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Long-lasting phosphorescence in oxygen-deficient Ge-doped silica glasses at room temperature
12 January 2001
Chemical Physics Letters 333 (2001) 236±241
www.elsevier.nl/locate/cplett
Long-lasting phosphorescence in oxygen-de®cient Ge-doped silica glasses at room temperature Jianrong Qiu a,b,*, Alexander L. Gaeta a, Kazuyuki Hirao c a
c
Department of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA b Photoncraft Project, JST, Keihanna-Plaza, Seika-cho, Kyoto 619-0237, Japan Department of Materials Chemistry, Graduate School of Engineering, Kyoto University, Kyoto, Japan Received 25 July 2000; in ®nal form 7 November 2000
Abstract We report on a novel phenomenon in oxygen-de®cient Ge-doped silica glasses at room temperature. Irradiation of focused 120 fs laser pulses at 800 nm induced long-lasting phosphorescence with peaks at 290 and 390 nm for oxygende®cient Ge-doped silica glass. The phosphorescence persisted for not less than 1 h after the removal of the irradiating light. The intensity of the phosphorescence at 390 nm increased with an increase in the concentration of oxygende®ciency associated with Ge ions. Based on the time dependence of the intensity of the phosphorescence, the longlasting phosphorescence in these glasses is considered to be due to the thermally activated electron±hole recombination at shallow traps. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Ge-doped glasses are one of the important optical materials. They have been used as ®ber materials for optical communication. In 1978, Hill et al. [1] discovered the photosensitivity and photoinduced holographic Bragg gratings in Gedoped glasses. Bragg gratings can be written in the Ge-doped silica glasses by printing periodical refractive index modulation with an excimer laser beam through a phase mask [2]. Since the photoinduced ®ber Bragg grating has wide applications in telecommunication technology for wavelength division multiplexing, signal shaping etc., many studies have been carried out on the optical
*
Corresponding author. Fax: +81-774-955205. E-mail address: [email protected] (J. Qiu).
properties of Ge-doped silica glasses to understand the mechanism of light induced refractive index change [3±5]. Usually, an absorption peak is observed at 5 eV in Ge-doped silica glass [3±5]. The peak is believed to relate to two oxygen de®ciencies: one is a neutral oxygen monovacancy (NOMV) and the other is a neutral oxygen divacancy (NODV) associated with Ge ions. NOMV is readily changed to GeE0 centers by illumination with a UV lamp emitting 5 eV light with photon density larger than 10 mW/ cm2 . On the other hand, NODV emits luminescence at 290 and 390 nm, but this type of defect is considered to be insensitive against illumination with a UV lamp. However, there were few reports on the long-lasting phosphorescence in Ge-doped silica glasses. In this Letter, we report on the observation of room-temperature long-lasting phosphorescence
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 3 6 2 - 2
J. Qiu et al. / Chemical Physics Letters 333 (2001) 236±241
237
in oxygen-de®cient Ge-doped silica glasses after the irradiation of 800 nm, 120 fs laser pulses. The mechanism of the phenomenon is also discussed. 2. Experimental Ge-doped silica glass samples were prepared by Shinetsu Chem., Japan as ®ber preforms by a vapor axial deposition (VAD) method. For comparison, a silica glass sample prepared using chemical vapor deposition (CVD) by Asahi Glass, Japan was also used in the experiment. The glass samples were cut, polished, and subjected to optical measurement. The absorption spectra of the samples were measured by a spectrophotometer (JASCO V-570). The normal ¯uorescence spectra of the samples were measured by a ¯uorescence spectrophotometer (Shimaduz RF-5300PC) using a xenon lamp (150 W) as an excitation source and passing the light through a monochromator to give 250 nm photons. A regeneratively ampli®ed 800 nm Ti:sapphire laser was used for our study to emit 120 fs, 1 kHz, mode-locked pulses. The laser beam, with an average power of 200 mW, was focused by a 100 mm focal-length lens on the interior of the glass samples. For the phosphorescence spectra and decay curves, the glass samples were ®rst irradiated by the focused laser for 30 s. All of the measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows the absorption spectra of the silica glass and the Ge-doped silica glasses. The GeO2 concentrations of the Ge-doped glasses were about 0.2 mol% (b) and 3 mol% (c). The thickness of the Ge-doped glass samples was 0:90 0:10 mm. No apparent absorption was observed at 250 nm in silica glass fabricated by the CVD method, while an absorption peak maximum at 250 nm due to the oxygen de®ciencies associated with Ge ions was observed in the glasses fabricated by the VAD method. After irradiation by the focused laser, no apparent phosphorescence was observed in the silica
Fig. 1. Absorption spectra of silica and oxygen-de®cient GeO2 doped silica glasses. (a): silica glass (b): glass cut from a ®ber preform (the GeO2 concentration is 0.2 mol%). (c): glass cut from a ®ber preform (the GeO2 concentration is 3 mol%).
