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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 1, Iss. 4 — Aug. 1, 2011
  • pp: 711–723
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Dependence of the femtosecond laser refractive index change thresholds on the chemical composition of doped-silica glasses

M. Lancry, B. Poumellec, A. Chahid-Erraji, M. Beresna, and P. G. Kazansky  »View Author Affiliations


Optical Materials Express, Vol. 1, Issue 4, pp. 711-723 (2011)
http://dx.doi.org/10.1364/OME.1.000711


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Abstract

The refractive index changes in doped silica are investigated. We observed that the permanent isotropic index change threshold (T1) is not significantly dependent on the doping. We show that strong birefringence (permanent linear birefringence) exists in doped silica but its threshold (T2) exhibits significant dependence on the used dopants. In our conditions, comparing with silica (0.31 μJ/pulse here), for 1.5 at% Ge-doped silica the T2 threshold is smaller (0.14 ± 0.05 μJ/pulse). For a silica doped with 0.3 at% of fluorine, T2 is close to 1.20 ± 0.05 μJ/pulse. An interpretation is given not only about threshold variation but also about RIC for energies beyond. It is based on the overcoming of relaxation time in the volume interaction.

© 2011 OSA

1. Introduction

Silica-based glasses are the backbone for many of today’s rapidly expanding photonics applications. Current advanced femtosecond laser systems offer a myriad of possibilities to modify glassy media; from surface ablation, annealing, to 3D refractive index modification (positive or negative index change with isotropic or anisotropic properties) depending on the laser parameters [1

1. L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, “Writing of permanent birefringent microlayers in bulk fused silica with femtosecond laser pulses,” Opt. Commun. 171(4-6), 279–284 (1999). [CrossRef]

]. Recently, new unique properties of laser induced glass modifications have been discovered, like chirality [2

2. B. Poumellec, M. Lancry, J. C. Poulin, and S. Ani-Joseph, “Non reciprocal writing and chirality in femtosecond laser irradiated silica,” Opt. Express 16(22), 18354–18361 (2008). [CrossRef] [PubMed]

,3

3. B. Poumellec, L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, “Femtosecond laser irradiation stress induced in pure silica,” Opt. Express 11(9), 1070–1079 (2003). [CrossRef] [PubMed]

], directional dependent writing [2

2. B. Poumellec, M. Lancry, J. C. Poulin, and S. Ani-Joseph, “Non reciprocal writing and chirality in femtosecond laser irradiated silica,” Opt. Express 16(22), 18354–18361 (2008). [CrossRef] [PubMed]

4

4. P. Kazansky, W. Yang, E. Bricchi, J. Bovatsek, A. Arai, Y. Shimotsuma, K. Miura, and K. Hirao, “‘Quill’ writing with ultrashort light pulses in transparent materials,” Appl. Phys. Lett. 90(15), 151120 (2007). [CrossRef]

], oxidation-reduction [5

5. J. Qiu, K. Miura, and K. Hirao, “Femtosecond laser-induced microfeatures in glasses and their applications,” J. Non-Cryst. Solids 354(12-13), 1100–1111 (2008). [CrossRef]

], glass decomposition [6

6. M. Lancry, F. Brisset, and B. Poumellec, “In the heart of nanogratings made up during femtosecond laser irradiation,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, OSA Technical Digest (CD), (Optical Society of America, 2010), ISBN 978–1-55752–896–4.

], nanocluster precipitation and shaping [7

7. M. Kaempfe, G. Seifert, K. Berg, H. Hofmeister, and H. Graener, “Polarization dependence of the permanent deformation of silver nanoparticles in glass by ultrashort laser pulses,” Eur. Phys. J. D 16(1), 237–240 (2001). [CrossRef]

] and elemental distribution with a sub-wavelength resolution [6

6. M. Lancry, F. Brisset, and B. Poumellec, “In the heart of nanogratings made up during femtosecond laser irradiation,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, OSA Technical Digest (CD), (Optical Society of America, 2010), ISBN 978–1-55752–896–4.

,8

8. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91(24), 247405 (2003). [CrossRef] [PubMed]

]. To our knowledge, no other technique holds the same potential to realize 3D multi-functional photonic devices, fabricated in a single step in such a wide range of transparent materials. It exhibits enormous potential in the development of a new generation of powerful components for micro-optics, telecommunications, 3D optical data storage, imaging, biophotonic and many more [5

5. J. Qiu, K. Miura, and K. Hirao, “Femtosecond laser-induced microfeatures in glasses and their applications,” J. Non-Cryst. Solids 354(12-13), 1100–1111 (2008). [CrossRef]

,9

9. M. Ams, G. Marshall, P. Dekker, M. Dubov, V. Mezentsev, I. Bennion, and M. Withford, “Investigation of ultrafast laser–photonic material interactions: challenges for directly written glass photonics,” IEEE J. Sel. Top. Quantum Electron. 14(5), 1370–1381 (2008). [CrossRef]

,10

10. K. Itoh, W. Watanabe, S. Nolte, and C. B. Schaffer, “Ultrafast processes for bulk modification of transparent materials,” MRS Bull. 31(08), 620–625 (2006). [CrossRef]

].

One of the most investigated aspects of ultrafast laser writing is refractive index modification in silica glass. The stability and anisotropy magnitude of the induced modification have been investigated as a function of the laser pulse energy and duration, numerical aperture, writing speed and etc. From these results, at least 3 “damage” thresholds in pure SiO2 glass can be identified according to the energy. Below the first threshold, T1, (e.g. 0.085 ± 0,015 μJ/pulse, 800 nm, 160 fs, 100 kHz, 0.5 NA [3

3. B. Poumellec, L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, “Femtosecond laser irradiation stress induced in pure silica,” Opt. Express 11(9), 1070–1079 (2003). [CrossRef] [PubMed]

,11

11. B. Poumellec and M. Lancry, “Damage thresholds in femtosecond laser processing of silica: a review,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, OSA Technical Digest (CD), (Optical Society of America, 2010), ISBN 978–1-55752–896–4.

,12

12. L. Sudrie, “Propagation non-linéaire des impulsions laser femtosecondes dans la silice,” (Université de Paris Sud XI Orsay, 2002).

