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Effect of Li and NH4 doping on the crystal perfection, second harmonic generation efficiency and laser damage threshold of potassium pentaborate crystals
Optics and Laser Technology 100 (2018) 153–156
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Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Effect of Li and NH4 doping on the crystal perfection, second harmonic generation efficiency and laser damage threshold of potassium pentaborate crystals A.N. Vigneshwaran a, S. Kalainathan b, C. Ramachandra Raja a,⇑ a b
Department of Physics, Government Arts College (Autonomous), Kumbakonam 612 002, India Center for Crystal Growth, School of Advanced Sciences, VIT University, Vellore 632 014, India
a r t i c l e
i n f o
Article history: Received 15 June 2017 Received in revised form 15 September 2017 Accepted 7 October 2017
Keywords: Dopants Borates High resolution X-ray diffraction Nonlinear optical materials
a b s t r a c t Potassium pentaborate (KB5) is an excellent nonlinear optical material especially in the UV region. In this work, Li and NH4 doped KB5 crystals were grown using slow evaporation solution growth method. The incorporation of dopant has been confirmed and analysed by Energy dispersive X-ray analysis (EDAX), Inductively coupled plasma (ICP) analysis and Raman spectroscopy. The crystalline perfection of pure and doped KB5 crystals was studied by High resolution X-ray diffraction (HRXRD) analysis. Structural grain boundaries were observed in doped crystals. Second harmonic generation was confirmed for pure and doped crystals and output values revealed the enhancement of SHG efficiency in doped crystals. Resistance against laser damage was carried out using 1064 nm Nd-YAG laser of pulse width 10 ns. The laser damage threshold value is increased in Li doped crystal and decreased in NH4 doped crystal when compared to pure KB5 crystal. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction In the last few decades enormous research work is going on the growth of single crystals for various applications. Various technologies require high quality single crystals. In device applications, miniaturization of modern devices depends on the high crystalline perfection [1–3]. Crystal perfection is one of the factor that decides the applicability of a crystal [4–6]. Many applications need crystals which are nearly close to the ideal crystal which is free from all defects. But no crystal in practice is ideal, there are some defects in all real crystals. Efforts are always going on to improve the crystalline quality of the single crystals [1–7]. Since the invention of first laser, its impact on various sectors like industrial, medicine and defense is growing enormously. For instance laser at Mid Infra red region have applications in many military and civil activities like remote sensing and biological tissues visualization [8,9]. Deep UV lasers are of great interest among others due to their variety of applications in various areas such as semiconductor manufacturing, photolithography, attosecond pulse generation and advanced instrument development [10–12]. Recently Liang et al. theoretically designed some possible deep UV nonlinear optical (NLO) crystals in fluorooxoborates combining ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (C.R. Raja). https://doi.org/10.1016/j.optlastec.2017.10.004 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
rational tailoring design and first principle calculations [13]. Shi et al. developed beryllium-free phase NH4B4O6F, a deep UV NLO crystal, which exhibits NLO properties superior to KBBF [14]. NLO crystals are capable to extend the frequency ranges of lasers through the frequency conversion process [15–22]. For an efficient NLO crystal, it could possess some properties like wide transparency, high resistance against laser damage, large NLO coefficients and fast response. Chen et al. have developed the anionic group theory to investigate the NLO effects in borate crystals [18–20]. According to this theory, the large difference in the electronegativities of the B and O atoms on the B-O bond will favour transmission of short wavelength UV radiation. They have developed several new borate based NLO crystals for use in the UV region. There are several structural units (anionic groups) available in borate series like the trigonal (BO3)3, the tetragonal (BO4)5 and the planar six-membered-ring (B3O6)3 anionic groups. In the case of Potassium pentaborate (KB5), the anionic group comprised of two nonplanar six-membered ring quintanions (B3O7)5. These rings are joined by sharing the tetrahedrally coordinated B atom, with the two six-membered ring planes perpendicular to each other [19]. KB5 is one of the few material which having operating frequency at deep UV region [23,24]. Lei Kang et al. reported that NH+4 cation is the suitable candidate to replace the K+ cation due to the similarity in ionic radius and valence electronic properties [25]. Based on this, in the present work K+ ions are partially
