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Kinetics of UV laser radiation defects in high performance glasses
Nuclear Instruments and Methods in Physics Research B 166±167 (2000) 470±475
www.elsevier.nl/locate/nimb
Kinetics of UV laser radiation defects in high performance glasses q U. Natura b
a,*
, T. Feurer b, D. Ehrt
a
a Otto-Schott-Institut, Friedrich-Schiller-Universit at Jena, Fraunhoferstr. 6, 07743 Jena, Germany Institut f ur Optik und Quantenelektronik, Friedrich-Schiller-Universit at Jena, Max-Wien-Platz 1, 07743 Jena, Germany
Abstract High purity ¯uoride phosphate glasses are attractive candidates as UV transmitting materials. The calculated values for the ultraviolet resonance wavelength are comparable with those of pure silica glass or ¯uoride single crystal CaF2 . The formation of radiation-induced defect centers leads to additional absorption bands in the VUV±UV±vis range. The damage and the healing behavior by lamps and lasers are investigated in dependence on phosphate content and the content of impurities, mainly transition metals. Experiments were carried out using pulsed lasers with a duration of femto- and nanoseconds at a wavelength of 248 nm. The initial slope of the induced absorption shows a nonlinear dependence on the pulse energy density. Resonant and non-resonant two-photon mechanisms were observed. Twophoton-absorption coecients at 248 nm for samples with dierent phosphate contents were measured. Models of the kinetics of the radiation-induced defects were developed. The inclusion of energy transfer was necessary to explain the dierence in the damage behavior for nanosecond (248 nm, 193 nm) and femtosecond (248 nm) laser pulses. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 61.80.Az; 61.80.Ba; 78.40.-q Keywords: Irradiation; Defect; Fluoride phosphate glass; Two-photon absorption; Two-step absorption; Energy transfer
1. Introduction The demand for high performance glasses as ultraviolet transmitting materials for lens systems in microlithography equipment and excimer laser
q Supported by foundation of Deutsche Forschungsgemeinschaft (No. Eh 140/2-1). * Corresponding author. Tel.: +49-3641-948523; fax: +493641-948502. E-mail address: [email protected] (U. Natura).
optics has increased [1,2]. Fluoride and ¯uoride phosphate (FP) glasses are attractive candidates as UV transmitting materials. The production of ¯uoride glasses is very dicult because they crystallize easily. Doping with small amounts of phosphate increases the glass forming ability of ¯uoro aluminate glasses drastically, however, the band gap becomes smaller with increasing phosphate content [3,4]. The ultraviolet transmission of glasses is limited by extrinsic absorption due to traces of impurities, mainly transition metals, such as Fe, Cu and Pb. The radiation-induced
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 9 8 - 9
U. Natura et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 470±475
formation of defect centers leads to a decrease in optical transmission in the ultraviolet (UV) and visible (vis) spectral ranges depending on phosphate content [5] and impurities [6]. Investigations of defect centers by EPR-spectroscopy were carried out by Hosono et al. [7] and Ebeling et al. [8]. In this work, the resistance of FP glasses to radiation damage due to lamp and excimer laser illumination is studied. In addition, the mechanisms of defect generation due to fs (248 nm)- and ns (248 nm, 193 nm)-pulses are investigated. 2. Experimental details Samples were melted under normal (air, Pt crucibles) and reducing (Ar atmosphere, carbon crucible) conditions. Samples melted under reducing conditions showed better UV-transmission than samples melted under normal conditions because the extinction coecient of Fe2 is much smaller than that of Fe3 . Further details are described in previous papers [9±11]. Table 1 gives an overview of the investigated samples. Raw materials of normal optical quality and raw materials of especially high purity (designated by s) were used. To investigate the in¯uence of phosphate content on defect generation, glasses of dierent phosphate content were used. The samples FP2, FP4, FP10 and FP20 were melted under normal conditions. The sample FP10 showed the best glass formation [3], therefore, this type was used to investigate the
471
in¯uence of the impurities on the defect generation. The main impurity was iron. Table 1 shows (in brackets) the normal content of impurities of lead and copper in glasses, melted under reducing conditions. To investigate the in¯uence of lead and copper on defect generation, samples of type FP10red were doped with 25 ppm Cu or Pb and melted under reducing conditions (designated by ``red'') in order to obtain Cu and Pb2 in the glass. Illumination was carried out with a mercury lamp (HOK-lamp) and excimer lasers. The spectrum of the lamp covers continuously the UV±vis± IR-range and starts at 190 nm. In the UV-region up to 280nm, the irradiation has an intensity of 1500 W/m2 . We used the ArF- and KrF-excimer lasers with a pulse width of 20 ns and energy densities up to 400 mJ/cm2 (KrF) and 100 mJ/cm2 (ArF) per pulse. We also used fs-pulses at a wavelength of 248 nm with energy densities up to 12 mJ/cm2 per pulse [12]. It was possible to vary the pulsewidth in the range of 300±1000 fs by a prism compressor. The intensity of the fs-pulses was approximately 1000 times the intensity of nspulses. The laser light was focused onto a 5 mm2 area of the sample. The optical absorption or transmission spectra in the range of 190±800 nm were recorded by a double beam spectrometer manufactured by Shimadzu (error <1%) before (transmission T0 ) and after (transmission T) various exposure times or several laser pulses. The induced extinction Ek log T0 =T k was normalized to a pathlength of 1 cm.
