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Optically induced defects in vitreous silica
Applied Surface Science 154–155 Ž2000. 696–700 www.elsevier.nlrlocaterapsusc
Optically induced defects in vitreous silica S. Juodkazis a
a,)
, M. Watanabe a , H.-B. Sun a , S. Matsuo a , J. Nishii b, H. Misawa
a
S-VBL & Graduate School of Engineering, Tokushima UniÕersity, 2-1 Minamijosanjima, 770-8506 Tokushima, Japan Optical Materials DiÕision, Osaka National Research Institute, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
b
Received 1 June 1999; accepted 29 July 1999
Abstract We report the observation of photoluminescence ŽPL. in optically damaged vitreous silica Žv-SiO 2 . and its gradual decrease by annealing at temperature range from room temperature to 773 K. Optical damage was induced by tightly focused picosecond or femtosecond irradiation inside v-SiO 2 . PL bands at 280, 470 and 650 nm were observed. The PL can be excited by 250 nm irradiation, which corresponds to the absorption band of the oxygen vacancy, VO . The decrease of PL with annealing is explained by structural modifications of the defects in the damaged area. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Silica; Light induced defects; Photoluminescence; Thermal annealing of defects; Three-dimensional optical memory; Laser fabrication
1. Introduction High purity silica is widely used as UV optical material because of its high optical transmission. Most of that silica are of the type-III, synthesized directly by flame hydrolysis of SiCl 4 in H 2rO 2 flame. Silica used in core and clad of optical fibers is generally produced in soot remelting processes such as vapor-phase axial deposition ŽVAD. w1x, where SOCl 2 and Cl 2 gases are used as a dehydration reagent. These methods can produce OH free Ž- 0.01 ppm. silica to transmit near-IR light and allows to control refractive index by doping. However, Cl contamination of these silica can reach 1000 ppm. )
Corresponding author. Tel.: q81-088-656-9512; fax: q81088-656-7598. E-mail address: [email protected] ŽS. Juodkazis..
The purification of silica widens its transmission window but increases the probability of bond breaking due to the augmentation of built-in stress. While terminal groups such as OH and Cl reduce some amount of the highly strained bonds, which can be precursors of paramagnetic defects, i.e., EX Ž.Si Ø . or non-bridging oxygen hole Ž.Si–O Ø . centers ŽNBOHC. and peroxy radicals ŽPOR. Ž.Si–O–O Ø .. Ž – and Ø denote bond and lone pair electron, respectively.. A higher susceptibility of stressed SiO 2 to the generation of defects was demonstrated w2x. New applications were proposed recently for an optically modified silica, where ‘optical damage’ in the transmission region of the glass was made by high intensity pulsed laser irradiation, ca. 10 TWrcm2rpulse. Here, the term ‘optical damage’ is used for the light induced dielectric breakdown of transparent material by the multiphoton and
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 3 0 - 4
S. Juodkazis et al.r Applied Surface Science 154–155 (2000) 696–700
avalanche ionization. Fabrication of three-dimensional memory w3x and optical waveguides w4x by this technique was reported. Optical damaging is usually achieved by femtosecond Žfs. or picosecond Žps. pulsed irradiation through a microscope equipped with high numerical aperture objective ŽNA ) 1.. This allows focusing of psrfs-pulses on diffraction limited spots of size 1.22 lrNA- 1 mm for visible range, where l is the wavelength of irradiation. Intensities employed are far beyond the dielectric breakdown of most dielectric materials Žtens of GWrcm2 . and can cause direct bond scission for short, - 100 fs, pulses or melting with subsequent thermal quenching for longer ones w5x. In spite of a solid database on the defects in silica Žrecent reviews, Refs. w6,7x., optically induced defects are not yet well understood. Our aim was to compare isochronal annealing of the light induced defects by measuring their photoluminescence ŽPL. spectra and electron spin resonance ŽESR.. This comparison shows annealing of both diamagnetic and paramagnetic defects.
