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Irradiation effects on the optical properties of a new NLO mixed borate crystal
Surface and Coatings Technology 158 – 159 (2002) 725–728
Irradiation effects on the optical properties of a new NLO mixed borate crystal S. Ishwar Bhata, P. Mohan Raoa,*, A.P. Ganesh Bhata, D.K. Avasthib a
Department of Physics, Mangalore University, Mangalagangotri 574199, Karnataka, India b Nuclear Science Centre, New Delhi 110 067, India
Abstract Single crystals of novel non-linear optical material, barium strontium borate, have been grown in our laboratory by employing a low temperature solution technique for the first time. These crystals are grown up to a size of 20 mm=10 mm=1 mm and they exhibit good non-linear optical properties. As grown faces of these mixed borate crystals are irradiated by 8-MeV electrons as well as silver ions of 120 MeV. The dielectric properties and the refractive indices of these crystals are studied before and after irradiation. It is found that there is an increase in the dielectric constant due to swift heavy ion irradiation as well as due to electron irradiation. A considerable change in the values of refractive indices is observed due to irradiation in both cases. The UV-visible spectra showed no new bands in both cases. The observed results and the possibility of fabricating optical waveguides in these crystals are discussed. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Irradiation effects; Borate crystals; Non-linear optical materials; SHI irradiation; Electron irradiation
1. Introduction Energetic ions are suitable means for the modification of the surface or bulk structure of solids. An ion, depending on its mass, nuclear charge and kinetic energy can create changes only within a thin surface layer, or is able to penetrate far into the bulk and to produce a long and narrow disordered zone along its trajectory. At energies of the order of MeV, the ion loses its energy almost exclusively by interaction with the electrons of the target atoms. The primary ionisations and excitation processes and the following electron cascades occur within a very short time of 10y17 to 10y14 seconds, which is much shorter than the time necessary to create defects via lattice relaxation w1x. Depending on the sensitivity of the solid, the degree of disorder can range from point defects to a continuous amorphised zone along the ion path commonly called latent track. On the other hand, irradiation by low ionising radiation, e.g. *Corresponding author. Tel.: q91-824-742363; fax: q91-824742367. E-mail address: [email protected] (P. Mohan Rao).
electrons produces a homogeneous distribution of the absorbed dose. While special ion-induced and electron-induced effects have been studied in a wide range of insulating materials w2–6x, no such investigations have been made on borate crystals. In this paper, we present the results of new experiments on electron irradiated and swift heavy ion irradiated barium strontium borate crystals obtained by various techniques such as UV-visible spectroscopy, dielectric studies and refractive index measurements. 2. Experimental procedures, results and discussion 2.1. Crystal growth Equal proportions (molar ratio) of AR grade barium borate and strontium borate are mixed together in double distilled water and a supersaturated solution is prepared. The solution is filtered three times to ascertain growth of pure crystals. The supersaturated solution is allowed to evaporate at constant temperature (30 8C) for approximately 1 week. This resulted in the formation of seed crystals. Good seed crystals are selected and are sus-
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 2 6 0 - 8
S. Ishwar Bhat et al. / Surface and Coatings Technology 158 – 159 (2002) 725–728
726
Fig. 1. Single crystals of barium strontium borate.
pended in the original solution and placed for approximately 1 month without disturbing. Good transparent single crystals of dimension up to 20 mm=10 mm=1 mm are formed. A few typically grown crystals are shown in Fig. 1. These crystals show good second harmonic generation (SHG) efficiency. The mixed barium strontium borate crystal is found to be a promising non-linear optical crystal, which possibly can be used for fabrication of photonic devices. 2.2. Swift heavy ion irradiation (SHI) Single crystals of thickness approximately 2 mm are irradiated by Agq13 ions of energy 120 MeV with a fluence of 5=1011 ionsycm2 using the Pelletron accelerator at the Nuclear Science Center, New Delhi. The samples are mounted on a target ladder inside a vacuum chamber (pressure(4=10y8 Pa) and are irradiated with Agq13 beam of 120 MeV energy at room temperature. The incident beam current is approximately 0.539 particles nano ampere (pnA).
