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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 2, Iss. 12 — Dec. 1, 2012
  • pp: 1751–1759
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Time resolved measurements of gratings photo-induced by femtosecond pulses in a lead doped glass

S. Chouli and E. Freysz  »View Author Affiliations


Optical Materials Express, Vol. 2, Issue 12, pp. 1751-1759 (2012)
http://dx.doi.org/10.1364/OME.2.001751


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Abstract

We report on the formation of gratings photo-induced by femtosecond laser pulses in SF59 glass. Depending on the number of pulses used to excite the sample and the pump power density, transient or permanent gratings are induced. We demonstrate that the grating formation is not instantaneous and is produced by laser-induced defects. This results in a change of both the real and imaginary part of the index of refraction. A simple set-up that records the temporal evolution of both parameters during the laser excitation is also presented. It makes it possible to evaluate the weight of both contributions to the grating diffraction efficiency.

© 2012 OSA

1. Introduction

Nowadays, femtosecond laser pulses are largely used to induce permanent index modulations inside bulk glass [1

1. H. Zhang, S. M. Eaton, J. Li, and P. R. Herman, “Femtosecond laser direct writing of multiwavelength Bragg grating waveguides in glass,” Opt. Lett. 31(23), 3495–3497 (2006). [CrossRef] [PubMed]

,2

2. H. Zhang, S. M. Eaton, and P. R. Herman, “Single-step writing of Bragg grating waveguides in fused silica with an externally modulated femtosecond fiber laser,” Opt. Lett. 32(17), 2559–2561 (2007). [CrossRef] [PubMed]

]. These methods rely on the use of multiphoton absorption for the ultra-short release of the laser energy into such transparent material. This makes it possible to photo-write isotropic or non-isotropic three dimensional structures inside bulk glass [3

3. W. Yang, P. G. Kazansky, and Y. P. Svirko, “Non-reciprocal ultrafast laser writing,” Nat. Photonics 2(2), 99–104 (2008). [CrossRef]

]. Different studies have shown that such femtosecond irradiations induce structural changes of the glass that modify, on the sub-micrometric scale, the index of refraction [4

4. M. Lancry, B. Poumellec, A. Chahid-Erraji, M. Beresna, and P. G. Kazansky, “Dependence of the femtosecond laser refractive index change thresholds on the chemical composition of doped-silica glasses,” Opt. Mater. Express 1(4), 711–723 (2011). [CrossRef]

]. The interaction of ultrafast laser pulses with soda-lime glasses and borosilicate glasses is also known for producing nonlinear effects including coloration. The latter also induces a change of the refractive index that is due to laser-induced color centers [5

5. J. B. Lonzaga, S. M. Avanesyan, S. C. Langford, and J. T. Dickinson, “Color center formation in soda-lime glass with femtosecond laser pulses,” J. Appl. Phys. 94(7), 4332–4340 (2003). [CrossRef]

,6

6. G. Lin, F. Luo, F. He, Q. Chen, D. Chen, Y. Cheng, L. Zhang, J. Qiu, and Q. Zhao, “Different refractive index change behavior in borosilicate glasses induced by 1 kHz and 250 kHz femtosecond lasers,” Opt. Mater. Express 1(4), 724–731 (2011). [CrossRef]

]. Hereafter, we have studied the impact of femtosecond laser-induced defects in a SF59 lead glass sample during the formation of photo-induced gratings. The grating is directly written into this glass sample by imaging a master grating, that is illuminated by the femtosecond laser pulses. With this technique it is possible to write directly a photo-induced grating [7

7. H. Guillet de Chatellus and E. Freysz, “Characterization and dynamics of gratings induced in glasses by femtosecond pulses,” Opt. Lett. 27(13), 1165–1167 (2002). [CrossRef] [PubMed]

]. The number of grooves per millimeter can be easily varied by changing the magnification of the optical device that images the master grating. Therefore, this technique offers many advantages compared to the direct write method [8