2. Experimental Samples of dry vitreous silica Žv-SiO 2 . with the hydroxyl group, OH, concentration less than 10 ppm ŽED samples. and silica with ca. 100 ppm of OH were selected for optical modification ŽTable 1.. They were irradiated in an optical microscope by focused pulsed laser shots through the oil-immersion objective of =100 magnification ŽOptiphot 2 Nikon with NA s 1.30 or IX70 Olympus with NA s 1.35.. Bits were fabricated by an amplified single femtosecond Žautocorrelation FWHMs 120 fs. or picosecond Ž30 ps. pulse. The light was tightly focused to induce an optical damage inside the volume of the sample to avoid post-damage chemical modifications
697
Fig. 1. PL spectra of optically damaged ED-C silica ŽOH -1 ppm. was excited by 250 nm illumination. Ža. Optical damage was introduced by ps-pulses 1 and 2 Ž30 ps, 532 nm. and fs-pulses 3 Ž120 fs, 800 nm. and 4, before optical damaging. Curve 1 is for single ps-shot damage while 2 is for 10 shots per damage. Žb. Filtered PL spectra of fs-damaged silica. The filtering by long-pass filters 370 and 540 nm Žtransmission scale in Žb.. was made to avoid detection of higher orders of short wavelength wing of spectra. Boxed areas denote second harmonic of fundamental at 375 nm and second diffraction order of excitation at 500 nm. The energy of single pulse used for optical damaging was 0.2 and 2 mJ for 30 ps and 120 fs pulses, respectively.
caused by atmosphere. The term ‘damage bit’ refers here to any observable silica transparency change recognizable by eye inspection with a conventional optical microscope. More details on the sample preparation can be found elsewhere w3x. ESR spectra were measured at 77 K at X-band frequency ŽBrucker Model 300 E. applying 100 kHz field modulation. The PL excitation light source was a Ti:sapphire fs-laser ŽTsunami. pumped by Nd:YVO4 laser ŽMillennia. with subsequent stages for frequency doubling and tripling Žall Spectra Physics.. We employed a spectrometer ŽSpectra-Pro 300i, Acton. with resolution better than 2 nm.
3. Results Irradiation of silica by a single psrfs-pulse of the energy of 0.05r0.2 mJ Žas evaluated at the plane of
Table 1 Sample list Sample
Preparation method
Impurities Žppm.
Company
OH
Cl
ED-C ED-B OH
VAD soot remetling VAD soot remetling Flame hydrolysis
-1 - 10 ) 100
830 1 not detected
Nippon Silica Glass Nippon Silica Glass Sumikin Sekiei
698
S. Juodkazis et al.r Applied Surface Science 154–155 (2000) 696–700
Fig. 2. Comparative PL spectra of different v-SiO 2 modified by fs -irradiation at 800 nm: 1, ED-C; 2, ED-B; and 3, OH. PL band at 560 nm is caused, in part, by second order diffraction of 280 nm.
incidence. is enough to create an observable alteration of the refractive index, when visible or near-IR Ž530r800 nm. laser irradiation is focused on a submicrometer sized spot by an objective of NA s 1.3 w3x. The most prominent change of the silica PL spectra is a strong 280 nm band attributable to oxygen vacancy, VO w8x, which we observed in all v-SiO 2 samples modified by the laser irradiation. An example of PL modification of ED-C sample is presented in Fig. 1. Initially, featureless PL spectra was changed after psrfs-irradiation. The spectra presented on Fig. 1Žb. were obtained using long-pass filters to observe separately PL bands at 470 and 650 nm. It is shown in Fig. 1Ža. that the spectral shape of PL depends on the pulse duration and on the dose of irradiation. Fs-irradiation induced modifications of PL of different v-SiO 2 are summarized in Fig. 2. The PL band at 470 nm is more intense in v-SiO 2 with higher content of OH. ESR spectra of Cl-free silica are the simplest for analysis, since there is no spin-coupling between free spins of dangling bonds and magnetic moment of Cl
Fig. 4. Temperature dependencies of integrated intensity of ESR signal and PL of oxygen vacancy ŽOH sample.. Samples were kept at each fixed temperature for 3 h. Data were collected on different samples Žfor every point on the plot., which were produced by the same 70 mW irradiation Ž1 kHz repetition rate. at 800 nm on the area of 4=4 mm2 . Power was measured at the entrance of microscope; pulse duration was 120 fs. Background level of ESR signal is plotted by dotted line. The lines are meant to guide the eye.