glass sample, while visible, phosphorescence from the focused area was observed in oxygen-de®cient Ge-doped silica glass samples in the dark after the removal of the femtosecond laser. In the case of a 10 mol% GeO2 -doped silica glass sample cut from a ®ber preform fabricated by the VAD method, the phosphorescence can still be seen with the naked eye in the dark even 1 h after the removal of the activating femtosecond laser. Fig. 2 shows the emission, excitation and phosphorescence spectra of the oxygen-de®cient 0.2 mol% GeO2 -doped silica glass sample. As can be seen from the ®gure, the phosphorescence spectrum induced by the femtosecond laser has the same appearance as the photoluminescence spectrum excited by the 250 nm UV light. Only the relative intensity at 290 nm is weaker than that in the normal photoluminescence spectrum. The emission peaks at 290 and 390 nm can be assigned to the singlet (S1 -S0 ) and triplet to singlet (T1 -S0 ) transitions of the NODV, respectively [5]. The appearance of the phosphorescence spectrum was con®rmed to remain unchanged, while the intensity of the phosphorescence decreased with time.
238
J. Qiu et al. / Chemical Physics Letters 333 (2001) 236±241
Fig. 2. Photoluminescence, excitation and phosphorescence spectra of oxygen-de®cient 0.2 mol% GeO2 -doped silica glass. For the measurement of the excitation spectrum, the luminescence at 390 nm is monitored.
Fig. 3 shows phosphorescence spectra of oxygen-de®cient Ge-doped silica glasses after the irrdiation of the focused infrared femtosecond laser. The emission peak at 290 nm decreased and the emission peak at 390 nm slightly shifted to longer wavelength with the increase of GeO2 concentration. Fig. 4 shows the intensity decay of the phosphorescence at 390 nm in the oxygen-de®cient 3 mol% GeO2 -doped silica glass sample. The intensity of the phosphorescence seems to decrease quickly at ®rst and then slowly with the passage of time. The time dependence of the inverse of the intensity of the phosphorescence is also shown in Fig. 4. The intensity of the phosphorescence decreases in inverse proportion to the time after the quick decrease of the intensity. We also measured the intensity decay of the phosphorescence at 390 and 290 nm in the oxygende®cient 0.2 mol% GeO2 -doped silica glass sample. Similar to the results observed in 3 mol% GeO2 doped silica glass, the intensity of the phosphorescence decreases quickly at ®rst and then slowly with the passage of time. The time dependence of
Fig. 3. Phosphorescence spectra of oxygen-de®cient Ge-doped silica glass after the irradiation of the femtosecond laser pulses. (a): 0.2 mol% GeO2 -doped silica glass. (b): 3 mol% GeO2 -doped silica glass. (c): 10 mol% GeO2 -doped silica glass.
Fig. 4. Decay curve of the phosphorescence at 390 nm in the oxygen-de®cient 3 mol% GeO2 -doped silica glass after the irradiation of the focused 800 nm femtosecond laser pulses.
the inverse of the intensity of the phosphorescence also shows that the intensity of the phosphorescence decreases in inverse proportion to the time after quick decrease of the intensity.