Table 1 Investigated samples Sample
Phosphate content (mol%)
Fluoride content (mol%)
FP2 FP4 FP10 FP20
2 4 10 20
98 96 90 80
Sample
Fe/Pb/Cu
Melting conditions
Redox state
FP10s/red FP10red
6 ppm Fe (1 ppm Pb; 0.5 ppm Cu) 18 ppm Fe (2.5 ppm Pb; 1 ppm Cu) +25 ppm Cu +25 ppm Pb
Reducing Reducing
6 ppm Fe2 18 ppm Fe2 +25 ppm Cu +25 ppm Pb2
472
U. Natura et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 470±475
3. Results and discussion Fig. 1 shows transmission spectra of the sample FP10s/red before and after irradiation nearly up to the saturation value with the lamp, the KrF-excimer laser, and the ArF-excimer laser. The lamp causes a decrease in transmission only in the UV region, whereas laser radiation causes additional absorption bands in the UV and the visible range. The defect generation by the ArF laser is much stronger than the defect generation by the KrF laser. To create the same decrease in transmission a much higher energy density of KrF-excimer laser is required. Fig. 2 shows the spectrum of induced extinction after irradiation by ArF-excimer laser. The spectrum was simulated by a separation of absorption bands. The decrease in transmission could be explained by photo oxidation of iron [11,13] and the generation of hole (HC) and electron (EC) centers shown in Fig. 2. The three absorption bands of HC2 associated with a small fraction of the EC1 , EC2 were observed only after laser irradiation. The other absorption bands were also created by irradiation with the HOK lamp. The formation of HC2 showed interesting eects. k 430 nm was chosen as a characteristic wavelength to show the defect-generation curves of the absorption bands of HC2 . To explain the mecha-
Fig. 2. Simulation and separation of radiation-induced absorption bands in FP10s/red after irradiation with the KrFexcimer laser (40 mJ/cm2 , 5000 pulses).
nisms of defect generation, the following experiments were carried out. 3.1. Experiments with fs-pulses Experiments with pulses of the same energy density (4 mJ/cm2 ), but dierent intensities varying the pulse length using pulses of 300 and 600 fs were carried out. A dependence of the initial slope of defect generation on intensity was observed. This is evidence of a two-photon process. The beam attenuation along the z-direction can be described according to dI ÿaI ÿ bI 2 ; dz
Fig. 1. Transmission spectra of the unirradiated (- - -) sample FP10s/red (thickness d 2 mm) and after irradiation nearly up to the saturation value with (--) HOK lamp (100 h), (Ð) KrFexcimer laser (400 mJ/cm2 , 10000 pulses), ( ) ArF-excimer laser (40 mJ/cm2 , 5000 pulses).