nucleus w9x. To trace a change in the defects concentration we made isochronal annealing of optically damaged v-SiO 2 Žsample OH. in the range of temperatures from room temperature ŽRT. to 773 K, keeping every sample at each fixed temperature for 3 h. ESR data are presented in Fig. 3 for different microwave power and modulation. The signal of EX center ŽFig. 3. was identified by its known form and spectroscopic splitting factors at g 2 s 2.0006 Žmaximum, zero-crossing and minimum of ESR signal of EX center are at g 1 s 2.0018, g 2 s 2.0006 and g 3 s 2.0003, respectively w10x.. Based on this assignment, another defect at g 2 s 2.0079 and g s 2.067 should
Fig. 3. Annealing of light induced defects. ESR spectrum of sample OH ŽOH concentration ; 100 ppm.. Microwave power and modulation were 1 mW and 0.2 mT Ža.; 0.1 mW and 0.1 mT Žb.. Annealing was carried out in atmospheric conditions for 3 h.
S. Juodkazis et al.r Applied Surface Science 154–155 (2000) 696–700
be POR Ž g s 2.067, g 1 s 2.0018, g 2 s 2.0079 w10x.. NBOHC with its g 2 s 2.0099 w10x was not observed or was superimposed with a stronger POR signal. In Fig. 4, intensity integrated ESR signal Žrecalculated to be proportional to the defects concentration. of EX and POR are plotted together with PL of 280 nm PL band of VO , main diamagnetic defect in an optically damaged v-SiO 2 . ŽFig. 4.. 4. Discussion The PL band at 360 nm ŽFig. 1. was found only in ps-modified v-SiO 2 and can not be attributed to the known defects. Most probably, it is the PL of Cl 2 that absorbs at ca. 350–310 nm w6x and was also detected by PL in fresh samples. Cl contamination of ED-C was the largest from all the samples we used. Ps-damaging is favorable for the formation of Cl 2 interstitials due to temperature increase at the damage area. Molization of Cl 2 from the Cl atoms, which appeared after the cleavage of Si–Cl bonds, is expected to occur by diffusion. As for NBOHC, it can be ESR inactive by electron or hole trapping w10x. In view of the fact that EX center can result from a hole trapping on neutral oxygen vacancy by the reaction w9x .Si–Si. .Si Ø qqSi. q ey the excess electron of initially excited Že–h. pair can charge an NBOHC defect. Such scenario is supported, also, by the fact that a weak PL of NBOHC at 650 nm w6x was observed in all the samples of optically damaged v-SiO 2 ŽFig. 1b.. A number of POR defects was found growing with the anealing up to 573 K after which the ESR signal decays. An increase in concentration of POR during thermal treatment can be understood from a consideration as follows. After optical damaging the creation of VO is accompanied with a complimentary Frenkel defect of O 2 interstitial orrand peroxy linkage ŽPO..Si–O–O–Si. w11x. Theoretical calculations on ideal b-cristobalite SiO 2 showed w12x that PO is supposed to relax into a POR state in which the undercoordinated O dangling bond is connected to an overcoordinated rather than a normal O atom, if a 0.8 eV activation energy is supplied. The primary process of the decay of concentration of all defects by annealing should be related to O 2 diffusion, which can be thermally activated at 473 K w11x. O 2 can be created as a result of direct light-induced
´
699
bond cleavage. Temperature activated diffusion of O 2 causes an increase of the POR by means of reaction w13x .Si Ø q O 2 .Si–O–O Ø , which is taking place at the expense of the concentration of EX center, a tendency observed experimentally ŽFig. 4. and, also, reported in silica exposed to 6.4 eV eximer laser light w14x. PL of NBOHC at 650 nm was measured in the annealed OH silica and was found following a similar tendency of the PL increase at 473–573 K and decrease at higher temperatures w15x. We found that the decay of 280 nm PL band and ESR signal of EX and POR defects starts at ca. 473 K, which could be expected for the standard massdensity silica where O 2 diffusion is not hampered w2x. Fourier