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replaced by Li+ and NH+4 ions in KB5 crystal. Transmission property and optical band gap of the pure, Li and NH4 doped KB5 crystals were reported by the authors [26]. In this work, effect of inclusion of Li and NH4 in crystal perfection, second harmonic generation efficiency and laser damage threshold of KB5 crystal was analyzed. 2. Experimental KB5 was synthesized by reacting potassium carbonate and boric acid in 1:10 M ratio. Saturated solution was housed in an unperturbed environment at 34 °C with the help of constant temperature bath (CTB) with accuracy of ±0.01 °C. After the supersaturation, good quality and transparent crystals were obtained in a period of 50 days. 5 mol% of Li2CO3, 95 mol% of K2CO3 and H3BO3 were dissolved in double distilled water and stirred for 8 h to get Li doped crystals. The saturated solution was kept at 34 °C in CTB for slow evaporation. The transparent crystals were harvested in a period of 50 days. The same approach was followed to grow NH4 doped KB5 crystals. Grown crystals are shown in Fig. 1. 3. Results and discussion 3.1. Elemental analysis EDAX analysis was carried out to confirm the presence of N in NH4 doped crystal. Results confirmed that 4.26% of nitrogen atoms entered into the crystal lattice of KB5. ICP analysis was employed to quantify the presence of Li ions. The results confirmed that 3.22% of Li ions are present in the doped crystal. 3.2. Raman analysis FT- Raman spectral analysis was done using BRUKER RFS 27 FTRaman spectrometer with Nd:YAG laser (1064 nm). The recorded FT- Raman spectrum of NH4 doped and pure KB5 are shown in Fig. 2(a) and (b) respectively. In NH4 doped crystals N-H vibrations are observed [27,28]. NH4 deformation peak was observed at 1447 cm1. The Peak observed at 1632 cm1 corresponds to NH4 asymmetric bending. NH4 oscillation was observed at 1762 cm1. From the spectra it is evident that the high intensity peak between the frequency range of 3060–3370 cm1 is broad and sharp in NH4 doped crystal. Vibrations of N-H stretching in NH4 ion and O-H symmetric stretching in water molecule occurring in the same frequency range could be the reason for this Raman intensity in NH4 doped crystal. There is no much variation in the Raman spectra of pure and Li doped KB5 crystals.
Fig. 2. Raman spectrum of (a) NH4 doped KB5, (b) pure KB5.
perfection of the grown crystals was analysed using PANalytical X-Pert PRO MRD high-resolution XRD system with Cu Ka1 radiation. 4-bounce Ge (220) monochromator was used to obtain the highly monochromatic X-ray beam. The diffracted X-ray beam was collected in the scintillation detector. To get all diffracted beam due to very low grain boundaries and dislocations, analyzer was not employed between specimen and detector. Fig. 3 shows the diffraction curve (DC) of (1 1 1) diffracting plane for KB5 crystal. The solid line (convoluted curve) is well fitted with the experimental points represented by the circles. As evident form HRXRD curve, the full width at half maximum (FWHM) of the peak is 56 arc sec and the peak is reasonably sharp. Though the value of FWHM value is higher than that expected from the plane wave theory of dynamical X-ray diffraction for an ideally perfect crystal but close to the value that expected for real crystals. Additional peaks are absent in DC. This shows that the crystal doesn’t contain any internal structural low angle boundaries and the relatively low value of FWHM shows that the crystal is of good quality. The HRXRD curve recorded for (1 1 1) diffraction plane of NH4 doped KB5 is shown in Fig. 4. On deconvolution of the DC, it is clear that the curve contains two additional peaks, one of them 37 arc sec away from the main peak (at zero glancing angle) on the higher angle side and the other one 23 arc sec away from the main peak on the lower angle side. Since tilt angles of the additional peaks
3.3. High resolution X-ray diffraction analysis HRXRD technique is a non-destructive analysis and can be used for direct observation of boundaries and dislocations. Crystalline
Fig. 1. Photograph of grown crystals.
Fig. 3. HRXRD curve of pure KB5 crystal.
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case of pure and NH4 doped crystal. The relatively high values of FWHM of the grains and the angular spread of the deconvoluted curve (3100 arc sec) indicate that the crystalline perfection is not up to the mark. The crystal contains some mosaic blocks which are formed due to the release of heavy stress in the crystal caused by Li doping. The low ionic radius of Li ion (0.9 Å) which occupies the K ion site causes stress in the crystal lattice [29,30]. 3.4. Second harmonic generation analysis
Fig. 4. HRXRD curve of NH4 doped KB5 crystal.