]), the increase of the index relaxes following third order kinetics typical for the Auger electron process [4

4. P. Kazansky, W. Yang, E. Bricchi, J. Bovatsek, A. Arai, Y. Shimotsuma, K. Miura, and K. Hirao, “‘Quill’ writing with ultrashort light pulses in transparent materials,” Appl. Phys. Lett. 90(15), 151120 (2007). [CrossRef]

]. The photo-induced refractive index change decays to the initial value within a few seconds followed by permanent transmission decrease [13

13. C. Schaffer, A. Brodeur, and E. Mazur, “Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses,” Meas. Sci. Technol. 12(11), 1784–1794 (2001). [CrossRef]

]. Above the first threshold T1, permanent changes are detected: transmission decreases, the refractive index is isotropically increasing until the second threshold T2 is reached [5

5. J. Qiu, K. Miura, and K. Hirao, “Femtosecond laser-induced microfeatures in glasses and their applications,” J. Non-Cryst. Solids 354(12-13), 1100–1111 (2008). [CrossRef]

]. The thermal stability is increased but moderate. In undoped fused silica the maximum refractive index change is up to 3-6 × 10−3, which is much larger value than typically induced by UV nanosecond lasers [14

14. M. Lancry, P. Niay, and M. Douay, “Comparing the properties of various sensitization methods in H2-loaded, UV hypersensitized or OH-flooded standard germanosilicate fibers,” Opt. Express 13(11), 4037–4043 (2005). [CrossRef] [PubMed]

,15

15. M. Lancry and B. Poumellec, “UV laser processing and multiphoton absorption processes in optical telecommunication fibers materials,” Phys. Rep. (in press).

]. Above the second damage threshold, T2, (e.g. 0.31 μJ/pulse, 800nm, 160fs, 100kHz, 0.5 NA, polarization parallel to the scanning direction [11

11. B. Poumellec and M. Lancry, “Damage thresholds in femtosecond laser processing of silica: a review,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, OSA Technical Digest (CD), (Optical Society of America, 2010), ISBN 978–1-55752–896–4.

]), the properties of the modified region are quite different (type II). The refractive index change magnitude can be as large as 10−2 [16

16. E. Bricchi, B. G. Klappauf, and P. G. Kazansky, “Form birefringence and negative index change created by femtosecond direct writing in transparent materials,” Opt. Lett. 29(1), 119–121 (2004). [CrossRef] [PubMed]

] and remain even after two hours of annealing at 1000°C [17

17. E. Bricchi and P. Kazansky, “Extraordinary stability of anisotropic femtosecond direct-written structures embedded in silica glass,” Appl. Phys. Lett. 88(11), 111119 (2006). [CrossRef]

]. The most striking feature is high anisotropy of this modification [16

16. E. Bricchi, B. G. Klappauf, and P. G. Kazansky, “Form birefringence and negative index change created by femtosecond direct writing in transparent materials,” Opt. Lett. 29(1), 119–121 (2004). [CrossRef] [PubMed]

] which originates from sub-wavelength atomic density nanogratings induced inside irradiated volume [8

8. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91(24), 247405 (2003). [CrossRef] [PubMed]

]. It has been shown by Bricchi et al. [16

16. E. Bricchi, B. G. Klappauf, and P. G. Kazansky, “Form birefringence and negative index change created by femtosecond direct writing in transparent materials,” Opt. Lett. 29(1), 119–121 (2004). [CrossRef] [PubMed]

] that both eigen index values decrease. We have proved recently that nanoplanes of these gratings consist of porous matter produced by a decomposition of SiO2 into SiO2(1-x) + x.O2 [6

6. M. Lancry, F. Brisset, and B. Poumellec, “In the heart of nanogratings made up during femtosecond laser irradiation,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, OSA Technical Digest (CD), (Optical Society of America, 2010), ISBN 978–1-55752–896–4.

].

2. Experimental details

The experiments were performed in an optical fiber preform plate of 1 mm thickness and 20 mm in diameter. The chemical analysis of the preform cross-section was made by EPMA (Electron Probe Micro Analyser) (Fig. 1
Fig. 1 Chemical analysis profile of the preform used in our experiments. Si and O contents are not shown. The Ge-doped part is the core of the optical fiber preform. The increase in F concentration indicates the preform center. Chlorine (in pink) is shown to remain at very low level. The analysis has been made by EPMA.
). The core is mainly germanium doped silica except the center which is doped predominantly with fluorine. The cladding is equally co-doped with phosphorus and fluorine. The fluctuations seen on the chemical composition profiles are well correlated with the MCVD layers. The homogeneous concentration of fluorine in core-cladding is caused by its higher diffusion compared with phosphorus, which as a result marks the layers. This occurs also for the cladding-tube interface which contains only fluorine. In the experiments we took advantage of these peculiarities for determining modification thresholds dependence with doping.

The laser used in the experiments was a Ti sapphire operating at 800 nm, 160 fs, and 100 kHz repetition rate. The × 20 microscope objective with NA 0.50 was used for writing in the preform plate. In the experiment 31 lines were drawn 200 μm below the entry surface with a pulse energy ranging from 2.6 μJ down to 55 nJ. The sample was moved along a direction perpendicular (let us say x) to the beam tracing continuous lines from the center (Ge doped core) to the peripheral (pure silica substrate) of the fiber preform. The polarization laid along x, i.e. parallel to the sample displacement. The distance between the lines was 50 μm.

After femtosecond laser irradiation, the photo-induced macroscopic structural modifications were inspected by quantitative phase microscopy (QPm) and retardance imaging system (ABRIO, CRI inc.) both based on the Olympus BX51 optical microscope. The circularly polarized light (515 nm), generated by passing the white light through the green band-pass filter and circular polarizer, was used for illuminating the sample. The transmitted light was collected by the microscope objective and passed through the liquid crystal (LC) based compensator, which can change the polarization state of the light with no moving parts and most importantly with no image shift. As a result four intensity images were recorded for various birefringence settings of the compensator and then processed using a special algorithm embedded in the commercial software. Birefringence information including retardance (R), which is defined as: R = Β·d (Β is the birefringence in the (x, y) plan and d is the thickness of the birefringent material) and azimuth angle (slow axis between horizontal) can be calculated. QPm measurements were realized using in natural light configuration with × 20 focusing objective mounted on a piezo stage. A quantitative phase mapping of the structures was produced using QPm software (Iatia vision sciences). The photo-induced refractive index change was estimated fromΔn=Δφλ2πL, where Δφ is the phase shift (in radians) of the non-polarized light at 550 nm and L – the thickness of the structure along the light propagation direction.