electron irradiation are shown in Fig. 2. It is observed that there is an increase in the dielectric constant due to SHI irradiation as well as due to electron irradiation. The dielectric constant of a material is composed of four types of contributions namely ionic, electronic, orientational and space charge polarisations. The nature of variations of the dielectric constant with frequency and temperature indicates the type of contributions that are predominant. The large value of the dielectric constant at low frequency is due to the presence of space charge polarisation w7x. The dielectric constant was found to increase by a factor of nearly 4 after SHI irradiation. The drastic increase in dielectric constant due to ion irradiation may be correlated to the defects created along the ion tracks w8x and the structural modifications induced in the surrounding regions w9– 11x. The range of 120 MeV Agq13 ions in barium strontium borate is found to be approximately 35 mm as determined by TRIM calculation w12x. Incident heavy ions get embedded in the crystal, lose energy by both the inelastic collisions dominant near the surface and the elastic collisions which dominate near the end of the range of the implanted ions. The increase in dielectric constant may be due to the disordering of the crystal lattice by the ion beam. Irradiation of materials with high energy electron bombardment is a proven method of production of Frenkel defects (vacancies and interstitials) without displacement cascades w13x. Controlled production of point defects in materials has been one of the early applications of energetic electron beams in the field of radiation damage and material science. It has also been observed that electron irradiation is superior to other methods for producing fast switching diodes through manipulative control of defect properties w14x. High energy electrons can cause atoms to be displaced from
2.3. Electron irradiation The single crystals of barium strontium borate are also irradiated by a 8-MeV electron beam at room temperature with a dose of 5.7=109 ycm2 using the microtron accelerator at Mangalore University. 2.4. Dielectric constant measurement Various parameters for the crystal like capacitance C, conductance G and dielectric loss D are measured at room temperature using a Hewlett Packard 4284 A (20 Hz–1 MHz) LCR meter before and after the irradiation of swift heavy ion and electron beam. From the data obtained, the dielectric constant ´ is calculated for the crystal before and after the irradiation. The variation of ´, in case of swift heavy ion irradiation (SHI) and
Fig. 2. Variation of dielectric constant with log f in case of unirradiated, SHI irradiated and electron irradiated crystals.
S. Ishwar Bhat et al. / Surface and Coatings Technology 158 – 159 (2002) 725–728 Table 1 Refractive indices of barium strontium borate single crystal before and after irradiation Sample
Wavelength (nm)
nx
ny
nz
Before irradiation With SHI irradiation With electron irradiation
633 633 633
1.528 1.463 1.489
1.539 1.485 1.499
1.603 1.513 1.577
normal lattice positions by the transfer of momentum w15x. In the present investigation, it was observed that the dielectric constant of electron irradiated barium strontium borate crystals increases by a factor of 2. This increase may be correlated to the defects produced at and near the surface of the crystals due to high energy electron irradiation. The increase in the dielectric constant may also be attributed to ionic polarisability induced in the crystal due to primary ionisation process. 2.5. Refractive index studies The bulk refractive index of the barium strontium borate crystal is measured along the three crystal axes by Brewster’s angle technique with a He–Ne laser (ls 633 nm). The reflected light is scanned by a photodetector. The refractive indices measured before and after SHI and electron irradiation are given in Table 1. It was observed that the refractive index decreases due to SHI irradiation and electron irradiation. Ion implantation has found a wide success in the construction of waveguides in insulating materials w16,17x. It may be noted that a decrease in refractive index has been reported in the case of inorganic NLO crystals like KTP and LBO due to He ion irradiation w18,19x. Similarly, the formation of optical waveguides in thin silicate glass films by electron beam irradiation has been reported w6x. The change in the refractive index due to ionyelectron irradiation shows the possibility of fabri-
727
cating planar waveguides in these crystals and increasing their applicability in various non-linear applications. 2.6. UV-visible spectral studies To determine the transmission range and hence to know the suitability of barium strontium borate single crystals for optical application the UV-visible spectrum is recorded over a wavelength range of 200–1000 nm. The UV-visible spectrum is recorded for heavy ion irradiated and electron irradiated sample also. Fig. 3 gives the UV-visible spectra corresponding to crystals irradiated by SHI, electron and for unirradiated crystals. The decrease in the absorbance in the case of electron irradiated samples may be due to the larger amount of scattering of high energy electron beam as it penetrates the sample. No new absorption peaks are observed in both cases. It may therefore be concluded that the irradiation either by electrons or SHI, would not change the transmittance properties of the crystals. 3. Conclusion Irradiation effects on single crystals of a novel nonlinear optical material barium strontium borate by MeV ions and electrons have been investigated. The dielectric constant of these crystals is increased by approximately a factor of 4 due to irradiation by silver ions of 120MeV energy. Electron irradiation also causes a slight increase in the dielectric constant. Refractive indices of these crystals are found to decrease due to both ion as well as electron irradiation which shows the possibility of fabricating optical waveguides by ionyelectron irradiation in barium strontium borate crystals. It is observed that there is no change in the transmittance properties of these crystals during ionyelectron irradiation. Further work in evaluating the observed changes in refractive indices and the related non-linear optical properties are in progress. References
Fig. 3. UV-visible spectrum of unirradiated, SHI irradiated and electron irradiated crystals.