transform IR spectroscopy Žoperated in reflection mode. on ED-B silica optically damaged by fs-pulses showed no evidence of spectral shift for Si–O–Si asymmetric stretching mode absorption at 1070 cmy1 w16x neither at 819 cmy1 w16x, the band which is caused by the rocking mode in bridging Si–O–Si bonds. This shows that there were no significant Si–O–Si angle changes, what was expected to occur in densified v-SiO 2 . Activation energy, D E, of PL decay can be evaluated from Arrhenius analysis ŽPL RT y PL T . s f Ž1rT . of the exponential decay of the concentration of a single defect. Identical values of activation energy D E f 0.77 eV were found for 280 and 470 nm PL bands, which implies that the same cause is involved in the concentration decay of both defects, supposedly a diffusion of O 2 . In spite of using a large simplification of the real interactions taking place during annealing, since more than two kinds of the defects are inter-reacting, this gives a clue on the activation energy of the process encountered. PL band at 470 nm is usually ascribed to the tripletŽT1 .-to-singletŽS 0 . transition of VO w6x with ms-time decay constant. We found that PL at 280 nm is decaying single exponentially with t s 3.7 ns time constant as it is expected for S 1 –S 0 transition of VO , but the 470 nm PL band decays by a stretched exponential law PL s PL 0 expŽyŽ trt . a . w17x. A new explanation is necessary for the 470 nm PL band in optically damaged v-SiO 2 . Stretched exponential shape of this decay implies hopping recombination of electron and hole separated on neighboring defects w18x, most probably on the vacancy and interstitial O 2 .
´
S. Juodkazis et al.r Applied Surface Science 154–155 (2000) 696–700
700
5. Conclusions
References
We described qualitatively the mechanisms that are responsible for the optically induced defect creation and annealing in v-SiO 2 . High intensity optical irradiation of v-SiO 2 forms Frenkel pairs Ž2VO –O 2 . along with EX , POR and NBOHC. Annealing up to 773 K showed that the reaction EX q O 2 POR is taking place by thermal activation of O 2 diffusion. The decay of PL of VO could be induced by the reaction 2VO q O 2 2Ž.Si–O–Si.. ŽPO formation, as a product of this reaction, is less probable due to higher atomic relaxation necessary.. On the minor scale, the formation of NBOHC and PO is possible as a result of O i thermally activated hopping. Total annealing of PL and ESR signal implies recovery of optically damaged material. This holds a promise for v-SiO 2 to be used as optically rewritable media.
w1x K. Chida, F. Hanawa, M. Nakahara, N. Inagaki, Electron. Lett. 15 Ž1979. 835. w2x R.A.B. Devine, J.J. Capponi, J. Arndt, Phys. Rev. B 35 Ž1987. 770. w3x M. Watanabe, H.-B. Sun, S. Juodkazis, T. Takahashi, S. Matsuo, Y. Suzuki, J. Nishii, H. Misawa, Jpn. J. Appl. Phys. 37 Ž1998. L1527. w4x K.M. Davis, K. Miura, N. Sugimoto, K. Hirao, Opt. Lett. 21 Ž1996. 1729. w5x B.C. Stiuart, M.D. Feit, S. Herman, A.M. Rubenchik, B.W. Shore, M.D. Perry, Phys. Rev. B 53 Ž1996. 1749. w6x L. Skuja, J. Non-Cryst. Solids 239 Ž1998. 16. w7x K. Shimakawa, A. Kolobov, S.R. Elliot, Adv. Phys. 44 Ž1995. 475. w8x L.N. Skuja, A.N. Streletsky, A.B. Pakovich, Solid State Commun. 50 Ž1984. 1069. w9x D.L. Griscom, E.J. Friebele, Phys. Rev. B 34 Ž1986. 7524. w10x H. Nishikawa, R. Nakamura, Y. Ohki, Phys. Rev. B 48 Ž1993. 15584. w11x A.H. Edwards, W.B. Fowler, Phys. Rev. B 26 Ž1982. 6649. w12x A.L. Shluger, M. Giorgiev, N. Itoh, Philos. Mag. B 63 Ž1991. 955. w13x H. Nishikawa, R. Nakamura, Y. Ohki, K. Nagasawa, Y. Hama, Phys. Rev. B 46 Ž1992. 8073. w14x T.E. Tsai, D.L. Griscom, Phys. Rev. Lett. 67 Ž1991. 2517. w15x H.-B. Sun, S. Juodkazis, M. Watanabe, S. Matsuo, H. Misawa, J. Nishii, to be published. w16x K. Kim, Phys. Rev. B 57 Ž1998. 13072. w17x M. Watanabe, S. Juodkazis, H.-B. Sun, S. Matsuo, H. Misawa, Phys. Rev. B 60 Ž1999. 9959. w18x L. Pavesi, J. Appl. Phys. 80 Ž1996. 216.
