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Growth and characterization of a novel nonlinear optical borate crystal – Yttrium calcium borate (YCB)
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 391–394
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Growth and characterization of a novel nonlinear optical borate crystal – Yttrium calcium borate (YCB) R. Arun Kumar a,b,⇑, M. Arivanandhan a, R. Dhanasekaran c, Y. Hayakawa a a
Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan Department of Basic Sciences (Physics), PSG College of Technology, Coimbatore, India c Crystal Growth Centre, Anna University, Chennai, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
New nonlinear optical borate-based
crystal – Yttrium calcium borate is grown and characterized. Good quality of the grown crystals is evidenced by HRXRD measurements. SHG efficiency of YCB crystal is twice that of KDP, and possesses high laser damage threshold value (10.5 GW/cm2). The crystal is also non-hygroscopic, favoring real-time device applications.
a r t i c l e
i n f o
Article history: Received 23 January 2013 Received in revised form 15 March 2013 Accepted 16 March 2013 Available online 25 March 2013 Keywords: Borates Crystal growth Characterization Lasers Nonlinear optics
a b s t r a c t A new nonlinear optical single crystal yttrium calcium borate Y2CaB10O19 (YCB) was grown for the first time from its melt. The starting materials were prepared by the solid-state reaction method. The melting point of the synthesized material was identified to be 967 °C. YCB crystal exhibits monoclinic crystal structure with the space group C2. The crystalline perfection of the grown YCB crystal was found to be good. From the UV–VIS–NIR studies, the lower cutoff wavelength of the crystal occurs below 200 nm. The functional groups of the grown crystal were assigned using the FTIR data. The second harmonic generation (SHG) of the YCB crystal was observed using a Nd:YAG laser with a fundamental wavelength of 1064 nm. The laser damage threshold value of the YCB crystal was found to be very high – 10.5 GW/cm2. Ó 2013 Elsevier B.V. All rights reserved.
Introduction The development of laser by Maiman in the year 1960 led to technological advancements in various fields. The idea of non-linear optics could not be possible without lasers. The observation of the non-linear optical phenomena was first demonstrated by Franken and his co-workers in 1961 [1]. Since then, research on devel⇑ Corresponding author at: Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan. Tel.: +81 53 478 1310. E-mail address: [email protected] (R. Arun Kumar). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.03.069
oping crystals for employing in non-linear optical (NLO) devices gained momentum. There are various nonlinear optical crystals reported till date [2]. The criteria for selecting a nonlinear optical crystal for device applications include good crystalline perfection, high non-linear optical coefficient, high transparency in the wavelength of interest, moderate birefringence, low walk-off effect, good mechanical, chemical and thermal stability. In addition, crystals with high laser-induced damage threshold (LDT) are highly desired for real-time laser applications. Keeping in view of the desirable parameters, several NLO crystals belonging to the organic, inorganic and semi-organic families are grown by many
392
R. Arun Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 391–394
research groups. Among the inorganic crystals KTP and LBO are employed extensively for the generation of green lasers by frequency doubling of Nd:YAG lasers. In the inorganic family of crystals, borate-based crystals have attracted attention owing to their intrinsic favorable properties. Few nonlinear optical borate-based crystals are LiB3O5 (LBO) [3], CsLiB6O10 (CLBO) [4], KBe2BO3F2 (KBBF) [5], K2Al2B2O7 (KAB) [6], K3YB6O12 (KYB) [7], La2CaB10O19 (LCB) [8], and so on. In addition to their suitability in nonlinear optical applications, a few borate crystals are also attractive candidates in self-frequency doubling (SFD) processes, due to their molecular structure which favors laser active ion doping. Examples of such crystals are Er:Yb:YCOB [9], Pr:GdCOB [10], Nd:YAB [11], and Yb:LCB [12]. Though there are several borate crystals available for laser applications, the present challenge is in developing a crystal with both high NLO efficiency and laser damage threshold. Moreover the crystal must be non-hygroscopic to be suitable for real-time applications. To prepare such a borate-based crystal, we have synthesized and grown a new NLO crystal yttrium calcium borate with the chemical formula Y2CaB10O19 (YCB). YCB, though a structural derivative of La2CaB10O19 (LCB), has several discrete and advantageous properties over it. YCB possess congruent melting point which facilitates its growth from melt, unlike the growth of LCB that is carried out by flux technique which retards its growth rate and poses the problem of fluxinclusions. Here we report the synthesis of the polycrystalline starting materials, single crystal growth from melt and the characterization performed on the grown crystals systematically. Experimental procedure Synthesis and crystal growth The polycrystalline YCB material was prepared by the solid state reaction method. The synthesis was carried out in two steps. The following chemical reaction shown in Eq. (1) was adopted.
