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Nuclear collision induced lattice swelling and refractive-index modification in ion-irradiated yttrium orthoaluminate crystal
Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Nuclear collision induced lattice swelling and refractive-index modification in ion-irradiated yttrium orthoaluminate crystal Y. Liu a, Q. Huang b, M. Qiao a, P. Liu a,⇑, X.L. Wang a a b
School of Physics, State Key Laboratory of Crystal Materials & Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Jinan 250100, China Shanghai Institute of Applied Physics, Chinese Academy of Sciences (CAS), Shanghai 201800, China
a r t i c l e
i n f o
Article history: Received 8 December 2016 Received in revised form 21 March 2017 Accepted 3 April 2017 Available online xxxx Keywords: Ion irradiation Nuclear collision Lattice swelling Refractive-index modification
a b s t r a c t This work reports the study of lattice damage behavior in yttrium orthoaluminate (YAlO3) crystal irradiated with medium-energy (6.0 MeV) and relatively high-energy (20.0 MeV) Si ions through complementary characterization techniques including Rutherford backscattering/channeling spectroscopy, transmission electron microscopy and X-ray diffraction. The results clearly demonstrate that under Siion irradiation over the energy range from a few MeV up to tens of MeV, the nuclear energy loss (elastic collisions between injected ions and target atoms) along ion trajectory would play a dominant role in lattice damage and swelling, which leads to the decrease of refractive index in the nuclear energy deposition region and the waveguide formation in YAlO3 crystal. By contrast, the electronic energy loss (ionization and electronic excitation) over the corresponding ion energy range would not produce obvious lattice damage, and therefore could not significantly modify the refractive index in YAlO3 crystal. Utilizing optical-coupling measurements and iWKB-procedure simulation, the modified refractive-index profile in ion irradiation region has been reconstructed, and the obtained corresponding relationship between the refractive-index profile and SRIM-simulated dpa profile further confirms the nuclear-energy-loss induced lattice swelling and refractive-index decrease behaviors in ion-irradiated YAlO3 crystal, consisting with the microstructure characterization results. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction During ion irradiation process, irradiating ion with relativelylow energy will mainly interact with target nuclei and lose energy via nuclear energy deposition (elastic collision) process, which could create a cascade of atomic collision events, displace atoms from initial sites and therefore produce permanent atomic-scale defects in crystal materials [1]; irradiating ion with relativelyhigh energy will primarily interact with target electrons and lose energy via electronic energy deposition (ionization and electronic excitation) process, which could induce the temperature rise along ion trajectory, and further produce the track containing partially or completely amorphous volume (lattice damage) in crystal materials [2–5]. Irradiation damage induced by nuclear or electronic energy losses could significantly change the physicochemical properties of crystal materials, and has been widely used to modify the optical properties of crystals and fabricate the micro- and nano-scale devices in integrated optics field [6,7]. Recently, for ⇑ Corresponding author. E-mail address: [email protected] (P. Liu).
some functional crystals, the physical mechanism of refractiveindex modification induced by the nuclear or electronic energy losses corresponding to different ion energies has been well understood [8,9]. In this work, yttrium orthoaluminate (YAlO3) crystal has been irradiated with medium-energy (6.0 MeV) and relatively high-energy (20.0 MeV) medium-mass (Si3+) ions to different fluences. Lattice damage production and refractive-index modification in YAlO3 crystal induced by ion irradiation over the energy range from a few MeV up to tens of MeV have been studied, and the effects of nuclear and electronic energy losses on irradiation damage have been discussed comparatively. 2. Experiment and simulation details Optically-polished YAlO3 crystal samples with (1 0 0) surface normal zone axis direction and dimensions of 10 10 0.5 mm3 were irradiated with 6.0 MeV and 20.0 MeV Si3+ at 300 K to different fluences using 2 1.7 MV and 2 6 MV tandem accelerators within the State Key Laboratory of Nuclear Physics and Technology at Peking University, respectively. The specific irradiation conditions of sample 1 (S1), sample 2 (S2), sample 3 (S3) and sample 4