w1x A.M. Stoneham, Nucl. Instrum. Meth. B48 (1990) 389. w2x N. Betz, A. LeMoel, E. Balanzat, et al., J. Polym. Sci. Polym. Phys. B 32 (1994) 1493. w3x T. Stecken Reiter, E. Balanzat, H. Fuess, C. Trautmann, Nucl. Instrum. Meth. B 131 (1997) 159. w4x M. Toulemonde, S. Bauffard, F. Studer, Nucl. Instrum Meth. B 91 (1994) 108. w5x P.D. Townsend, MAPS Vacuum 4 (44) (1993) 267. w6x M. Nakanishi, O. Sugihara, N. Okamoto, J. Appl. Phys. 86 (5) (1999) 2393. w7x P. Suryanaryana, H.N Acharya, K. Rao, J. Mater. Sci. Lett. 321 (1984). w8x G.W. Arndt, Nucl. Instrum. Meth. 339 (1989) 708. w9x P.J. Chandler, P.P. Townsend, Nucl. Instrum. Meth. B 80y81 (1993) 1135. w10x P.D. Townsend, Nucl. Instrum. Meth. B89 (1994) 270.
728
S. Ishwar Bhat et al. / Surface and Coatings Technology 158 – 159 (2002) 725–728
w11x H. Karge, G. Totz, U. Jahn, S. Schmidt, Nucl. Instrum. Meth. 182y183 (1981) 777. w12x J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, Pergamon, New York, 1988. w13x D.S. Billington, J.H. Crawford, Radiation Damage in Solids, JR. Princeton University Press, Princeton, New Jersey, 1961. w14x P.G. Fuochi, Radiat. Phys. Chem. 44 (4) (1994) 431.
w15x R.P. Agarwala, Irr. induced physical property changes in materials. Proceedings of the Symposium on radiation effects in solids, Nov. 23–25, BARC Bombay, 1983, 2. w16x P.D. Townsend, Rep. Prog. Phys. 50 (1987) 501. w17x P.D. Townsend, Cryst. Lattice Defects 18 (1989) 377. w18x E.L. Babsil, P.D. Townsend, Appl. Phys. Lett. 59 (1991) 384. w19x D. Fluck, P. Gunter, Opt. Commun. 90 (1992) 304.
Irradiation effects on the optical properties of a new NLO mixed borate crystal S. Ishwar Bhata, P. Mohan Raoa,*, A.P. Ganesh Bhata, D.K. Avasthib a
Department of Physics, Mangalore University, Mangalagangotri 574199, Karnataka, India b Nuclear Science Centre, New Delhi 110 067, India
Abstract Single crystals of novel non-linear optical material, barium strontium borate, have been grown in our laboratory by employing a low temperature solution technique for the first time. These crystals are grown up to a size of 20 mm=10 mm=1 mm and they exhibit good non-linear optical properties. As grown faces of these mixed borate crystals are irradiated by 8-MeV electrons as well as silver ions of 120 MeV. The dielectric properties and the refractive indices of these crystals are studied before and after irradiation. It is found that there is an increase in the dielectric constant due to swift heavy ion irradiation as well as due to electron irradiation. A considerable change in the values of refractive indices is observed due to irradiation in both cases. The UV-visible spectra showed no new bands in both cases. The observed results and the possibility of fabricating optical waveguides in these crystals are discussed. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Irradiation effects; Borate crystals; Non-linear optical materials; SHI irradiation; Electron irradiation
1. Introduction Energetic ions are suitable means for the modification of the surface or bulk structure of solids. An ion, depending on its mass, nuclear charge and kinetic energy can create changes only within a thin surface layer, or is able to penetrate far into the bulk and to produce a long and narrow disordered zone along its trajectory. At energies of the order of MeV, the ion loses its energy almost exclusively by interaction with the electrons of the target atoms. The primary ionisations and excitation processes and the following electron cascades occur within a very short time of 10y17 to 10y14 seconds, which is much shorter than the time necessary to create defects via lattice relaxation w1x. Depending on the sensitivity of the solid, the degree of disorder can range from point defects to a continuous amorphised zone along the ion path commonly called latent track. On the other hand, irradiation by low ionising radiation, e.g. *Corresponding author. Tel.: q91-824-742363; fax: q91-824742367. E-mail address: [email protected] (P. Mohan Rao).