8. G. D. Marshall, R. J. Williams, N. Jovanovic, M. J. Steel, and M. J. Withford, “Point-by-point written fiber-Bragg gratings and their application in complex grating designs,” Opt. Express 18(19), 19844–19859 (2010). [CrossRef] [PubMed]

,9

9. A. Martinez, M. Dubov, I. Khrushchev, and I. Bennion, “Direct writing of fibre Bragg gratings by femtosecond laser,” Electron. Lett. 40(19), 1170–1172 (2004). [CrossRef]

] and the side-illumination with a phase mask [10

10. S. J. Mihailov, C. W. Smelser, D. Grobnic, R. B. Walker, P. Lu, H. Ding, and J. Unruh, “Bragg gratings written in all-SiO2 and ge-doped core fibers with 800-nm femtosecond radiation and a phase mask,” J. Lightwave Technol. 22(1), 94–100 (2004). [CrossRef]

14

14. M. Bernier, S. Gagnon, and R. Vallée, “Role of the 1D optical filamentation process in the writing of first order fiber Bragg gratings with femtosecond pulses at 800nm,” Opt. Mater. Express 1(5), 832–844 (2011). [CrossRef]

]. Moreover, it is also possible to record the evolution of the grating during the writing process. Hereafter we apply this technique to demonstrate that the formation of the photo-induced grating has a non-instantaneous contribution that can be easily recorded on the microsecond time scale and that results from laser-induced defects. These defects impact both the real and the imaginary part of the index of refraction of the glass. Their contributions to the diffraction grating efficiency are evaluated. Under our experimental conditions, it is mainly the real part of the index of refraction that contributes to reflectivity the grating.

2. The experimental set-up

To study the temporal evolution of gratings photo-induced by femtosecond laser pulses in glass, we used the experimental set-up sketched in Fig. 1
Fig. 1 The experimental set-up. D.M. stands for dichroic mirror.
. It has been previously described [7

7. H. Guillet de Chatellus and E. Freysz, “Characterization and dynamics of gratings induced in glasses by femtosecond pulses,” Opt. Lett. 27(13), 1165–1167 (2002). [CrossRef] [PubMed]

]. In short, a femtosecond pump beam delivered by a regenerative amplifier (1 mJ, ~50 fs, 1 kHz, ~800 nm) is used to photo-induce the grating. The pump-pulse is spatially overlapped with a continuous wave (CW) He-Ne laser beam that is used as a probe beam. Both beams are incident on a master phase-grating (spatial step L1 = 30 µm) that is optimized to diffract ~65% of the intensity at 800 nm along + 1 and −1 diffraction orders. These two latter diffraction orders, that have almost the same intensity, are focused by a 100 mm focal-length lens to a beam spot of ~180 µm in a 2 mm thick SF59 glass plate. Only the + 1 diffraction order of the He-Ne beam was focused on the sample. The direction of the diffracted He-Ne beam is along the −1 diffraction order of the grating. The intensity of the He-Ne beam diffracted by the photo-induced grating (spatial step L2 = 2.5 µm.) is focused on a fast photodiode that has a rising time of ~1 ns and that is either connected to a lock-in amplifier or a 10 GHz sampling oscilloscope. The latter devices are used to record the intensity reflected by the grating on the second and the millisecond time-scale respectively. The 10 GHz sampling oscilloscope has 1 GHz bandwidth. The pump and probe beam were chopped independently.

