Journal of Non-Crystalline Solids 385 (2014) 153–159

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Journal of Non-Crystalline Solids

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Refractive index modification in fluoro-borate glasses containing WO3

induced by femtosecond laser

Yannick Ledemi ⁎, Jean-Philippe Bérubé, Réal Vallée, Younès MessaddeqCentre d'optique, photonique et laser (COPL), Université Laval, Québec G1V 0A6, Canada

⁎ Corresponding author.E-mail address: yannick.ledemi@copl.ulaval.ca (Y. Led

0022-3093/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.jnoncrysol.2013.11.029

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 October 2013Received in revised form 14 November 2013Available online 2 December 2013

Keywords:Borate glass;Tungsten;Raman spectroscopy;Photoinduced change;Refractive index modification;Femtosecond laser exposure

New glasses belonging to the NaF–Na2B4O7–WO3 system have been studied in order to inscribe optical wave-guides within their volume through local refractive index modification after irradiation by ultrafast laser pulses.Appropriate addition of NaF to glasses belonging to the pseudo-binary vitreous system Na2B4O7–WO3 hasallowed to obtain striae-free bulk glasses of large volume with high content of WO3 (35 mol%). Their physicaland structural properties have been investigated throughUV–visible spectroscopy, differential scanning calorim-etry, densitometry and Raman spectroscopy. The glass of composition (NaF)15–(Na2B4O7)50–(WO3)35 has beenthen exposed to 800 nm femtosecond pulses of 70 fs width at a repetition rate of 100 kHz from a Ti-sapphire re-generative amplifier system. By varying the input pulse energy and scanning speed, homogeneous optical wave-guides were inscribed and a positive refractive index change of the order of 5 × 10−3 has been measured.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Photoinduced change of the refractive index in glassy materialresulting from femtosecond laser (fs-laser) exposition has been first re-ported by Davis et al. [1]. This pioneer work has attracted a lot of interestdue to the vast range of applications for optical components such aswaveguides, Bragg-gratings [2], optical couplers [3], optical storage [4]and other devices based on inscribed micro- and nano-structures [5,6].By tailoring the fs-laser parameters (wavelength, pulse energy, duration,repetition rate) and writing conditions (focalization, scanning speed), itis indeed possible to achieve very localized and permanent refractiveindex modifications. Over the past decade, one of the most explored av-enues arising from this phenomenon is the inscription of channel wave-guides within the volume of a transparent optical material.

Numerous optical materials have been investigated for thispurpose. Although some works have been reported on crystals such asTi3+:sapphire [7] or Nd3+:YAG [8], the vast majority of publishedwork is focused on glassy materials. Among them, the most studiedare commercial fused silica [6,9,10] and derivatives such as borosilicate[10,11] or soda lime silicate [12]. Photo-response to 800 nm fs-laser ex-posure has also been investigated on commercial sodium-aluminophosphate glasses [13,14], zinc phosphate glasses [15], phospho-tellurite glasses [16], niobium tellurite glasses [17], and on severalnon-oxide glasses: chalcogenide glasses such as As2S3 [18], GexS1 − x

[19] or GaLaS [20], fluoride glasses such as ZBLAN [21], fluoro-phosphate glasses [22]. Many reviews have been published on this sub-ject [14,23,24]. Nevertheless, there are few reports in the literature

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dealing with waveguide inscribed by fs-laser in borate glasses, except-ing those based onborosilicate. But for the latter, the B2O3 content rarelyexceeds 20 mol% and is always accompanied with silica SiO2 as themain glass-former. Regarding the studies on glasses containing B2O3

as glass-former, Ehrt et al. have reported qualitatively on refractiveindex change in glasses of composition (B2O3)74–(Al2O3)9–(PbO)17and (B2O3)50–(Bi2O3)50 without concluding about the sign of theindex change [22]. More recently, Yang et al. have successfully writtenwith a 800 nm fs-laser single mode waveguides in a (Bi2O3)12.5–(ZnO)43.75–(B2O3)43.75 glass substrate with a maximum value ofΔn = +4.5 · 10−3 for a 280 nJ pulse energy [25].