electrons produces a homogeneous distribution of the absorbed dose. While special ion-induced and electron-induced effects have been studied in a wide range of insulating materials w2–6x, no such investigations have been made on borate crystals. In this paper, we present the results of new experiments on electron irradiated and swift heavy ion irradiated barium strontium borate crystals obtained by various techniques such as UV-visible spectroscopy, dielectric studies and refractive index measurements. 2. Experimental procedures, results and discussion 2.1. Crystal growth Equal proportions (molar ratio) of AR grade barium borate and strontium borate are mixed together in double distilled water and a supersaturated solution is prepared. The solution is filtered three times to ascertain growth of pure crystals. The supersaturated solution is allowed to evaporate at constant temperature (30 8C) for approximately 1 week. This resulted in the formation of seed crystals. Good seed crystals are selected and are sus-
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 2 6 0 - 8
S. Ishwar Bhat et al. / Surface and Coatings Technology 158 – 159 (2002) 725–728
726
Fig. 1. Single crystals of barium strontium borate.
pended in the original solution and placed for approximately 1 month without disturbing. Good transparent single crystals of dimension up to 20 mm=10 mm=1 mm are formed. A few typically grown crystals are shown in Fig. 1. These crystals show good second harmonic generation (SHG) efficiency. The mixed barium strontium borate crystal is found to be a promising non-linear optical crystal, which possibly can be used for fabrication of photonic devices. 2.2. Swift heavy ion irradiation (SHI) Single crystals of thickness approximately 2 mm are irradiated by Agq13 ions of energy 120 MeV with a fluence of 5=1011 ionsycm2 using the Pelletron accelerator at the Nuclear Science Center, New Delhi. The samples are mounted on a target ladder inside a vacuum chamber (pressure(4=10y8 Pa) and are irradiated with Agq13 beam of 120 MeV energy at room temperature. The incident beam current is approximately 0.539 particles nano ampere (pnA).
electron irradiation are shown in Fig. 2. It is observed that there is an increase in the dielectric constant due to SHI irradiation as well as due to electron irradiation. The dielectric constant of a material is composed of four types of contributions namely ionic, electronic, orientational and space charge polarisations. The nature of variations of the dielectric constant with frequency and temperature indicates the type of contributions that are predominant. The large value of the dielectric constant at low frequency is due to the presence of space charge polarisation w7x. The dielectric constant was found to increase by a factor of nearly 4 after SHI irradiation. The drastic increase in dielectric constant due to ion irradiation may be correlated to the defects created along the ion tracks w8x and the structural modifications induced in the surrounding regions w9– 11x. The range of 120 MeV Agq13 ions in barium strontium borate is found to be approximately 35 mm as determined by TRIM calculation w12x. Incident heavy ions get embedded in the crystal, lose energy by both the inelastic collisions dominant near the surface and the elastic collisions which dominate near the end of the range of the implanted ions. The increase in dielectric constant may be due to the disordering of the crystal lattice by the ion beam. Irradiation of materials with high energy electron bombardment is a proven method of production of Frenkel defects (vacancies and interstitials) without displacement cascades w13x. Controlled production of point defects in materials has been one of the early applications of energetic electron beams in the field of radiation damage and material science. It has also been observed that electron irradiation is superior to other methods for producing fast switching diodes through manipulative control of defect properties w14x. High energy electrons can cause atoms to be displaced from
2.3. Electron irradiation The single crystals of barium strontium borate are also irradiated by a 8-MeV electron beam at room temperature with a dose of 5.7=109 ycm2 using the microtron accelerator at Mangalore University. 2.4. Dielectric constant measurement Various parameters for the crystal like capacitance C, conductance G and dielectric loss D are measured at room temperature using a Hewlett Packard 4284 A (20 Hz–1 MHz) LCR meter before and after the irradiation of swift heavy ion and electron beam. From the data obtained, the dielectric constant ´ is calculated for the crystal before and after the irradiation. The variation of ´, in case of swift heavy ion irradiation (SHI) and
Fig. 2. Variation of dielectric constant with log f in case of unirradiated, SHI irradiated and electron irradiated crystals.