Y2 O3 þ CaCO3 þ 5B2 O3 ! Y2 CaB10 O19 þ CO2 "
ð1Þ
The starting materials of Y2O3, CaCO3, and B2O3 with 5N purity were used for synthesizing the material. The starting materials were mixed in appropriate quantities and the mixture was heated at 450 °C for 12 h in a platinum crucible. The obtained product was again well-mixed by grinding in an agate mortar, and heated to 950 °C for 24 h. The resultant product was the polycrystalline YCB
material. Since there are no reports available on YCB, thermal analysis was carried out to identify its melting point. The recorded DTA pattern is presented in Fig. 1. From the DTA pattern, the following characteristics of the YCB material were understood – the material does not decompose before melting (possess congruently melting behavior) and the endothermic peak at 967 °C was attributed to the melting of the material. To perform single crystal growth of YCB, the polycrystalline material was taken in a platinum crucible and was heated to 1025 °C. The melt was maintained at high temperature for several hours for homogenization, since borate melts are highly viscous. The temperature of the growth zone was slowly reduced at an optimized cooling rate. Several transparent YCB single crystals with the dimensions of 10 10 5 mm3 were obtained from the melt. The photograph of one of the as-grown single crystals is shown in Fig. 2.
Results and discussion X-ray diffraction analyses To determine the crystal structure, single crystal XRD data of the YCB crystal was collected using the Enraf Nonius X-ray diffractometer. The YCB crystal belongs to the monoclinic crystal system with C2 space group. The lattice parameters of the crystal are a = 11.456 Å, b = 6.340 Å, c = 9.217 Å and a = c = 90° and b = 91.68°. From the grown YCB single crystals, few tiny crystals were crushed and ground to powder. The powder was subjected to X-ray diffraction study using the Rich Seifert diffractometer with Cu Ka (k = 1.540 Å) radiation source at a scan rate of 0.5 °/min. The obtained powder XRD pattern of the YCB crystal is shown in the Fig. 3. The powder X-ray diffraction pattern was indexed and the lattice parameters were verified using Shell X diffraction software. The crystalline perfection of YCB was analyzed by high resolution X-ray diffraction studies. Multi-crystal X-ray diffractometer was used for the present investigation. The MoKa1 X-ray beam was used as the exploring X-ray beam. The diffraction curve (DC) was recorded in the x scan mode. Before recording the diffraction curve, in order to remove few layers from the surface of the crystal and also to ensure surface planarity, the YCB specimen was lapped and chemically etched. The diffraction curve recorded on the (0 0 3) plane of the YCB crystal is presented in Fig. 4. The full width at half maximum (FWHM) of the DC was very lesser (46 arc sec), which indicated that the grown crystal has good crystalline perfection.
1.4 1.2
DTA signal (µV)
967 1.0 0.8 0.6 0.4 0.2 0.0 700
800
900
1000
1100
1200
Temperature (°C) Fig. 1. DTA pattern recorded for polycrystalline YCB material.
Fig. 2. Photograph of as-grown YCB crystal.
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100 90 80
Transmission (%)
70 60 50 40 30 20 10 0 200
400
600
800
1000
wavelength (nm)
Fig. 3. Powder XRD pattern of YCB.
