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Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 5 — Mar. 7, 2005
  • pp: 1696–1701
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Characterization of a highly photorefractive RF-sputtered SiO2-GeO2 waveguide

S. Sebastiani, G. Nunzi Conti, S. Pelli, G. C. Righini, A. Chiasera, M. Ferrari, and C. Tosello  »View Author Affiliations


Optics Express, Vol. 13, Issue 5, pp. 1696-1701 (2005)
http://dx.doi.org/10.1364/OPEX.13.001696


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Abstract

We present the characterization of highly photorefractive Er3+/Yb3+-doped 75SiO2-25GeO2 planar waveguides, single mode at 1550 nm, deposited by radio-frequency-magnetron-sputtering (RFMS) technique. Details of the deposition process are reported. The material presents an intense absorption band (α≈103÷104 cm-1) in the UV region. Irradiations by a KrF excimer laser source at λ=248 nm have produced large positive (up to 3·10-3) refractive index changes, without the need of particular sensitization procedures. Direct measurements of UV photo-induced volume densification demonstrates that glass compaction accounts for large part of the refractive index change. Highly efficient photo-induced phase gratings have thus been fabricated in the waveguide.

© 2005 Optical Society of America

1. Introduction

Despite the substantial experimental effort in the characterization of the photorefractive effect, the origin of the UV laser-induced refractive index change has not been fully understood. Even if particular treatments such as H2 loading, boron co-doping and OH flooding can increase considerably the photorefractive proprieties of the glass, large UV-induce refractive index changes have been measured also in sputter deposited [5

5. J. Nishii, H. Yamanaka, H. Hosono, and K. Kawazoe, “Preparation of Bragg gratings in sputter-deposited GeO2-SiO2 glasses by excimer-laser irradiation,” Opt. Lett. 21, 1369–1362 (1996). [CrossRef]

,6

6. H. Hosono and J. Nishii, “High photosensitivity and nanometer-scale phase separation in GeO2SiO2 glass thin films,” Opt. Lett. 24, 1352–1354 (1999). [CrossRef]

] and plasma-enhanced-chemical-vapor deposited [2

2. M. Svalgaard, C.V. Poulsen, A. Bjarklev, and O. Poulsen, “Direct UV writing of buried single mode channel waveguides in Ge-doped silica films,” Electron. Lett. 30, 1401–1403 (1994). [CrossRef]

,7

7. M. Takahashi, A. Sakoh, K. Ishii, Y. Tokuda, T. Yoko, and J. Nishii, “Photosensitive GeO2-SiO2 films for ultraviolet laser writing of channel waveguides and Bragg gratings with Cr-loaded waveguide structure,” Appl. Opt. 42, 4594–4598 (2003). [CrossRef] [PubMed]

] thin films, without the need of particular sensitization procedures. The present films represent an excellent opportunity for the investigation of the UV-induced phenomena of the SiO2-GeO2 system, since the experimental evidences of the photo-induced structural modifications are large enough to allow us measurements with good accuracy.

Here we present the characterization of highly photorefractive planar waveguides, single-mode at 1550 nm, prepared by conventional RFMS technique. The positive refractive index and the absorption changes have been explored as a function of the laser irradiated energy. The entity of the index changes was found to be sufficient for the fabrication of photo-induced channel waveguides. Direct measurements of UV-induced volume densification were performed and their contribution to the index change evaluated. A highly efficient phase grating was also demonstrated.

2. Experimental

Thin films with 75SiO2-25GeO2 (molar %) composition, co-doped with 0.27 mol% each of Er2O3 and Yb2O3, were deposited onto silica substrates by radio-frequency-magnetron-sputtering technique, in order to produce planar waveguides single-mode at 1550 nm. Sputtering deposition of the film was performed by using a 4” silica target on which pieces of GeO2, metallic ytterbium and metallic erbium pieces were placed. The residual pressure, before deposition, was about 2×10-5 Pa. During the deposition process the substrates were not heated. The sputtering was carried out with an Ar gas at a pressure of 0.7 Pa and an applied RF power of 150 W, with a reflected power of 18 W. The deposition time was 4 h 15 min, producing a film thickness of about 3.35±0.2 µm. The as-deposited films were acting as waveguides, but they were showing rather high propagation losses; thus, following a procedure similar to the one that had demonstrated quite effective with silica-titania sputtered waveguides [8