J. Qiu et al. / Chemical Physics Letters 333 (2001) 236±241
Fig. 5 shows the absorption spectra of the oxygen-de®cient 0.2 mol% GeO2 -doped silica glass sample before and after laser irradiation. The
Fig. 5. Absorption spectra of the oxygen-de®cient 0.2 mol% GeO2 -doped silica glass before (a) and after the femtosecond laser irradiation. The time shown in the ®gure is the duration after the removal of the activating laser. The inset is the difference absorption spectrum of the oxygen-de®cient 0.2 mol% GeO2 -doped silica glass after and before the femtosecond laser irradiation.
239
thickness of the used glass sample was 9:9 0:1 mm. Absorption in the wavelength region from 190 to 230 nm, and 260 to 400 nm increased after the laser irradiation. Hence, some new defects were induced after the laser irradiation. No apparent decay was observed in absorbance due to the laserinduced defects. The dierence absorption spectrum of the glass after and before the focused femtosecond laser irradiation is shown in Fig. 5. Three apparent peaks were observed at 4.7 eV (264 nm), 5.8 eV (214 nm) and 6.3 eV (197 nm). A slight decrease in the absorbance was observed near 5.2 eV (240 nm). The result agrees well with the results observed in Ge-doped silica glass after the KrF laser irradiation [4]. The peaks at 4.7, 5.8 and 6.3 eV can be assigned to Ge(1), Ge(2) of Ge electron trapped centers (GEC) and GeE0 centers [4], respectively. The GEC is abbreviated as Ge(1) or Ge(2), depending on the number of nearestneighbor Ge ions. The formation of GeE0 centers was also con®rmed by electron spin resonance spectroscopy. Focused femtosecond laser induced localized long-lasting phosphorescence has been observed in rare-earth ion and transition metal ion doped glasses [6±8]. Usually, the absorption band due to the laser-induced defects faded at room temperature and was in agreement with the decay in the phosphorescence, though phosphorescence and absorption could not be simply related [6,8]. The mechanism of the long-lasting phosphorescence has been suggested as follows. After the irradiation by the focused femtosecond pulsed laser, free electrons and holes were formed in the glass samples through multiphoton absorption processes. The holes or electrons were trapped by defect centers, released by heat at room temperature, and recombined with electrons or holes trapped by other defect centers. The recombination of holes and electrons or the energy transfer due to the recombination of holes and electrons to the active ions results in the characteristic rare-earth ion and transition metal ion emissions. We observed the long-lasting phosphorescence phenomenon in the oxygen-de®cient Ge-doped silica glass samples. However, we did not observe the same phenomenon in the silica glass sample without oxygen de®ciencies associated with Ge
240
J. Qiu et al. / Chemical Physics Letters 333 (2001) 236±241
ions. Therefore, the NODV type oxygen de®ciency acted as a luminescent center in the long-lasting phosphorescence. On the other hand, long-lasting phosphorescence occurs together with the disappearance of some defects due to the thermal recombination of electron and hole. However, as shown in Fig. 5, no apparent decay in the absorption due to the laser-induced defects was observed in the 0.2 mol% GeO2 -doped silica glass. We suppose that there are two reasons for this. One is that the laser-induced defects, which contribute to the long-lasting phosphorescence, may have absorptions in the IR or UV region that are outside of the measured wavelength region. The other is that the cross section of the absorption due to the laser-induced defects is very small and cannot be detected by the regular spectrophotometer. The decrease in the relative intensity of the phosphorescence at 290 nm as shown in Fig. 2 can be considered to be due to the absorption at 260 to 350 nm arising from the laser-induced Ge electron trapped centers. In addition, energy transfer or energy excursion between the NODVs is the main reason for the disappearance of emission at 290 nm since the emission at 290 nm may be further absorbed by the nearby NODVs. The probability of the energy transfer between NOMVs is inversely proportional to nth (n P 6) power of the distance between NODVs. Thus, the intensity of the phosphorescence at 290 nm decreased quickly with an increase of GeO2 (i.e., oxygen-de®cient defects associated with Ge ions) concentration. The slight shift of emission at about 390 nm is due to the energy transfer between NODVs. A similar phenomenon has been observed in rare-earth-doped glasses [9]. The time dependence of the intensity of the phosphorescence was not a simple exponential but can be approximately expressed as I t / 1=t in oxygen-de®cient Ge-doped silica glass when t is larger than a certain time (300 s). It is the same as has been observed in Ag- and Tl-doped KCl crystals [10] and various active ion-doped glasses [6±8]. A radiation tunneling recombination model was proposed by Delbecq et al. [10], to explain the observed phenomenon. We suggest that similar thermally assisted tunneling recombination at
