1184 OPTICS LETTERS / Vol. 20, No. 10 / May 15, 1995

Ultraviolet-radiation-induced chemical reactions through one-and two-photon absorption processes in GeO2–SiO2 glasses

Junji Nishii, Naoyuki Kitamura, and Hiroshi Yamanaka

Optical Material Division, Osaka National Research Institute, Agency of Industrial Science and Technology,1-8-31 Midorigaoka, Ikeda, Osaka 563, Japan

Hideo Hosono and Hiroshi Kawazoe

Research Laboratory of Engineering Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 227, Japan

Received November 1, 1994

Photochemical reactions in 10 GeO2 –90 SiO2 glass induced by irradiation with excimer lasers (KrF, 5.0 eV, XeCl,4.0 eV) and a Hg lamp (4.9 eV) were examined. The irradiation with excimer lasers generated two types ofparamagnetic defect, an electron-trapped center associated with fourfold coordinated Ge ions and a self-trappedhole center on bridging oxygen. Taking the optical band gap (,7.1 eV) of the glass obtained in this work and thepower density of laser pulses [10–90 mJy(cm2 pulse), pulse duration 20 ns] into account, we concluded that thesecenters were formed by band-to-band excitation by two-photon absorption process. On the other hand, the lampillumination (,16 mWycm2) caused the formation of Ge E0 centers from preexisting oxygen-vacancy-type defects bythe one-photon absorption process. These two kinds of reaction proceed independently, depending on the powerdensities of UV beams, at least in our experimental condition.

Oxygen-deficient defects causing intense absorptionbands near 5 eV (the 5-eV band) in GeO2 –SiO2 opticalfibers are closely related to the formation of distributedBragg reflectors.1 The 5-eV band originates from twooxygen-deficient-type defects with a different natures:a neutral oxygen monovacancy (Ge–Ge or Ge–Si) anda neutral oxygen divacancy of 2-coordinated Ge21

(Ref. 2) or 4-coordinated Ge41.3 The former can bebleached by illumination with a UV emission lampsuch as a filtered Hg–Xe lamp, and a new band isinduced near 6.4 eV. The latter (oxygen divacancy)emits intense luminescence bands peaking at 3.3 and4.5 eV, but no significant change in their intensities isobserved by irradiation with the UV lamp.

Distinct spectral changes, which differ from thoseinduced by the lamp illumination, are observed byirradiation with KrF laser pulses. Not only thebleaching of the 5-eV band but the formation of abroad absorption band can be recognized distinctlyabove 3 eV.4 Laser pulses exceeding 100 mJycm2,corresponding to .5 MWy(cm2 pulse) (pulse duration20 ns), in power density are irradiated for the forma-tion of Bragg reflectors.5,6 Therefore it is importantto elucidate the photochemical reactions, thereby tak-ing the multiphoton absorption processes into accountas in the case of SiO2 glass.7 In this Letter we re-port on two photochemical reaction channels that arecontrolled by the photon flux density in GeO2 –SiO2

glasses.An optical fiber preform (10 GeO2 –90 SiO2 in mol. %)

was cut and polished into plates 0.1 mm thick. TheUV light irradiation was conducted at room tempera-ture with a KrF laser [10 mJy(cm2 pulse) at 5.0 eV],a XeCl laser [90 mJy(cm2 pulse) at 4.0 eV], and a Hglamp (,16 mWycm2 at 4.9 eV). The repetition rateand the pulse duration of the lasers are 10 Hz and20 ns, respectively. Electron-spin-resonance (ESR)spectra were measured at 300 K at X-band frequency

0146-9592/95/101184-03$6.00/0

with a Brucker Model 300 E applying 100-kHz fieldmodulation.2 The optical band gap (Eg) was esti-mated by GeO2 –SiO2 thin glass films (3.8 mm thick)deposited onto a SiO2 substrate by the rf sputteringmethod.8 The chemical composition of the thin filmwas analyzed by x-ray photoelectron spectroscopy (ex-citation source, Mg Ka). The spectra in the VUVregion were measured with a Seya-Namioka-typespectrometer.