8. A. Chiasera, M. Montagna, C. Tosello, S. Pelli, G.C. Righini, M. Ferrari, L. Zampedri, A. Monteil, and P. Lazzeri, “Enhanced spectroscopic properties at 1.5 µm in Er/Yb activated silica-titania planar waveguides fabricated by rf-sputtering,” Opt. Mat. 25, 117–122 (2004). [CrossRef]

], the deposited films were annealed in air at 600 °C for 6 h, with a heating/cooling rate of 6 °C/minute.

Subsequent irradiations of the samples were performed by using a Lambda Physik Compex KrF excimer laser source, with emission at λ=248 nm. Single pulse fluency was 36 mJ/cm2 and repetition rate was set at 10 Hz. Transmission spectroscopy in the UV region was performed by using a Perkin-Elmer λ19 spectrophotometer.

3. Results and discussion

The guiding properties of the deposited films were tested by using an m-line apparatus developed in house, based on the prism coupling technique. The annealed films had a refractive index at 632.8 nm around 1.495, less than 1% smaller than that of the corresponding as-deposited films. Refractive index variations were less than 0.2% in a same sample and smaller than 1% from sample to sample. The value of 1.495 fits quite well the expected refractive index, calculated by the Lorentz-Lorenz equation, of a bulk glass having the same stoichiometric ratio between silica and germania. The optical quality of the annealed films was good, as testified by the propagation losses of the TE0 mode, measured at 633 nm (0.8±0.2 dB/cm) and at 1300nm (<0.3 dB/cm) by the photometric detection of the light intensity scattered out of the waveguide plane. The photoluminescence properties of these films are quite good as well, and will be reported elsewhere.

Figure 1 illustrates the UV absorption band of the annealed film, for different values of cumulative exposure dose. The pristine film (curve a) presents an intense absorption peak centered at 5.16 eV (α5.16eV=4.75·103 cm-1). The first exposure, with energy density 1.08 kJ/cm2 (curve b), has strongly modified the absorption spectrum: part of the 5 eV band was bleached and the absorption increased for higher photon energies. Subsequent irradiations, with 2.16 kJ/cm2 (curve c) and 3.25 kJ/cm2 (curve d) cumulative exposure doses, produced an additional increase of the absorption above 5.3 eV. The spectral changes in the range between 4.0 and 6.2 eV saturated for higher irradiated energies. Spectroscopic features in this UV region are similar to those reported by Nishii et al. [9

9. J. Nishii and H. Yamanaka, “Characteristics of 5-eV band in sputter deposited GeO2-SiO2 thin films glass films,” Appl. Phys. Lett. 64, 282–284 (1994). [CrossRef]

11

11. H. Hosono, M. Mizuguchi, H. Kawazoe, and J. Nishii, “Correlation between GeE’ centres and optical absorption bands in SiO2:GeO2 glasses,” Jpn. J. Appl. Phys. 35 (2B), Part2, 234–236 (1996). [CrossRef]

] for SiO2-GeO2 sputter-deposited thin films. The UV-light absorption has been recognized to be produced by the presence of germanium-oxygen-defect-centers (GODC’s) [12

12. H. Hosono, Y. Abe, D.L. Kinser, R.A. Weeks, K. Muta, and H. Kawazoe, “Nature and origin of the 5-eV band in SiO2:GeO2 glasses,” Phys. Rev. B 46, 11445–11450 (1992). [CrossRef]

15

15. T. Uchino, M. Takahashi, K. Ichii, and T. Yoko, “Microscopic model of photoinduced and pressure-induced UV spectral changes in germanosilicate glass,” Phys. Rev. B 65, 172202-1 (2002). [CrossRef]

] that are formed, depending on the sintering conditions, during the glass matrix growth. It appears that, due to the reduced pressure and pure oxygen-free argon atmosphere, sputtering favors the generation of the oxygen vacancies, so increasing the photosensitive proprieties of the film.