room temperature was the main mechanism for the observed long-lasting phosphorescence in oxygen-de®cient Ge-doped silica glasses. From the results of Fig. 5, the oxygen-de®cient centers associated with Ge were transformed to GeE0 and Ge electron trapped centers (GEC) after the focused infrared femtosecond laser irradiation. The results are in good agreement with those observed using an excimer laser [4]. Therefore, we suggest that the lone pair electron on bridging oxygen (BO), which occupies the uppermost level of the valence band, is excited to the conduction band via the multiphoton excitation during the focused femtosecond laser irradiation. In the present case, the excitation process should be a 4 or 5 photon excitation process, considering the band gap (7.1 eV) of the Ge-doped silica glass. Other processes such as the absorption or multiphoton absorption of supercontinuum due to the phase modulation of the ultrashort pulsed laser cannot be fully denied at the present stage. The electron is trapped on the fourfold-coordinated Ge ions (GeO4 ). BO is converted into a self-trapped hole center (STH). Some of the GEC centers can be further converted into the GeE' centres and nonbridging oxygen which trapped an electron during the laser irradiation. The reaction can be expressed as: nhm
GeO4 BO!GEC GeO4 e STH GEC ! GeE0 NBO: STH is not stable at room temperature [4]. Therefore, the hole trapped by STH can be released by the heat energy at room temperature and recombine with the trapped electron. The energy due to the recombination of the hole-electron pair may excite the electron at the ground state of the NODV, thus resulting in the emission from the NODV. On the other hand, we observed the dim phosphorescence from the oxygen-de®cient 10 mol% GeO2 -doped silica glasses after the irradiation of UV light at 254 nm using a UV lamp with a power density of 5 mW/cm2 . It means that the electron in the ground state of a NODV can be excited to the excitation level via single photon absorption, and further trapped by an electrontrapping center. The electron can ®nally be released and returned to its original state, resulting
J. Qiu et al. / Chemical Physics Letters 333 (2001) 236±241
in the characteristic NODV emission. Therefore, direct excitation of the NODV via multiphoton absorption is also a possible mechanism for the long-lasting phosphorescence. The electron density excited by the focused femtosecond laser can be very high due to the multiphoton absorption and avalanche ionization, thus resulting in the visible long-lasting phosphorescence.
241
neering Physics, Cornell University, Dr. Jinhai Si and Mr K. Nouchi of Hirao Active Glass Project, ERATO, JST, for their kind help in some of the experiments. J.Q. also acknowledges partial support from Cornell Center for Materials Research.
References 4. Conclusion In conclusion, we have observed femtosecond infrared laser-induced long-lasting phosphorescence in oxygen-de®cient GeO2 -doped silica glasses. The intensity of phosphorescence increased with an increase in the GeO2 concentration. The long-lasting phosphorescence is considered to be due to the thermally stimulated recombination of holes and electrons at traps induced via multiphoton absorption and avalanche ionization at room temperature. Acknowledgements J.Q. is grateful to Dr. Xiang Liu and Prof. F. Wise of the Department of Applied and Engi-
[1] K.O. Hill, Y. Fujii, D.C. Johnson, B.S. Kawasaki, Appl. Phys. Lett. 32 (1978) 647. [2] F. Bilodeau, D.C. Johnson, S. Theriault, B. Malo, T. Albert, K.O. Hill, IEEE Photonics Technol. Lett. 7 (1995) 388. [3] H. Hosono, Y. Abe, D.L. Kinser, R.A. Weeks, K. Muta, H. Kawazoe, Phys. Rev. B 46 (1992) 11445. [4] J. Nishi, K. Fukumi, H. Yamanaka, K. Kawamura, H. Hosono, H. Kawazoe, Phys. Rev. B 52 (1995) 1661. [5] L. Skuja, J. Non-Cryst. Solids 239 (1998) 16. [6] J. Qiu, K. Miura, H. Inouye, Y. Kondo, T. Mitsuyu, K. Hirao, Appl. Phys. Lett. 28 (1998) 1763. [7] J. Qiu, K. Miura, H. Inouye, H. Fujiwara, T. Mitsuyu, K. Hirao, J. Non-Cryst. Solids 244 (1999) 185. [8] J. Qiu, Y. Kondo, K. Miura, T. Mitsuyu, K. Hirao, Jpn. J. Appl. Phys. 38 (1999) 649. [9] J. Qiu, K. Miura, N. Sugimoto, K. Hirao, J. Non-Cryst. Solids 213&214 (1997) 266. [10] C.J. Delbecq, Y. Toyozawa, P.H. Yuster, Phys. Rev. B 9 (1974) 4497.