http://dx.doi.org/10.1016/j.nimb.2017.04.013 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Liu et al., Nuclear collision induced lattice swelling and refractive-index modification in ion-irradiated yttrium orthoaluminate crystal, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.013
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Table 1 SRIM-simulated electronic energy loss, nuclear energy loss and dpa values, and RBS/channeling-measured disorder level corresponding to different irradiation conditions. Sample
S1 S2 S3 S4
Si3+ energy (MeV)
Si3+ fluence (cm2)
Damage peak region dpa
Near surface region Nuclear energy loss (keV/nm)
Electronic energy loss (keV/nm)
dpa
Disorder level
20.0 6.0 6.0 20.0
1.0 1013 4.4 1014 6.6 1014 1.0 1015
0.003 0.15 0.22 0.33
0.007 0.04 0.04 0.007
6.6 5.2 5.2 6.6
0.0001 0.011 0.017 0.01
0 0.06 ± 0.01 0.08 ± 0.01 0.14 ± 0.02
(S4) are indicated in Table 1. Lattice damage and swelling in YAlO3 crystal induced by ion irradiation were characterized through Rutherford backscattering/channeling spectroscopy, transmission electron microscopy and X-ray diffraction using 2 1.7 MV tandem accelerator, 200 kV Tecnai G2 F20 transmission electron microscope and Bruker D8 Advance diffractometer, respectively. Dark-mode spectra of ion irradiation region and near-field intensity distribution of guided mode were measured by prism and end-face coupling techniques, and used to discuss the refractiveindex modification behavior induced by ion irradiation process. SRIM 2013 code [10], SIMNRA code [11] and inverse WentzelKramers-Brillouin (iWKB) procedure [12,13] were used to determine the nuclear and electronic energy losses along ion trajectory, fit the measured RBS spectrum and reconstruct the refractiveindex profile in ion irradiation region, respectively.
3. Results and discussion 3.1. Lattice damage and swelling The nuclear and electronic energy losses induced by 6.0 MeV and 20.0 MeV Si-ion in YAlO3 (density: 5.35 g cm3) have been determined utilizing SRIM 2013 full-cascade simulation code. As indicated in Table 1, the electronic energy loss is dominant (5.2 keV/nm for 6.0 MeV Si3+ and 6.6 keV/nm for 20.0 MeV Si3+) in the near surface region. The nuclear energy loss would produce lattice defects through the cascades of atomic collision events, which could be characterized by displacement per atom (dpa). For Si3+-irradiated S1, S2, S3 and S4, the dpa values are 0.0001,
0.011, 0.017 and 0.01 in the surface region, and 0.003, 0.15, 0.22 and 0.33 in the heavily damaged dpa-peak region, respectively. Utilizing 2.0 MeV He+ beam, the measured RBS/channeling spectra of YAlO3 samples are shown in Fig. 1, and SIMNRA-fitting curve is also indicated. The disorder levels on Y sublattice at the surface of S1, S2, S3 and S4 have been determined through a classical approximate expression [14], which are 0, 0.06 ± 0.01, 0.08 ± 0.01 and 0.14 ± 0.02, respectively, and have been summarized in Table 1. The results indicate that in the near surface region, the dominant electronic energy loss would not produce obvious irradiation damage, and the measured lattice damage should be attributed to the nuclear collision process. Compared to S2 and S3, the surface region in S4 has lower dpa, and the measured relatively-high disorder level would be ascribed to the measurement error (slight inaccuracy of channel direction), which could be supported by the change of refractive index at the sample surface (Fig. 4). TEM observations have been performed on the cross section of S4, which has relatively high disorder level in the damage peak region owing to the highest ion fluence. Compared to the un-irradiated region (Fig. 2(a)), high resolution TEM image and electron diffraction pattern taken from the surface region (Fig. 2(b)) indicate that the near-surface region still remains relatively complete crystal structure, and the dominant electronic energy loss would not produce obvious lattice damage. Fig. 2(c)–(e) show the TEM images taken from the heavily damaged dpa-peak region under different magnifications, confirming the highly-disordered and amorphous domains. As shown in the electron diffraction pattern (Fig. 2(f)) taken from the dpa-peak region, the disappearance and deformation of diffraction spots, and the appearance of ring
Fig. 1. RBS/channeling spectra of YAlO3 crystal samples corresponding to different Si3+-irradiation conditions.