are less than 10 , these are corresponding to two internal structural very low angle boundaries. The FWHM of the main peak and very low angle boundaries are 22, 21 and 14 arc sec. These low values reveal that the grains of the crystal are nearly perfect as one can expect such low values only for crystals with reasonable quality. Although the specimen contains very low angle grain boundaries, the low FWHM values and the relatively low angular spread of DC of around 62 arc sec indicate that the crystalline perfection is reasonably good. The low angular spread of DC (62 arc sec) is in comparison with the FWHM of pure KB5 (56 arc sec). This is expected because the ionic radius of K (1.49 Å) and the NH4 (1.48 Å) ion are comparable and may induce minimum stress [29,30]. Thermal fluctuation, mechanical disturbances or segregation of the solvent molecules during the growth process are the reasons reported for the observed very low angle boundaries [4]. It may be mentioned here that such low angle boundaries could be detected in the DC only because of the four bounce monochromators used in the present study. Such defects may not have much influence on the physical properties. The HRXRD curve recorded for (1 1 1) diffraction plane of Li doped KB5 crystal is shown in Fig. 5. The solid line (convoluted curve) is well fitted with the experimental points represented by the circles. On deconvolution of the DC, it is clear that the curve contain three additional peaks at 500, 1800 and 3050 arc sec away from the main peak on the higher angle side. These three additional peaks correspond to three internal structural low angle boundaries (tilt angle >10 but less than a degree). The tilt angles of these grain boundaries are 500, 1300 and 1250 arc sec from their adjoining regions. The FWHM of main peak and low angle boundaries are 222.8, 291.7, 387.5 and 471.6 arc sec. The tilt angles and FWHM of grain boundaries are larger than that are observed in the
SHG of the grown crystal was confirmed by Kurtz-Perry powder technique. Grown crystals are uniformly powdered and tightly packed in the capillary tube. The sample was placed on the path of Nd-YAG laser of 1064 nm of having input pulse energy 1.6 mJ/ pulse. Green light emitted from the sample confirmed the SHG in the grown crystals. The emitted beam was fed into the photomultiplier tube which converts the output beam into the corresponding electric pulses. KDP was chosen as the standard material for the comparison of SHG efficiency. The output values of pure, doped KB5 crystals and KDP are given in Table 1. Results of SHG output are in good agreement with anionic group theory proposed by Chen et al. which states that the overall SHG of a crystal is contributed only by the anionic groups in the crystal [19]. So it is expected that doping of cationic groups doesn’t significantly affect the overall SHG of the pure KB5 crystal. 3.5. Laser damage threshold study Since NLO process involves high optical intensities, NLO material should have high resistance against the laser involved. Resistance against irradiated laser is one of the key factor for device applications. Because NLO crystal having low laser damage threshold (LDT) value could limit the device applications of that crystal even though it possess excellent optical transmission properties. For the long-pulse regime s > 100 ps, the damage process occur mainly by the rate of thermal conduction through the atomic lattice and for the short-pulse regime s 10 ps, the optical breakdown is a nonthermal process and various nonlinear ionization mechanisms (multiphoton, avalanche multiplication, and tunnelling) become important [31]. To determine LDT, Pure, Li and NH4 doped crystals with well flat surface having thickness 2 mm were mounted on the crystal holder on the path of 1064 nm NdYAG laser. The pulse width of irradiating laser is 10 ns. For this measurement, a 1 mm diameter beam was focused onto the sample along a 35 cm focal length lens. LDT was calculated by the relation
E ¼ P=s0 A where P is the laser energy that cause damage, s0 is pulse width (10 ns) and A area of spot size. The calculated values of LDT are given in Table 2. The observed values indicate doping influences the LDT values irrespective of crystalline perfection [32]. In the present work, 10 ns pulsed laser has been used and therefore thermal effects are prominent. As LDT of a crystal for long pulses depends mainly on the thermal conductivity of material, Li doped crystal posses high LDT because of the higher thermal conductivity of Li (84.8 W/m K) compared to K (57.8 W/m K).
Table 1 SHG output of pure and doped crystals.
Fig. 5. HRXRD curve of Li doped KB5 crystal.
Crystal
Output (mV)
KDP KB5 Li doped KB5 NH4 doped KB5
150 135 141 138
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Table 2 LDT values of pure and doped crystals. Crystal
LDT value (GW/cm2)
KB5 Li doped KB5 NH4 doped KB5
8.194 8.592 7.938