The UV-induced refractive index change of these films was estimated by measuring, before and after the irradiations, the effective index of the single mode supported at 1550 nm by the planar waveguides. Much care was taken in order to repeat the measurement in a same position inside the waveguide, to avoid errors due to film inhomogeneities. Figure 2 shows the effective index changes Δn obtained for increasing values of cumulative irradiated energy. For doses less than 5 kJ/cm2 the refractive index of the film increases linearly with irradiated energy. The saturation value of the index change, of the order of 3·10-3, is reached at a dose of about 20 kJ/cm2 (see inset of Fig. 2). The results shown were obtained in two samples and indicate a very good repeatability and reliability of measurements. Such a positive value of the index change is high enough to achieve a good lateral confinement of the radiation: channel waveguides’ direct writing on similar films is therefore now under way in our laboratory.

Fig. 1. UV absorption spectra of the film (a) after deposition and annealing; (b) irradiated with 1.08 kJ/cm2; (c) irradiated with 2.16 kJ/cm2 ; and (d) irradiated with 3.25 kJ/cm2.
Fig. 2. UV laser-induced changes of the effective index at 1550 nm of a single mode waveguide for increasing values of the cumulative exposure dose. The inset shows the saturation behavior of the index change measured in another, nominally equal, waveguide.

In order to test the contribution of the UV photo-induced volume densification [16

16. P. Cordier, S. Dupont, M. Douay, G. Martinelli, P. Bernage, P. Niay, J. F. Bayon, and L. Dong, “Evidence by transmission electron microscopy of densification associated to Bragg grating photoimprinting in germanosilicate optical fibers,” Appl. Phys. Lett. 70, 1204–1206 (1997). [CrossRef]

,17

17. N. F. Borrelli, D. C. Allan, and R. A. Modavis, “Direct measurement of 248- and 193-nm excimer-induced densification in silica-germania waveguide blanks,” JOSA B 16, 1672–1679 (1999). [CrossRef]

] to the index change, we placed a thin metallic wire of diameter 150 µm, as a simple exposure mask, onto the sample surface. The sample was then irradiated by the excimer laser; after exposure we removed the wire and scanned the sample surface with a Tencor P-10 profilometer. Fig. 3 shows the profile produced by the instrument, and the inset shows a 3D map of a 550 µm×180 µm area. The step in the profile corresponds to the position of the wire during the exposure, namely to the not-irradiated area. The spike is an artifact, likely due to a point defect or a debris grain. For 16.2 kJ/cm2 of irradiated energy we measured a decrease ΔT=-16±2 nm of the film thickness T.

Fig. 3. Profilometer scan of the sample surface after the UV-exposure, using a metal wire as a simple mask. The step reveals the position assumed by the wire during the irradiation. The inset shows a 3D image of a 550 µm×180 µm area, where the masking effect of the wire appears evident.

The contribution of the photo-induced volume densification to Δn can be evaluated quantitatively by differentiating the Lorentz-Lorenz formula [18

18. M. Born and E. Wolf, “The mean polarizability: the Lorentz-Lorenz formula,” in Principles of Optics, (Pergamon Press, Oxford, 1980), pp. 87.

]:

Δnn=(n2+2)·(n21)6·n2·(ΔNN+Δαα),
(1)

where n is the refractive index, N is the number of molecules per unit volume and α’ is the mean polarizability. After the irradiation with 16.2 kJ/cm2 energy density we measured a change of the effective index

ΔnnEXP=1.7±0.2.103.

On the other hand, from Eq. (1), by considering only the volume contribution and introducing the measured value of thickness decrease, we obtain:

ΔnnCALC=(n2+2)·(n21)6·n2·ΔNN=(n2+2)·(n21)6·n2·(ΔT)T=1.8±0.3.103.

We can therefore conclude that, by taking into account the errors, the UV-induced glass compaction accounts for a very large part, if not all, of the positive refractive index change. Additional measurements, carried out onto similar samples, confirmed this indication.

Fig. 4. Deflection of the guided light at 633 nm produced by a highly efficient photo-induced Bragg grating.

