Permanent refractive-index changes in GeO2 glass slabsinduced by irradiation with sub band gap light

Yuichi Watanabe a,*, Junji Nishii b, Hiroyuki Moriwaki a, Genjiro Furuhashi a,Hideo Hosono c, Hiroshi Kawazoe c

a Department of Materials Science and Technology, Faculty of Industrial Science and Technology, Science University of Tokyo, 2641

Yamazaki, Noda, Chiba 278, Japanb Department of Optical Materials, Osaka National Research Institute, AIST, 1-8-31, Midorigaoka, Ikeda, Osaka 563, Japan

c Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta, 4259, Midori-ku, Yokohama 227, Japan

Abstract

Permanent, positive refractive-index changes in pure GeO2 glass slabs are induced by illumination with light having

energy far below that of the fundamental absorption edge. The refractive-index change is tentatively explained by the

Kramers±Kr�onig relationship for an additional absorption band induced in the glass via two competitive photo-

chemical reactions. Ó 1998 Elsevier Science B.V. All rights reserved.

PACS: 42.70.Ce; 78.20.-e

1. Introduction

Photosensitivity in germanosilicate glass hasattracted much attention because the photo-in-duced permanent refractive-index change can beused to form Bragg gratings in optical ®bers and/or waveguides [1±3]. It is generally accepted thatthe GeO2 dopant is responsible for the photosen-sitivity of the glass [4]; the Ge-associated absorp-tion band at 5 eV plays a key role. In spite of thefact that (i) the same band is observed in pureGeO2 glass, and (ii) the band possesses photosen-sitivity as in binary germanosilicate glass, therehave been few studies of the possible formation ofBragg gratings in pure GeO2 glass [5]. Furtherexamination of the photosensitivity of pure GeO2

glass is of importance from both scienti®c (mech-anisms of refractive-index change) and practical(optimization of glass composition) points of view.

Here we report on permanent, positive refrac-tive-index changes in pure GeO2 glass slabs inducedby illumination with light having a photon energyless than that of the fundamental absorption edge.The z-scan technique [6] has been used to detectrefractive-index changes with a sensitivity of 10ÿ5.The mechanism for the formation of permanentrefractive-index change is explained tentatively as aconsequence of Kramers±Kr�onig principle for anadditional absorption band induced in the glass viatwo competitive photochemical reactions.

2. Experimental procedures

Single component GeO2 glass was prepared bythe conventional melt-quenching method [7]. The

Journal of Non-Crystalline Solids 239 (1998) 104±107

* Corresponding author. Tel.: +81-471 241501; fax: +81-471

239362; e-mail: ywlabo@rs.noda.sut.ac.jp.

0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 7 2 5 - X

sample glass was cut and polished into a slab 2.3mm thick. Optical transmittance of the sample wasmore than 80% from 300 to 800 nm. The loss of16% at 355 nm (3.51 eV) was comparable to that ofthe Fresnel re¯ection, i.e., 12% [7], at both incidentand exit surface.

The z-scan technique [6] was used to observethe formation of permanent refractive-indexchange. In practice, third-harmonic light(hm� 3.51 eV) of a Q-switched Nd:YAG laser wasfocused with a lens. At the focal point, the radiusof the Gaussian beam was determined to be �14lm. First, a closed-aperture (S� 0.5) z-scan mea-surement was carried out with the ultra violet(UV) light at a low irradiance level (1 lJ perpulse). Then, the sample was placed at the focusand was irradiated with UV light at a larger irra-diance level (7 lJ per pulse, pulse repetition fre-quency 10 Hz, pulse duration 4 ns (FWHM), andexposure time 1±30 min). After the irradiation, theclosed aperture z-scan transmittance was remea-sured at the low irradiance level. Consequently,formation of permanent refractive-index changewas observed as changes in z-scan transmittancebefore and after UV-irradiation. It should benoted that the 3.51 eV photon energy of the UVlight is less than the energy gap of GeO2 glass,5.63 eV [8].

3. Experimental results

Fig. 1 shows the results of closed-aperture z-scan measurements. The minimum transmittanceat the pre-focal position followed by the post-focalmaximum indicates that a positive refractive indexchange has been induced in the glass by the UV-irradiation. As seen in Fig. 1, the di�erence be-tween minimum and maximum transmittance in-creased with increasing irradiation time.

A theoretical calculation for the z-scan trans-mittance was carried out with the assumption thatthe radial distribution of induced refractive-indexchange is proportional to the nth power of beamirradiance of the Gaussian beam, i.e.,

Dn�r� � c I0 expÿ2r2

x20

� �� �n

; �1�

where I0 and x0 represent peak irradiance andradius of the Gaussian beam at the focal plane,respectively. In Eq. (1), the nth power of irradi-ance represents the refractive-index changes in-duced by a light pulse, while C represents thecumulative e�ect associated with the total numberof light pulses. The exponent n was introduced torepresent the non-linear optical e�ect: the changein refractive-index is assumed to be an n-photonprocess.