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Fig. 2. Bright-field TEM micrographs of YAlO3 crystal irradiated by 20.0 MeV Si3+ with the fluence of 1.0 1015 cm2. (a) un-irradiated region; (b) irradiated surface region; (c), (d) and (e) damage peak region under different magnifications; (f) electron diffraction pattern taken from damage peak region.
pattern further prove the existence of lattice damage and amorphous region induced by the nuclear collisions. Based on the spatial distance between the diffraction spots in the electron diffraction patterns (Fig. 2(a) and (f)), compared to the unirradiated region, the increase of lattice constant in the heavily damaged dpa-peak region could be confirmed qualitatively. Fig. 3 presents the measured XRD patterns (h–2h scan) of virgin and Si3+-irradiated YAlO3 samples. In this work, XRD measurements have been carried out several times, and the results have good stability and repeatability. YAlO3 crystal with (1 0 0) surface normal zone axis direction exhibits the characteristic peak at 2h = 73.1°, resulted from the reflection from (4 0 0) plane and corresponding to the lattice constant 5.17 Å. Besides the main
diffraction peak coming from the undamaged region, an additional scattered intensity appears on the lower angle side of the main peak, which corresponds to the heavily damaged region and reflects the dilatation strain along the direction perpendicular to sample surface [15–17]. The dilatation strain would be remarkably enhanced accompanying with the increasing dpa, indicating the first step of damage accumulation within the dpa range from 0.003 to 0.33 [18,19]. Considering the orthorhombic symmetry of YAlO3 crystal, the variation e of lattice constant (lattice strain) as a function of measured 2h could be given by the expression e ¼ ðdi d0 Þ=d0 , where di is the lattice constant calculated through Bragg equation di ¼ k=2sinh, and d0 is the initial lattice constant 5.17 Å. Thus, the final determined conversion from measured 2h
Please cite this article in press as: Y. Liu et al., Nuclear collision induced lattice swelling and refractive-index modification in ion-irradiated yttrium orthoaluminate crystal, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.013
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to e has been indicated through the top horizontal axis in Fig. 3, which clearly presents the lattice swelling in ion irradiation region. 3.2. Refractive-index modification
Fig. 3. Experimental h–2h scan curves recorded in the vicinity of (400) Bragg reflection on virgin and Si3+-irradiated YAlO3 crystal samples.
Under Si-ion irradiation over the energy range from a few MeV up to tens of MeV, the nuclear collision process could lead to the lattice damage and swelling in YAlO3 crystal. In order to study the refractive-index modification in the nuclear energy deposition region, dark-mode spectra of ion irradiation region have been measured utilizing prism coupling technique (633 nm laser with TM polarization state), and are shown in Fig. 4. Compared to the refractive index (1.948) of virgin sample, the refractive indices at the surface of S1, S2, S3 and S4 are 1.948, 1.947, 1.945 and 1.947, respectively, consisting with the dpa values (0.0001, 0.011, 0.017 and 0.01 at the surface of S1, S2, S3 and S4) and proving the refractive-index decrease behavior induced by the lattice damage and swelling. In this work, the maximum of refractive index decrease (buried low-index layer acting as optical barrier) appears at the heavily
Fig. 4. Dark mode spectra of Si3+-irradiated YAlO3 crystal samples measured by 633 nm laser under TM polarization.
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Fig. 5. (a) iWKB-reconstructed refractive-index profile in ion irradiation region at 633 nm utilizing the effective refractive indices of guided modes; (b) near-field light intensity distribution of fundamental TM-polarized mode (TM0); (c) SRIM-simulated dpa profile for YAlO3 crystal irradiated by 20.0 MeV Si3+ with the fluence of 1.0 1015 cm2.