4. Conclusion Pure, Li and NH4 doped KB5 crystals were grown by slow evaporation solution growth method. EDAX and Raman spectral study confirms the incorporation of NH4 in KB5 crystal. ICP analysis confirms the presence of Li in KB5 crystal. HRXRD analysis of pure KB5 indicates that the crystalline perfection is good and the crystals are without any internal structural grain boundaries. HRXRD analysis of NH4 doped and Li doped KB5 crystal reveals that the crystal contains very low and low angle boundaries, respectively. Second harmonic generation in pure and doped crystals is confirmed using Kurtz-Perry powder technique. The laser damage threshold value for pure, Li and NH4 doped crystal is calculated as 8.194 GW/cm2, 8.592 GW/cm2 and 7.938 GW/cm2 respectively. Acknowledgement Author C.R acknowledges Council of Scientific and Industrial Research (CSIR), New Delhi for financial support (scheme number: 03(1301)/13/EMR II) and gratefully acknowledge the authorities of SAIF, IIT, Chennai for ICP and Raman analysis, National Physical Laboratory, New Delhi for HRXRD analysis, Prof. P.K. Das, IISc, Bangalore for providing SHG testing facility, and National College, Trichy for EDAX analysis. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.optlastec.2017.10. 004. References [1] Anuj Krishna, N. Vijayan, Chandan Bagdia, Kanika Thukral, Sonia, D. Haranath, K.K. Maurya, G. Bhagavannarayana, Effect of ampoule support on the growth of organic benzimidazole single crystals by vertical Bridgman technique for nonlinear optical applications, Cryst. Eng. Comm. 18 (2016) 4844–4850. [2] G. Bhagavannarayana, R.V. Ananthamurthy, G.C. Budakoti, B. Kumar, K.S. Bartwal, A study of the effect of annealing on Fe-doped LiNbO3 by HRXRD, XRT and FT–IR, J. Appl. Cryst. 38 (2005) 768–771. [3] Krishnamurthy Senthil Kumar, Sridharan Moorthy Babu, G. Bhagavannarayana, Study of the influence of dopants on the crystalline perfection of ferroelectric glycine phosphite single crystals using high-resolution X-ray diffraction analysis, J. Appl. Cryst. 44 (2011) 313–318. [4] G. Bhagavannarayana, S.K. Kushwaha, Enhancement of SHG efficiency by urea doping in ZTS single crystals and its correlation with crystalline perfection as revealed by Kurtz powder and high-resolution X-ray diffraction methods, J. Appl. Cryst. 43 (2010) 154–162. [5] S.K. Kushwaha, Mohd. Shakir, K.K. Maurya, A.L. Shah, M.A. Wahab, G. Bhagavannarayana, Remarkable enhancement in crystalline perfection, second harmonic generation efficiency, optical transparency, and laser damage threshold in potassium dihydrogen phosphate crystals by Lthreonine doping, J. Appl. Phys. 108 (2010) 033506. [6] A. Krishna, N. Vijayan, S. Gupta, K. Thukral, V. Jayaramakrishnan, B. Singh, J. Philip, S. Das, K.K. Maurya, G. Bhagavannarayana, Key aspects of L-threoninium picrate single crystal: an excellent organic nonlinear optical material with a high laser-induced damage threshold, RSC Adv. 4 (2014) 56188–56199.
[7] Mohd. Shakir, S.K. Kushawaha, K.K. Maurya, Sumeet Kumar, M.A. Wahab, G. Bhagavannarayana, Enhancement of second harmonic generation, optical and dielectric properties in L-asparagine monohydrate single crystals due to an improvement in crystalline perfection by annealing, J. Appl. Cryst. 43 (2010) 491–497. [8] Fei Liang, Lei Kang, Zheshuai Lin, Yicheng Wu, Mid-infrared nonlinear optical materials based on metal chalcogenides: structure-property relationship, Cryst. Growth Des. 17 (2017) 2254–2289. [9] Fei Liang, Lei Kang, Zheshuai Lin, Yicheng Wu, Chuangtian Chen, Analysis and prediction of mid-IR nonlinear optical metal sulfides with diamond-like structures, Coord. Chem. Rev. 333 (2017) 57–70. [10] T.Thao Tran, Hongwei Yu, James M. Rondinelli, Kenneth R. Poeppelmeier, P. Shiv Halasyamani, Deep ultraviolet nonlinear optical materials, Chem. Mater. 28 (2016) 5238–5258. [11] Ying Wang, Shilie Pan, Recent development of metal borate halides: crystal chemistry and application in second-order NLO materials, Coord. Chem. Rev. 323 (2016) 15–35. [12] Sangen Zhao, Lei Kang, Yaoguo Shen, Xiaodong Wang, Muhammad Adnan Asghar, Zheshuai Lin, Yingying Xu, Siyuan Zeng, Maochun Hong, Junhua Luo, Designing a Beryllium-free deep-ultraviolet nonlinear optical material without a structural instability problem, J. Am. Chem. Soc. 138 (2016) 2961–2964. [13] Fei Liang, Lei Kang, Pifu Gong, Zheshuai Lin, Yicheng Wu, Rational design of deep-ultraviolet nonlinear optical materials in fluorooxoborates: towards to optimal planar configuration, Chem. Mater. 