3. Results

As described in the introduction, we define the first damage threshold T1 as the pulse energy above which appears a permanent isotropic index change. Notice that we performed multiple (typ. 1000) pulses threshold measurements. Usually, it gives rise to a converging lens effect and thus to a higher luminosity. The second threshold T2 is (as defined in this paper) the pulse energy above which a strong linear birefringence appears and can be easily seen in crossed Nicol polarizer configuration or using retardance imaging system.

By correlating, the energy damage thresholds determined from Figs. 2a and 2b as a function of material chemical composition profile (as shown in Fig. 1), we are then able to determine the T2 threshold according to the doping. Comparing with silica (0.31 μJ/pulse here), for 1.5 at% Ge-doped silica the threshold T2 is several times smaller (0.14 ± 0.05 μJ/pulse) as shown in Fig. 2a. For a silica doped with 0.3 at% of fluorine (see arrows in Fig. 2b), the second threshold T2 is close to 1.20 ± 0.05 μJ/pulse. The addition of 0.3 at% of phosphorus to 0.3 at% F doped silica (see left arrows in Fig. 2b) degrades the second damage threshold from 1.20 to 0.25 ± 0.05 μJ/pulse. Under optical microscope working in transmission mode the lines written below the second threshold could be also detected. The contrast in the optical image arises from an isotropic variation of refractive index. In this way, we deduce that the first threshold is not influenced significantly by the doping and is always fixed at 0.095 ± 0.005 μJ/pulse.

Subsequently, from these quantitative cartographies, the retardance dependencies on the writing pulse energy for different composition are plotted in Fig. 3
Fig. 3 Average retardance (in nm) over the interaction length t (i.e. the laser trace) according to the writing pulse energy for various chemical compositions.
from 0.055 up to 2.6 μJ. In pure silica (blue curve), there is a sharp increase of the birefringence around 0.31 μJ that indicates the T2 threshold already observed in Fig. 2a. In Ge-doped silica (black curve), we can see a continuous increase of retardance even at large pulse energies. We note that the maximum photo-induced retardance in slightly (1.5 at%) Ge-doped SiO2 is much higher (230 nm retardance at 2.6 μJ) than in pure SiO2 (140 nm at 2 μJ).

Average index changes have been also inspected by quantitative phase microscopy (QPm) (Fig. 4
Fig. 4 Optical image of the Ge-doped, (F, P) co-doped, F-doped and silica in the (x, y) plane (i.e. through the sample) recorded with quantitative phase measurement (QPM) microscope. The black lines correspond to negative index changes whereas the bright ones are for positive index changes.
). As it can be seen, at high pulse energy (above 0.31 μJ/pulse in silica) we can observe dark lines indicating that the average refractive index value (in the xy plan) has decreased when compared to the pristine silica background. It is worth noticing that the threshold of negative index change appearance coincides with the T2 threshold whatever the doping may be (see Fig. 5
Fig. 5 Pulse energy versus numerical aperture diagram in log-log scale defining regions with different kinds of laser interaction with silica. Laser parameters: 0.1-17 μJ, NA = 0.01-1, 160 fs, 200 kHz, 800 nm. N.B.: the blue discontinuous line marks the position of our today experiments.
).

4. Discussion

T1 is independent on doping, T2 decreases strongly on Ge, P doping, increases on F doping. An important role in the interaction process might be played by self-focusing (SF) [13

13. C. Schaffer, A. Brodeur, and E. Mazur, “Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses,” Meas. Sci. Technol. 12(11), 1784–1794 (2001). [CrossRef]

], in particular when the pulse energy overcomes the SF threshold and a filament appears. For comparison purpose, the corresponding SF values are reported in the right column of Table 1. As it is well known self-focusing is defined by the linear and the non-linear n 2 indices. It should also be noted that self-focusing depends only on the peak power in the beam [13

13. C. Schaffer, A. Brodeur, and E. Mazur, “Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses,” Meas. Sci. Technol. 12(11), 1784–1794 (2001). [CrossRef]

], but not intensity; thus the critical self-focusing threshold does not depend on the focusing strength. As it can be seen, n 2 increases only slightly with the presence of either Ge or F [25

25. T. Kato, Y. Suetsugu, and M. Nishimura, “Estimation of nonlinear refractive index in various silica-based glasses for optical fibers,” Opt. Lett. 20(22), 2279–2281 (1995). [CrossRef] [PubMed]

] when compared to pure SiO2. The self focusing effect is roughly the same whatever the doping (at the level used in our sample). On the contrary, the appearance of a linear birefringence is strongly dependent on doping. Especially in F-doped silica, the threshold energy for self-focusing is 3 times lower than T2, indicating that self-focusing does not play a detectable role in the formation of index anisotropy.

4.1 T1 interpretation

For understanding the independence of T1 on doping for large number of pulses (typ. 1000 pulses) we have to analyze precisely what is the basis of T1, which is defined here through the appearance of permanent isotropic index change. In other papers [13

13. C. Schaffer, A. Brodeur, and E. Mazur, “Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses,” Meas. Sci. Technol. 12(11), 1784–1794 (2001). [CrossRef]

] it is defined through attenuation permanent variation in the near IR but it is actually the same threshold. What we call threshold corresponds to the clear intersection between two lines in a given plot of the property according to the pulse energy. Isotropic index changes or attenuation decrease are both arising from extended glass modifications. On the other hand, in the first femtoseconds after the pump pulse beginning, the energy absorbed from the femtosecond laser pulse leads to formation of excited electrons that relax first into self-trapped excitons (STE) on the 100’s femtosecond timescale and then annihilate or transform into point defects later on [31

31. M. Lancry, N. Groothoff, S. Guizard, W. Yang, B. Poumellec, P. Kazansky, and J. Canning, “Femtosecond laser direct processing in wet and dry silica glass,” J. Non-Cryst. Solids 355(18-21), 1057–1061 (2009). [CrossRef]

], following the reaction scheme sketched for the Ge doped silica case:
STEGe|radiative or non-radiative recombination|structural modification like densificationεGeE'+NbOHCGeODC(II)+O2        
Glass modifications are the result of STE relaxation. Two ways of STE decay are possible: a simple radiative recombination with a photon emission (lifetime of nanoseconds at room temperature [31

31. M. Lancry, N. Groothoff, S. Guizard, W. Yang, B. Poumellec, P. Kazansky, and J. Canning, “Femtosecond laser direct processing in wet and dry silica glass,” J. Non-Cryst. Solids 355(18-21), 1057–1061 (2009). [CrossRef]