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Acknowledgements The present work was partly supported by a Grant-in-Aid for Scientific Research ŽA.Ž2. from the Ministry of Education, Science, Sports and Culture Ž09355008. and Satellite Venture Business Laboratory of the University of Tokushima. The authors thank Prof. H. Hosono for useful discussions.
Optically induced defects in vitreous silica S. Juodkazis a
a,)
, M. Watanabe a , H.-B. Sun a , S. Matsuo a , J. Nishii b, H. Misawa
a
S-VBL & Graduate School of Engineering, Tokushima UniÕersity, 2-1 Minamijosanjima, 770-8506 Tokushima, Japan Optical Materials DiÕision, Osaka National Research Institute, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
b
Received 1 June 1999; accepted 29 July 1999
Abstract We report the observation of photoluminescence ŽPL. in optically damaged vitreous silica Žv-SiO 2 . and its gradual decrease by annealing at temperature range from room temperature to 773 K. Optical damage was induced by tightly focused picosecond or femtosecond irradiation inside v-SiO 2 . PL bands at 280, 470 and 650 nm were observed. The PL can be excited by 250 nm irradiation, which corresponds to the absorption band of the oxygen vacancy, VO . The decrease of PL with annealing is explained by structural modifications of the defects in the damaged area. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Silica; Light induced defects; Photoluminescence; Thermal annealing of defects; Three-dimensional optical memory; Laser fabrication
1. Introduction High purity silica is widely used as UV optical material because of its high optical transmission. Most of that silica are of the type-III, synthesized directly by flame hydrolysis of SiCl 4 in H 2rO 2 flame. Silica used in core and clad of optical fibers is generally produced in soot remelting processes such as vapor-phase axial deposition ŽVAD. w1x, where SOCl 2 and Cl 2 gases are used as a dehydration reagent. These methods can produce OH free Ž- 0.01 ppm. silica to transmit near-IR light and allows to control refractive index by doping. However, Cl contamination of these silica can reach 1000 ppm. )
Corresponding author. Tel.: q81-088-656-9512; fax: q81088-656-7598. E-mail address: [email protected] ŽS. Juodkazis..
The purification of silica widens its transmission window but increases the probability of bond breaking due to the augmentation of built-in stress. While terminal groups such as OH and Cl reduce some amount of the highly strained bonds, which can be precursors of paramagnetic defects, i.e., EX Ž.Si Ø . or non-bridging oxygen hole Ž.Si–O Ø . centers ŽNBOHC. and peroxy radicals ŽPOR. Ž.Si–O–O Ø .. Ž – and Ø denote bond and lone pair electron, respectively.. A higher susceptibility of stressed SiO 2 to the generation of defects was demonstrated w2x. New applications were proposed recently for an optically modified silica, where ‘optical damage’ in the transmission region of the glass was made by high intensity pulsed laser irradiation, ca. 10 TWrcm2rpulse. Here, the term ‘optical damage’ is used for the light induced dielectric breakdown of transparent material by the multiphoton and
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 4 3 0 - 4
S. Juodkazis et al.r Applied Surface Science 154–155 (2000) 696–700
avalanche ionization. Fabrication of three-dimensional memory w3x and optical waveguides w4x by this technique was reported. Optical damaging is usually achieved by femtosecond Žfs. or picosecond Žps. pulsed irradiation through a microscope equipped with high numerical aperture objective ŽNA ) 1.. This allows focusing of psrfs-pulses on diffraction limited spots of size 1.22 lrNA- 1 mm for visible range, where l is the wavelength of irradiation. Intensities employed are far beyond the dielectric breakdown of most dielectric materials Žtens of GWrcm2 . and can cause direct bond scission for short, - 100 fs, pulses or melting with subsequent thermal quenching for longer ones w5x. In spite of a solid database on the defects in silica Žrecent reviews, Refs. w6,7x., optically induced defects are not yet well understood. Our aim was to compare isochronal annealing of the light induced defects by measuring their photoluminescence ŽPL. spectra and electron spin resonance ŽESR.. This comparison shows annealing of both diamagnetic and paramagnetic defects.