In this work, we describe the photo-induced modification of refrac-tive indexΔn following near infrared 800 nm fs-laser exposition on so-dium borotungstate glass belonging to the NaF–Na2B4O7–WO3 system.First, we present the physical and structural characterizations of theprepared glasses from which the candidate for fs-laser writing was se-lected. Then the material photo-sensitivity to 800 nm femtosecondpulses for various laser exposure conditions was investigated. The re-fractive index change was measured using the quantitative phase mi-croscopy technique (QPM).

2. Experimental details

2.1. Glass preparation

The sodiumborotungstate glasseswere prepared from the tradition-al melt-casting technique. In a first step, Na2B4O7 (so-called borax-glass) was prepared by thermal decomposition for 1 h at 500 °C of theraw materials Na2CO3 (J.T. Baker, 3 N) and H3BO3 (Alfa Aesar, 4 N)and consecutive melting for 3 h at 1000 °C in a platinum crucible. The

0 500 1000 1500 2000 2500 3000 3500 4000 4500

0

20

40

60

80

100

Na2B

4O

7

(Na2B

4O

7)65

-(WO3)35

(NaF)15

-(Na2B

4O

7)50

-(WO3)35

Tra

nsm

issi

on (

% T

)

Wavelength (nm)

(thickness: 0.8 mm)

Fig. 1. Transmission spectra of the (NaF)15–(Na2B4O7)50–(WO3)35, (Na2B4O7)65–(WO3)35and Na2B4O7 glass samples (sample thickness is 0.8 mm).

154 Y. Ledemi et al. / Journal of Non-Crystalline Solids 385 (2014) 153–159

obtained Na2B4O7 glass was then crushed and used as starting materialaswell as NaF (Alfa Aesar, 4 N) andWO3 (Alfa Aesar, 3 N). Theweighedbatches were melted in platinum crucible in an electric furnace at1000 °C for 60 min after a plateau of 60 min at 500 °C to remove mois-ture from the powders. The glass was then poured in a stainless steelmold pre-heated at 300 °C, annealed at the same temperature for 3 hand slowly cooled down to room temperature to remove residual me-chanical stress induced by the quenching. The typical dimensions afteroptical polishing of the bulks are 45 × 20 × 2 mm3 for the pseudo-binary Na2B4O7–WO3 glasses and 45 × 2 × 7 mm3 for the (NaF)15–(Na2B4O7)50–(WO3)35 glass sample.

2.2. Glass characterizations

The UV–visible transmission spectra were recorded on a Cary 500(Varian) double beam spectrophotometer while the near infrared spec-tra were recorded on a Perkin Elmer FTIR Frontier spectrometer. To de-termine the glass characteristic temperatures such as glass transitiontemperature Tg and onset crystallization temperature Tx, differentialscanning calorimetric (DSC) measurements were performed by usinga Netzsch DSC Pegasus 404F3 apparatus on glass pieces into Pt pans ata heating rate of 10 °C/min. The thermo-mechanical analysis (TMA)were performed by a using a Netzsch TMA Hyperion 402F1 equipmenton glass rods of 15 mm length at a heating of 5 °C/min and a load of0.02 N. The linear thermal expansion coefficient (TEC) was then deter-mined in the temperature range from 50 to 300 °C. The density ρ wasdetermined by the Archimedes' method with an Alfa-Mirage MD-300Sdensimeter using deionized water as buoyant liquid. Refractive indexhas beenmeasured by employing theM-lines prism coupling technique(Metricon 2010) at 543, 633 and 1553 nm. Raman spectra were record-ed using a LABRAM 800HR Raman spectrometer (Horiba Jobin Yvon)and the 632.8 nm line of a He–Ne laser (Melles Griot).

