: Prentice-Hall) [19] Kurtz S K and Perry T T 1968 J. Appl. Phys.39 3798–813
[20] Dougherty J P and Kurtz S K 1976 J. Appl. Crystallogr.9 145–58
[21] Prasad P N and Williams D J 1991 Introduction to Nonlinear Optical Effects in Molecules and Polymers (New York: Wiley) chapter 6
[22] Chen W K, Cheng C M, Huang J Y, Hsieh W F and Tseng T Y 2000 J. Phys. Chem. Solids61 969–77