damaged dpa-peak region, and the refractive index in the layer between the surface and the heavily damaged dpa-peak region will not significantly change owing to the relatively low lattice damage and swelling in this region. Due to the refractive index of air layer above the sample surface is a constant value of 1, the modified refractive-index profile in ion irradiation region could lead to the waveguide formation between the air layer and the heavily damaged dpa-peak region. Effective refractive index (neff) is defined as a number quantifying the phase delay per unit length in the waveguide relative to the phase delay in vacuum, and has the meaning for light propagation in the waveguide with restricted transverse extension: b value (phase constant) of waveguide is the effective index times the vacuum wavenumber: b ¼ neff 2kp. For prepared multimode waveguide, the effective refractive index depends on the guided mode in which the light propagates. Actually, as shown in Fig. 4, each dip of light intensity measured by prism coupling technique corresponds to one specific guided mode, and the value of horizontal axis indicates the effective refractive index corresponding to this specific guided mode. Based on two-dimensional step-index waveguide model [20,21], the refractive index in the heavily damaged dpa-peak (optical barrier) region will affect the waveguide-supported mode number, and accompanying with the increasing irradiation damage, the decreasing refractive index in the optical barrier region could effectively promote the waveguide formation and increase the guided mode number. Therefore, compared to Fig. 4(a)–(c), the number of the dips (guided modes) in Fig. 4(d) will obviously increase owing to the increasing irradiation damage and the decreasing refractive index in the heavily damaged dpa-peak (optical barrier) region. Based on the effective refractive indices of guided modes in S4 (light intensity dips in Fig. 4(d)) and polynomial-fitted effective refractive index function (red curve in Fig. 5(a)), the refractiveindex profile (black curve in Fig. 5(a)) in ion irradiation region has been reconstructed utilizing iWKB procedure. The reconstructed refractive-index profile presents the approximate inverse relationship with SRIM-simulated dpa profile (Fig. 5(c)), further proving the refractive-index decrease behavior induced by the nuclear collision and lattice swelling. As shown in Fig. 5(b), the near-field intensity distribution of fundamental TM-polarized
mode (TM0) was measured by end-face coupling technique, indicating the dimension of formed waveguide and the effective confinement of light propagation. 4. Summary and conclusions In this work, the lattice damage, volume swelling and refractive-index modification in YAlO3 crystal irradiated with 6.0 MeV and 20.0 MeV Si3+ ions have been studied utilizing structure characterization techniques and optical measurements, and the effects of nuclear and electronic energy losses along ion trajectory have been discussed comparatively. Under Si-ion irradiation over the energy range from a few MeV up to tens of MeV, the intense electronic energy loss would not produce observable lattice damage, and therefore could not modify the refractive index in the electronic energy deposition region. By contrast, the measured irradiation damage and lattice swelling should be attributed to the nuclear energy loss, which will further induce the refractiveindex decrease in the nuclear energy deposition region and promote the waveguide formation in YAlO3 crystal. The results obtained from microstructure characterization techniques and optical measurements have presented reasonable consistency. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 11405097, U1432120), Natural Science Foundation of Shandong Province of China (No. ZR2014AQ021), and the Special Financial Grant (No. 2016T90624) from the China Postdoctoral Science Foundation. This work was also supported by the State Key Laboratory of Nuclear Physics and Technology, Peking University. References [1] W.J. Weber, D.M. Duffy, L. Thomé, Y. Zhang, Curr. Opin. Solid State Mater. Sci. 19 (2015) 1–11. [2] Y. Zhang, R. Sachan, O.H. Pakarinen, M.F. Chisholm, P. Liu, H. Xue, W.J. Weber, Nat. Commun. 6 (2015) 8049.
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[3] A. Meftah, F. Brisard, J.M. Costantini, E. Dooryhee, M. Hage-Ali, M. Hervieu, J.P. Stoquert, F. Studer, M. Toulemonde, Phys. Rev. B 49 (1994) 12457. [4] M. Toulemonde, J.M. Costantini, Ch. Dufour, A. Meftah, E. Paumier, F. Studer, Nucl. Instrum. Methods Phys. Res., Sect. B 116 (1996) 37–42. [5] W.J. Weber, E. Zarkadoula, O.H. Pakarinen, R. Sachan, M.F. Chisholm, P. Liu, H. Xue, K. Jin, Y. Zhang, Sci. Rep. 5 (2015) 7726. [6] F. Chen, Laser Photon. Rev. 6 (2012) 622–640. [7] F. Chen, X.L. Wang, K.M. Wang, Opt. Mater. 29 (2007) 1523–1542. [8] F. Chen, J. Appl. Phys. 106 (2009) 081101. [9] P. Liu, Y. Zhang, H. Xue, K. Jin, M.L. Crespillo, X. Wang, W.J. Weber, Acta Mater. 105 (2016) 429–437. [10] J.F. Ziegler, M.D. Ziegler, J.P. Biersack, Nucl. Instrum. Methods Phys. Res., Sect. B 268 (2010) 1818–1823. [11] M. Mayer, SIMNRA user’s guide (1997). [12] P.J. Chandler, F.L. Lama, Opt. Acta 33 (1986) 127–143. [13] Q. Huang, P. Liu, T. Liu, L. Zhang, X.L. Wang, Opt. Express 20 (2012) 4213–4218.