1
where I z is the beam intensity and a and b are the one- and two-photon absorption coecients. The dependence of the transmittance T on the input intensity is then obtained by integrating (1) [14]. The two-photon absorption coecients b were determined from intensity-dependent transmission experiments: bFP4 1:3 10ÿ10 cm/W, bFP10s=red 2:4 10ÿ10 cm/W, bFP10red 2:7 10ÿ10 cm=W; bFP20 4:5 10ÿ10 cm=W: b strongly depends on phosphate content of the samples according to the initial slope of the defect-generation curves (Fig. 3). As a reference, the two-photon absorption coe-
U. Natura et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 470±475
473
Fig. 3. Defect generation at k 430 nm in FP-glasses of different phosphate content by irradiation with fs-pulses (248 nm): (1) FP20, (2) FP10s/red, (3) FP4.
cient of SiO2 was measured (bSiO2 5:0 10ÿ11 cm=W). This value is very similar to bSiO2 4:5 10ÿ11 cm=W published in [15]. 3.2. Experiments with ns-pulses The in¯uence of the impurities was investigated in addition to the in¯uence of phosphate content on defect generation. The samples doped with copper showed a similar defect generation as the undoped sample. But the samples doped with lead showed the strong in¯uence of lead on both the initial slope and the saturation value of defect generation. The defect generation was recorded depending on energy density F (Fig. 4(a)). Samples doped with lead (FP10red/25 ppm Pb) showed a dependence that was less than quadratic. A radiation-induced recovery of the induced extinction was observed after irradiation (KrF) of a sample, preirradiated by F1 400 mJ/cm2 , by an energy density F2 200 mJ/cm2 less than F1 . The defectgeneration curve (1) in Fig. 4(a) shows a decrease of the induced extinction after a maximum is passed. Consequently, the sample preirradiated with F1 reaches a smaller apparent saturation value after irradiation with the lower energy density F2 than the non-preirradiated sample (Fig. 4(b)). The solid lines in Fig. 4 are calculated defectgeneration curves. The calculation was based on the model shown in Fig. 5.
Fig. 4. Defect generation at k 430 nm in the lead doped sample FP10red/25 ppm Pb depending on energy density (248 nm, ns-pulses): (a) defect generation curves for (1) sample 1 (F1 400 mJ=cm2 ), (2) sample 2 (F2 200 mJ=cm2 ), (3) sample 3 (F3 100 mJ=cm2 ); (b) defect generation curves for samples irradiated at F2 200 mJ=cm2 : (1)* sample 1 after preirradiation by 10000 pulses at F1 400 mJ=cm2 (as shown in (a)), (2) sample 2 without preirradiation.
3.3. Modeling of defect generation The defect generation due to ns-pulses can be explained by a two-step absorption process introducing an energy transfer process (Fig. 5). The existence of Pb2 is very important for the defect generation. Pb2 absorbs a photon, and the excited state of lead has a lifetime sPb . Pb2 disappears with increasing exposure time, described by a rate of disappearance k1 . An energy transfer takes place from the excited Pb2 -ions to the precursors VL of defect centers (described by an energy
474
U. Natura et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 470±475
Fig. 5. Model of defect generation for nanosecond and femtosecond cases.
transfer rate k2 ). The excited state has the lifetime sS . Excited precursors are able to absorb a photon and so the defect D is created. The one-photon cross-section r1 describes the radiation-induced recovery of defect centers. The disappearance of Pb2 -ions explains the loss of induced extinction after passing a maximum and the dierence in the apparent saturation values in the case of irradiation of a pre- and a non-preirradiated sample. The reciprocal rate of energy transfer is similar to the laser pulse duration. It explains the dierence in the mechanisms of defect generation for ns- and fspulses. Energy transfer is more appreciable with input beams with longer pulse width. The same cross-sections determining the two-step process and the same lifetimes sPb and sS were used for modeling the defect generation in glasses of type FP10 with diering content of Pb2 . The initial concentration of Pb2 was put in the calculation and the energy transfer rate was varied. The defect generation curves of FP10s/red (1 ppm Pb2 ) and FP10red (2.5 ppm Pb2 ) were calculated and showed a good agreement with the measured values. The dependence of the initial slope of defect generation on energy density is quadratic in the case of irradiation of FP10s/red by KrF-excimer laser. The defect generation by ArF-excimer laser is much stronger than by KrFexcimer laser and the dependence on the energy density becomes less than quadratic. Fig. 6 shows the calculated defect generation curves and measured values of the sample FP10s/red irradiated
Fig. 6. Defect generation at k 430 nm in the sample FP10s/ red irradiated by ArF-excimer laser (ns-pulses) depending on energy density (F) and calculated defect-generation curves.
with the ArF-excimer laser (ns-pulses). Both the initial slope and the saturation values of the defect generation of FP10s/red by ArF-excimer laser are very similar to the defect generation in the leaddoped sample by KrF-excimer laser (Fig. 5(a)). We can explain this with the wavelength-dependent absorption of Pb2 . The absorption of 25 ppm at 248 nm is similar to the absorption of 1 ppm at 193 nm (Gaussian shape of absorption band with a maximum at 207 nm, band width of 6400 cmÿ1 ). The calculation of the defect generation curves was based on the described model.