29 (2017) 7098–7102. [14] Guoqiang Shi, Ying Wang, Fangfang Zhang, Bingbing Zhang, Zhihua Yang, Xueling Hou, Shilie Pan, Kenneth R. Poeppelmeier, Finding the next deepultraviolet onlinear optical material: NH4B4O6F, J. Am. Chem. Soc. 139 (2017) 10645–10648. [15] Chuangtian Chen, Takatamo Sasaki, Rukang Li, Wu. Yincheng, Zheshuai Lin, Yusuke Mori, Zhangui Hu, Jiyang Wang, Gerard Aka, Masashi Yoshimura, Yushi Kaneda, Nonlinear Optical Borate: Crystals Principals and Applications, WileyVCH, Berlin, Germany, 2012. [16] C.T. Chen, L. Bai, Z.Z. Wang, R.K. Li, Development of new NLO crystals for UV and IR applications, J. Cryst. Growth 292 (2006) 169–178. [17] T. Sasaki, Y. Mori, M. Yoshimura, Y.K. Yap, T. Kamimura, Recent development of nonlinear optical borate crystals: key materials for generation of visible and UV light, Mater. Sci. Eng. 30 (2000) 1–54. [18] C.T. Chen, G.Z. Liu, Recent advances in nonlinear optical and electro-optical materials, Ann. Rev. Mater. Sci. 16 (1986) 203–243. [19] Chuangtian Chen, W.U. Yicheng, Rukang Li, The anionic group theory of the non-linear optical effect and its applications in the development of new highquality NLO crystals in the borate series, Int. Rev. Phys. Chem. 8 (1989) 65–91. [20] C.T. Chen, Z. Lin, Z. Wang, The development of new borate-based UV nonlinear optical crystals, Appl. Phys. B 80 (2005) 1–25. [21] C.Ramachandra Raja, R. Gobinathan, F.D. Gnanam, Dielectric properties of beta barium borate and potassium pentaborate single crystals, Cryst. Res. Tech. 28 (1993) 737–743. [22] D. Xue, K. Betzler, H. Hesse, D. Lammers, Nonlinear optical properties of borate crystals, Solid State Commun. 114 (2000) 21–25. [23] C.F. Dewey Jr., W.R. Cook Jr., R.T. Hodgson, J.J. Wynne, Frequency doubling in KB5O84H2O and NH4B5O84H2O to 217.3 nm, Appl. Phys. Lett. 26 (1975) 714– 716. [24] Y. Mori, Y.K. Yap, T. Kamimura, M. Yoshimura, T. Sasaki, Recent development of nonlinear optical borate crystals for UV generation, Opt. Mater. 19 (2002) 1–5. [25] Lei Kang, Siyang Luo, Guang Peng, Ning Ye, Yicheng Wu, Chuangtian Chen, Zheshuai Lin, First-principles design of a deep-ultraviolet nonlinear-optical crystal from KBe2BO3F2 to NH4Be2BO3F2, Inorg. Chem. 54 (2015) 10533– 10535. [26] A.N. Vigneshwaran, P. Paramasivam, C. Ramachandra Raja, A study on the linear and nonlinear optical properties of Li and NH4 doped potassium pentaborate crystals, Mater. Lett. 178 (2016) 100–103. [27] T. Balakrishnan, G. Bhagavannarayana, K. Ramamurthi, Growth, structural, optical, thermal and mechanical properties of ammonium pentaborate single crystal, Spectrochim. Acta A 71 (2008) 578–589. [28] R.S. Katiyar, Raman and infra-red spectra of crystalline ammonium sulphamate, Proc. Indian Acad. Sci. 63 (1966) 20–25. [29] M. Susan Howitt, K. Michael Udvardi, Structure, function and regulation of ammonium transporters in plants, BBA-Biomembr. 1465 (2000) 152–170. [30] O.S. Ksenzhek, A.G. Volkov, Plant Energies, Academic Press, San Diego, CA, 1998. [31] B.C. Stuart, M.D. Feit, A.M. Rubenchik, B.W. Shore, M.D. Perry, Laser-induced damage in dielectrics with nanosecond to sub pico second pulses, Phys. Rev. Lett. 74 (1995) 2248–2251. [32] H. Newkirk, J. Swain, S. Stokowski, D. Milam, D. Smith, H. Klapper, X-ray topography of laser-induced damage in potassium dihydrogen phosphate crystals, J. Cryst. Growth 65 (1983) 651–659.
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Effect of Li and NH4 doping on the crystal perfection, second harmonic generation efficiency and laser damage threshold of potassium pentaborate crystals A.N. Vigneshwaran a, S. Kalainathan b, C. Ramachandra Raja a,⇑ a b
Department of Physics, Government Arts College (Autonomous), Kumbakonam 612 002, India Center for Crystal Growth, School of Advanced Sciences, VIT University, Vellore 632 014, India
a r t i c l e
i n f o
Article history: Received 15 June 2017 Received in revised form 15 September 2017 Accepted 7 October 2017
Keywords: Dopants Borates High resolution X-ray diffraction Nonlinear optical materials
a b s t r a c t Potassium pentaborate (KB5) is an excellent nonlinear optical material especially in the UV region. In this work, Li and NH4 doped KB5 crystals were grown using slow evaporation solution growth method. The incorporation of dopant has been confirmed and analysed by Energy dispersive X-ray analysis (EDAX), Inductively coupled plasma (ICP) analysis and Raman spectroscopy. The crystalline perfection of pure and doped KB5 crystals was studied by High resolution X-ray diffraction (HRXRD) analysis. Structural grain boundaries were observed in doped crystals. Second harmonic generation was confirmed for pure and doped crystals and output values revealed the enhancement of SHG efficiency in doped crystals. Resistance against laser damage was carried out using 1064 nm Nd-YAG laser of pulse width 10 ns. The laser damage threshold value is increased in Li doped crystal and decreased in NH4 doped crystal when compared to pure KB5 crystal. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction In the last few decades enormous research work is going on the growth of single crystals for various applications. Various technologies require high quality single crystals. In device applications, miniaturization of modern devices depends on the high crystalline perfection [1–3]. Crystal perfection is one of the factor that decides the applicability of a crystal [4–6]. Many applications need crystals which are nearly close to the ideal crystal which is free from all defects. But no crystal in practice is ideal, there are some defects in all real crystals. Efforts are always going on to improve the crystalline quality of the single crystals [1–7]. Since the invention of first laser, its impact on various sectors like industrial, medicine and defense is growing enormously. For instance laser at Mid Infra red region have applications in many military and civil activities like remote sensing and biological tissues visualization [8,9]. Deep UV lasers are of great interest among others due to their variety of applications in various areas such as semiconductor manufacturing, photolithography, attosecond pulse generation and advanced instrument development [10–12]. Recently Liang et al. theoretically designed some possible deep UV nonlinear optical (NLO) crystals in fluorooxoborates combining ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (C.R. Raja). https://doi.org/10.1016/j.optlastec.2017.10.004 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
rational tailoring design and first principle calculations [13]. Shi et al. developed beryllium-free phase NH4B4O6F, a deep UV NLO crystal, which exhibits NLO properties superior to KBBF [14]. NLO crystals are capable to extend the frequency ranges of lasers through the frequency conversion process [15–22]. For an efficient NLO crystal, it could possess some properties like wide transparency, high resistance against laser damage, large NLO coefficients and fast response. Chen et al. have developed the anionic group theory to investigate the NLO effects in borate crystals [18–20]. According to this theory, the large difference in the electronegativities of the B and O atoms on the B-O bond will favour transmission of short wavelength UV radiation. They have developed several new borate based NLO crystals for use in the UV region. There are several structural units (anionic groups) available in borate series like the trigonal (BO3)3, the tetragonal (BO4)5 and the planar six-membered-ring (B3O6)3 anionic groups. In the case of Potassium pentaborate (KB5), the anionic group comprised of two nonplanar six-membered ring quintanions (B3O7)5. These rings are joined by sharing the tetrahedrally coordinated B atom, with the two six-membered ring planes perpendicular to each other [19]. KB5 is one of the few material which having operating frequency at deep UV region [23,24]. Lei Kang et al. reported that NH+4 cation is the suitable candidate to replace the K+ cation due to the similarity in ionic radius and valence electronic properties [25]. Based on this, in the present work K+ ions are partially
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replaced by Li+ and NH+4 ions in KB5 crystal. Transmission property and optical band gap of the pure, Li and NH4 doped KB5 crystals were reported by the authors [26]. In this work, effect of inclusion of Li and NH4 in crystal perfection, second harmonic generation efficiency and laser damage threshold of KB5 crystal was analyzed. 2. Experimental KB5 was synthesized by reacting potassium carbonate and boric acid in 1:10 M ratio. Saturated solution was housed in an unperturbed environment at 34 °C with the help of constant temperature bath (CTB) with accuracy of ±0.01 °C. After the supersaturation, good quality and transparent crystals were obtained in a period of 50 days. 5 mol% of Li2CO3, 95 mol% of K2CO3 and H3BO3 were dissolved in double distilled water and stirred for 8 h to get Li doped crystals. The saturated solution was kept at 34 °C in CTB for slow evaporation. The transparent crystals were harvested in a period of 50 days. The same approach was followed to grow NH4 doped KB5 crystals. Grown crystals are shown in Fig. 1. 3. Results and discussion 3.1. Elemental analysis EDAX analysis was carried out to confirm the presence of N in NH4 doped crystal. Results confirmed that 4.26% of nitrogen atoms entered into the crystal lattice of KB5. ICP analysis was employed to quantify the presence of Li ions. The results confirmed that 3.22% of Li ions are present in the doped crystal. 3.2. Raman analysis FT- Raman spectral analysis was done using BRUKER RFS 27 FTRaman spectrometer with Nd:YAG laser (1064 nm). The recorded FT- Raman spectrum of NH4 doped and pure KB5 are shown in Fig. 2(a) and (b) respectively. In NH4 doped crystals N-H vibrations are observed [27,28]. NH4 deformation peak was observed at 1447 cm1. The Peak observed at 1632 cm1 corresponds to NH4 asymmetric bending. NH4 oscillation was observed at 1762 cm1. From the spectra it is evident that the high intensity peak between the frequency range of 3060–3370 cm1 is broad and sharp in NH4 doped crystal. Vibrations of N-H stretching in NH4 ion and O-H symmetric stretching in water molecule occurring in the same frequency range could be the reason for this Raman intensity in NH4 doped crystal. There is no much variation in the Raman spectra of pure and Li doped KB5 crystals.