[14] W.K. Chu, J.W. Mayer, M.A. Nicolet, Back-Scattering Spectrometry, Academic Press, New York, 1978. [15] S. Leclerc, A. Declémy, M.F. Beaufort, C. Tromas, J.F. Barbot, J. Appl. Phys. 98 (2005) 113506. [16] S. Leclerc, M.F. Beaufort, A. Declémy, J.F. Barbot, Appl. Phys. Lett. 93 (2008) 122101. [17] T. Yang, X. Huang, Y. Gao, C. Wang, Y. Zhang, J. Xue, S. Yan, Y. Wang, J. Nucl. Mater. 420 (2012) 430–436. [18] F. Garrido, L. Vincent, L. Nowicki, G. Sattonnay, L. Thomé, Nucl. Instrum. Methods Phys. Res., Sect. B 266 (2008) 2842–2847. [19] S. Moll, L. Thomé, G. Sattonnay, A. Debelle, F. Garrido, L. Vincent, J. Jagielski, J. Appl. Phys. 106 (2009) 073509. [20] C.L. Chen, Foundations for Guided-Wave Optics, John Wiley and Sons, Hoboken, 2007. [21] Y. Liu, Q. Huang, M.L. Crespillo, M. Qiao, P. Liu, X.L. Wang, Opt. Mater. 64 (2017) 391–400.
Please cite this article in press as: Y. Liu et al., Nuclear collision induced lattice swelling and refractive-index modification in ion-irradiated yttrium orthoaluminate crystal, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.013
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Nuclear collision induced lattice swelling and refractive-index modification in ion-irradiated yttrium orthoaluminate crystal Y. Liu a, Q. Huang b, M. Qiao a, P. Liu a,⇑, X.L. Wang a a b
School of Physics, State Key Laboratory of Crystal Materials & Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Jinan 250100, China Shanghai Institute of Applied Physics, Chinese Academy of Sciences (CAS), Shanghai 201800, China
a r t i c l e
i n f o
Article history: Received 8 December 2016 Received in revised form 21 March 2017 Accepted 3 April 2017 Available online xxxx Keywords: Ion irradiation Nuclear collision Lattice swelling Refractive-index modification
a b s t r a c t This work reports the study of lattice damage behavior in yttrium orthoaluminate (YAlO3) crystal irradiated with medium-energy (6.0 MeV) and relatively high-energy (20.0 MeV) Si ions through complementary characterization techniques including Rutherford backscattering/channeling spectroscopy, transmission electron microscopy and X-ray diffraction. The results clearly demonstrate that under Siion irradiation over the energy range from a few MeV up to tens of MeV, the nuclear energy loss (elastic collisions between injected ions and target atoms) along ion trajectory would play a dominant role in lattice damage and swelling, which leads to the decrease of refractive index in the nuclear energy deposition region and the waveguide formation in YAlO3 crystal. By contrast, the electronic energy loss (ionization and electronic excitation) over the corresponding ion energy range would not produce obvious lattice damage, and therefore could not significantly modify the refractive index in YAlO3 crystal. Utilizing optical-coupling measurements and iWKB-procedure simulation, the modified refractive-index profile in ion irradiation region has been reconstructed, and the obtained corresponding relationship between the refractive-index profile and SRIM-simulated dpa profile further confirms the nuclear-energy-loss induced lattice swelling and refractive-index decrease behaviors in ion-irradiated YAlO3 crystal, consisting with the microstructure characterization results. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction During ion irradiation process, irradiating ion with relativelylow energy will mainly interact with target nuclei and lose energy via nuclear energy deposition (elastic collision) process, which could create a cascade of atomic collision events, displace atoms from initial sites and therefore produce permanent atomic-scale defects in crystal materials [1]; irradiating ion with relativelyhigh energy will primarily interact with target electrons and lose energy via electronic energy deposition (ionization and electronic excitation) process, which could induce the temperature rise along ion trajectory, and further produce the track containing partially or completely amorphous volume (lattice damage) in crystal materials [2–5]. Irradiation damage induced by nuclear or electronic energy losses could significantly change the physicochemical properties of crystal materials, and has been widely used to modify the optical properties of crystals and fabricate the micro- and nano-scale devices in integrated optics field [6,7]. Recently, for ⇑ Corresponding author. E-mail address: [email protected] (P. Liu).