4. Conclusions

The silica-germania films we deposited and characterized exhibited interesting properties. Low propagation losses and good index matching with fibers are two of the fulfilled requirements for the development of integrated optical circuits. The rare-earth-doping increases the material’s functionality. Large positive refractive index changes were produced and easily controlled by the exposure from a KrF excimer laser source. It was demonstrated that the photo-induced volume densification accounted for a very large part of the photorefractive index change. The amounts of the index changes were sufficient for the fabrication of highly efficient sub-micrometric phase gratings, and photo-induced channel waveguides are now being written and characterized.

Acknowledgments

This work was partially supported by MIUR, Italy, through the FIRB project “Sistemi Miniaturizzati per Elettronica e Fotonica”. We are grateful to Mr. Roberto Calzolai (Optical shop, IFAC-CNR) for technological assistance.

References and links

1.

V. Mizrahi, P. J. Lemaire, T. Erdogan, W. A. Reed, D. J. Di Giovanni, and R. M. Atkins, “Ultraviolet laser fabrication of ultrastrong optical fiber gratings and of germania-doped channel waveguides,” Appl. Phys. Lett. 63, 1727–1729 (1993). [CrossRef]

2.

M. Svalgaard, C.V. Poulsen, A. Bjarklev, and O. Poulsen, “Direct UV writing of buried single mode channel waveguides in Ge-doped silica films,” Electron. Lett. 30, 1401–1403 (1994). [CrossRef]

3.

G. D. Emmerson, S. P. Watts, C. B. E. Gawith, V. Albanis, M. Ibsen, R.B. Williams, and P.G.R. Smith, “Fabrication of directly UV-written channel waveguides with simultaneously defined integral Bragg gratings,” Electron. Lett. 38, 1531–1532 (2002). [CrossRef]

4.

H. Nishiyama, I. Miyamoto, S. Matsumoto, M. Saito, K. Kintaka, and J. Nishii, “Direct laser writing of thermally stabilized channel waveguides with Bragg gratings,” Opt. Express12, 4589–4595 (2004). http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-19-4589 [CrossRef] [PubMed]

5.

J. Nishii, H. Yamanaka, H. Hosono, and K. Kawazoe, “Preparation of Bragg gratings in sputter-deposited GeO2-SiO2 glasses by excimer-laser irradiation,” Opt. Lett. 21, 1369–1362 (1996). [CrossRef]

6.

H. Hosono and J. Nishii, “High photosensitivity and nanometer-scale phase separation in GeO2SiO2 glass thin films,” Opt. Lett. 24, 1352–1354 (1999). [CrossRef]

7.

M. Takahashi, A. Sakoh, K. Ishii, Y. Tokuda, T. Yoko, and J. Nishii, “Photosensitive GeO2-SiO2 films for ultraviolet laser writing of channel waveguides and Bragg gratings with Cr-loaded waveguide structure,” Appl. Opt. 42, 4594–4598 (2003). [CrossRef] [PubMed]

8.

A. Chiasera, M. Montagna, C. Tosello, S. Pelli, G.C. Righini, M. Ferrari, L. Zampedri, A. Monteil, and P. Lazzeri, “Enhanced spectroscopic properties at 1.5 µm in Er/Yb activated silica-titania planar waveguides fabricated by rf-sputtering,” Opt. Mat. 25, 117–122 (2004). [CrossRef]

9.

J. Nishii and H. Yamanaka, “Characteristics of 5-eV band in sputter deposited GeO2-SiO2 thin films glass films,” Appl. Phys. Lett. 64, 282–284 (1994). [CrossRef]

10.

J. Nishii, N. Kitamura, H. Yamanaka, H. Hosono, and H. Kawazoe, “Ultraviolet-radiation-induced chemical reactions through one- and two-photon absorption processes in GeO2-SiO2 glasses,” Opt. Lett. 20, 1184 (1995). [CrossRef] [PubMed]

11.

H. Hosono, M. Mizuguchi, H. Kawazoe, and J. Nishii, “Correlation between GeE’ centres and optical absorption bands in SiO2:GeO2 glasses,” Jpn. J. Appl. Phys. 35 (2B), Part2, 234–236 (1996). [CrossRef]

12.