Fig. 1. Closed-aperture (S� 0.5) z-scan transmittance for GeO2

glass irradiated with UV-light (355 nm) for (a) 3.5 min, (b) 7.5

min, and (c) 15.5 min. Filled circles represent the experimental

data whereas solid lines are the theoretical ®ts.

Y. Watanabe et al. / Journal of Non-Crystalline Solids 239 (1998) 104±107 105

The calculated z-scan transmittances shown assolid lines in Fig. 1, agreed with the data for n� 3.The induced refractive-index change could be de-termined from the theoretical ®ts of the z-scandata. Fig. 2 shows the induced refractive-indexchange, Dn, at the center of Gaussian beam as afunction of irradiation time. As seen in Fig. 2, Dnincreases linearly with irradiation time for t < 10min. Prolonged irradiation caused further increasein Dn to 10ÿ4 after irradiation for 30 min.

4. Discussion

Here we discuss the mechanism for formationof permanent refractive-index changes in pureGeO2 glass on the basis of a representative model:In the Kramers±Kr�onig model [4,9], the changes inrefractive-index are explained as a consequence ofthe appearance of an additional absorption band

due to photo-induced color centers in the glass.We observed the formation of Ge-E0 paramagneticdefect centers in the UV-irradiated glass. Theirconcentration was measured by electron spin res-onance (ESR) to be 3.1 ´ 1018 spins/cm3 at thepeak of Gaussian beam for the glass irradiatedwith UV-light for 10 min. We have previously re-ported that the Ge-E0 center gives rise to an ab-sorption band centered at 6.3 eV with an FWHMof 1.1 eV [10]. The absorption coe�cient a (cmÿ1)at the peak of the band is proportional to thedensity N(cmÿ3) of Ge-E0 centers, i.e., a/N� 7.2 ´ 10ÿ17 cm2. By substituting the abovefactors into the formula proposed by Hand et al.[11], in which the refractive-index change is eval-uated from the changes in absorption coe�cientthrough the Kramers±Kr�onig relationship, theexpected refractive-index change for the irradiatedglass of this study was calculated to be 1.0 ´ 10ÿ4.This magnitude is comparable to that observed inthe present z-scan measurements, i.e.,Dn� 3.2 ´ 10ÿ5 for the glass irradiated for 7.5 min.Thus, the color center model is valid for thepresent observations.

With regard to the formation of Ge-E' centersby irradiation with sub band gap light, it should benoted that the radial distribution of Dn assumed inEq. (1) is proportional to the third power of theirradiance. This dependence can be explained byassuming that the Ge-E0 centers are generated viatwo competitive photochemical reactions [12] un-der UV-irradiation: One reaction is formation ofmetastable defect species associated with Ge via atwo-photon absorption (TPA) process, while theother is a structural relaxation of the metastabledefect into the Ge-E0 center through a one-photonabsorption process. GEC (Ge electron trappedcenter) [13] is a possible candidate for the meta-stable defect because: (i) The defect is known torelax into the Ge-E0 center through a thermalstabilization process [13]. (ii) The GEC has anabsorption band centered at 4.4 eV withFWHM� 1.97 eV [14]. Then, the 3.51 eV UV lightshould be absorbed by a portion of the GECs. (iii)The GEC is the dominant defect species inGeO2:SiO2 glass irradiated with an ArF excimerlaser [15]; The photon energy of ArF, hm� 6.5 eV,is comparable to the two-photon energy, 2 ´ 3.51

Fig. 2. Change in the index of refraction, Dn at 355 nm versus

irradiation time. Filled squares represent the experimental data.

The straight line is drawn as a guide for the eye.

106 Y. Watanabe et al. / Journal of Non-Crystalline Solids 239 (1998) 104±107

eV� 7.02 eV for the light used in the present study.These facts are consistent with the two competitivephotochemical reaction model proposed for theformation of Ge-E0 centers, resulting in a forma-tion of permanent, positive refractive-index chan-ges in GeO2 glass.

5. Conclusion

The formation mechanism of the refractive-in-dex change is tentatively explained by a twocompetitive photochemical reaction process, i.e.,formation of a metastable defect via two-photonabsorption and conversion of the defect into an-other stable defect via a one-photon process.

Acknowledgements

The work was supported in part by the NipponSheet Glass Foundation for Materials Science andEngineering (1997).

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