Fig. 5. UV–VIS–NIR spectrum of YCB.
100
Transmission (%)
80
60
40
711 20
0 Fig. 4. HRXRD curve obtained for YCB crystal.
1200
There is a lesser degree of asymmetry observed in the curve. This may be due to the presence of low-angle grain boundaries in the grown crystal. In general, crystals grown from melt possess higher values of FWHM, owing to the thermal effects occurring on the material during growth. But the FWHM observed for YCB crystal was found to be lesser indicating superior crystalline quality. Optical studies UV–VIS–NIR studies YCB single crystal was subjected to UV–VIS–NIR study to find its suitability in NLO and laser applications. For this purpose, thin plate-like sample with 2 mm thickness was prepared and used for optical transmission studies. The recorded spectrum is shown in Fig. 5. The lower cutoff wavelength of the YCB crystal is below 200 nm (deep UV region), and it is highly transparent (T > 60%) over a wide wavelength range. Moreover, there are no absorptions in the visible region. From the spectrum, it is inferred that the YCB crystal is a potential candidate for UV laser applications. FTIR analysis The YCB crystal was subjected to the Fourier transform infrared (FTIR) spectroscopic analysis by adopting the KBr pellet method. The FTIR spectrum recorded in the wavenumber region ranging from 1200 to 450 cm1, is shown in Fig. 6. The corresponding bond assignments were made. The absorptions at 920 and 711 cm1 are attributed to the symmetric stretching of B–O bonds [13]. The
1026 1000
920 871 800
600 -1
wavenumber (cm ) Fig. 6. FTIR spectrum recorded for YCB crystal.
absorption at 1026 cm1 is due to the Ca–O bond vibration and 871 cm1 is due to the Y–O bond vibrations. Nonlinear optical study The YCB crystal possesses the monoclinic crystal structure with the non-centrosymmetric C2 space group. In order to analyze the NLO property, the grown YCB crystal was subjected to second harmonic generation (SHG) studies by Kurtz powder technique [14]. The sample was ground into very fine powder and tightly packed in a micro-capillary tube. Then it was mounted in the path of a Nd:YAG laser beam of energy 3.60 mJ/pulse. When KH2PO4 (KDP) crystal was used as a reference material, it produced 155 mV as output beam voltage. But the output beam voltage was 314 mV for the grown YCB sample. Hence it is confirmed that the YCB crystal has NLO efficiency about twice that of the KDP crystal. Laser damage threshold measurements YCB crystal can be employed for NLO applications till the UV region, as it exhibits cutoff wavelength below 200 nm (Fig. 5). In general, UV lasers possess higher energies due to lower wavelengths. In order to determine the tolerable limit of the YCB crystal when exposed to high power lasers, the LDT measurements were carried
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R. Arun Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 391–394
out. The YCB crystal was exposed to a Q-switched Nd:YAG laser for 20 ns laser pulses at the wavelength of 1064 nm. The pulse repetition rate was 10 Hz. The laser beam divergence is 2 m rad. The output intensity of the laser was controlled with a variable attenuator and delivered to the test sample (YCB crystal) located at the focus of the converging lens. The lens with a focal length of 20.5 cm was used, which determines the spot size on the sample. Laser damage threshold value of the YCB crystal was calculated to be 10.5 GW/ cm2. The result indicates that the grown YCB single crystals have superior LDT value when compared with a few other borate crystals such as LiB3O5 (LBO, 0.9 GW/cm2), b-BaB2O4 (b-BBO, 5 GW/ cm2), LaCa4O(BO3)3 (LCOB, 2.206 GW/cm2), GdCa4O(BO3)3 (GdCOB, 1.0 GW/cm2), and Nd0.275La0.725Ca4O(BO3)3 (NdLCOB, 2.209 GW/ cm2), and hence can be employed in high power laser applications [2,15]. Conclusions In conclusion, polycrystalline YCB materials were prepared adopting a two-step process by the solid-state reaction method. The synthesized materials were subjected to powder X-ray diffraction studies. The material melts congruently at 967 °C. Single crystal growth of YCB was carried out from its melt after optimizing the thermal cycle. The crystal structure of YCB was obtained from single crystal XRD and powder XRD analyses. The non-centrosymmetric crystal structure of the crystal was identified from the single crystal XRD data. The quality of the grown crystal was ascertained using HRXRD analysis. The degree of crystallinity was found to be good. The crystal had UV cutoff lesser than 200 nm and is highly transparent over a wide wavelength range. All the functional groups of the YCB crystal were identified and assigned by the FTIR analysis. The powder SHG test was carried out on the YCB crystal and it was found that the grown YCB crystal had SHG efficiency twice that of KDP. The laser induced damage tolerance of the crystal was also very high of the order of 10.5 GW/cm2. Since the grown
YCB crystal possess high crystalline quality, transparent in the visible and UV region (till 200 nm), and capable of frequency doubling, non-hygroscopic and exhibits high LDT values, it acts as a versatile material for device applications. Acknowledgements One of the authors (R.A.) thanks the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India, for awarding major research project to work on borate-based crystals. Authors are grateful to the Central Instrumentation Facility Centre, Shizuoka University, Hamamatsu, for extending characterization facilities. References [1] P.A. Franken, A.E. Hill, C.W. Peters, G. Weinreich, Phys. Rev. Lett. 7 (1961) 118– 119. [2] Petra Becker, Adv. Mater. 10 (1998) 979–992. [3] S.C. Sabharwal, Babita Tiwari, Sangeeta, J. Cryst. Growth 263 (2004) 327–331. [4] Xuesong Yu, Zhang-Gui Hu, J. Cryst. Growth 318 (2011) 1171–1174. [5] Baichang Wu, Dingyuen Tang, Ning Ye, Chuangtian Chen, Opt. Mater. 5 (1996) 105–109. [6] Takatomo Sasaki, Yusuke Mori, Masashi Yoshimura, Yoke Khin Yap, Tomosumi Kamimura, Mater. Sci Eng. R 30 (2000) 1–54. [7] Sangen Zhao, Guochun Zhang, Jiyong Yao, Yicheng Wu, Mater. Res. Bull. 47 (2012) 3810–3813. [8] Fangli Jing, Peizhen Fu, Yicheng Wu, Yanlei Zu, Xian Wang, Opt. Mater. 30 (2008) 1867–1872. [9] Pu Wang, Judith M. Dawes, Phillip Burns, James A. Piper, Huaijin Zhang, Li Zhu, et al., Opt. Mater. 19 (2002) 383–387. [10] M.G. Brik, A. Majchrowski, I.V. Kityk, T. Łukasiewicz, M. Piasecki, J. Alloys Compd. 465 (2008) 24–34. [11] I. Schütz, I. Freitag, R. Wallenstein, Opt. Commun. 77 (1990) 221–225. [12] Rui Guo, Yicheng Wu, Peizhen Fu, Fangli Jing, Opt. Commun. 244 (2005) 321– 325. [13] V. Krishnakumar, R. Nagalakshmi, Spectrochim. Acta A 60 (2004) 2733–2739. [14] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798–3814. [15] R. Arun Kumar, R. Dhanasekaran, J. Cryst. Growth 318 (2011) 636–641.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Growth and characterization of a novel nonlinear optical borate crystal – Yttrium calcium borate (YCB) R. Arun Kumar a,b,⇑, M. Arivanandhan a, R. Dhanasekaran c, Y. Hayakawa a a
Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan Department of Basic Sciences (Physics), PSG College of Technology, Coimbatore, India c Crystal Growth Centre, Anna University, Chennai, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
New nonlinear optical borate-based
crystal – Yttrium calcium borate is grown and characterized. Good quality of the grown crystals is evidenced by HRXRD measurements. SHG efficiency of YCB crystal is twice that of KDP, and possesses high laser damage threshold value (10.5 GW/cm2). The crystal is also non-hygroscopic, favoring real-time device applications.