]) or a non-radiative one by coupling with lattice phonons. Alternatively, STE can transform into point defects (e.g. GeODC(II)) [32

32. S. Mao, F. Quéré, S. Guizard, X. Mao, R. Russo, G. Petite, and P. Martin, “Dynamics of femtosecond laser interactions with dielectrics,” Appl. Phys., A Mater. Sci. Process. 79(7), 1695–1709 (2004). [CrossRef]

] but its yield is small [33

33. S. Guizard, P. Martin, G. Petite, P. D'Oliveira, and P. Meynadier, “Time-resolved study of laser-induced colour centres in SiO2,” J. Phys. Condens. Matter 8(9), 1281–1290 (1996). [CrossRef]

]. On the other hand, the permanent attenuation in the IR lies in a wavelength range where there is no point defect absorption [34

34. L. Skuja, M. Hirano, H. Hosono, and K. Kajihara, “Defects in oxide glasses,” Phys. Status Solidi C 2(1), 15–24 (2005). [CrossRef]

] and thus the transparency loss is thus resulting of long or medium range structural modifications but not from point defects. We speculate that such glass transformation results from non-radiative relaxation of excited electrons by coupling with the lattice. This coupling corresponds to an increase of non-equilibrium temperature of the lattice that can reach several thousands of degrees in the illuminated area just limited by thermal conduction. The temperature increase during a time long enough can lead to glass melting and to a change of average disorder i.e. a change of glass fictive temperature [35

35. M. Lancry, E. Régnier, and B. Poumellec, “Fictive temperature in silica-based glasses and its application to optical fiber manufacturing,” Prog. Mater. Sci. 57(1), 63–94 (2012). [CrossRef]

]. We can define this temperature (Tc) by saying that the relaxation time defined by the ratio between the viscosity (η) and the shear modulus (G) [36

36. A. Tool, “Relation between inelastic deformability and theram expansion of glass in its annealing range,” J. Am. Ceram. Soc. 29(9), 240–253 (1946). [CrossRef]

] should be smaller than the time during which the matter overcome this temperature. This can be written as:
η(Tc)/G(Tc)=δt(T>Tc)
(1)
On the other hand, the time-temperature profile of the irradiated area is mainly defined by the amount of energy density released in the lattice during the laser pulse divided by the specific heat capacity [37

37. S. Eaton, H. Zhang, P. Herman, F. Yoshino, L. Shah, J. Bovatsek, and A. Arai, “Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate,” Opt. Express 13(12), 4708–4716 (2005). [CrossRef] [PubMed]

]. This energy comes from excited electron-relaxation and not directly from pump pulse that is not completely absorbed by the matter through electron excitation. As a matter of fact, the energy releases by a STE is at least equal to Eg but less than 2Eg as no avalanche is detected in our conditions [32

32. S. Mao, F. Quéré, S. Guizard, X. Mao, R. Russo, G. Petite, and P. Martin, “Dynamics of femtosecond laser interactions with dielectrics,” Appl. Phys., A Mater. Sci. Process. 79(7), 1695–1709 (2004). [CrossRef]

,38

38. M. Lancry, N. Groothoff, B. Poumellec, S. Guizard, N. Fedorov, and J. Canning, “Time-resolved plasma measurements in Ge-doped silica exposed to IR femtosecond laser,” Phys. Rev. B (in press).

]. Let assume that this energy is roughly the same whatever the doping (so a = constant and 1<a<2). So, finally the released energy density (W) is expressed as the product of (a), of the forbidden energy gap Eg of the glass and of the STE concentration at the end of the pulse i.e. W=aEg[STE].

The STE concentration is obtained by time integration of the MPI (multiphoton ionization) rate. It yields approximately [STE]=σnIthnτNO where σn is the multiphoton absorption cross section, τ is the duration of the pulse, NO the state density at the top of valence band and Ith the light intensity corresponding to T1 energy. It comes thus:

W=aEg[STE]=aEgσn.Ithn.τ.NO.
(2)

As mentioned above, only a fraction of the incident pulse energy (T1) is absorbed by the matter i.e. a part is reflected by the plasma [39

39. F. Quéré, S. Guizard, and P. Martin, “Time-resolved study of laser-induced breakdown in dielectrics,” Europhys. Lett. 56(1), 138–144 (2001). [CrossRef]

]. Let us write αT1. Due to focusing and taking into account the absorption length, we have: T1=2.3Ithα.τ.S(NA,Ithα)where S(NA, Ith) is the average cross section of the focal volume alike the expression given by Schaffer et al. [13

13. C. Schaffer, A. Brodeur, and E. Mazur, “Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses,” Meas. Sci. Technol. 12(11), 1784–1794 (2001). [CrossRef]

] S(NA,Ithα)=λ2π(NA)2+Ithα2πn0n2 valid for NA>0.1. α, is a priori unknown but assumed constant in this paper. It has been determined by considering that the Eq. (1) is fulfilled with the minimum energy T1 = 0.1µJ in pure silica. We found α = 23%. This appears consistent with time-resolved absorption measurements performed in similar experimental conditions [40

40. A. Couairon, L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, “Filamentation and damage in fused silica induced by tightly focused femtosecond laser pulses,” Phys. Rev. B 71(12), 125435 (2005). [CrossRef]

].

Equation (1) itself has been solved by taking into account the data available in the literature for the glass considered in this paper and by solving the time dependent Fourier equation after a time long enough after the pulse. More details will be published elsewhere. Equation (1) is thus the intersection of the two curves types of curves plotted in Fig. 7
Fig. 7 This graph compares the glass relaxation time and the cooling profile after single pulse femtosecond laser irradiation for pure and doped silica. The cooling profiles with varying incident energy are for pure silica only.
. This graph compares the temperature dependence of the relaxation time for the different sample compositions of this paper based on viscosity measurements and shear modulus data [41

41. M. Yamane and J. Mackenzie, “Vicker's hardness of glass,” J. Non-Cryst. Solids 15(2), 153–164 (1974). [CrossRef]

] change and the cooling profile modeling after single pulse irradiation [37

37. S. Eaton, H. Zhang, P. Herman, F. Yoshino, L. Shah, J. Bovatsek, and A. Arai, “Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate,” Opt. Express 13(12), 4708–4716 (2005). [CrossRef] [PubMed]

]. For pure silica, the estimate of the fictive temperature is thus around 2490°C. If we decrease the incident pulse energy, there is of course no more intersection, but if we increase the pulse energy, the fictive temperature decreases slightly to 2250°C from 0.1 to 0.3 μJ.