2.3. Femtosecond laser exposition and characterization

The (NaF)15–(Na2B4O7)50–(WO3)35 glass sample was exposed tofemtosecond pulses generated by a chirped-pulse-amplification (CPA)Ti:sapphire laser system (Coherent RegA) operating at a repetitionrate of 100 kHz. Pulses have a central wavelength of 792 nm and theirtemporal FWHMwasmeasured to be ~60 fs at the laser output and es-timated at ~70 fs on the sample. The beamwas focused in the bulk at adepth of 270 μm by a 40× (f = 4.5 mm, 0.55 NA) aspheric microscopeobjective. The sample was translated perpendicularly to the laser beam(x direction) at different scanning speeds from 50 μm/s to 50 mm/s byusing a motorized mechanical stage (Newport XML210). A cylindricallens telescope was used to produce an astigmatic beam and shape thefocal volume in suchway as to obtain focal volume and therefore wave-guides with near circular cross sections [26]. The size of the resultingbeam at focus, in a plane transverse to the laser propagation direction(y–z), is estimated as 6.2 μm (2wy) by 4.8 μm (2zRx).

After the inscription process, the end faces of the samples werepolished and the photo-inscribed structures were examined under aphase contrast optical microscope (Olympus IX71). The quantitativephase microscopy (QPM) method was used to obtain the radial indexprofiles of the waveguides [27]. The QPM commercial software (IatiaLtd.) proceeds from slightly defocused bright field images of the wave-guide to extract a corresponding phase image. The radial refractiveindex profile is retrieved by applying an inverse Abel transform onthis phase image [28].

3. Results

3.1. Glass structure and properties

The glass composition selected for the fs-laser inscription experi-ments described later in this work, is (NaF)15–(Na2B4O7)50–(WO3)35.

This composition was chosen after a two-step approach. In the firststep, the glass-forming region of the pseudo-binary (Na2B4O7)100 − x–

(WO3)x glass system was explored, leading to a maximum incorpora-tion of WO3 into the glass of 30 mol% when the glass melt is poured inthe heated mold, as described above. The maximum WO3 contentin the glass was further increased to 35 mol% by increasing theglass cooling rate through a quenching between two stainless steelplates (yielding thus to a glass plate of 0.5 mm thickness). Then, in thesecond step, sodium fluoride NaF was substituted for Na2B4O7 in the(Na2B4O7)65–(WO3)35 composition, allowing to prepare glass bulks oflarger volume by simple pouring of the glassmelt. An optically homoge-neous and striae-free glass sample of nominal composition (NaF)15–(Na2B4O7)50–(WO3)35 and 45 × 20 × 7 mm3 dimensions has been fab-ricated and then chosen as best candidate for the fs-laser writingexperiments.

The transmission spectra of the (NaF)15–(Na2B4O7)50–(WO3)35,(Na2B4O7)65–(WO3)35 and Na2B4O7 glass samples are presented inFig. 1. Their transmission window extends from about 0.4 μm to2.8 μm. The addition of WO3 to Na2B4O7 glass results in: (i) a decreaseof the maximum transmission from ~90% to ~85% which can be attrib-uted to the increase of refractive index reported in Table 1; (ii) a red-shift of about 150 nmof the UV cut-off wavelength and; (iii) a slight de-crease of both absorption bands around 2.8 μmand 3.7 μm, respectivelyascribed to the vibrations of \OH− groups and to the B\O multi-phonon absorption. Then, the addition of NaF to (Na2B4O7)65–(WO3)35did not affect significantly the glass transmission window.