a r t i c l e
i n f o
Article history: Received 23 January 2013 Received in revised form 15 March 2013 Accepted 16 March 2013 Available online 25 March 2013 Keywords: Borates Crystal growth Characterization Lasers Nonlinear optics
a b s t r a c t A new nonlinear optical single crystal yttrium calcium borate Y2CaB10O19 (YCB) was grown for the first time from its melt. The starting materials were prepared by the solid-state reaction method. The melting point of the synthesized material was identified to be 967 °C. YCB crystal exhibits monoclinic crystal structure with the space group C2. The crystalline perfection of the grown YCB crystal was found to be good. From the UV–VIS–NIR studies, the lower cutoff wavelength of the crystal occurs below 200 nm. The functional groups of the grown crystal were assigned using the FTIR data. The second harmonic generation (SHG) of the YCB crystal was observed using a Nd:YAG laser with a fundamental wavelength of 1064 nm. The laser damage threshold value of the YCB crystal was found to be very high – 10.5 GW/cm2. Ó 2013 Elsevier B.V. All rights reserved.
Introduction The development of laser by Maiman in the year 1960 led to technological advancements in various fields. The idea of non-linear optics could not be possible without lasers. The observation of the non-linear optical phenomena was first demonstrated by Franken and his co-workers in 1961 [1]. Since then, research on devel⇑ Corresponding author at: Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan. Tel.: +81 53 478 1310. E-mail address: [email protected] (R. Arun Kumar). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.03.069
oping crystals for employing in non-linear optical (NLO) devices gained momentum. There are various nonlinear optical crystals reported till date [2]. The criteria for selecting a nonlinear optical crystal for device applications include good crystalline perfection, high non-linear optical coefficient, high transparency in the wavelength of interest, moderate birefringence, low walk-off effect, good mechanical, chemical and thermal stability. In addition, crystals with high laser-induced damage threshold (LDT) are highly desired for real-time laser applications. Keeping in view of the desirable parameters, several NLO crystals belonging to the organic, inorganic and semi-organic families are grown by many
392
R. Arun Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 391–394
research groups. Among the inorganic crystals KTP and LBO are employed extensively for the generation of green lasers by frequency doubling of Nd:YAG lasers. In the inorganic family of crystals, borate-based crystals have attracted attention owing to their intrinsic favorable properties. Few nonlinear optical borate-based crystals are LiB3O5 (LBO) [3], CsLiB6O10 (CLBO) [4], KBe2BO3F2 (KBBF) [5], K2Al2B2O7 (KAB) [6], K3YB6O12 (KYB) [7], La2CaB10O19 (LCB) [8], and so on. In addition to their suitability in nonlinear optical applications, a few borate crystals are also attractive candidates in self-frequency doubling (SFD) processes, due to their molecular structure which favors laser active ion doping. Examples of such crystals are Er:Yb:YCOB [9], Pr:GdCOB [10], Nd:YAB [11], and Yb:LCB [12]. Though there are several borate crystals available for laser applications, the present challenge is in developing a crystal with both high NLO efficiency and laser damage threshold. Moreover the crystal must be non-hygroscopic to be suitable for real-time applications. To prepare such a borate-based crystal, we have synthesized and grown a new NLO crystal yttrium calcium borate with the chemical formula Y2CaB10O19 (YCB). YCB, though a structural derivative of La2CaB10O19 (LCB), has several discrete and advantageous properties over it. YCB possess congruent melting point which facilitates its growth from melt, unlike the growth of LCB that is carried out by flux technique which retards its growth rate and poses the problem of fluxinclusions. Here we report the synthesis of the polycrystalline starting materials, single crystal growth from melt and the characterization performed on the grown crystals systematically. Experimental procedure Synthesis and crystal growth The polycrystalline YCB material was prepared by the solid state reaction method. The synthesis was carried out in two steps. The following chemical reaction shown in Eq. (1) was adopted.