Fig. 2. Raman spectrum of (a) NH4 doped KB5, (b) pure KB5.
perfection of the grown crystals was analysed using PANalytical X-Pert PRO MRD high-resolution XRD system with Cu Ka1 radiation. 4-bounce Ge (220) monochromator was used to obtain the highly monochromatic X-ray beam. The diffracted X-ray beam was collected in the scintillation detector. To get all diffracted beam due to very low grain boundaries and dislocations, analyzer was not employed between specimen and detector. Fig. 3 shows the diffraction curve (DC) of (1 1 1) diffracting plane for KB5 crystal. The solid line (convoluted curve) is well fitted with the experimental points represented by the circles. As evident form HRXRD curve, the full width at half maximum (FWHM) of the peak is 56 arc sec and the peak is reasonably sharp. Though the value of FWHM value is higher than that expected from the plane wave theory of dynamical X-ray diffraction for an ideally perfect crystal but close to the value that expected for real crystals. Additional peaks are absent in DC. This shows that the crystal doesn’t contain any internal structural low angle boundaries and the relatively low value of FWHM shows that the crystal is of good quality. The HRXRD curve recorded for (1 1 1) diffraction plane of NH4 doped KB5 is shown in Fig. 4. On deconvolution of the DC, it is clear that the curve contains two additional peaks, one of them 37 arc sec away from the main peak (at zero glancing angle) on the higher angle side and the other one 23 arc sec away from the main peak on the lower angle side. Since tilt angles of the additional peaks
3.3. High resolution X-ray diffraction analysis HRXRD technique is a non-destructive analysis and can be used for direct observation of boundaries and dislocations. Crystalline
Fig. 1. Photograph of grown crystals.
Fig. 3. HRXRD curve of pure KB5 crystal.
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case of pure and NH4 doped crystal. The relatively high values of FWHM of the grains and the angular spread of the deconvoluted curve (3100 arc sec) indicate that the crystalline perfection is not up to the mark. The crystal contains some mosaic blocks which are formed due to the release of heavy stress in the crystal caused by Li doping. The low ionic radius of Li ion (0.9 Å) which occupies the K ion site causes stress in the crystal lattice [29,30]. 3.4. Second harmonic generation analysis
Fig. 4. HRXRD curve of NH4 doped KB5 crystal.
are less than 10 , these are corresponding to two internal structural very low angle boundaries. The FWHM of the main peak and very low angle boundaries are 22, 21 and 14 arc sec. These low values reveal that the grains of the crystal are nearly perfect as one can expect such low values only for crystals with reasonable quality. Although the specimen contains very low angle grain boundaries, the low FWHM values and the relatively low angular spread of DC of around 62 arc sec indicate that the crystalline perfection is reasonably good. The low angular spread of DC (62 arc sec) is in comparison with the FWHM of pure KB5 (56 arc sec). This is expected because the ionic radius of K (1.49 Å) and the NH4 (1.48 Å) ion are comparable and may induce minimum stress [29,30]. Thermal fluctuation, mechanical disturbances or segregation of the solvent molecules during the growth process are the reasons reported for the observed very low angle boundaries [4]. It may be mentioned here that such low angle boundaries could be detected in the DC only because of the four bounce monochromators used in the present study. Such defects may not have much influence on the physical properties. The HRXRD curve recorded for (1 1 1) diffraction plane of Li doped KB5 crystal is shown in Fig. 5. The solid line (convoluted curve) is well fitted with the experimental points represented by the circles. On deconvolution of the DC, it is clear that the curve contain three additional peaks at 500, 1800 and 3050 arc sec away from the main peak on the higher angle side. These three additional peaks correspond to three internal structural low angle boundaries (tilt angle >10 but less than a degree). The tilt angles of these grain boundaries are 500, 1300 and 1250 arc sec from their adjoining regions. The FWHM of main peak and low angle boundaries are 222.8, 291.7, 387.5 and 471.6 arc sec. The tilt angles and FWHM of grain boundaries are larger than that are observed in the
SHG of the grown crystal was confirmed by Kurtz-Perry powder technique. Grown crystals are uniformly powdered and tightly packed in the capillary tube. The sample was placed on the path of Nd-YAG laser of 1064 nm of having input pulse energy 1.6 mJ/ pulse. Green light emitted from the sample confirmed the SHG in the grown crystals. The emitted beam was fed into the photomultiplier tube which converts the output beam into the corresponding electric pulses. KDP was chosen as the standard material for the comparison of SHG efficiency. The output values of pure, doped KB5 crystals and KDP are given in Table 1. Results of SHG output are in good agreement with anionic group theory proposed by Chen et al. which states that the overall SHG of a crystal is contributed only by the anionic groups in the crystal [19]. So it is expected that doping of cationic groups doesn’t significantly affect the overall SHG of the pure KB5 crystal. 3.5. Laser damage threshold study Since NLO process involves high optical intensities, NLO material should have high resistance against the laser involved. Resistance against irradiated laser is one of the key factor for device applications. Because NLO crystal having low laser damage threshold (LDT) value could limit the device applications of that crystal even though it possess excellent optical transmission properties. For the long-pulse regime s > 100 ps, the damage process occur mainly by the rate of thermal conduction through the atomic lattice and for the short-pulse regime s 10 ps, the optical breakdown is a nonthermal process and various nonlinear ionization mechanisms (multiphoton, avalanche multiplication, and tunnelling) become important [31]. To determine LDT, Pure, Li and NH4 doped crystals with well flat surface having thickness 2 mm were mounted on the crystal holder on the path of 1064 nm NdYAG laser. The pulse width of irradiating laser is 10 ns. For this measurement, a 1 mm diameter beam was focused onto the sample along a 35 cm focal length lens. LDT was calculated by the relation
E ¼ P=s0 A where P is the laser energy that cause damage, s0 is pulse width (10 ns) and A area of spot size. The calculated values of LDT are given in Table 2. The observed values indicate doping influences the LDT values irrespective of crystalline perfection [32]. In the present work, 10 ns pulsed laser has been used and therefore thermal effects are prominent. As LDT of a crystal for long pulses depends mainly on the thermal conductivity of material, Li doped crystal posses high LDT because of the higher thermal conductivity of Li (84.8 W/m K) compared to K (57.8 W/m K).