3. Results

3.1 The grating formation

3.2 Relaxation of the grating

3.3 Evolution of the grating in the transient and the steady state regime

Figure 4
Fig. 4 (a) Evolution of the intensity diffracted He-Ne beam at millisecond time scale for 140, 315 and 700 GW.cm−2 pump peak power density. The inset presents the evolution of diffraction efficiency versus the order of pulses within the exciting pump pulse train for peak power density of 600 and 700 GW.cm−2 respectively. (b) (○) Evolution of the diffracted intensity recorded at steady state during pump pulse excitation. The femtosecond pump pulses are applied at t = 0 and t = 1 ms. (−−−): Fit of this data using the equation R(t)=R0[1exp(tτ)][1AtB]
presents the evolution of the diffracted intensity on the ten millisecond time-scales when the sample is excited by a sequence of pump pulses with different power density. One should notice that the increase of the reflectivity is not instantaneous indicating that the grating continues to evolve in between two subsequent femtosecond pulses. The overall increase of the diffracted intensity depends on pump power density (Fig. 4(a)) but also on the number of pump pulses exciting the sample. In general, as one increases the number of exciting pulses, one records an increase of the overall diffraction efficiency. As presented in Fig. 4(b), we can also notice the occurrence of a steady state regime, where in between two successive exciting pulses, we first record an increase of diffracted intensity that relaxes back towards its initial value.

Figure 4 presents the evolution of the grating on the millisecond time-scale when the steady state regime presented in Fig. 1 is reached. On this time-scale, our data were fitted using the following equation:
R(t)=R0[1exp(tτ)][1AtB].
(2)
This equation accounts for both the formation and the relaxation of the grating. We have studied in more detail the evolution of the characteristic time constant (t, A−1). These parameters account for the temporal increase and decrease of the reflectivity of the grating respectively. We found that the characteristic time t is almost independent of the number of pump pulses used to excite the sample whereas the characteristic time constant A−1 can strongly change between two subsequent exciting pump pulses. These latter results are presented in Fig. 5
Fig. 5 (a) Evolution of the constant time t versus the number of exciting pulses. (b) Evolution of the constant time A−1 versus the number of exciting pulses. The data have been recorded for a pump peak power density of 245 GW.cm−2.
. This figure also indicates that A decreases and then saturates as the number of exciting pulses increases. Finally we stress that the value of characteristic time A−1 for the last exciting pulse agreed well with that found for the relaxation of the grating when the pump beam was off. For instance in Fig. 5(a), for a power density of 245 GW.cm−2 and after exposure of the sample to 10 exciting pulses, the relaxation parameter is A = 7x10−10 s−1, whereas for the same pump pulse peak, when the pump beam is stopped after 11 exciting pulses this relaxation parameter is A = 6.7x10−10 s−1.

These latter results make it possible to account for the saturation of the grating reflectivity presented in Fig. 3. As we previously mentioned, when the pump intensity increases, the number of defects induced increases as well. We therefore expect an increase of grating reflectivity with the pump intensity. Moreover, when the number of pulses that excites the sample increases, the number of defects also increases. While these two parameters result in an increase of the grating reflectivity, they also result in a decrease of the relaxation constant A. As a consequence, the photo-induced grating relaxes more and more rapidly as its reflectivity increases. Hence, in agreement with Fig. 3, the photo-induced grating reaches a saturation regime in which the increase of the grating reflectivity, induced by two successive exciting pulses, is compensated by the relaxation of the grating between these two exciting pulses.

3.4 Measurement of the photo-induced index variations at the steady state regime

3.5 Kinetics of the photo-induced index variations

The sign of Dnr was inferred from the thermal index variation of the SF59 sample. In the latter case, we recorded the direction in which the fringes were shifted upon a temperature increase of the sample and compared it to the direction in which the fringes are shifted upon femtosecond laser excitation. We found that both were moving in the same direction. Knowing dnr/dT~-5 10−7 K−1 for SF59, we concluded Dnr was negative.