4. Summary The generation of the defect centres HC2 requires high intensities. Defects are generated by laser radiation. A radiation-induced recovery was observed. The defect generation by fs-pulses was dierent from the ns-results and could be explained by a two-photon process. We measured the two-photon-absorption-coecients and obtained a dependence on the phosphate content. The defect generation by ns-pulses could be explained by a two-step-process introducing an energy transfer. The defect generation increases with increasing phosphate content or increasing content of Pb2 . Pb2 disappears with irradiation time. The inclusion of energy transfer between the ex-
U. Natura et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 470±475
cited Pb2 -ions and the precursors was necessary to explain the dierence in the defect-generation mechanisms for ns- and fs-laser-pulses. A model was developed. Based on this model, defect-generation curves for samples of dierent content of lead could be calculated with constant cross-sections of the two-step process and constant lifetimes of the excited states, varying the content of lead and the rate of energy transfer. There was good agreement between the measurements and the model calculations. References [1] D. Ehrt, W. Seeber, J. Non-Cryst. Solids 129 (1991) 19. [2] N. Kitamura, J. Hayakawa, H. Yamashita, J. Non-Cryst. Solids 126 (1990) 155. [3] D. Ehrt, Habilitation, Friedrich-Schiller-Universit at Jena, 1984.
475
[4] D. Ehrt, SPIE 1761 (1992) 213. [5] H. Toratani, X. Zou, Y. Matsumoto, Appl. Phys. 35 (1996) 6351. [6] H. Ebendor-Heidepriem, D. Ehrt, J. Non-Cryst. Solids 196 (1996) 113. [7] H. Hosono, M. Mizuguchi, H. Kawazoe, T. Ichimura, Y. Watanabe, Y. Shinkuma, T. Ogawa, J. Appl. Phys. 85 (6) (1999) 3038. [8] P. Ebeling, D. Ehrt, M. Friedrich, in: 73rd Meeting of DGG, Halle, May 1999. [9] D. Ehrt, J. Non-Cryst. Solids 196 (1996) 304. [10] D. Ehrt, M. Carl, T. Kittel, M. M uller, W. Seeber, J. NonCryst. Solids 177 (1994) 405. [11] D. Ehrt, U. Natura, P. Ebeling, M. M uller, in: Proceedings of the Eighteenth International Congress on Glass C 10 (1998) 1. [12] Stephen Paul le Blanc, Ph.D., Rice University Houston, April 1991. [13] X. Zou, H. Toratani, J. Appl. Phys. 81 (8) (1997) 3354. [14] Y.P. Kim, M.H.R. Hutchinson, Appl. Phys. B 49 (1989) 469. [15] A.J. Taylor, R.B. Gibson, J.P. Roberts, Optics Lett. 13 (10) (1988) 814.
www.elsevier.nl/locate/nimb
Kinetics of UV laser radiation defects in high performance glasses q U. Natura b
a,*
, T. Feurer b, D. Ehrt
a
a Otto-Schott-Institut, Friedrich-Schiller-Universit at Jena, Fraunhoferstr. 6, 07743 Jena, Germany Institut f ur Optik und Quantenelektronik, Friedrich-Schiller-Universit at Jena, Max-Wien-Platz 1, 07743 Jena, Germany
Abstract High purity ¯uoride phosphate glasses are attractive candidates as UV transmitting materials. The calculated values for the ultraviolet resonance wavelength are comparable with those of pure silica glass or ¯uoride single crystal CaF2 . The formation of radiation-induced defect centers leads to additional absorption bands in the VUV±UV±vis range. The damage and the healing behavior by lamps and lasers are investigated in dependence on phosphate content and the content of impurities, mainly transition metals. Experiments were carried out using pulsed lasers with a duration of femto- and nanoseconds at a wavelength of 248 nm. The initial slope of the induced absorption shows a nonlinear dependence on the pulse energy density. Resonant and non-resonant two-photon mechanisms were observed. Twophoton-absorption coecients at 248 nm for samples with dierent phosphate contents were measured. Models of the kinetics of the radiation-induced defects were developed. The inclusion of energy transfer was necessary to explain the dierence in the damage behavior for nanosecond (248 nm, 193 nm) and femtosecond (248 nm) laser pulses. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.43.Fs; 61.80.Az; 61.80.Ba; 78.40.-q Keywords: Irradiation; Defect; Fluoride phosphate glass; Two-photon absorption; Two-step absorption; Energy transfer
1. Introduction The demand for high performance glasses as ultraviolet transmitting materials for lens systems in microlithography equipment and excimer laser
q Supported by foundation of Deutsche Forschungsgemeinschaft (No. Eh 140/2-1). * Corresponding author. Tel.: +49-3641-948523; fax: +493641-948502. E-mail address: [email protected] (U. Natura).