Figure 1 shows changes in optical absorption of thespecimens with UV irradiation. Curve (a) shows theabsorption spectrum of as-polished glass. The ab-sorption band peaking at 5.15 eV is due to theoxygen-deficient defects.2 As shown by curve (b), thebleaching of the 5.15-eV band and the positive inten-sity change above 5.7 eV were caused by illuminationwith the Hg lamp, as reported in Ref. 2. No spec-tral change was perceived by illumination longer than100 h. The irradiation with KrF laser pulses inducednew broad and intense bands above 3 eV [see curve(c)]. An increase in the intensity of these bands wassaturated after irradiation of several hundred shots.Curve (d) denotes the spectrum of the sample af-ter irradiation with the Hg lamp followed by KrFlaser pulses.

Difference spectra of the specimen before and afterUV irradiation are shown in the inset of Fig. 1. Theband bleached and the band induced with the lampillumination are located near 5.1 and 6.3 eV, respec-tively [see curve (e)]. The positions of these bandsdid not change after subsequent irradiation with KrFlaser pulses of 60 shots [see curve (f)]. Curve (g) rep-resents the difference spectrum between curves (e) and(f). Curve (h), on the other hand, shows the differencespectrum before and after irradiation with KrF laserpulses (60 shots). Note that the shapes of curves (f)and (h) are almost identical. This indicates that theformation reactions of relevant color centers appar-

1995 Optical Society of America

May 15, 1995 / Vol. 20, No. 10 / OPTICS LETTERS 1185

Fig. 1. Absorption spectra of 10 GeO2 –90 SiO2 glasses.(a) As polished; after illumination with (b) Hg lamp radi-ation (140 h), (c) KrF laser pulses (60 shots), (d) Hg lamp(140 h) ! KrF laser pulses (60 shots). The inset showsthe difference spectra of the specimens before and after(e) illumination with Hg lamp radiation (140 h) and (f )irradiation with Hg lamp radiation (140 h) followed by KrFlaser pulses (60 shots). (g) Difference spectrum betweencurves (e) and (f). The difference spectrum before andafter irradiation with only KrF laser pulses (60 shots) isrepresented by curve (h).

Fig. 2. ESR spectra of 10 GeO2 –90 SiO2 glasses (a) afterillumination of Hg lamp radiation (140 h) and (b) subse-quent irradiation with Hg lamp radiation (140 h) followedby KrF laser pulses (60 shots). Spectrometer gain waskept constant for all the measurements.

ently proceed independently, depending on the powerdensity of UV light sources.

Figure 2 show the ESR spectra of the specimen af-ter UV irradiation. The concentration of Ge E0 inthe specimen after prolonged illumination with a Hglamp was 4 3 1015 spinsycm3. The irradiation with

KrF laser induced intense resonances near that ofthe Ge E0 center. Curve (b) is the resonance signalsafter irradiation with the Hg lamp (140 h) and thenKrF laser pulses (60 shots). The total spin concen-tration reached 4.5 3 1017 spinsycm3. The resonanceappearing in the region g , 2 can be assigned tothe electron-trapped centers associated with fourfold-coordinated Ge ions (GEC’s).9 – 11 Two kinds of GEC,i.e., Ge(1) and Ge(2), were reported, depending onthe number of the next nearest neighbor Ge ions,one or two, respectively.10 The downward peaks atg ­ 1.9930 and 1.9866 agree with those of Ge(1) andGe(2). The resonance at g ­ 2.01 is attributed tohole-trapped centers judging from its positive g shift.There are three plausible hole-trapped ESR centers inSiO2 and GeO2 glasses: nonbridging oxygen hole cen-ter (g2 ­ 2.0076),12 peroxy radical (g2 ­ 2.0065),12 andself-trapped hole (STH; g2 ­ 2.008–2.009).13 Becausethe g value in question in very close to that of the thirdone, we assigned the resonance signal at g ­ 2.01 tothe STH. Neither a GEC nor a STH was observed forthe specimen irradiated with the Hg lamp.

It was reported that the optical absorption bandsthat are due to Ge(1) and Ge(2) are located at 4.4 and5.8 eV.11 Thus the two absorption bands (peaking at4.4 and 5.8 eV) generated by irradiation with the KrFlaser in Fig. 1, curves (c) and (d), are attributed to theGe(1) and Ge(2) centers.