4. Discussion

Hereafter, we briefly discuss the origin of the photo-induced defects. The two photon absorption at 800 nm is well into the absorption edge of the SF59 glass. Hence sufficiently intense ultrafast laser pulses will produce a high density of electron-hole pairs. It was previously proposed that the trapping, detrapping and recombination of these excitation accounts for the behavior of the darkening and partial recovery on both slow and fast time scales [6

6. G. Lin, F. Luo, F. He, Q. Chen, D. Chen, Y. Cheng, L. Zhang, J. Qiu, and Q. Zhao, “Different refractive index change behavior in borosilicate glasses induced by 1 kHz and 250 kHz femtosecond lasers,” Opt. Mater. Express 1(4), 724–731 (2011). [CrossRef]

,7

7. H. Guillet de Chatellus and E. Freysz, “Characterization and dynamics of gratings induced in glasses by femtosecond pulses,” Opt. Lett. 27(13), 1165–1167 (2002). [CrossRef] [PubMed]

]. In fact femtosecond laser irradiation, x rays and γ rays induced similar darkening in many different glasses. For instance it has been shown that the formation of hole centers (such as H2+ or OHC1 centers and H3+or OHC2 centers) induces absorption peaks in the visible spectral range. Similarly the production of electron traps (denoted E3- and E4- centers) results in an enhancement of the absorption in the near UV. Therefore, both defects could contribute to the index modification of our photo-excited sample. As previously shown, the partial bleaching or recovery of the sample is attributed to the electron-hole recombination that occurs between laser pulses [6

6. G. Lin, F. Luo, F. He, Q. Chen, D. Chen, Y. Cheng, L. Zhang, J. Qiu, and Q. Zhao, “Different refractive index change behavior in borosilicate glasses induced by 1 kHz and 250 kHz femtosecond lasers,” Opt. Mater. Express 1(4), 724–731 (2011). [CrossRef]

].

5. Conclusion

We have investigated the interaction of ultrafast laser pulses with SF59 glass. Although this glass is not considered as photosensitive, the high peak power density of the used femtosecond pulses makes it possible to photo-induce transient or permanent gratings within this material. We have shown that the latter grating results from laser induced defects that induce a coloration of the glass on the whole visible spectral range. We have shown the defects are induced by a single femtosecond pulse but continue to increase on the millisecond timescale. Considering the optical properties of these gratings, it would be useful to characterize completely the index variation at smaller timescale. However this work presents promising results for the understanding of optical data storage in glasses.

Acknowledgments

The authors thank the Aquitaine Region for supporting this project with the development of the COLA Platform and the ANR Agency (FLAG-09-BLAN-495203).

References and links

1.

H. Zhang, S. M. Eaton, J. Li, and P. R. Herman, “Femtosecond laser direct writing of multiwavelength Bragg grating waveguides in glass,” Opt. Lett. 31(23), 3495–3497 (2006). [CrossRef] [PubMed]

2.

H. Zhang, S. M. Eaton, and P. R. Herman, “Single-step writing of Bragg grating waveguides in fused silica with an externally modulated femtosecond fiber laser,” Opt. Lett. 32(17), 2559–2561 (2007). [CrossRef] [PubMed]

3.

W. Yang, P. G. Kazansky, and Y. P. Svirko, “Non-reciprocal ultrafast laser writing,” Nat. Photonics 2(2), 99–104 (2008). [CrossRef]

4.

M. Lancry, B. Poumellec, A. Chahid-Erraji, M. Beresna, and P. G. Kazansky, “Dependence of the femtosecond laser refractive index change thresholds on the chemical composition of doped-silica glasses,” Opt. Mater. Express 1(4), 711–723 (2011). [CrossRef]

5.

J. B. Lonzaga, S. M. Avanesyan, S. C. Langford, and J. T. Dickinson, “Color center formation in soda-lime glass with femtosecond laser pulses,” J. Appl. Phys. 94(7), 4332–4340 (2003). [CrossRef]

6.

G. Lin, F. Luo, F. He, Q. Chen, D. Chen, Y. Cheng, L. Zhang, J. Qiu, and Q. Zhao, “Different refractive index change behavior in borosilicate glasses induced by 1 kHz and 250 kHz femtosecond lasers,” Opt. Mater. Express 1(4), 724–731 (2011). [CrossRef]

7.

H. Guillet de Chatellus and E. Freysz, “Characterization and dynamics of gratings induced in glasses by femtosecond pulses,” Opt. Lett. 27(13), 1165–1167 (2002). [CrossRef] [PubMed]

8.