optics has increased [1,2]. Fluoride and ¯uoride phosphate (FP) glasses are attractive candidates as UV transmitting materials. The production of ¯uoride glasses is very dicult because they crystallize easily. Doping with small amounts of phosphate increases the glass forming ability of ¯uoro aluminate glasses drastically, however, the band gap becomes smaller with increasing phosphate content [3,4]. The ultraviolet transmission of glasses is limited by extrinsic absorption due to traces of impurities, mainly transition metals, such as Fe, Cu and Pb. The radiation-induced
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 9 8 - 9
U. Natura et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 470±475
formation of defect centers leads to a decrease in optical transmission in the ultraviolet (UV) and visible (vis) spectral ranges depending on phosphate content [5] and impurities [6]. Investigations of defect centers by EPR-spectroscopy were carried out by Hosono et al. [7] and Ebeling et al. [8]. In this work, the resistance of FP glasses to radiation damage due to lamp and excimer laser illumination is studied. In addition, the mechanisms of defect generation due to fs (248 nm)- and ns (248 nm, 193 nm)-pulses are investigated. 2. Experimental details Samples were melted under normal (air, Pt crucibles) and reducing (Ar atmosphere, carbon crucible) conditions. Samples melted under reducing conditions showed better UV-transmission than samples melted under normal conditions because the extinction coecient of Fe2 is much smaller than that of Fe3 . Further details are described in previous papers [9±11]. Table 1 gives an overview of the investigated samples. Raw materials of normal optical quality and raw materials of especially high purity (designated by s) were used. To investigate the in¯uence of phosphate content on defect generation, glasses of dierent phosphate content were used. The samples FP2, FP4, FP10 and FP20 were melted under normal conditions. The sample FP10 showed the best glass formation [3], therefore, this type was used to investigate the
471
in¯uence of the impurities on the defect generation. The main impurity was iron. Table 1 shows (in brackets) the normal content of impurities of lead and copper in glasses, melted under reducing conditions. To investigate the in¯uence of lead and copper on defect generation, samples of type FP10red were doped with 25 ppm Cu or Pb and melted under reducing conditions (designated by ``red'') in order to obtain Cu and Pb2 in the glass. Illumination was carried out with a mercury lamp (HOK-lamp) and excimer lasers. The spectrum of the lamp covers continuously the UV±vis± IR-range and starts at 190 nm. In the UV-region up to 280nm, the irradiation has an intensity of 1500 W/m2 . We used the ArF- and KrF-excimer lasers with a pulse width of 20 ns and energy densities up to 400 mJ/cm2 (KrF) and 100 mJ/cm2 (ArF) per pulse. We also used fs-pulses at a wavelength of 248 nm with energy densities up to 12 mJ/cm2 per pulse [12]. It was possible to vary the pulsewidth in the range of 300±1000 fs by a prism compressor. The intensity of the fs-pulses was approximately 1000 times the intensity of nspulses. The laser light was focused onto a 5 mm2 area of the sample. The optical absorption or transmission spectra in the range of 190±800 nm were recorded by a double beam spectrometer manufactured by Shimadzu (error <1%) before (transmission T0 ) and after (transmission T) various exposure times or several laser pulses. The induced extinction Ek log T0 =T k was normalized to a pathlength of 1 cm.