Chemical Physics Letters 333 (2001) 236±241
www.elsevier.nl/locate/cplett
Long-lasting phosphorescence in oxygen-de®cient Ge-doped silica glasses at room temperature Jianrong Qiu a,b,*, Alexander L. Gaeta a, Kazuyuki Hirao c a
c
Department of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA b Photoncraft Project, JST, Keihanna-Plaza, Seika-cho, Kyoto 619-0237, Japan Department of Materials Chemistry, Graduate School of Engineering, Kyoto University, Kyoto, Japan Received 25 July 2000; in ®nal form 7 November 2000
Abstract We report on a novel phenomenon in oxygen-de®cient Ge-doped silica glasses at room temperature. Irradiation of focused 120 fs laser pulses at 800 nm induced long-lasting phosphorescence with peaks at 290 and 390 nm for oxygende®cient Ge-doped silica glass. The phosphorescence persisted for not less than 1 h after the removal of the irradiating light. The intensity of the phosphorescence at 390 nm increased with an increase in the concentration of oxygende®ciency associated with Ge ions. Based on the time dependence of the intensity of the phosphorescence, the longlasting phosphorescence in these glasses is considered to be due to the thermally activated electron±hole recombination at shallow traps. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Ge-doped glasses are one of the important optical materials. They have been used as ®ber materials for optical communication. In 1978, Hill et al. [1] discovered the photosensitivity and photoinduced holographic Bragg gratings in Gedoped glasses. Bragg gratings can be written in the Ge-doped silica glasses by printing periodical refractive index modulation with an excimer laser beam through a phase mask [2]. Since the photoinduced ®ber Bragg grating has wide applications in telecommunication technology for wavelength division multiplexing, signal shaping etc., many studies have been carried out on the optical
*
Corresponding author. Fax: +81-774-955205. E-mail address: [email protected] (J. Qiu).
properties of Ge-doped silica glasses to understand the mechanism of light induced refractive index change [3±5]. Usually, an absorption peak is observed at 5 eV in Ge-doped silica glass [3±5]. The peak is believed to relate to two oxygen de®ciencies: one is a neutral oxygen monovacancy (NOMV) and the other is a neutral oxygen divacancy (NODV) associated with Ge ions. NOMV is readily changed to GeE0 centers by illumination with a UV lamp emitting 5 eV light with photon density larger than 10 mW/ cm2 . On the other hand, NODV emits luminescence at 290 and 390 nm, but this type of defect is considered to be insensitive against illumination with a UV lamp. However, there were few reports on the long-lasting phosphorescence in Ge-doped silica glasses. In this Letter, we report on the observation of room-temperature long-lasting phosphorescence
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 3 6 2 - 2
J. Qiu et al. / Chemical Physics Letters 333 (2001) 236±241
237
in oxygen-de®cient Ge-doped silica glasses after the irradiation of 800 nm, 120 fs laser pulses. The mechanism of the phenomenon is also discussed. 2. Experimental Ge-doped silica glass samples were prepared by Shinetsu Chem., Japan as ®ber preforms by a vapor axial deposition (VAD) method. For comparison, a silica glass sample prepared using chemical vapor deposition (CVD) by Asahi Glass, Japan was also used in the experiment. The glass samples were cut, polished, and subjected to optical measurement. The absorption spectra of the samples were measured by a spectrophotometer (JASCO V-570). The normal ¯uorescence spectra of the samples were measured by a ¯uorescence spectrophotometer (Shimaduz RF-5300PC) using a xenon lamp (150 W) as an excitation source and passing the light through a monochromator to give 250 nm photons. A regeneratively ampli®ed 800 nm Ti:sapphire laser was used for our study to emit 120 fs, 1 kHz, mode-locked pulses. The laser beam, with an average power of 200 mW, was focused by a 100 mm focal-length lens on the interior of the glass samples. For the phosphorescence spectra and decay curves, the glass samples were ®rst irradiated by the focused laser for 30 s. All of the measurements were carried out at room temperature. 