G. D. Marshall, R. J. Williams, N. Jovanovic, M. J. Steel, and M. J. Withford, “Point-by-point written fiber-Bragg gratings and their application in complex grating designs,” Opt. Express 18(19), 19844–19859 (2010). [CrossRef] [PubMed]

9.

A. Martinez, M. Dubov, I. Khrushchev, and I. Bennion, “Direct writing of fibre Bragg gratings by femtosecond laser,” Electron. Lett. 40(19), 1170–1172 (2004). [CrossRef]

10.

S. J. Mihailov, C. W. Smelser, D. Grobnic, R. B. Walker, P. Lu, H. Ding, and J. Unruh, “Bragg gratings written in all-SiO2 and ge-doped core fibers with 800-nm femtosecond radiation and a phase mask,” J. Lightwave Technol. 22(1), 94–100 (2004). [CrossRef]

11.

D. Grobnic, C. W. Smelser, S. J. Mihailov, R. B. Walker, and P. Lu, “Fiber Bragg gratings with suppressed cladding modes made in SMF-28 with a femtosecond IR laser and a phase mask,” IEEE Photon. Technol. Lett. 16(8), 1864–1866 (2004). [CrossRef]

12.

S. J. Mihailov, D. Grobnic, C. W. Smelser, P. Lu, R. B. Walker, and H. Ding, “Bragg grating inscription in various optical fibers with femtosecond infrared lasers and a phase mask,” Opt. Mater. Express 1(4), 754–765 (2011). [CrossRef]

13.

Ch. Voigtländer, R. G. Becker, J. Thomas, D. Richter, A. Singh, A. Tünnermann, and S. Nolte, “Ultrashort pulse inscription of tailored fiber Bragg gratings with a phase mask and a deformed wavefront,” Opt. Mater. Express 1(4), 633–642 (2011). [CrossRef]

14.

M. Bernier, S. Gagnon, and R. Vallée, “Role of the 1D optical filamentation process in the writing of first order fiber Bragg gratings with femtosecond pulses at 800nm,” Opt. Mater. Express 1(5), 832–844 (2011). [CrossRef]

15.

M. Ghanassi, M. C. Schanne-Klein, F. Hache, A. I. Ekimov, D. Ricard, and C. Flytzanis, “Time-resolved measurements of carrier recombination in experimental seconductor-doped glasses: confirmation of the role of Auger recombination,” Appl. Phys. Lett. 62(1), 78–80 (1993). [CrossRef]

16.

H. Kogelnik, “Coupled wave theory for thick hologram gratings,” Bell Syst. Tech. J. 48, 2909–2948 (1969).

OCIS Codes
(140.3390) Lasers and laser optics : Laser materials processing
(140.3440) Lasers and laser optics : Laser-induced breakdown
(160.2750) Materials : Glass and other amorphous materials
(320.2250) Ultrafast optics : Femtosecond phenomena
(320.7130) Ultrafast optics : Ultrafast processes in condensed matter, including semiconductors

ToC Category:
Laser Materials Processing

History
Original Manuscript: September 11, 2012
Revised Manuscript: October 22, 2012
Manuscript Accepted: November 6, 2012
Published: November 8, 2012

Citation
S. Chouli and E. Freysz, "Time resolved measurements of gratings photo-induced by femtosecond pulses in a lead doped glass," Opt. Mater. Express 2, 1751-1759 (2012)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-2-12-1751