some functional crystals, the physical mechanism of refractiveindex modification induced by the nuclear or electronic energy losses corresponding to different ion energies has been well understood [8,9]. In this work, yttrium orthoaluminate (YAlO3) crystal has been irradiated with medium-energy (6.0 MeV) and relatively high-energy (20.0 MeV) medium-mass (Si3+) ions to different fluences. Lattice damage production and refractive-index modification in YAlO3 crystal induced by ion irradiation over the energy range from a few MeV up to tens of MeV have been studied, and the effects of nuclear and electronic energy losses on irradiation damage have been discussed comparatively. 2. Experiment and simulation details Optically-polished YAlO3 crystal samples with (1 0 0) surface normal zone axis direction and dimensions of 10 10 0.5 mm3 were irradiated with 6.0 MeV and 20.0 MeV Si3+ at 300 K to different fluences using 2 1.7 MV and 2 6 MV tandem accelerators within the State Key Laboratory of Nuclear Physics and Technology at Peking University, respectively. The specific irradiation conditions of sample 1 (S1), sample 2 (S2), sample 3 (S3) and sample 4
http://dx.doi.org/10.1016/j.nimb.2017.04.013 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Liu et al., Nuclear collision induced lattice swelling and refractive-index modification in ion-irradiated yttrium orthoaluminate crystal, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.013
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Table 1 SRIM-simulated electronic energy loss, nuclear energy loss and dpa values, and RBS/channeling-measured disorder level corresponding to different irradiation conditions. Sample
S1 S2 S3 S4
Si3+ energy (MeV)
Si3+ fluence (cm2)
Damage peak region dpa
Near surface region Nuclear energy loss (keV/nm)
Electronic energy loss (keV/nm)
dpa
Disorder level
20.0 6.0 6.0 20.0
1.0 1013 4.4 1014 6.6 1014 1.0 1015
0.003 0.15 0.22 0.33
0.007 0.04 0.04 0.007
6.6 5.2 5.2 6.6
0.0001 0.011 0.017 0.01
0 0.06 ± 0.01 0.08 ± 0.01 0.14 ± 0.02
(S4) are indicated in Table 1. Lattice damage and swelling in YAlO3 crystal induced by ion irradiation were characterized through Rutherford backscattering/channeling spectroscopy, transmission electron microscopy and X-ray diffraction using 2 1.7 MV tandem accelerator, 200 kV Tecnai G2 F20 transmission electron microscope and Bruker D8 Advance diffractometer, respectively. Dark-mode spectra of ion irradiation region and near-field intensity distribution of guided mode were measured by prism and end-face coupling techniques, and used to discuss the refractiveindex modification behavior induced by ion irradiation process. SRIM 2013 code [10], SIMNRA code [11] and inverse WentzelKramers-Brillouin (iWKB) procedure [12,13] were used to determine the nuclear and electronic energy losses along ion trajectory, fit the measured RBS spectrum and reconstruct the refractiveindex profile in ion irradiation region, respectively.
3. Results and discussion 3.1. Lattice damage and swelling The nuclear and electronic energy losses induced by 6.0 MeV and 20.0 MeV Si-ion in YAlO3 (density: 5.35 g cm3) have been determined utilizing SRIM 2013 full-cascade simulation code. As indicated in Table 1, the electronic energy loss is dominant (5.2 keV/nm for 6.0 MeV Si3+ and 6.6 keV/nm for 20.0 MeV Si3+) in the near surface region. The nuclear energy loss would produce lattice defects through the cascades of atomic collision events, which could be characterized by displacement per atom (dpa). For Si3+-irradiated S1, S2, S3 and S4, the dpa values are 0.0001,
0.011, 0.017 and 0.01 in the surface region, and 0.003, 0.15, 0.22 and 0.33 in the heavily damaged dpa-peak region, respectively. Utilizing 2.0 MeV He+ beam, the measured RBS/channeling spectra of YAlO3 samples are shown in Fig. 1, and SIMNRA-fitting curve is also indicated. The disorder levels on Y sublattice at the surface of S1, S2, S3 and S4 have been determined through a classical approximate expression [14], which are 0, 0.06 ± 0.01, 0.08 ± 0.01 and 0.14 ± 0.02, respectively, and have been summarized in Table 1. The results indicate that in the near surface region, the dominant electronic energy loss would not produce obvious irradiation damage, and the measured lattice damage should be attributed to the nuclear collision process. Compared to S2 and S3, the surface region in S4 has lower dpa, and the measured relatively-high disorder level would be ascribed to the measurement error (slight inaccuracy of channel direction), which could be supported by the change of refractive index at the sample surface (Fig. 4). TEM observations have been performed on the cross section of S4, which has relatively high disorder level in the damage peak region owing to the highest ion fluence. Compared to the un-irradiated region (Fig. 2(a)), high resolution TEM image and electron diffraction pattern taken from the surface region (Fig. 2(b)) indicate that the near-surface region still remains relatively complete crystal structure, and the dominant electronic energy loss would not produce obvious lattice damage. Fig. 2(c)–(e) show the TEM images taken from the heavily damaged dpa-peak region under different magnifications, confirming the highly-disordered and amorphous domains. As shown in the electron diffraction pattern (Fig. 2(f)) taken from the dpa-peak region, the disappearance and deformation of diffraction spots, and the appearance of ring
Fig. 1. RBS/channeling spectra of YAlO3 crystal samples corresponding to different Si3+-irradiation conditions.