The photochemical reaction2 caused by illuminationwith the Hg lamp is described as

Ge22Ge sor Ge22Sid!Ge E0 1GeO31 sor SiO3

1d1e2 .(1)

UV photons excite the preexisting neutral oxygen va-cancy to give Ge E0 by the one-photon absorptionprocess.2 The power density of the KrF laser pulseused in this study s,500 kWycm2d is higher by ,7orders of magnitude than that of the Hg lamp. There-fore we have to consider the possibility of the two-photon absorption processes for the formation reactionof GEC’s and STH’s. Figure 3 shows the absorptionspectrum in the VUV region of the 5 GeO2 –95 SiO2

thin glass films. The Eg was estimated as 7.1 eV fromthe Tauc plot shown in the inset of Fig. 3 [cf. Eg(SiO2

glass) ­ 9.3 eV,14 Eg(GeO2 glass) ­ 5.6 eV (Ref. 15)].Because the two-photon energy of a photon (5 eV) fromthe KrF laser is sufficiently larger than the Eg of thespecimen, it is reasonable to consider that photochemi-cal reactions triggered by the two-photon absorptionprocess occur by irradiation with the KrF laser. Aprominent result substantiating the key role of thetwo-photon absorption process in this case was ob-tained by the exposure of the specimen to the XeCllaser (4-eV) radiation. Figure 4 shows the absorptionspectrum of 10 GeO2 –90 SiO2 glass irradiated withXeCl laser pulses (5 3 104 shots). As noted in thedifference spectrum [curve (c)], the change in spec-tral shape was almost in agreement with curve (f)(KrF laser irradiation) in Fig. 1, indicating that GEC’s,not Ge E0, are created by irradiation with XeCl laser.The formation of GEC was also confirmed by themeasurement of ESR spectra, and its concentration

1186 OPTICS LETTERS / Vol. 20, No. 10 / May 15, 1995

Fig. 3. VUV absorption spectra of (a) 5 GeO2 –95 SiO2thin glass film (3.8 mm thick) deposited onto the SiO2substrate [curve (a)] and the SiO2 substrate (1 mm thick)[curve (b)]. The film was annealed at 450 ±C for severalhours in air before the measurement. The inset is a Taucplot for the thin film.

Fig. 4. Absorption spectra of 10 GeO2 –90 SiO2 glassesbefore [curve (a)] and after [curve (b)] irradiation with XeCllaser pulses (5 3 104 shots). Curve (c) is the differencespectra between curves (a) and (b).

increased approximately with the square of power den-sity of laser light under the condition of 5000 shots,10–50 mWycm2. These results indicate that GEC’sand STH’s are generated by band-to-band excitationby the two-photon absorption of XeCl laser pulses.16

On the basis of the results obtained here, we proposethe following scheme as a photochemical reaction inGeO2 –SiO2 glasses by excimer laser irradiation:

Ge41 sfourfold coordinatedd 1 O22 sbridging oxygend !

GEC 1 STH . (2)

The lone pair of electrons on the bridging oxygens,which occupies the uppermost level of the valenceband, is excited to the conduction band through thetwo-photon absorption processes and then trapped on

the fourfold-coordinated Ge ions, giving the GEC. Incontrast, the bridging oxygens are converted into theSTH’s after releasing an electron by band-to-band exci-tation. In the framework of the color-center model,17

therefore, the large photorefractive index change inGeO2 –SiO2 glasses is expected by the formation ofGEC’s.

In summary, phenomenologically, two kinds of reac-tion proceeded independently, depending on the powerdensities of the UV light: (i) conversion of the neutraloxygen monovacancy into a Ge E0 center by illumina-tion with a UV lamp and (ii) formation of the electron-trapped center associated with GEC and STH byirradiation with excimer laser pulses. The Ge E0 cen-ters and GEC’sySTH’s are formed by one-photon andtwo-photon absorption processes, respectively. Theformer center was derived from the precursor (neu-tral oxygen monovacancy), whereas the latter wascreated from intrinsic (at least in a sense of chemi-cal order) structural units by band-to-band excitation.The spectral change caused by irradiation with XeCllaser pulses strongly supports this model.

We thank K. Muta and M. Kato of Showa Elec-tric Wire and Cable Company, Ltd., for supplying theGeO2 –SiO2 glass rod. The measurement of the ab-sorption spectra in the VUV region was supported bythe Joint Studies Program of the Institute for Molecu-lar Science, Okazaki, Japan.

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