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References

  1. H. Zhang, S. M. Eaton, J. Li, and P. R. Herman, “Femtosecond laser direct writing of multiwavelength Bragg grating waveguides in glass,” Opt. Lett.31(23), 3495–3497 (2006). [CrossRef] [PubMed]
  2. H. Zhang, S. M. Eaton, and P. R. Herman, “Single-step writing of Bragg grating waveguides in fused silica with an externally modulated femtosecond fiber laser,” Opt. Lett.32(17), 2559–2561 (2007). [CrossRef] [PubMed]
  3. W. Yang, P. G. Kazansky, and Y. P. Svirko, “Non-reciprocal ultrafast laser writing,” Nat. Photonics2(2), 99–104 (2008). [CrossRef]
  4. M. Lancry, B. Poumellec, A. Chahid-Erraji, M. Beresna, and P. G. Kazansky, “Dependence of the femtosecond laser refractive index change thresholds on the chemical composition of doped-silica glasses,” Opt. Mater. Express1(4), 711–723 (2011). [CrossRef]
  5. J. B. Lonzaga, S. M. Avanesyan, S. C. Langford, and J. T. Dickinson, “Color center formation in soda-lime glass with femtosecond laser pulses,” J. Appl. Phys.94(7), 4332–4340 (2003). [CrossRef]
  6. G. Lin, F. Luo, F. He, Q. Chen, D. Chen, Y. Cheng, L. Zhang, J. Qiu, and Q. Zhao, “Different refractive index change behavior in borosilicate glasses induced by 1 kHz and 250 kHz femtosecond lasers,” Opt. Mater. Express1(4), 724–731 (2011). [CrossRef]
  7. H. Guillet de Chatellus and E. Freysz, “Characterization and dynamics of gratings induced in glasses by femtosecond pulses,” Opt. Lett.27(13), 1165–1167 (2002). [CrossRef] [PubMed]
  8. G. D. Marshall, R. J. Williams, N. Jovanovic, M. J. Steel, and M. J. Withford, “Point-by-point written fiber-Bragg gratings and their application in complex grating designs,” Opt. Express18(19), 19844–19859 (2010). [CrossRef] [PubMed]
  9. A. Martinez, M. Dubov, I. Khrushchev, and I. Bennion, “Direct writing of fibre Bragg gratings by femtosecond laser,” Electron. Lett.40(19), 1170–1172 (2004). [CrossRef]
  10. S. J. Mihailov, C. W. Smelser, D. Grobnic, R. B. Walker, P. Lu, H. Ding, and J. Unruh, “Bragg gratings written in all-SiO2 and ge-doped core fibers with 800-nm femtosecond radiation and a phase mask,” J. Lightwave Technol.22(1), 94–100 (2004). [CrossRef]
  11. D. Grobnic, C. W. Smelser, S. J. Mihailov, R. B. Walker, and P. Lu, “Fiber Bragg gratings with suppressed cladding modes made in SMF-28 with a femtosecond IR laser and a phase mask,” IEEE Photon. Technol. Lett.16(8), 1864–1866 (2004). [CrossRef]
  12. S. J. Mihailov, D. Grobnic, C. W. Smelser, P. Lu, R. B. Walker, and H. Ding, “Bragg grating inscription in various optical fibers with femtosecond infrared lasers and a phase mask,” Opt. Mater. Express1(4), 754–765 (2011). [CrossRef]
  13. Ch. Voigtländer, R. G. Becker, J. Thomas, D. Richter, A. Singh, A. Tünnermann, and S. Nolte, “Ultrashort pulse inscription of tailored fiber Bragg gratings with a phase mask and a deformed wavefront,” Opt. Mater. Express1(4), 633–642 (2011). [CrossRef]
  14. M. Bernier, S. Gagnon, and R. Vallée, “Role of the 1D optical filamentation process in the writing of first order fiber Bragg gratings with femtosecond pulses at 800nm,” Opt. Mater. Express1(5), 832–844 (2011). [CrossRef]
  15. M. Ghanassi, M. C. Schanne-Klein, F. Hache, A. I. Ekimov, D. Ricard, and C. Flytzanis, “Time-resolved measurements of carrier recombination in experimental seconductor-doped glasses: confirmation of the role of Auger recombination,” Appl. Phys. Lett.62(1), 78–80 (1993). [CrossRef]
  16. H. Kogelnik, “Coupled wave theory for thick hologram gratings,” Bell Syst. Tech. J.48, 2909–2948 (1969).

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