S. Ishwar Bhat et al. / Surface and Coatings Technology 158 – 159 (2002) 725–728 Table 1 Refractive indices of barium strontium borate single crystal before and after irradiation Sample
Wavelength (nm)
nx
ny
nz
Before irradiation With SHI irradiation With electron irradiation
633 633 633
1.528 1.463 1.489
1.539 1.485 1.499
1.603 1.513 1.577
normal lattice positions by the transfer of momentum w15x. In the present investigation, it was observed that the dielectric constant of electron irradiated barium strontium borate crystals increases by a factor of 2. This increase may be correlated to the defects produced at and near the surface of the crystals due to high energy electron irradiation. The increase in the dielectric constant may also be attributed to ionic polarisability induced in the crystal due to primary ionisation process. 2.5. Refractive index studies The bulk refractive index of the barium strontium borate crystal is measured along the three crystal axes by Brewster’s angle technique with a He–Ne laser (ls 633 nm). The reflected light is scanned by a photodetector. The refractive indices measured before and after SHI and electron irradiation are given in Table 1. It was observed that the refractive index decreases due to SHI irradiation and electron irradiation. Ion implantation has found a wide success in the construction of waveguides in insulating materials w16,17x. It may be noted that a decrease in refractive index has been reported in the case of inorganic NLO crystals like KTP and LBO due to He ion irradiation w18,19x. Similarly, the formation of optical waveguides in thin silicate glass films by electron beam irradiation has been reported w6x. The change in the refractive index due to ionyelectron irradiation shows the possibility of fabri-
727
cating planar waveguides in these crystals and increasing their applicability in various non-linear applications. 2.6. UV-visible spectral studies To determine the transmission range and hence to know the suitability of barium strontium borate single crystals for optical application the UV-visible spectrum is recorded over a wavelength range of 200–1000 nm. The UV-visible spectrum is recorded for heavy ion irradiated and electron irradiated sample also. Fig. 3 gives the UV-visible spectra corresponding to crystals irradiated by SHI, electron and for unirradiated crystals. The decrease in the absorbance in the case of electron irradiated samples may be due to the larger amount of scattering of high energy electron beam as it penetrates the sample. No new absorption peaks are observed in both cases. It may therefore be concluded that the irradiation either by electrons or SHI, would not change the transmittance properties of the crystals. 3. Conclusion Irradiation effects on single crystals of a novel nonlinear optical material barium strontium borate by MeV ions and electrons have been investigated. The dielectric constant of these crystals is increased by approximately a factor of 4 due to irradiation by silver ions of 120MeV energy. Electron irradiation also causes a slight increase in the dielectric constant. Refractive indices of these crystals are found to decrease due to both ion as well as electron irradiation which shows the possibility of fabricating optical waveguides by ionyelectron irradiation in barium strontium borate crystals. It is observed that there is no change in the transmittance properties of these crystals during ionyelectron irradiation. Further work in evaluating the observed changes in refractive indices and the related non-linear optical properties are in progress. References
Fig. 3. UV-visible spectrum of unirradiated, SHI irradiated and electron irradiated crystals.
w1x A.M. Stoneham, Nucl. Instrum. Meth. B48 (1990) 389. w2x N. Betz, A. LeMoel, E. Balanzat, et al., J. Polym. Sci. Polym. Phys. B 32 (1994) 1493. w3x T. Stecken Reiter, E. Balanzat, H. Fuess, C. Trautmann, Nucl. Instrum. Meth. B 131 (1997) 159. w4x M. Toulemonde, S. Bauffard, F. Studer, Nucl. Instrum Meth. B 91 (1994) 108. w5x P.D. Townsend, MAPS Vacuum 4 (44) (1993) 267. w6x M. Nakanishi, O. Sugihara, N. Okamoto, J. Appl. Phys. 86 (5) (1999) 2393. w7x P. Suryanaryana, H.N Acharya, K. Rao, J. Mater. Sci. Lett. 321 (1984). w8x G.W. Arndt, Nucl. Instrum. Meth. 339 (1989) 708. w9x P.J. Chandler, P.P. Townsend, Nucl. Instrum. Meth. B 80y81 (1993) 1135. w10x P.D. Townsend, Nucl. Instrum. Meth. B89 (1994) 270.
728
S. Ishwar Bhat et al. / Surface and Coatings Technology 158 – 159 (2002) 725–728
w11x H. Karge, G. Totz, U. Jahn, S. Schmidt, Nucl. Instrum. Meth. 182y183 (1981) 777. w12x J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, Pergamon, New York, 1988. w13x D.S. Billington, J.H. Crawford, Radiation Damage in Solids, JR. Princeton University Press, Princeton, New Jersey, 1961. w14x P.G. Fuochi, Radiat. Phys. Chem. 44 (4) (1994) 431.
w15x R.P. Agarwala, Irr. induced physical property changes in materials. Proceedings of the Symposium on radiation effects in solids, Nov. 23–25, BARC Bombay, 1983, 2. w16x P.D. Townsend, Rep. Prog. Phys. 50 (1987) 501. w17x P.D. Townsend, Cryst. Lattice Defects 18 (1989) 377. w18x E.L. Babsil, P.D. Townsend, Appl. Phys. Lett. 59 (1991) 384. w19x D. Fluck, P. Gunter, Opt. Commun. 90 (1992) 304.