Now, if we consider the case of Ge doping at a level of 1,5 at % in silica, specific heat capacity increases by less than 2% [42

42. P. Richet, D. de Ligny, and E. Westrum, “Low-temperature heat capacity of GeO2 and B2O3 glasses: thermophysical and structural implications,” J. Non-Cryst. Solids 315(1-2), 20–30 (2003). [CrossRef]

], viscosity decreases by less than 3% based on our own measurements and [43

43. K. Tajima, M. Tateda, and M. Ohashi, “Viscosity of GeO2-doped silica glasses,” J. Lightwave Technol. 12(3), 411–414 (1994). [CrossRef]

], shear modulus decreases by less than 4% [41

41. M. Yamane and J. Mackenzie, “Vicker's hardness of glass,” J. Non-Cryst. Solids 15(2), 153–164 (1974). [CrossRef]

]. The thermal diffusivity is also increasing by not more than 2%. The same is for P and F doping [44

44. M. Kyoto, Y. Ohoga, S. Ishikawa, and Y. Ishiguro, “Characterization of fluorine-doped silica glasses,” J. Mater. Sci. 28(10), 2738–2744 (1993). [CrossRef]

]. Therefore, the critical temperature for structural modification as defined above is expected not to change significantly (less than 50°C) with doping and thus the corresponding energy density appearing in Eq. (2), from those ones we can deduce the change in intensity threshold. We have Ith,GeIth,Si=(EgSiσSiIT1,SiEgGeσGe)1/5

The band gap seems to significantly decrease as the STE localize on Ge atoms even with the weak doping used in this paper [45

45. M. Lancry and B. Poumellec, “Femtosecond laser direct writing in P, Ge doped silica glasses: time resolved plasma measurements,” in Femtosecond Laser Microfabrication, OSA Technical Digest (CD) (Optical Society of America, 2009), ISBN 978–1–55752–879–7, paper LMTuA5 (2009).

]. As a result, the band gap decreases from 9 eV to 7.5 eV i.e. by about 17%. On the other hand, the frequency-based spectral interferometry measurements indicate that the STE concentration increases by about a factor 3 [45

45. M. Lancry and B. Poumellec, “Femtosecond laser direct writing in P, Ge doped silica glasses: time resolved plasma measurements,” in Femtosecond Laser Microfabrication, OSA Technical Digest (CD) (Optical Society of America, 2009), ISBN 978–1–55752–879–7, paper LMTuA5 (2009).

]. Here we used the multiphoton cross-sections for pure silica (6 photons absorption) and Ge-doped silica (5 photons absorption) that we measured using plasma density measurements in similar experimental conditions (σGe = 1.8 ± 0.2 10−55 s−1cm10W−5 and σSi = 4.5 ± 0.2 10−69 s−1cm12W−6 [38

38. M. Lancry, N. Groothoff, B. Poumellec, S. Guizard, N. Fedorov, and J. Canning, “Time-resolved plasma measurements in Ge-doped silica exposed to IR femtosecond laser,” Phys. Rev. B (in press).

]) and the silica intensity threshold corresponding to T1 threshold in silica (≈15 TW/cm2). As a result, the variation of the intensity threshold is less than 10%. Further, as the cross section of the focal volume is not varying significantly on doping, we deduce that T1 is not varying either. This is thus consistent with the observations.

The ratios calculated for other doped silica leads to the same conclusions: the energy thresholds for glass structural change are about the same. As a matter of fact, comparing relaxation time and cooling curve in doped samples, see Fig. 7 assuming the same proportion of pulse energy absorption, leads to a pulse energy threshold of 0.10μJ for Ge doped sample, of 0.095μJ of P and F-doped samples. The corresponding fictive temperatures are 2490°C and 2450°C respectively.

4.2 T2 interpretation

It is worth noticing that point defects in Ge-doped silica is easier achieved through reduction process [50

50. R. Araujo, “Oxygen vacancies in silica and germania glasses,” J. Non-Cryst. Solids 197(2-3), 164–169 (1996). [CrossRef]

,51

51. M. Schurman and M. Tomozawa, “Equilibrium oxygen vacancy concentrations and oxidant diffusion in germania, silica, and germania-silica glasses,” J. Non-Cryst. Solids 202(1-2), 93–106 (1996). [CrossRef]

] and Ge itself can easily behave as a trapping centers forming Ge(1) centers [52

52. T. E. Tsai, E. J. Friebele, and D. L. Griscom, “Thermal stability of photoinduced gratings and paramagnetic centers in Ge- and Ge/P-doped silica optical fibers,” Opt. Lett. 18(12), 935–937 (1993). [CrossRef] [PubMed]

]. We have to note also that GeODC(II), inherent to Ge doped silica is an electron source that gives rise to Ge(1) through UV absorption. The same properties stands for phosphorus doped silica [52

52. T. E. Tsai, E. J. Friebele, and D. L. Griscom, “Thermal stability of photoinduced gratings and paramagnetic centers in Ge- and Ge/P-doped silica optical fibers,” Opt. Lett. 18(12), 935–937 (1993). [CrossRef] [PubMed]

] but not for fluorine doping, which is also recognized for hardening the glass against the ionizing radiation [53

53. K. Sanada, N. Shamoto, and K. Inada, “Radiation resistance of fluorine-doped silica-core fibers,” J. Non-Cryst. Solids 179, 339–344 (1994). [CrossRef]

]. On the other hand, decomposition of the glass that depends on the bound energies is easier with Ge and P but not with F-doping.

Finally, as we have seen for T1, the electron density or energy in the plasma seems not being the step that determines the doping dependence of the nanoplanes formation, it is rather the ability of the glass to be decomposed and to record the plasma density spatial structure. In such a way, the branching ratio for STE’s to transform into point defects is larger for Ge and P than for F. The number of pulses of a given energy is thus varying accordingly and thus the energy for a given number of pulses. On the other hand, as the energy required for decomposing Ge doped silica is less than for pure silica, the onset energy for nanoplanes appearance is smaller. Similar remarks stand for P doping. On the contrary, doping silica with fluorine reduces defects concentrations and the structural disorder [54

54. K. Saito and A. Ikushima, “Effects of fluorine on structure, structural relaxation, and absorption edge in silica glass,” J. Appl. Phys. 91(8), 4886–4890 (2002). [CrossRef]

,55

55. L. Skuja, K. Kajihara, Y. Ikuta, M. Hirano, and H. Hosono, “Urbach absorption edge of silica: reduction of glassy disorder by fluorine doping,” J. Non-Cryst. Solids 345-346, 328–331 (2004). [CrossRef]

], this result in a more difficult glass decomposition and thus a higher T2 threshold.