Please cite this article in press as: Y. Liu et al., Nuclear collision induced lattice swelling and refractive-index modification in ion-irradiated yttrium orthoaluminate crystal, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.013
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Fig. 2. Bright-field TEM micrographs of YAlO3 crystal irradiated by 20.0 MeV Si3+ with the fluence of 1.0 1015 cm2. (a) un-irradiated region; (b) irradiated surface region; (c), (d) and (e) damage peak region under different magnifications; (f) electron diffraction pattern taken from damage peak region.
pattern further prove the existence of lattice damage and amorphous region induced by the nuclear collisions. Based on the spatial distance between the diffraction spots in the electron diffraction patterns (Fig. 2(a) and (f)), compared to the unirradiated region, the increase of lattice constant in the heavily damaged dpa-peak region could be confirmed qualitatively. Fig. 3 presents the measured XRD patterns (h–2h scan) of virgin and Si3+-irradiated YAlO3 samples. In this work, XRD measurements have been carried out several times, and the results have good stability and repeatability. YAlO3 crystal with (1 0 0) surface normal zone axis direction exhibits the characteristic peak at 2h = 73.1°, resulted from the reflection from (4 0 0) plane and corresponding to the lattice constant 5.17 Å. Besides the main
diffraction peak coming from the undamaged region, an additional scattered intensity appears on the lower angle side of the main peak, which corresponds to the heavily damaged region and reflects the dilatation strain along the direction perpendicular to sample surface [15–17]. The dilatation strain would be remarkably enhanced accompanying with the increasing dpa, indicating the first step of damage accumulation within the dpa range from 0.003 to 0.33 [18,19]. Considering the orthorhombic symmetry of YAlO3 crystal, the variation e of lattice constant (lattice strain) as a function of measured 2h could be given by the expression e ¼ ðdi d0 Þ=d0 , where di is the lattice constant calculated through Bragg equation di ¼ k=2sinh, and d0 is the initial lattice constant 5.17 Å. Thus, the final determined conversion from measured 2h
Please cite this article in press as: Y. Liu et al., Nuclear collision induced lattice swelling and refractive-index modification in ion-irradiated yttrium orthoaluminate crystal, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.013
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to e has been indicated through the top horizontal axis in Fig. 3, which clearly presents the lattice swelling in ion irradiation region. 3.2. Refractive-index modification
Fig. 3. Experimental h–2h scan curves recorded in the vicinity of (400) Bragg reflection on virgin and Si3+-irradiated YAlO3 crystal samples.
Under Si-ion irradiation over the energy range from a few MeV up to tens of MeV, the nuclear collision process could lead to the lattice damage and swelling in YAlO3 crystal. In order to study the refractive-index modification in the nuclear energy deposition region, dark-mode spectra of ion irradiation region have been measured utilizing prism coupling technique (633 nm laser with TM polarization state), and are shown in Fig. 4. Compared to the refractive index (1.948) of virgin sample, the refractive indices at the surface of S1, S2, S3 and S4 are 1.948, 1.947, 1.945 and 1.947, respectively, consisting with the dpa values (0.0001, 0.011, 0.017 and 0.01 at the surface of S1, S2, S3 and S4) and proving the refractive-index decrease behavior induced by the lattice damage and swelling. In this work, the maximum of refractive index decrease (buried low-index layer acting as optical barrier) appears at the heavily
Fig. 4. Dark mode spectra of Si3+-irradiated YAlO3 crystal samples measured by 633 nm laser under TM polarization.