The measured glass transition temperature Tg, onset crystallizationtemperature Tx as well as their difference ΔT = Tx − Tg, which definesthe glass thermal stability against crystallization, are reported in Table 1.The linear thermal expansion coefficient (TEC), the density ρ, therefractive indices measured at 543, 633 and 1553 nm, and the UV cut-off wavelength λ0 calculated for a linear absorption coefficientα = 10 cm−1 are also summarized in Table 1. The addition of WO3 toglassy Na2B4O7 leads to an increase of the glass density, cut-off wave-length and refractive indices. Then, the characteristic temperaturessuch as glass transition temperature Tg and the thermal stability ΔT,strongly decrease with WO3 addition. One can also observe in Table 1an increase of the TEC measured on the (NaF)15–(Na2B4O7)50–(WO3)35glass sample compared to that of glassy Na2B4O7. Note that the TECvalue measured for the latter is consistent with the literature [29]whereas nomeasurementwas performed on the (Na2B4O7)65–(WO3)35sample due to its too small thickness to get accurate data. Last, while theaddition of NaF to (Na2B4O7)65–(WO3)35 has low influence on the glass

Table 1Physical properties of the prepared glasses.

(NaF)15–(Na2B4O7)50–(WO3)35

(Na2B4O7)65–(WO3)35

Na2B4O7

Tg (±2 °C) 387 411 472Tx (±2 °C) 447 435 613ΔT = Tx − Tg (±4 °C) 60 24 141TEC [50–300 °C](±0.2 × 10−6 K−1)

16.0 n/a 12.4

ρ (±0.02 g/cm3) 4.04 3.95 2.35UV-cut-off λ0 (nm)a 359 387 233@ 543 nm 1.7009 1.6947 1.5159n @ 633 nm 1.6910 1.6801 1.5122@ 1553 nm 1.6590 1.6490 1.4938

a Calculated for a linear absorption coefficient α = 10 cm−1.

155Y. Ledemi et al. / Journal of Non-Crystalline Solids 385 (2014) 153–159

density and refractive indices, a blue-shift of about 30 nmof theUV cut-off wavelength λ0 is observed.

The Raman spectra recorded on the (NaF)15–(Na2B4O7)50–(WO3)35,(Na2B4O7)65–(WO3)35 and Na2B4O7 glass samples are presented inFig. 2. Raman spectra recorded on other glass compositions belongingto the (Na2B4O7)100 − x–(WO3)x pseudo-binary system are also de-picted in Fig. 2 to provide some insights to help interpreting the spectra.

One can observe in Fig. 2 that the Raman spectrum of the Na2B4O7

glass strongly differs from that of WO3-containing glasses. The mainband observed at 764 cm−1 has been assigned to the symmetric ringbreathing vibration of six-membered ring arrangements containingone or two BO4 tetrahedra, i.e. triborate, tetraborate, or pentaborateunits, as described in [30,31]. Firstly, triborate and tetraborate groupscan be identified through the presence of the two bands at 764 and980 cm−1. Secondly, the simultaneous presence of bands at 500, 630,764, and 980 cm−1 in the Na2B4O7 glass spectrum confirms the exis-tence of pentaborate groups in the network. Finally, the slight band at630 cm−1 indicates the presence of ring-type metaborate units [31].

Then, the addition of intermediate glass-formerWO3 to the Na2B4O7

glass causes major changes of the Raman spectra. Firstly, themain bandis now observed at 935 cm−1 for the sample containing 12.5% of WO3

and shifted up to 958 cm−1 with increasing WO3 content up to 35%.Then, the shoulder located at 874 cm−1 increases and extends overlower wavenumbers whereas the intensity of the band at 775 cm−1

slowly diminishes with increasing WO3 content. Secondly, one can ob-serve a band centered at 337 cm−1 which decreases as well as shifts

200 400 600 800 1000 1200 1400

874

terminal

W=O or W-O-

Na2WO

4

764

361

337

NaF-Na2B

4O

7-WO

3

630

O-W-O

x =18x = 25

x = 12.5

Nor

mal

ized

inte

nsity

(a.

u.)