H. Hosono, Y. Abe, D.L. Kinser, R.A. Weeks, K. Muta, and H. Kawazoe, “Nature and origin of the 5-eV band in SiO2:GeO2 glasses,” Phys. Rev. B 46, 11445–11450 (1992). [CrossRef]

13.

B. L. Zhang and K. Raghavachari, “Photoabsorption and photoluminescence of divalent defects in silicate and germanosilicate glasses: First-principles calculations,” Phys. Rev. B 55, R15993–R15996 (1997). [CrossRef]

14.

G. Pacchioni and I. Ieranò, “Ab initio formation energies of point defects in pure and Ge-doped SiO2,” Phys. Rev. B 56, 7304–7312 (1997). [CrossRef]

15.

T. Uchino, M. Takahashi, K. Ichii, and T. Yoko, “Microscopic model of photoinduced and pressure-induced UV spectral changes in germanosilicate glass,” Phys. Rev. B 65, 172202-1 (2002). [CrossRef]

16.

P. Cordier, S. Dupont, M. Douay, G. Martinelli, P. Bernage, P. Niay, J. F. Bayon, and L. Dong, “Evidence by transmission electron microscopy of densification associated to Bragg grating photoimprinting in germanosilicate optical fibers,” Appl. Phys. Lett. 70, 1204–1206 (1997). [CrossRef]

17.

N. F. Borrelli, D. C. Allan, and R. A. Modavis, “Direct measurement of 248- and 193-nm excimer-induced densification in silica-germania waveguide blanks,” JOSA B 16, 1672–1679 (1999). [CrossRef]

18.

M. Born and E. Wolf, “The mean polarizability: the Lorentz-Lorenz formula,” in Principles of Optics, (Pergamon Press, Oxford, 1980), pp. 87.

19.

K.O. Hill, B. Malo, F. Bilodeau, D.C. Johnson, and J. Albert, “Bragg gratings fabricated in monomode photosensitive optical fiber by exposure through a phase mask”, Appl. Phys. Lett. 62, 1035–1037 (1993). [CrossRef]

OCIS Codes
(160.2750) Materials : Glass and other amorphous materials
(160.5320) Materials : Photorefractive materials
(310.2790) Thin films : Guided waves

ToC Category:
Research Papers

History
Original Manuscript: December 16, 2004
Revised Manuscript: February 25, 2005
Published: March 7, 2005

Citation
S. Sebastiani, G. Nunzi Conti, S. Pelli, G. Righini, A. Chiasera, M. Ferrari, and C. Tosello, "Characterization of a highly photorefractive RF-sputtered SiO2-GeO2 waveguide," Opt. Express 13, 1696-1701 (2005)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-5-1696