More quantitatively, we may consider that birefringence is always detected above the same quantity (quoted as [B]). This minimal quantity is certainly proportional to an amount of glass decomposition which is itself proportional to the branching ratio (ε) from STE and to STE concentration ([STE]) produced by a series of pulses (N). We may write [B]~N·ε·[STE]. On the other hand, [STE]~σ·Ik where k is the number of photon in the MPI process. So, finally we get:
Ith2,GeIth2,Si=(NSiεSiσSiIth2,SiNGeεGeσGe)1/5with IT2, Si  40TW/cm².
Again referring to the focal volume cross section, invariant on doping and to the Table 1, we find that the left term of the above equation reaches about 0.5. We deduce that N·ε ratio reaches about 0.1, meaning that the branching ratio is in the order of 10 in favor of Ge if we assume that the number of pulses is the same for both glasses.

Another point to discuss is the birefringence beyond the energy thresholds. In Fig. 3, we see that retardance after a sudden increase (what is consistent with multiphoton production of birefringence in volume) the birefringence saturates. In addition, RIC is larger for Ge and smaller for F-doped SiO2. We can note that above a certain energy (0.2 μJ for Ge-doped silica or P/F doped samples but 2 μJ for F-doped silica), the plasma saturates [45

45. M. Lancry and B. Poumellec, “Femtosecond laser direct writing in P, Ge doped silica glasses: time resolved plasma measurements,” in Femtosecond Laser Microfabrication, OSA Technical Digest (CD) (Optical Society of America, 2009), ISBN 978–1–55752–879–7, paper LMTuA5 (2009).

] indicating that the coupled energy in the matter is thus limited. Another limitation is that glass decomposition is in itself limited and that form birefringence can increases only by increasing the number of nanoplanes in the cross section of the laser tracks (perpendicular to the propagation axis) but it is observed that their distance remain roughly the same and so this number rapidly saturates. Finally, only the increase of their length along the propagation axis might be possible but this needs light intensity increase beyond the focus. As the larger intensity is in the focus, the plasma formed reflects more and more the incoming pulse as its energy increases. A fraction of light going through to the tail of the structure is thus becoming smaller and smaller thus justifying the birefringence curve saturation.

For, isotropic index change displayed in Fig. 6, the sudden increase for low energy is consistent with multiphoton absorption. As a matter of fact, taking again the mechanism described at the beginning of the discussion, beyond the T1 threshold, the relationη(Tc)/G(Tc)=δt(T>Tc) defines the critical temperature for the beginning of glass modification. When pulse energy increases, it is easy to see in Fig. 7 that fictive temperature does not change significantly in the interaction region but the volume satisfying the above condition increases. Then, depending on the glass modification (some examples are reported in [56

56. B. Poumellec and F. Kherbouche, “The photorefractive Bragg gratings in the fibers for telecommunications,” J. Phys. III 6(12), 1595–1624 (1996). [CrossRef]

]) the phase recorded through the laser track increases accordingly. However, beyond T2, the curves in Fig. 4 drop dramatically before leveling off. This corresponds to the mechanism change and the appearance of nanoplanes with lower index. The leveling is thus consistent with the one observed in Fig. 3 and the associated mechanism described above.

5. Conclusion

From a practical point of view, it appears that silica doped with small amounts of either germanium or phosphorous is a better choice for birefringence tailoring than pure silica glass, since the processing window is wider and the maximum photo-induced retardance (typ. 230nm at 550nm in one layer) is larger than in pure silica. In contrast, it is the reverse influence for fluorine doping. It is thus judicious to use Ge or P doped silica for discrete optical device fabrication when birefringence is required, such as for a retardation plate or Fresnel lens. As the retardance can be accumulated over many layers, quarter and half waveplates can be easily realized over a typical 100 microns thickness. F-doped silica is well adapted for devices based on isotropic index change such as 3D waveguides or volume Bragg gratings. Indeed, the pulse energy processing window is quite large (from 0.1 up to 1.20 μJ/pulse) which allows us to tailor the photo-induced index change profile. In addition the phase change can be as high as -π/2. This means that we can realized phase lens such as annular phase ring in two or three layers (i.e. less than 150 microns thickness).

Acknowledgments

This work has been performed in the frame of FLAG (Femtosecond Laser Application in Glasses) consortium project with the support of several organisations: the Agence Nationale pour la Recherche (ANR-09-BLAN-0172-01), the RTRA Triangle de la Physique (Réseau Thématique de Recherche Avancée, 2008-056T), the Essonne administrative Department (ASTRE2007), the Ministry of the Foreign Affairs (PHC Alliance) and FP7-PEOPLE-IRSES e-FLAG247635.

References and links

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L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, “Writing of permanent birefringent microlayers in bulk fused silica with femtosecond laser pulses,” Opt. Commun. 171(4-6), 279–284 (1999). [CrossRef]

2.

B. Poumellec, M. Lancry, J. C. Poulin, and S. Ani-Joseph, “Non reciprocal writing and chirality in femtosecond laser irradiated silica,” Opt. Express 16(22), 18354–18361 (2008). [CrossRef] [PubMed]

3.

B. Poumellec, L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, “Femtosecond laser irradiation stress induced in pure silica,” Opt. Express 11(9), 1070–1079 (2003). [CrossRef] [PubMed]

4.

P. Kazansky, W. Yang, E. Bricchi, J. Bovatsek, A. Arai, Y. Shimotsuma, K. Miura, and K. Hirao, “‘Quill’ writing with ultrashort light pulses in transparent materials,” Appl. Phys. Lett. 90(15), 151120 (2007). [CrossRef]

5.

J. Qiu, K. Miura, and K. Hirao, “Femtosecond laser-induced microfeatures in glasses and their applications,” J. Non-Cryst. Solids 354(12-13), 1100–1111 (2008). [CrossRef]

6.