Raman shift (cm-1)

-WO3

775

500980

935-958

358

Na2B

4O

7

terminal

W=O or W-O-

x = 35

γ

Fig. 2. Raman spectra of the (NaF)15–(Na2B4O7)50–(WO3)35 glass and (Na2B4O7)100 − x–

(WO3)x glass compositions (with x = 0, 12.5, 18, 25 and 35). Raman spectra of crystallineγ-WO3 andNa2WO4 powders are also depicted. (The spectra have been vertically translat-ed for better clarity.)

up to 361 cm−1 with increasing WO3 content. By comparison with theRaman spectrum of the Na2B4O7 glass, only the Raman peak rising at775 cm−1 is present for the WO3-containing samples. This band,whose relative intensity is almost unaffected by the addition of WO3,was assigned to diborate groups [32]. Such result suggests thus a break-down of the penta-, tetra- and tri-borate units toward simpler diborateunits when WO3 is incorporated into the network.

3.2. Femtosecond laser exposition

The (NaF)15–(Na2B4O7)50–(WO3)35 glass has been irradiated by800 nm femtosecond laser pulses varying two parameters: the pulseenergy from 0.2 to 2.0 μJ and the scanning speed from 50 μm/s to50 mm/s. Phase contrast microscope images of photo-induced trackswritten with different pulse energies and for various translation speedsare illustrated in Fig. 3.

For a pulse energy of 0.7 μJ or less, smooth tracks with a diametervarying between 5 and 15 μm are inscribed. Pulse energies of 1 μJ ormore yield structures with a cross section that extends significantly be-yond the irradiated region which are typical of tracks inscribed in theheat accumulation regime [11]. Tracks inscribed under those exposureconditions andwith a translation speed of 5 mm/s or less are character-ized by a strong modulation of the refractive index change along thewriting direction. The spacing and amplitude of themodulation dependon the deposited energy and translation speed. A similar phenomenonhas been observed by other groups in multicomponent glasses for in-scriptionsmade at high repetition rate [14,33,34] andmay be attributedto an interplay of heat-induced material modifications and subsequentdefocusing of the laser beam.

Fig. 4 presents the evolution of the diameter of the inscribed struc-tures as a function of both the input pulse energy and the translationspeed.

We note that the general trend is for the diameter of the inscribedtracks to increase with pulse energy and decrease with translationspeed. Accordingly, one also notes that the structures inscribed at lowpulse energy (b500 nJ) have a diameter approaching that of the laserbeam in the transverse y–z plane at the beamwaist. For higher pulse en-ergy, the heat accumulation effect dominates the inscription process[11] so that the diameter of the inscribed structures increases up to58 μm for a pulse energy of 2 μJ and a scanning speed of 0.05 mm/s.

Fig. 5 presents the photo-induced refractive index modificationassessed via the QPM technique for different exposure parameters.Fig. 5(a) shows the radial refractive index profiles of tracks inscribedat low (0.5 mm/s) and high translation speed (50 mm/s) for differentpulse energies. Fig. 5(b) presents the induced refractive index changemeasured at the center of the exposed region plotted versus pulse ener-gy for different translation speed.

Depending mainly on laser fluence, two distinct irradiation regimesare apparent. The laser cumulated fluence on a length corresponding to2wx is calculated using F = 2ETrep / πwyvx where E is the pulse energy,Γrep = 100 kHz is the repetition rate, 2wy = 6.2 μm is the estimatedbeam waist in the y direction and v is the translation speed. At hightranslation speed (50 mm/s), the fluence ranges between 12.5 and45 J/cm2 for a pulse energy of 0.6 μJ and 2 μJ respectively. In this regime,thermal effects are minimal and the induced refractive index changeis negative. The index at the center of the exposed area decreaseswith increasing pulse energy and a maximum index change ofΔn = −8.5 × 10−3 is reached for E = 2 μJ. Tracks inscribed at a trans-lation speed of 5 mm/s (F = 100 − 165 J/cm2) show a weak indexchange (b1 × 10−3), that can be either negative or positive at the cen-ter of the irradiated region. For a scanning speed of 0.5 mm/s or less, thelaser fluence reaches several kJ/cm2 and the heat accumulation effectdominates the inscription process. Under those irradiation conditions,the refractive index change is mostly positive. For a pulse energy of0.5 μJ or less, the index change grows with increasing pulse energyand the resulting structures are characterized by a smooth refractive

Fig. 3. Phase contrast microscope images of the side view (x–y plane) of tracks written with different translation speeds and pulse energies.