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References

  1. V. Mizrahi, P. J. Lemaire, T. Erdogan, W. A. Reed, D. J. Di Giovanni, R. M. Atkins, �??Ultraviolet laser fabrication of ultrastrong optical fiber gratings and of germania-doped channel waveguides,�?? Appl. Phys. Lett. 63, 1727-1729 (1993). [CrossRef]
  2. M. Svalgaard, C.V. Poulsen, A. Bjarklev, O. Poulsen, �??Direct UV writing of buried single mode channel waveguides in Ge-doped silica films,�?? Electron. Lett. 30, 1401-1403 (1994). [CrossRef]
  3. G. D. Emmerson, S. P. Watts, C. B. E. Gawith, V. Albanis, M. Ibsen, R.B. Williams, P.G.R. Smith, �??Fabrication of directly UV-written channel waveguides with simultaneously defined integral Bragg gratings,�?? Electron. Lett. 38, 1531-1532 (2002). [CrossRef]
  4. H. Nishiyama, I. Miyamoto, S. Matsumoto, M. Saito, K. Kintaka, J. Nishii, �??Direct laser writing of thermally stabilized channel waveguides with Bragg gratings,�?? Opt. Express 12, 4589-4595 (2004). <a href= "http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-19-4589">http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-19-4589</a> [CrossRef] [PubMed]
  5. J. Nishii, H. Yamanaka, H. Hosono, K. Kawazoe, �??Preparation of Bragg gratings in sputter-deposited GeO2-SiO2 glasses by excimer-laser irradiation,�?? Opt. Lett. 21, 1369-1362 (1996). [CrossRef]
  6. H. Hosono, J. Nishii, �??High photosensitivity and nanometer-scale phase separation in GeO2SiO2 glass thin films,�?? Opt. Lett. 24, 1352-1354 (1999). [CrossRef]
  7. M. Takahashi, A. Sakoh, K. Ishii, Y. Tokuda, T. Yoko, J. Nishii, �??Photosensitive GeO2-SiO2 films for ultraviolet laser writing of channel waveguides and Bragg gratings with Cr-loaded waveguide structure,�?? Appl. Opt. 42, 4594-4598 (2003). [CrossRef] [PubMed]
  8. A. Chiasera, M. Montagna, C. Tosello, S. Pelli, G.C. Righini, M. Ferrari, L. Zampedri, A. Monteil, P. Lazzeri, �??Enhanced spectroscopic properties at 1.5 µm in Er/Yb activated silica-titania planar waveguides fabricated by rf-sputtering,�?? Opt. Mat. 25, 117-122 (2004). [CrossRef]
  9. J. Nishii, H. Yamanaka, �??Characteristics of 5-eV band in sputter deposited GeO2-SiO2 thin films glass films,�?? Appl. Phys. Lett. 64, 282-284 (1994). [CrossRef]
  10. J. Nishii, N. Kitamura, H. Yamanaka, H. Hosono, H. Kawazoe, �??Ultraviolet-radiation-induced chemical reactions through one- and two-photon absorption processes in GeO2-SiO2 glasses,�?? Opt. Lett. 20, 1184 (1995). [CrossRef] [PubMed]
  11. H. Hosono, M. Mizuguchi, H. Kawazoe, J. Nishii, �??Correlation between GeE�?? centres and optical absorption bands in SiO2:GeO2 glasses,�?? Jpn. J. Appl. Phys. 35 (2B), Part2, 234-236 (1996). [CrossRef]
  12. H. Hosono, Y. Abe, D.L. Kinser, R.A. Weeks, K. Muta, H. Kawazoe, �??Nature and origin of the 5-eV band in SiO2:GeO2 glasses,�?? Phys. Rev. B 46, 11445-11450 (1992). [CrossRef]
  13. B. L. Zhang, K. Raghavachari, �??Photoabsorption and photoluminescence of divalent defects in silicate and germanosilicate glasses: First-principles calculations,�?? Phys. Rev. B 55, R15993-R15996 (1997). [CrossRef]
  14. G. Pacchioni, I. Ieranò, �??Ab initio formation energies of point defects in pure and Ge-doped SiO2,�?? Phys. Rev. B 56, 7304-7312 (1997). [CrossRef]
  15. T. Uchino, M. Takahashi, K. Ichii, T. Yoko, �??Microscopic model of photoinduced and pressure-induced UV spectral changes in germanosilicate glass,�?? Phys. Rev. B 65, 172202-1 (2002). [CrossRef]
  16. P. Cordier, S. Dupont, M. Douay, G. Martinelli, P. Bernage, P. Niay, J. F.Bayon, L. Dong, �??Evidence by transmission electron microscopy of densification associated to Bragg grating photoimprinting in germanosilicate optical fibers,�?? Appl. Phys. Lett. 70, 1204-1206 (1997). [CrossRef]
  17. N. F. Borrelli, D. C. Allan, R. A. Modavis, �??Direct measurement of 248- and 193-nm excimer-induced densification in silica-germania waveguide blanks,�?? JOSA B 16, 1672-1679 (1999). [CrossRef]
  18. M. Born, E. Wolf, �??The mean polarizability: the Lorentz-Lorenz formula,�?? in Principles of Optics, (Pergamon Press, Oxford, 1980), pp. 87.
  19. K.O. Hill, B. Malo, F. Bilodeau, D.C. Johnson, J. Albert, �??Bragg gratings fabricated in monomode photosensitive optical fiber by exposure through a phase mask�??, Appl. Phys. Lett. 62, 1035-1037 (1993). [CrossRef]

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