3. Results and discussion Fig. 1 shows the absorption spectra of the silica glass and the Ge-doped silica glasses. The GeO2 concentrations of the Ge-doped glasses were about 0.2 mol% (b) and 3 mol% (c). The thickness of the Ge-doped glass samples was 0:90 0:10 mm. No apparent absorption was observed at 250 nm in silica glass fabricated by the CVD method, while an absorption peak maximum at 250 nm due to the oxygen de®ciencies associated with Ge ions was observed in the glasses fabricated by the VAD method. After irradiation by the focused laser, no apparent phosphorescence was observed in the silica
Fig. 1. Absorption spectra of silica and oxygen-de®cient GeO2 doped silica glasses. (a): silica glass (b): glass cut from a ®ber preform (the GeO2 concentration is 0.2 mol%). (c): glass cut from a ®ber preform (the GeO2 concentration is 3 mol%).
glass sample, while visible, phosphorescence from the focused area was observed in oxygen-de®cient Ge-doped silica glass samples in the dark after the removal of the femtosecond laser. In the case of a 10 mol% GeO2 -doped silica glass sample cut from a ®ber preform fabricated by the VAD method, the phosphorescence can still be seen with the naked eye in the dark even 1 h after the removal of the activating femtosecond laser. Fig. 2 shows the emission, excitation and phosphorescence spectra of the oxygen-de®cient 0.2 mol% GeO2 -doped silica glass sample. As can be seen from the ®gure, the phosphorescence spectrum induced by the femtosecond laser has the same appearance as the photoluminescence spectrum excited by the 250 nm UV light. Only the relative intensity at 290 nm is weaker than that in the normal photoluminescence spectrum. The emission peaks at 290 and 390 nm can be assigned to the singlet (S1 -S0 ) and triplet to singlet (T1 -S0 ) transitions of the NODV, respectively [5]. The appearance of the phosphorescence spectrum was con®rmed to remain unchanged, while the intensity of the phosphorescence decreased with time.
238
J. Qiu et al. / Chemical Physics Letters 333 (2001) 236±241
Fig. 2. Photoluminescence, excitation and phosphorescence spectra of oxygen-de®cient 0.2 mol% GeO2 -doped silica glass. For the measurement of the excitation spectrum, the luminescence at 390 nm is monitored.
Fig. 3 shows phosphorescence spectra of oxygen-de®cient Ge-doped silica glasses after the irrdiation of the focused infrared femtosecond laser. The emission peak at 290 nm decreased and the emission peak at 390 nm slightly shifted to longer wavelength with the increase of GeO2 concentration. Fig. 4 shows the intensity decay of the phosphorescence at 390 nm in the oxygen-de®cient 3 mol% GeO2 -doped silica glass sample. The intensity of the phosphorescence seems to decrease quickly at ®rst and then slowly with the passage of time. The time dependence of the inverse of the intensity of the phosphorescence is also shown in Fig. 4. The intensity of the phosphorescence decreases in inverse proportion to the time after the quick decrease of the intensity. We also measured the intensity decay of the phosphorescence at 390 and 290 nm in the oxygende®cient 0.2 mol% GeO2 -doped silica glass sample. Similar to the results observed in 3 mol% GeO2 doped silica glass, the intensity of the phosphorescence decreases quickly at ®rst and then slowly with the passage of time. The time dependence of
Fig. 3. Phosphorescence spectra of oxygen-de®cient Ge-doped silica glass after the irradiation of the femtosecond laser pulses. (a): 0.2 mol% GeO2 -doped silica glass. (b): 3 mol% GeO2 -doped silica glass. (c): 10 mol% GeO2 -doped silica glass.