2. Experimental Samples of dry vitreous silica Žv-SiO 2 . with the hydroxyl group, OH, concentration less than 10 ppm ŽED samples. and silica with ca. 100 ppm of OH were selected for optical modification ŽTable 1.. They were irradiated in an optical microscope by focused pulsed laser shots through the oil-immersion objective of =100 magnification ŽOptiphot 2 Nikon with NA s 1.30 or IX70 Olympus with NA s 1.35.. Bits were fabricated by an amplified single femtosecond Žautocorrelation FWHMs 120 fs. or picosecond Ž30 ps. pulse. The light was tightly focused to induce an optical damage inside the volume of the sample to avoid post-damage chemical modifications
697
Fig. 1. PL spectra of optically damaged ED-C silica ŽOH -1 ppm. was excited by 250 nm illumination. Ža. Optical damage was introduced by ps-pulses 1 and 2 Ž30 ps, 532 nm. and fs-pulses 3 Ž120 fs, 800 nm. and 4, before optical damaging. Curve 1 is for single ps-shot damage while 2 is for 10 shots per damage. Žb. Filtered PL spectra of fs-damaged silica. The filtering by long-pass filters 370 and 540 nm Žtransmission scale in Žb.. was made to avoid detection of higher orders of short wavelength wing of spectra. Boxed areas denote second harmonic of fundamental at 375 nm and second diffraction order of excitation at 500 nm. The energy of single pulse used for optical damaging was 0.2 and 2 mJ for 30 ps and 120 fs pulses, respectively.
caused by atmosphere. The term ‘damage bit’ refers here to any observable silica transparency change recognizable by eye inspection with a conventional optical microscope. More details on the sample preparation can be found elsewhere w3x. ESR spectra were measured at 77 K at X-band frequency ŽBrucker Model 300 E. applying 100 kHz field modulation. The PL excitation light source was a Ti:sapphire fs-laser ŽTsunami. pumped by Nd:YVO4 laser ŽMillennia. with subsequent stages for frequency doubling and tripling Žall Spectra Physics.. We employed a spectrometer ŽSpectra-Pro 300i, Acton. with resolution better than 2 nm.
3. Results Irradiation of silica by a single psrfs-pulse of the energy of 0.05r0.2 mJ Žas evaluated at the plane of
Table 1 Sample list Sample
Preparation method
Impurities Žppm.
Company
OH
Cl
ED-C ED-B OH
VAD soot remetling VAD soot remetling Flame hydrolysis
-1 - 10 ) 100
830 1 not detected
Nippon Silica Glass Nippon Silica Glass Sumikin Sekiei
698
S. Juodkazis et al.r Applied Surface Science 154–155 (2000) 696–700
Fig. 2. Comparative PL spectra of different v-SiO 2 modified by fs -irradiation at 800 nm: 1, ED-C; 2, ED-B; and 3, OH. PL band at 560 nm is caused, in part, by second order diffraction of 280 nm.