Y2 O3 þ CaCO3 þ 5B2 O3 ! Y2 CaB10 O19 þ CO2 "
ð1Þ
The starting materials of Y2O3, CaCO3, and B2O3 with 5N purity were used for synthesizing the material. The starting materials were mixed in appropriate quantities and the mixture was heated at 450 °C for 12 h in a platinum crucible. The obtained product was again well-mixed by grinding in an agate mortar, and heated to 950 °C for 24 h. The resultant product was the polycrystalline YCB
material. Since there are no reports available on YCB, thermal analysis was carried out to identify its melting point. The recorded DTA pattern is presented in Fig. 1. From the DTA pattern, the following characteristics of the YCB material were understood – the material does not decompose before melting (possess congruently melting behavior) and the endothermic peak at 967 °C was attributed to the melting of the material. To perform single crystal growth of YCB, the polycrystalline material was taken in a platinum crucible and was heated to 1025 °C. The melt was maintained at high temperature for several hours for homogenization, since borate melts are highly viscous. The temperature of the growth zone was slowly reduced at an optimized cooling rate. Several transparent YCB single crystals with the dimensions of 10 10 5 mm3 were obtained from the melt. The photograph of one of the as-grown single crystals is shown in Fig. 2.
Results and discussion X-ray diffraction analyses To determine the crystal structure, single crystal XRD data of the YCB crystal was collected using the Enraf Nonius X-ray diffractometer. The YCB crystal belongs to the monoclinic crystal system with C2 space group. The lattice parameters of the crystal are a = 11.456 Å, b = 6.340 Å, c = 9.217 Å and a = c = 90° and b = 91.68°. From the grown YCB single crystals, few tiny crystals were crushed and ground to powder. The powder was subjected to X-ray diffraction study using the Rich Seifert diffractometer with Cu Ka (k = 1.540 Å) radiation source at a scan rate of 0.5 °/min. The obtained powder XRD pattern of the YCB crystal is shown in the Fig. 3. The powder X-ray diffraction pattern was indexed and the lattice parameters were verified using Shell X diffraction software. The crystalline perfection of YCB was analyzed by high resolution X-ray diffraction studies. Multi-crystal X-ray diffractometer was used for the present investigation. The MoKa1 X-ray beam was used as the exploring X-ray beam. The diffraction curve (DC) was recorded in the x scan mode. Before recording the diffraction curve, in order to remove few layers from the surface of the crystal and also to ensure surface planarity, the YCB specimen was lapped and chemically etched. The diffraction curve recorded on the (0 0 3) plane of the YCB crystal is presented in Fig. 4. The full width at half maximum (FWHM) of the DC was very lesser (46 arc sec), which indicated that the grown crystal has good crystalline perfection.
1.4 1.2
DTA signal (µV)
967 1.0 0.8 0.6 0.4 0.2 0.0 700
800
900
1000
1100
1200
Temperature (°C) Fig. 1. DTA pattern recorded for polycrystalline YCB material.
Fig. 2. Photograph of as-grown YCB crystal.
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100 90 80
Transmission (%)
70 60 50 40 30 20 10 0 200
400
600
800
1000
wavelength (nm)
Fig. 3. Powder XRD pattern of YCB.
Fig. 5. UV–VIS–NIR spectrum of YCB.
100
Transmission (%)