Table 1 Investigated samples Sample
Phosphate content (mol%)
Fluoride content (mol%)
FP2 FP4 FP10 FP20
2 4 10 20
98 96 90 80
Sample
Fe/Pb/Cu
Melting conditions
Redox state
FP10s/red FP10red
6 ppm Fe (1 ppm Pb; 0.5 ppm Cu) 18 ppm Fe (2.5 ppm Pb; 1 ppm Cu) +25 ppm Cu +25 ppm Pb
Reducing Reducing
6 ppm Fe2 18 ppm Fe2 +25 ppm Cu +25 ppm Pb2
472
U. Natura et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 470±475
3. Results and discussion Fig. 1 shows transmission spectra of the sample FP10s/red before and after irradiation nearly up to the saturation value with the lamp, the KrF-excimer laser, and the ArF-excimer laser. The lamp causes a decrease in transmission only in the UV region, whereas laser radiation causes additional absorption bands in the UV and the visible range. The defect generation by the ArF laser is much stronger than the defect generation by the KrF laser. To create the same decrease in transmission a much higher energy density of KrF-excimer laser is required. Fig. 2 shows the spectrum of induced extinction after irradiation by ArF-excimer laser. The spectrum was simulated by a separation of absorption bands. The decrease in transmission could be explained by photo oxidation of iron [11,13] and the generation of hole (HC) and electron (EC) centers shown in Fig. 2. The three absorption bands of HC2 associated with a small fraction of the EC1 , EC2 were observed only after laser irradiation. The other absorption bands were also created by irradiation with the HOK lamp. The formation of HC2 showed interesting eects. k 430 nm was chosen as a characteristic wavelength to show the defect-generation curves of the absorption bands of HC2 . To explain the mecha-
Fig. 2. Simulation and separation of radiation-induced absorption bands in FP10s/red after irradiation with the KrFexcimer laser (40 mJ/cm2 , 5000 pulses).
nisms of defect generation, the following experiments were carried out. 3.1. Experiments with fs-pulses Experiments with pulses of the same energy density (4 mJ/cm2 ), but dierent intensities varying the pulse length using pulses of 300 and 600 fs were carried out. A dependence of the initial slope of defect generation on intensity was observed. This is evidence of a two-photon process. The beam attenuation along the z-direction can be described according to dI ÿaI ÿ bI 2 ; dz
Fig. 1. Transmission spectra of the unirradiated (- - -) sample FP10s/red (thickness d 2 mm) and after irradiation nearly up to the saturation value with (--) HOK lamp (100 h), (Ð) KrFexcimer laser (400 mJ/cm2 , 10000 pulses), ( ) ArF-excimer laser (40 mJ/cm2 , 5000 pulses).
1
where I z is the beam intensity and a and b are the one- and two-photon absorption coecients. The dependence of the transmittance T on the input intensity is then obtained by integrating (1) [14]. The two-photon absorption coecients b were determined from intensity-dependent transmission experiments: bFP4 1:3 10ÿ10 cm/W, bFP10s=red 2:4 10ÿ10 cm/W, bFP10red 2:7 10ÿ10 cm=W; bFP20 4:5 10ÿ10 cm=W: b strongly depends on phosphate content of the samples according to the initial slope of the defect-generation curves (Fig. 3). As a reference, the two-photon absorption coe-
U. Natura et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 470±475
473
Fig. 3. Defect generation at k 430 nm in FP-glasses of different phosphate content by irradiation with fs-pulses (248 nm): (1) FP20, (2) FP10s/red, (3) FP4.
cient of SiO2 was measured (bSiO2 5:0 10ÿ11 cm=W). This value is very similar to bSiO2 4:5 10ÿ11 cm=W published in [15]. 3.2. Experiments with ns-pulses The in¯uence of the impurities was investigated in addition to the in¯uence of phosphate content on defect generation. The samples doped with copper showed a similar defect generation as the undoped sample. But the samples doped with lead showed the strong in¯uence of lead on both the initial slope and the saturation value of defect generation. The defect generation was recorded depending on energy density F (Fig. 4(a)). Samples doped with lead (FP10red/25 ppm Pb) showed a dependence that was less than quadratic. A radiation-induced recovery of the induced extinction was observed after irradiation (KrF) of a sample, preirradiated by F1 400 mJ/cm2 , by an energy density F2 200 mJ/cm2 less than F1 . The defectgeneration curve (1) in Fig. 4(a) shows a decrease of the induced extinction after a maximum is passed. Consequently, the sample preirradiated with F1 reaches a smaller apparent saturation value after irradiation with the lower energy density F2 than the non-preirradiated sample (Fig. 4(b)). The solid lines in Fig. 4 are calculated defectgeneration curves. The calculation was based on the model shown in Fig. 5.