incidence. is enough to create an observable alteration of the refractive index, when visible or near-IR Ž530r800 nm. laser irradiation is focused on a submicrometer sized spot by an objective of NA s 1.3 w3x. The most prominent change of the silica PL spectra is a strong 280 nm band attributable to oxygen vacancy, VO w8x, which we observed in all v-SiO 2 samples modified by the laser irradiation. An example of PL modification of ED-C sample is presented in Fig. 1. Initially, featureless PL spectra was changed after psrfs-irradiation. The spectra presented on Fig. 1Žb. were obtained using long-pass filters to observe separately PL bands at 470 and 650 nm. It is shown in Fig. 1Ža. that the spectral shape of PL depends on the pulse duration and on the dose of irradiation. Fs-irradiation induced modifications of PL of different v-SiO 2 are summarized in Fig. 2. The PL band at 470 nm is more intense in v-SiO 2 with higher content of OH. ESR spectra of Cl-free silica are the simplest for analysis, since there is no spin-coupling between free spins of dangling bonds and magnetic moment of Cl
Fig. 4. Temperature dependencies of integrated intensity of ESR signal and PL of oxygen vacancy ŽOH sample.. Samples were kept at each fixed temperature for 3 h. Data were collected on different samples Žfor every point on the plot., which were produced by the same 70 mW irradiation Ž1 kHz repetition rate. at 800 nm on the area of 4=4 mm2 . Power was measured at the entrance of microscope; pulse duration was 120 fs. Background level of ESR signal is plotted by dotted line. The lines are meant to guide the eye.
nucleus w9x. To trace a change in the defects concentration we made isochronal annealing of optically damaged v-SiO 2 Žsample OH. in the range of temperatures from room temperature ŽRT. to 773 K, keeping every sample at each fixed temperature for 3 h. ESR data are presented in Fig. 3 for different microwave power and modulation. The signal of EX center ŽFig. 3. was identified by its known form and spectroscopic splitting factors at g 2 s 2.0006 Žmaximum, zero-crossing and minimum of ESR signal of EX center are at g 1 s 2.0018, g 2 s 2.0006 and g 3 s 2.0003, respectively w10x.. Based on this assignment, another defect at g 2 s 2.0079 and g s 2.067 should
Fig. 3. Annealing of light induced defects. ESR spectrum of sample OH ŽOH concentration ; 100 ppm.. Microwave power and modulation were 1 mW and 0.2 mT Ža.; 0.1 mW and 0.1 mT Žb.. Annealing was carried out in atmospheric conditions for 3 h.
S. Juodkazis et al.r Applied Surface Science 154–155 (2000) 696–700
be POR Ž g s 2.067, g 1 s 2.0018, g 2 s 2.0079 w10x.. NBOHC with its g 2 s 2.0099 w10x was not observed or was superimposed with a stronger POR signal. In Fig. 4, intensity integrated ESR signal Žrecalculated to be proportional to the defects concentration. of EX and POR are plotted together with PL of 280 nm PL band of VO , main diamagnetic defect in an optically damaged v-SiO 2 . ŽFig. 4.. 4. Discussion The PL band at 360 nm ŽFig. 1. was found only in ps-modified v-SiO 2 and can not be attributed to the known defects. Most probably, it is the PL of Cl 2 that absorbs at ca. 350–310 nm w6x and was also detected by PL in fresh samples. Cl contamination of ED-C was the largest from all the samples we used. Ps-damaging is favorable for the formation of Cl 2 interstitials due to temperature increase at the damage area. Molization of Cl 2 from the Cl atoms, which appeared after the cleavage of Si–Cl bonds, is expected to occur by diffusion. As for NBOHC, it can be ESR inactive by electron or hole trapping w10x. In view of the fact that EX center can result from a hole trapping on neutral oxygen vacancy by the reaction w9x .Si–Si. .Si Ø qqSi. q ey the excess electron of initially excited Že–h. pair can charge an NBOHC defect. Such scenario is supported, also, by the fact that a weak PL of NBOHC at 650 nm w6x was observed in all the samples of optically damaged v-SiO 2 ŽFig. 1b.. A number of POR defects was found growing with the anealing up to 573 K after which the ESR signal decays. An increase in concentration of POR during thermal treatment can be understood from a consideration as follows. After optical damaging the creation of VO is accompanied with a complimentary Frenkel defect of O 2 interstitial orrand peroxy linkage ŽPO..Si–O–O–Si. w11x. Theoretical calculations on ideal b-cristobalite SiO 2 showed w12x that PO is supposed to relax into a POR state in which the undercoordinated O dangling bond is connected to an overcoordinated rather than a normal O atom, if a 0.8 eV activation energy is supplied. The primary process of the decay of concentration of all defects by annealing should be related to O 2 diffusion, which can be thermally activated at 473 K w11x. O 2 can be created as a result of direct light-induced