Table 1 SHG output of pure and doped crystals.
Fig. 5. HRXRD curve of Li doped KB5 crystal.
Crystal
Output (mV)
KDP KB5 Li doped KB5 NH4 doped KB5
150 135 141 138
156
A.N. Vigneshwaran et al. / Optics and Laser Technology 100 (2018) 153–156
Table 2 LDT values of pure and doped crystals. Crystal
LDT value (GW/cm2)
KB5 Li doped KB5 NH4 doped KB5
8.194 8.592 7.938
4. Conclusion Pure, Li and NH4 doped KB5 crystals were grown by slow evaporation solution growth method. EDAX and Raman spectral study confirms the incorporation of NH4 in KB5 crystal. ICP analysis confirms the presence of Li in KB5 crystal. HRXRD analysis of pure KB5 indicates that the crystalline perfection is good and the crystals are without any internal structural grain boundaries. HRXRD analysis of NH4 doped and Li doped KB5 crystal reveals that the crystal contains very low and low angle boundaries, respectively. Second harmonic generation in pure and doped crystals is confirmed using Kurtz-Perry powder technique. The laser damage threshold value for pure, Li and NH4 doped crystal is calculated as 8.194 GW/cm2, 8.592 GW/cm2 and 7.938 GW/cm2 respectively. Acknowledgement Author C.R acknowledges Council of Scientific and Industrial Research (CSIR), New Delhi for financial support (scheme number: 03(1301)/13/EMR II) and gratefully acknowledge the authorities of SAIF, IIT, Chennai for ICP and Raman analysis, National Physical Laboratory, New Delhi for HRXRD analysis, Prof. P.K. Das, IISc, Bangalore for providing SHG testing facility, and National College, Trichy for EDAX analysis. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.optlastec.2017.10. 004. References [1] Anuj Krishna, N. Vijayan, Chandan Bagdia, Kanika Thukral, Sonia, D. Haranath, K.K. Maurya, G. Bhagavannarayana, Effect of ampoule support on the growth of organic benzimidazole single crystals by vertical Bridgman technique for nonlinear optical applications, Cryst. Eng. Comm. 18 (2016) 4844–4850. [2] G. Bhagavannarayana, R.V. Ananthamurthy, G.C. Budakoti, B. Kumar, K.S. Bartwal, A study of the effect of annealing on Fe-doped LiNbO3 by HRXRD, XRT and FT–IR, J. Appl. Cryst. 38 (2005) 768–771. [3] Krishnamurthy Senthil Kumar, Sridharan Moorthy Babu, G. Bhagavannarayana, Study of the influence of dopants on the crystalline perfection of ferroelectric glycine phosphite single crystals using high-resolution X-ray diffraction analysis, J. Appl. Cryst. 44 (2011) 313–318. [4] G. Bhagavannarayana, S.K. Kushwaha, Enhancement of SHG efficiency by urea doping in ZTS single crystals and its correlation with crystalline perfection as revealed by Kurtz powder and high-resolution X-ray diffraction methods, J. Appl. Cryst. 43 (2010) 154–162. [5] S.K. Kushwaha, Mohd. Shakir, K.K. Maurya, A.L. Shah, M.A. Wahab, G. Bhagavannarayana, Remarkable enhancement in crystalline perfection, second harmonic generation efficiency, optical transparency, and laser damage threshold in potassium dihydrogen phosphate crystals by Lthreonine doping, J. Appl. Phys. 108 (2010) 033506. [6] A. Krishna, N. Vijayan, S. Gupta, K. Thukral, V. Jayaramakrishnan, B. Singh, J. Philip, S. Das, K.K. Maurya, G. Bhagavannarayana, Key aspects of L-threoninium picrate single crystal: an excellent organic nonlinear optical material with a high laser-induced damage threshold, RSC Adv. 4 (2014) 56188–56199.
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