Fig. 4. Defect generation at k 430 nm in the lead doped sample FP10red/25 ppm Pb depending on energy density (248 nm, ns-pulses): (a) defect generation curves for (1) sample 1 (F1 400 mJ=cm2 ), (2) sample 2 (F2 200 mJ=cm2 ), (3) sample 3 (F3 100 mJ=cm2 ); (b) defect generation curves for samples irradiated at F2 200 mJ=cm2 : (1)* sample 1 after preirradiation by 10000 pulses at F1 400 mJ=cm2 (as shown in (a)), (2) sample 2 without preirradiation.
3.3. Modeling of defect generation The defect generation due to ns-pulses can be explained by a two-step absorption process introducing an energy transfer process (Fig. 5). The existence of Pb2 is very important for the defect generation. Pb2 absorbs a photon, and the excited state of lead has a lifetime sPb . Pb2 disappears with increasing exposure time, described by a rate of disappearance k1 . An energy transfer takes place from the excited Pb2 -ions to the precursors VL of defect centers (described by an energy
474
U. Natura et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 470±475
Fig. 5. Model of defect generation for nanosecond and femtosecond cases.
transfer rate k2 ). The excited state has the lifetime sS . Excited precursors are able to absorb a photon and so the defect D is created. The one-photon cross-section r1 describes the radiation-induced recovery of defect centers. The disappearance of Pb2 -ions explains the loss of induced extinction after passing a maximum and the dierence in the apparent saturation values in the case of irradiation of a pre- and a non-preirradiated sample. The reciprocal rate of energy transfer is similar to the laser pulse duration. It explains the dierence in the mechanisms of defect generation for ns- and fspulses. Energy transfer is more appreciable with input beams with longer pulse width. The same cross-sections determining the two-step process and the same lifetimes sPb and sS were used for modeling the defect generation in glasses of type FP10 with diering content of Pb2 . The initial concentration of Pb2 was put in the calculation and the energy transfer rate was varied. The defect generation curves of FP10s/red (1 ppm Pb2 ) and FP10red (2.5 ppm Pb2 ) were calculated and showed a good agreement with the measured values. The dependence of the initial slope of defect generation on energy density is quadratic in the case of irradiation of FP10s/red by KrF-excimer laser. The defect generation by ArF-excimer laser is much stronger than by KrFexcimer laser and the dependence on the energy density becomes less than quadratic. Fig. 6 shows the calculated defect generation curves and measured values of the sample FP10s/red irradiated
Fig. 6. Defect generation at k 430 nm in the sample FP10s/ red irradiated by ArF-excimer laser (ns-pulses) depending on energy density (F) and calculated defect-generation curves.
with the ArF-excimer laser (ns-pulses). Both the initial slope and the saturation values of the defect generation of FP10s/red by ArF-excimer laser are very similar to the defect generation in the leaddoped sample by KrF-excimer laser (Fig. 5(a)). We can explain this with the wavelength-dependent absorption of Pb2 . The absorption of 25 ppm at 248 nm is similar to the absorption of 1 ppm at 193 nm (Gaussian shape of absorption band with a maximum at 207 nm, band width of 6400 cmÿ1 ). The calculation of the defect generation curves was based on the described model.
4. Summary The generation of the defect centres HC2 requires high intensities. Defects are generated by laser radiation. A radiation-induced recovery was observed. The defect generation by fs-pulses was dierent from the ns-results and could be explained by a two-photon process. We measured the two-photon-absorption-coecients and obtained a dependence on the phosphate content. The defect generation by ns-pulses could be explained by a two-step-process introducing an energy transfer. The defect generation increases with increasing phosphate content or increasing content of Pb2 . Pb2 disappears with irradiation time. The inclusion of energy transfer between the ex-
U. Natura et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 470±475
cited Pb2 -ions and the precursors was necessary to explain the dierence in the defect-generation mechanisms for ns- and fs-laser-pulses. A model was developed. Based on this model, defect-generation curves for samples of dierent content of lead could be calculated with constant cross-sections of the two-step process and constant lifetimes of the excited states, varying the content of lead and the rate of energy transfer. There was good agreement between the measurements and the model calculations. References [1] D. Ehrt, W. Seeber, J. Non-Cryst. Solids 129 (1991) 19. [2] N. Kitamura, J. Hayakawa, H. Yamashita, J. Non-Cryst. Solids 126 (1990) 155. [3] D. Ehrt, Habilitation, Friedrich-Schiller-Universit at Jena, 1984.
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