80
60
40
711 20
0 Fig. 4. HRXRD curve obtained for YCB crystal.
1200
There is a lesser degree of asymmetry observed in the curve. This may be due to the presence of low-angle grain boundaries in the grown crystal. In general, crystals grown from melt possess higher values of FWHM, owing to the thermal effects occurring on the material during growth. But the FWHM observed for YCB crystal was found to be lesser indicating superior crystalline quality. Optical studies UV–VIS–NIR studies YCB single crystal was subjected to UV–VIS–NIR study to find its suitability in NLO and laser applications. For this purpose, thin plate-like sample with 2 mm thickness was prepared and used for optical transmission studies. The recorded spectrum is shown in Fig. 5. The lower cutoff wavelength of the YCB crystal is below 200 nm (deep UV region), and it is highly transparent (T > 60%) over a wide wavelength range. Moreover, there are no absorptions in the visible region. From the spectrum, it is inferred that the YCB crystal is a potential candidate for UV laser applications. FTIR analysis The YCB crystal was subjected to the Fourier transform infrared (FTIR) spectroscopic analysis by adopting the KBr pellet method. The FTIR spectrum recorded in the wavenumber region ranging from 1200 to 450 cm1, is shown in Fig. 6. The corresponding bond assignments were made. The absorptions at 920 and 711 cm1 are attributed to the symmetric stretching of B–O bonds [13]. The
1026 1000
920 871 800
600 -1
wavenumber (cm ) Fig. 6. FTIR spectrum recorded for YCB crystal.
absorption at 1026 cm1 is due to the Ca–O bond vibration and 871 cm1 is due to the Y–O bond vibrations. Nonlinear optical study The YCB crystal possesses the monoclinic crystal structure with the non-centrosymmetric C2 space group. In order to analyze the NLO property, the grown YCB crystal was subjected to second harmonic generation (SHG) studies by Kurtz powder technique [14]. The sample was ground into very fine powder and tightly packed in a micro-capillary tube. Then it was mounted in the path of a Nd:YAG laser beam of energy 3.60 mJ/pulse. When KH2PO4 (KDP) crystal was used as a reference material, it produced 155 mV as output beam voltage. But the output beam voltage was 314 mV for the grown YCB sample. Hence it is confirmed that the YCB crystal has NLO efficiency about twice that of the KDP crystal. Laser damage threshold measurements YCB crystal can be employed for NLO applications till the UV region, as it exhibits cutoff wavelength below 200 nm (Fig. 5). In general, UV lasers possess higher energies due to lower wavelengths. In order to determine the tolerable limit of the YCB crystal when exposed to high power lasers, the LDT measurements were carried
394
R. Arun Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 391–394
out. The YCB crystal was exposed to a Q-switched Nd:YAG laser for 20 ns laser pulses at the wavelength of 1064 nm. The pulse repetition rate was 10 Hz. The laser beam divergence is 2 m rad. The output intensity of the laser was controlled with a variable attenuator and delivered to the test sample (YCB crystal) located at the focus of the converging lens. The lens with a focal length of 20.5 cm was used, which determines the spot size on the sample. Laser damage threshold value of the YCB crystal was calculated to be 10.5 GW/ cm2. The result indicates that the grown YCB single crystals have superior LDT value when compared with a few other borate crystals such as LiB3O5 (LBO, 0.9 GW/cm2), b-BaB2O4 (b-BBO, 5 GW/ cm2), LaCa4O(BO3)3 (LCOB, 2.206 GW/cm2), GdCa4O(BO3)3 (GdCOB, 1.0 GW/cm2), and Nd0.275La0.725Ca4O(BO3)3 (NdLCOB, 2.209 GW/ cm2), and hence can be employed in high power laser applications [2,15]. Conclusions In conclusion, polycrystalline YCB materials were prepared adopting a two-step process by the solid-state reaction method. The synthesized materials were subjected to powder X-ray diffraction studies. The material melts congruently at 967 °C. Single crystal growth of YCB was carried out from its melt after optimizing the thermal cycle. The crystal structure of YCB was obtained from single crystal XRD and powder XRD analyses. The non-centrosymmetric crystal structure of the crystal was identified from the single crystal XRD data. The quality of the grown crystal was ascertained using HRXRD analysis. The degree of crystallinity was found to be good. The crystal had UV cutoff lesser than 200 nm and is highly transparent over a wide wavelength range. All the functional groups of the YCB crystal were identified and assigned by the FTIR analysis. The powder SHG test was carried out on the YCB crystal and it was found that the grown YCB crystal had SHG efficiency twice that of KDP. The laser induced damage tolerance of the crystal was also very high of the order of 10.5 GW/cm2. Since the grown
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