Please cite this article in press as: Y. Liu et al., Nuclear collision induced lattice swelling and refractive-index modification in ion-irradiated yttrium orthoaluminate crystal, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.013
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Fig. 5. (a) iWKB-reconstructed refractive-index profile in ion irradiation region at 633 nm utilizing the effective refractive indices of guided modes; (b) near-field light intensity distribution of fundamental TM-polarized mode (TM0); (c) SRIM-simulated dpa profile for YAlO3 crystal irradiated by 20.0 MeV Si3+ with the fluence of 1.0 1015 cm2.
damaged dpa-peak region, and the refractive index in the layer between the surface and the heavily damaged dpa-peak region will not significantly change owing to the relatively low lattice damage and swelling in this region. Due to the refractive index of air layer above the sample surface is a constant value of 1, the modified refractive-index profile in ion irradiation region could lead to the waveguide formation between the air layer and the heavily damaged dpa-peak region. Effective refractive index (neff) is defined as a number quantifying the phase delay per unit length in the waveguide relative to the phase delay in vacuum, and has the meaning for light propagation in the waveguide with restricted transverse extension: b value (phase constant) of waveguide is the effective index times the vacuum wavenumber: b ¼ neff 2kp. For prepared multimode waveguide, the effective refractive index depends on the guided mode in which the light propagates. Actually, as shown in Fig. 4, each dip of light intensity measured by prism coupling technique corresponds to one specific guided mode, and the value of horizontal axis indicates the effective refractive index corresponding to this specific guided mode. Based on two-dimensional step-index waveguide model [20,21], the refractive index in the heavily damaged dpa-peak (optical barrier) region will affect the waveguide-supported mode number, and accompanying with the increasing irradiation damage, the decreasing refractive index in the optical barrier region could effectively promote the waveguide formation and increase the guided mode number. Therefore, compared to Fig. 4(a)–(c), the number of the dips (guided modes) in Fig. 4(d) will obviously increase owing to the increasing irradiation damage and the decreasing refractive index in the heavily damaged dpa-peak (optical barrier) region. Based on the effective refractive indices of guided modes in S4 (light intensity dips in Fig. 4(d)) and polynomial-fitted effective refractive index function (red curve in Fig. 5(a)), the refractiveindex profile (black curve in Fig. 5(a)) in ion irradiation region has been reconstructed utilizing iWKB procedure. The reconstructed refractive-index profile presents the approximate inverse relationship with SRIM-simulated dpa profile (Fig. 5(c)), further proving the refractive-index decrease behavior induced by the nuclear collision and lattice swelling. As shown in Fig. 5(b), the near-field intensity distribution of fundamental TM-polarized
mode (TM0) was measured by end-face coupling technique, indicating the dimension of formed waveguide and the effective confinement of light propagation. 4. Summary and conclusions In this work, the lattice damage, volume swelling and refractive-index modification in YAlO3 crystal irradiated with 6.0 MeV and 20.0 MeV Si3+ ions have been studied utilizing structure characterization techniques and optical measurements, and the effects of nuclear and electronic energy losses along ion trajectory have been discussed comparatively. Under Si-ion irradiation over the energy range from a few MeV up to tens of MeV, the intense electronic energy loss would not produce observable lattice damage, and therefore could not modify the refractive index in the electronic energy deposition region. By contrast, the measured irradiation damage and lattice swelling should be attributed to the nuclear energy loss, which will further induce the refractiveindex decrease in the nuclear energy deposition region and promote the waveguide formation in YAlO3 crystal. The results obtained from microstructure characterization techniques and optical measurements have presented reasonable consistency. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 11405097, U1432120), Natural Science Foundation of Shandong Province of China (No. ZR2014AQ021), and the Special Financial Grant (No. 2016T90624) from the China Postdoctoral Science Foundation. This work was also supported by the State Key Laboratory of Nuclear Physics and Technology, Peking University. References [1] W.J. Weber, D.M. Duffy, L. Thomé, Y. Zhang, Curr. Opin. Solid State Mater. Sci. 19 (2015) 1–11. [2] Y. Zhang, R. Sachan, O.H. Pakarinen, M.F. Chisholm, P. Liu, H. Xue, W.J. Weber, Nat. Commun. 6 (2015) 8049.
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Please cite this article in press as: Y. Liu et al., Nuclear collision induced lattice swelling and refractive-index modification in ion-irradiated yttrium orthoaluminate crystal, Nucl. Instr. Meth. B (2017), http://dx.doi.org/10.1016/j.nimb.2017.04.013