M. Lancry, F. Brisset, and B. Poumellec, “In the heart of nanogratings made up during femtosecond laser irradiation,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, OSA Technical Digest (CD), (Optical Society of America, 2010), ISBN 978–1-55752–896–4.

7.

M. Kaempfe, G. Seifert, K. Berg, H. Hofmeister, and H. Graener, “Polarization dependence of the permanent deformation of silver nanoparticles in glass by ultrashort laser pulses,” Eur. Phys. J. D 16(1), 237–240 (2001). [CrossRef]

8.

Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91(24), 247405 (2003). [CrossRef] [PubMed]

9.

M. Ams, G. Marshall, P. Dekker, M. Dubov, V. Mezentsev, I. Bennion, and M. Withford, “Investigation of ultrafast laser–photonic material interactions: challenges for directly written glass photonics,” IEEE J. Sel. Top. Quantum Electron. 14(5), 1370–1381 (2008). [CrossRef]

10.

K. Itoh, W. Watanabe, S. Nolte, and C. B. Schaffer, “Ultrafast processes for bulk modification of transparent materials,” MRS Bull. 31(08), 620–625 (2006). [CrossRef]

11.

B. Poumellec and M. Lancry, “Damage thresholds in femtosecond laser processing of silica: a review,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, OSA Technical Digest (CD), (Optical Society of America, 2010), ISBN 978–1-55752–896–4.

12.

L. Sudrie, “Propagation non-linéaire des impulsions laser femtosecondes dans la silice,” (Université de Paris Sud XI Orsay, 2002).

13.

C. Schaffer, A. Brodeur, and E. Mazur, “Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses,” Meas. Sci. Technol. 12(11), 1784–1794 (2001). [CrossRef]

14.

M. Lancry, P. Niay, and M. Douay, “Comparing the properties of various sensitization methods in H2-loaded, UV hypersensitized or OH-flooded standard germanosilicate fibers,” Opt. Express 13(11), 4037–4043 (2005). [CrossRef] [PubMed]

15.

M. Lancry and B. Poumellec, “UV laser processing and multiphoton absorption processes in optical telecommunication fibers materials,” Phys. Rep. (in press).

16.

E. Bricchi, B. G. Klappauf, and P. G. Kazansky, “Form birefringence and negative index change created by femtosecond direct writing in transparent materials,” Opt. Lett. 29(1), 119–121 (2004). [CrossRef] [PubMed]

17.

E. Bricchi and P. Kazansky, “Extraordinary stability of anisotropic femtosecond direct-written structures embedded in silica glass,” Appl. Phys. Lett. 88(11), 111119 (2006). [CrossRef]

18.

K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef] [PubMed]

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K. Hirao and K. Miura, “Writing waveguides and gratings in silica and related materials by a femtosecond laser,” J. Non-Cryst. Solids 239(1-3), 91–95 (1998). [CrossRef]

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C. Hnatovsky, R. S. Taylor, E. Simova, P. P. Rajeev, D. M. Rayner, V. R. Bhardwaj, and P. B. Corkum, “Fabrication of microchannels in glass using focused femtosecond laser irradiation and selective chemical etching,” Appl. Phys., A Mater. Sci. Process. 84(1-2), 47–61 (2006). [CrossRef]

22.

A. Paleari, E. Franchina, N. Chiodini, A. Lauria, E. Bricchi, and P. Kazansky, “SnO2 nanoparticles in silica: Nanosized tools for femtosecond-laser machining of refractive index patterns,” Appl. Phys. Lett. 88(13), 131912 (2006). [CrossRef]

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M. Lancry, B. Poumellec, and M. Douay, “Anisotropic luminescence photo-excitation in H2-loaded Ge-doped silica exposed to polarized 193 nm laser light,” J. Non-Cryst. Solids 355(18-21), 1062–1065 (2009). [CrossRef]

27.

J. Chan, T. Huser, S. Risbud, and D. Krol, “Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses,” Appl. Phys., A Mater. Sci. Process. 76(3), 367–372 (2003). [CrossRef]

28.

H. Hosono, K. Kawamura, S. Matsuishi, and M. Hirano, “Holographic writing of micro-gratings and nanostructures on amorphous SiO2 by near infrared femtosecond pulses,” Nucl. Instrum. Methods Phys. Res. B 191(1-4), 89–97 (2002). [CrossRef]

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30.

H. Sun, S. Juodkazis, M. Watanabe, S. Matsuo, H. Misawa, and J. Nishii, “Generation and recombination of defects in vitreous silica induced by irradiation with a near-infrared femtosecond laser,” J. Phys. Chem. B 104(15), 3450–3455 (2000). [CrossRef]

31.

M. Lancry, N. Groothoff, S. Guizard, W. Yang, B. Poumellec, P. Kazansky, and J. Canning, “Femtosecond laser direct processing in wet and dry silica glass,” J. Non-Cryst. Solids 355(18-21), 1057–1061 (2009). [CrossRef]

32.

S. Mao, F. Quéré, S. Guizard, X. Mao, R. Russo, G. Petite, and P. Martin, “Dynamics of femtosecond laser interactions with dielectrics,” Appl. Phys., A Mater. Sci. Process. 79(7), 1695–1709 (2004). [CrossRef]

33.

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35.

M. Lancry, E. Régnier, and B. Poumellec, “Fictive temperature in silica-based glasses and its application to optical fiber manufacturing,” Prog. Mater. Sci. 57(1), 63–94 (2012). [CrossRef]

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38.

M. Lancry, N. Groothoff, B. Poumellec, S. Guizard, N. Fedorov, and J. Canning, “Time-resolved plasma measurements in Ge-doped silica exposed to IR femtosecond laser,” Phys. Rev. B (in press).

39.

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40.

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45.

M. Lancry and B. Poumellec, “Femtosecond laser direct writing in P, Ge doped silica glasses: time resolved plasma measurements,” in Femtosecond Laser Microfabrication, OSA Technical Digest (CD) (Optical Society of America, 2009), ISBN 978–1–55752–879–7, paper LMTuA5 (2009).

46.

M. Lancry, K. Cook, J. Canning, and B. Poumellec, “Nanogratings and molecular oxygen formation during femtosecond laser irradiation in silica,” in The International Quantum Electronics Conference (IQEC)/The Conference on Lasers and Electro-Optics (CLEO) Pacific Rim (2011).

47.