´
699
bond cleavage. Temperature activated diffusion of O 2 causes an increase of the POR by means of reaction w13x .Si Ø q O 2 .Si–O–O Ø , which is taking place at the expense of the concentration of EX center, a tendency observed experimentally ŽFig. 4. and, also, reported in silica exposed to 6.4 eV eximer laser light w14x. PL of NBOHC at 650 nm was measured in the annealed OH silica and was found following a similar tendency of the PL increase at 473–573 K and decrease at higher temperatures w15x. We found that the decay of 280 nm PL band and ESR signal of EX and POR defects starts at ca. 473 K, which could be expected for the standard massdensity silica where O 2 diffusion is not hampered w2x. Fourier transform IR spectroscopy Žoperated in reflection mode. on ED-B silica optically damaged by fs-pulses showed no evidence of spectral shift for Si–O–Si asymmetric stretching mode absorption at 1070 cmy1 w16x neither at 819 cmy1 w16x, the band which is caused by the rocking mode in bridging Si–O–Si bonds. This shows that there were no significant Si–O–Si angle changes, what was expected to occur in densified v-SiO 2 . Activation energy, D E, of PL decay can be evaluated from Arrhenius analysis ŽPL RT y PL T . s f Ž1rT . of the exponential decay of the concentration of a single defect. Identical values of activation energy D E f 0.77 eV were found for 280 and 470 nm PL bands, which implies that the same cause is involved in the concentration decay of both defects, supposedly a diffusion of O 2 . In spite of using a large simplification of the real interactions taking place during annealing, since more than two kinds of the defects are inter-reacting, this gives a clue on the activation energy of the process encountered. PL band at 470 nm is usually ascribed to the tripletŽT1 .-to-singletŽS 0 . transition of VO w6x with ms-time decay constant. We found that PL at 280 nm is decaying single exponentially with t s 3.7 ns time constant as it is expected for S 1 –S 0 transition of VO , but the 470 nm PL band decays by a stretched exponential law PL s PL 0 expŽyŽ trt . a . w17x. A new explanation is necessary for the 470 nm PL band in optically damaged v-SiO 2 . Stretched exponential shape of this decay implies hopping recombination of electron and hole separated on neighboring defects w18x, most probably on the vacancy and interstitial O 2 .
´
S. Juodkazis et al.r Applied Surface Science 154–155 (2000) 696–700
700
5. Conclusions
References
We described qualitatively the mechanisms that are responsible for the optically induced defect creation and annealing in v-SiO 2 . High intensity optical irradiation of v-SiO 2 forms Frenkel pairs Ž2VO –O 2 . along with EX , POR and NBOHC. Annealing up to 773 K showed that the reaction EX q O 2 POR is taking place by thermal activation of O 2 diffusion. The decay of PL of VO could be induced by the reaction 2VO q O 2 2Ž.Si–O–Si.. ŽPO formation, as a product of this reaction, is less probable due to higher atomic relaxation necessary.. On the minor scale, the formation of NBOHC and PO is possible as a result of O i thermally activated hopping. Total annealing of PL and ESR signal implies recovery of optically damaged material. This holds a promise for v-SiO 2 to be used as optically rewritable media.
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Acknowledgements The present work was partly supported by a Grant-in-Aid for Scientific Research ŽA.Ž2. from the Ministry of Education, Science, Sports and Culture Ž09355008. and Satellite Venture Business Laboratory of the University of Tokushima. The authors thank Prof. H. Hosono for useful discussions.