Fig. 4. Decay curve of the phosphorescence at 390 nm in the oxygen-de®cient 3 mol% GeO2 -doped silica glass after the irradiation of the focused 800 nm femtosecond laser pulses.
the inverse of the intensity of the phosphorescence also shows that the intensity of the phosphorescence decreases in inverse proportion to the time after quick decrease of the intensity.
J. Qiu et al. / Chemical Physics Letters 333 (2001) 236±241
Fig. 5 shows the absorption spectra of the oxygen-de®cient 0.2 mol% GeO2 -doped silica glass sample before and after laser irradiation. The
Fig. 5. Absorption spectra of the oxygen-de®cient 0.2 mol% GeO2 -doped silica glass before (a) and after the femtosecond laser irradiation. The time shown in the ®gure is the duration after the removal of the activating laser. The inset is the difference absorption spectrum of the oxygen-de®cient 0.2 mol% GeO2 -doped silica glass after and before the femtosecond laser irradiation.
239
thickness of the used glass sample was 9:9 0:1 mm. Absorption in the wavelength region from 190 to 230 nm, and 260 to 400 nm increased after the laser irradiation. Hence, some new defects were induced after the laser irradiation. No apparent decay was observed in absorbance due to the laserinduced defects. The dierence absorption spectrum of the glass after and before the focused femtosecond laser irradiation is shown in Fig. 5. Three apparent peaks were observed at 4.7 eV (264 nm), 5.8 eV (214 nm) and 6.3 eV (197 nm). A slight decrease in the absorbance was observed near 5.2 eV (240 nm). The result agrees well with the results observed in Ge-doped silica glass after the KrF laser irradiation [4]. The peaks at 4.7, 5.8 and 6.3 eV can be assigned to Ge(1), Ge(2) of Ge electron trapped centers (GEC) and GeE0 centers [4], respectively. The GEC is abbreviated as Ge(1) or Ge(2), depending on the number of nearestneighbor Ge ions. The formation of GeE0 centers was also con®rmed by electron spin resonance spectroscopy. Focused femtosecond laser induced localized long-lasting phosphorescence has been observed in rare-earth ion and transition metal ion doped glasses [6±8]. Usually, the absorption band due to the laser-induced defects faded at room temperature and was in agreement with the decay in the phosphorescence, though phosphorescence and absorption could not be simply related [6,8]. The mechanism of the long-lasting phosphorescence has been suggested as follows. After the irradiation by the focused femtosecond pulsed laser, free electrons and holes were formed in the glass samples through multiphoton absorption processes. The holes or electrons were trapped by defect centers, released by heat at room temperature, and recombined with electrons or holes trapped by other defect centers. The recombination of holes and electrons or the energy transfer due to the recombination of holes and electrons to the active ions results in the characteristic rare-earth ion and transition metal ion emissions. We observed the long-lasting phosphorescence phenomenon in the oxygen-de®cient Ge-doped silica glass samples. However, we did not observe the same phenomenon in the silica glass sample without oxygen de®ciencies associated with Ge
240
J. Qiu et al. / Chemical Physics Letters 333 (2001) 236±241
ions. Therefore, the NODV type oxygen de®ciency acted as a luminescent center in the long-lasting phosphorescence. On the other hand, long-lasting phosphorescence occurs together with the disappearance of some defects due to the thermal recombination of electron and hole. However, as shown in Fig. 5, no apparent decay in the absorption due to the laser-induced defects was observed in the 0.2 mol% GeO2 -doped silica glass. We suppose that there are two reasons for this. One is that the laser-induced defects, which contribute to the long-lasting phosphorescence, may have absorptions in the IR or UV region that are outside of the measured wavelength region. The other is that the cross section of the absorption due to the laser-induced defects is very small and cannot be detected by the regular spectrophotometer. The decrease in the relative intensity of the phosphorescence at 290 nm as shown in Fig. 2 can be considered to be due to the absorption at 260 to 350 nm arising from the laser-induced Ge electron trapped centers. In addition, energy transfer or energy excursion between the NODVs is the main reason for the disappearance of emission at 290 nm since the emission at 290 nm may be further