P. Rajeev, M. Gertsvolf, C. Hnatovsky, E. Simova, R. Taylor, P. Corkum, D. Rayner, and V. Bhardwaj, “Transient nanoplasmonics inside dielectrics,” J. Phys. At. Mol. Opt. Phys. 40(11), S273–S282 (2007). [CrossRef]

48.

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49.

M. Lancry, B. Poumellec, W. Yang, and B. Bourguignon, “Oriented creation of anisotropic defects by IR femtosecond laser scanning in silica,” Opt. Express (in press).

50.

R. Araujo, “Oxygen vacancies in silica and germania glasses,” J. Non-Cryst. Solids 197(2-3), 164–169 (1996). [CrossRef]

51.

M. Schurman and M. Tomozawa, “Equilibrium oxygen vacancy concentrations and oxidant diffusion in germania, silica, and germania-silica glasses,” J. Non-Cryst. Solids 202(1-2), 93–106 (1996). [CrossRef]

52.

T. E. Tsai, E. J. Friebele, and D. L. Griscom, “Thermal stability of photoinduced gratings and paramagnetic centers in Ge- and Ge/P-doped silica optical fibers,” Opt. Lett. 18(12), 935–937 (1993). [CrossRef] [PubMed]

53.

K. Sanada, N. Shamoto, and K. Inada, “Radiation resistance of fluorine-doped silica-core fibers,” J. Non-Cryst. Solids 179, 339–344 (1994). [CrossRef]

54.

K. Saito and A. Ikushima, “Effects of fluorine on structure, structural relaxation, and absorption edge in silica glass,” J. Appl. Phys. 91(8), 4886–4890 (2002). [CrossRef]

55.

L. Skuja, K. Kajihara, Y. Ikuta, M. Hirano, and H. Hosono, “Urbach absorption edge of silica: reduction of glassy disorder by fluorine doping,” J. Non-Cryst. Solids 345-346, 328–331 (2004). [CrossRef]

56.

B. Poumellec and F. Kherbouche, “The photorefractive Bragg gratings in the fibers for telecommunications,” J. Phys. III 6(12), 1595–1624 (1996). [CrossRef]

OCIS Codes
(160.6030) Materials : Silica
(320.2250) Ultrafast optics : Femtosecond phenomena
(320.7130) Ultrafast optics : Ultrafast processes in condensed matter, including semiconductors
(350.3450) Other areas of optics : Laser-induced chemistry

ToC Category:
Laser Materials Processing

History
Original Manuscript: April 7, 2011
Revised Manuscript: June 30, 2011
Manuscript Accepted: July 1, 2011
Published: July 27, 2011

Virtual Issues
Femtosecond Direct Laser Writing and Structuring of Materials (2011) Optical Materials Express

Citation
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, 711-723 (2011)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-1-4-711


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References

  1. L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, “Writing of permanent birefringent microlayers in bulk fused silica with femtosecond laser pulses,” Opt. Commun. 171(4-6), 279–284 (1999). [CrossRef]
  2. B. Poumellec, M. Lancry, J. C. Poulin, and S. Ani-Joseph, “Non reciprocal writing and chirality in femtosecond laser irradiated silica,” Opt. Express 16(22), 18354–18361 (2008). [CrossRef] [PubMed]
  3. B. Poumellec, L. Sudrie, M. Franco, B. Prade, and A. Mysyrowicz, “Femtosecond laser irradiation stress induced in pure silica,” Opt. Express 11(9), 1070–1079 (2003). [CrossRef] [PubMed]
  4. P. Kazansky, W. Yang, E. Bricchi, J. Bovatsek, A. Arai, Y. Shimotsuma, K. Miura, and K. Hirao, “‘Quill’ writing with ultrashort light pulses in transparent materials,” Appl. Phys. Lett. 90(15), 151120 (2007). [CrossRef]
  5. J. Qiu, K. Miura, and K. Hirao, “Femtosecond laser-induced microfeatures in glasses and their applications,” J. Non-Cryst. Solids 354(12-13), 1100–1111 (2008). [CrossRef]
  6. M. Lancry, F. Brisset, and B. Poumellec, “In the heart of nanogratings made up during femtosecond laser irradiation,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, OSA Technical Digest (CD), (Optical Society of America, 2010), ISBN 978–1-55752–896–4.
  7. M. Kaempfe, G. Seifert, K. Berg, H. Hofmeister, and H. Graener, “Polarization dependence of the permanent deformation of silver nanoparticles in glass by ultrashort laser pulses,” Eur. Phys. J. D 16(1), 237–240 (2001). [CrossRef]
  8. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, “Self-organized nanogratings in glass irradiated by ultrashort light pulses,” Phys. Rev. Lett. 91(24), 247405 (2003). [CrossRef] [PubMed]
  9. M. Ams, G. Marshall, P. Dekker, M. Dubov, V. Mezentsev, I. Bennion, and M. Withford, “Investigation of ultrafast laser–photonic material interactions: challenges for directly written glass photonics,” IEEE J. Sel. Top. Quantum Electron. 14(5), 1370–1381 (2008). [CrossRef]
  10. K. Itoh, W. Watanabe, S. Nolte, and C. B. Schaffer, “Ultrafast processes for bulk modification of transparent materials,” MRS Bull. 31(08), 620–625 (2006). [CrossRef]
  11. B. Poumellec and M. Lancry, “Damage thresholds in femtosecond laser processing of silica: a review,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides, OSA Technical Digest (CD), (Optical Society of America, 2010), ISBN 978–1-55752–896–4.
  12. L. Sudrie, “Propagation non-linéaire des impulsions laser femtosecondes dans la silice,” (Université de Paris Sud XI Orsay, 2002).
  13. C. Schaffer, A. Brodeur, and E. Mazur, “Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses,” Meas. Sci. Technol. 12(11), 1784–1794 (2001). [CrossRef]
  14. M. Lancry, P. Niay, and M. Douay, “Comparing the properties of various sensitization methods in H2-loaded, UV hypersensitized or OH-flooded standard germanosilicate fibers,” Opt. Express 13(11), 4037–4043 (2005). [CrossRef] [PubMed]
  15. M. Lancry and B. Poumellec, “UV laser processing and multiphoton absorption processes in optical telecommunication fibers materials,” Phys. Rep. (in press).
  16. E. Bricchi, B. G. Klappauf, and P. G. Kazansky, “Form birefringence and negative index change created by femtosecond direct writing in transparent materials,” Opt. Lett. 29(1), 119–121 (2004). [CrossRef] [PubMed]
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