absorbed by the nearby NODVs. The probability of the energy transfer between NOMVs is inversely proportional to nth (n P 6) power of the distance between NODVs. Thus, the intensity of the phosphorescence at 290 nm decreased quickly with an increase of GeO2 (i.e., oxygen-de®cient defects associated with Ge ions) concentration. The slight shift of emission at about 390 nm is due to the energy transfer between NODVs. A similar phenomenon has been observed in rare-earth-doped glasses [9]. The time dependence of the intensity of the phosphorescence was not a simple exponential but can be approximately expressed as I t / 1=t in oxygen-de®cient Ge-doped silica glass when t is larger than a certain time (300 s). It is the same as has been observed in Ag- and Tl-doped KCl crystals [10] and various active ion-doped glasses [6±8]. A radiation tunneling recombination model was proposed by Delbecq et al. [10], to explain the observed phenomenon. We suggest that similar thermally assisted tunneling recombination at
room temperature was the main mechanism for the observed long-lasting phosphorescence in oxygen-de®cient Ge-doped silica glasses. From the results of Fig. 5, the oxygen-de®cient centers associated with Ge were transformed to GeE0 and Ge electron trapped centers (GEC) after the focused infrared femtosecond laser irradiation. The results are in good agreement with those observed using an excimer laser [4]. Therefore, we suggest that the lone pair electron on bridging oxygen (BO), which occupies the uppermost level of the valence band, is excited to the conduction band via the multiphoton excitation during the focused femtosecond laser irradiation. In the present case, the excitation process should be a 4 or 5 photon excitation process, considering the band gap (7.1 eV) of the Ge-doped silica glass. Other processes such as the absorption or multiphoton absorption of supercontinuum due to the phase modulation of the ultrashort pulsed laser cannot be fully denied at the present stage. The electron is trapped on the fourfold-coordinated Ge ions (GeO4 ). BO is converted into a self-trapped hole center (STH). Some of the GEC centers can be further converted into the GeE' centres and nonbridging oxygen which trapped an electron during the laser irradiation. The reaction can be expressed as: nhm
GeO4 BO!GEC GeO4 e STH GEC ! GeE0 NBO: STH is not stable at room temperature [4]. Therefore, the hole trapped by STH can be released by the heat energy at room temperature and recombine with the trapped electron. The energy due to the recombination of the hole-electron pair may excite the electron at the ground state of the NODV, thus resulting in the emission from the NODV. On the other hand, we observed the dim phosphorescence from the oxygen-de®cient 10 mol% GeO2 -doped silica glasses after the irradiation of UV light at 254 nm using a UV lamp with a power density of 5 mW/cm2 . It means that the electron in the ground state of a NODV can be excited to the excitation level via single photon absorption, and further trapped by an electrontrapping center. The electron can ®nally be released and returned to its original state, resulting
J. Qiu et al. / Chemical Physics Letters 333 (2001) 236±241
in the characteristic NODV emission. Therefore, direct excitation of the NODV via multiphoton absorption is also a possible mechanism for the long-lasting phosphorescence. The electron density excited by the focused femtosecond laser can be very high due to the multiphoton absorption and avalanche ionization, thus resulting in the visible long-lasting phosphorescence.
241
neering Physics, Cornell University, Dr. Jinhai Si and Mr K. Nouchi of Hirao Active Glass Project, ERATO, JST, for their kind help in some of the experiments. J.Q. also acknowledges partial support from Cornell Center for Materials Research.
References 4. Conclusion In conclusion, we have observed femtosecond infrared laser-induced long-lasting phosphorescence in oxygen-de®cient GeO2 -doped silica glasses. The intensity of phosphorescence increased with an increase in the GeO2 concentration. The long-lasting phosphorescence is considered to be due to the thermally stimulated recombination of holes and electrons at traps induced via multiphoton absorption and avalanche ionization at room temperature. Acknowledgements J.Q. is grateful to Dr. Xiang Liu and Prof. F. Wise of the Department of Applied and Engi-
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