156 Y. Ledemi et al. / Journal of Non-Crystalline Solids 385 (2014) 153–159

index profile that shows great potential for waveguiding applications. Amaximum positive index change of Δn = +5 × 10−3 is reached for achannel inscribedwith a translation speed of 0.05 mm/s and a pulse en-ergy of 0.5 μJ. A further increase of the pulse energy results in a dip atthe center of the exposed area and a decrease of the overall indexchange. This decreasing trend can be explained by the large size of theinduced structures with high pulse energy that limits the refractiveindex change. Also, for a translation speed of 5 mm/s or less and highpulse energy (N700–800 nJ), intense thermal effects lead to the forma-tion of large channels that exhibit a strong and irregular modulation ofthe refractive index along both longitudinal and transverse directions.The onset for the formation of this type of modification is identified inFig. 5(b) (labeled: Irregular Δn).

4. Discussion

4.1. Glass structure and properties

Tungsten oxide WO3 is known to be an intermediate glass-former,i.e. it doesn't form a glass by itself but it may act as glass-former or asnetwork modifier, depending on the glass system and composition.Here, when added to the Na2B4O7 glass network, WO3 clearly acts asnetworkmodifier and decreases the network connectivity, as evidencedby the decrease of glass transition temperature Tg and the strong de-crease of thermal stability against crystallization ΔT criterion (seeTable 1) when increasing its concentration in the glass. Such behavior

Fig. 4. Diameter of the tracks plotted versus a) pulse energy for different tr

contrasts with the effect of the addition of WO3 into tellurite or phos-phate glass networks whose glass transition temperature is usually in-creased, revealing thereby its glass-forming role by forming newbonds and increasing the glass connectivity. Here, the modifier roleplayed byWO3 is also supported by the increase of glass-forming criticalcooling rate upon its addition to Na2B4O7, i.e. a faster cooling is requiredto form a glass. Further addition of glass-modifier NaF by substitutingfor Na2B4O7 in the (Na2B4O7)65–(WO3)35 composition has then im-proved the glass-forming ability, as evidenced by the pouring of 7 mmthick glass bulks without suffering any devitrification whereas onlythin glass plates could be obtained from the (Na2B4O7)65–(WO3)35 com-position. The increase of ΔT criterion reported in Table 1 also supportsthis observation. This may result from the addition of NaF whichenters within the network as non-bridging sodium Na+ cations andfluorine F− anions, causing its depolymerization, or in other words, abreakage of its cross-linking, as confirmed by both the decrease ofglass transition temperature Tg and the increase of coefficient of thermalexpansion α reported in Table 1.

As mentioned above, the addition of WO3 to Na2B4O7 glass has alsoresulted in changes of optical properties (see Fig. 1 and Table 1) like:(i) an increase of refractive index (and thus a lowering a maximum op-tical transmission) due to the large molecular weight and polarizabilityof tungsten ions; (ii) a red-shift of the UV cut-off wavelength relatedto the lower optical band gap energy of WO3 (Eopt ~ 3.0 eV whichcorresponds to λ ~ 413 nm [35]) compared to that of Na2B4O7

(Eopt ~ 3.85 eV which corresponds to λ ~ 322 nm [36]) and; (iii) a

anslation speeds and b) translation speed for different pulse energies.

Fig. 5. Radial refractive index change profiles at a) fixed translation speeds of 50 and 0.5 mm/s and different pulse energies, and b) the refractive index change measured at the center(r = 0) of the exposed area.

157Y. Ledemi et al. / Journal of Non-Crystalline Solids 385 (2014) 153–159

slight decrease of bothOH− groups and B\Omulti-phonon infrared ab-sorptions due to the lower relative content of Na2B4O7 within thenetwork.

Then, as observed in Table 1, the addition of NaF to (Na2B4O7)65–(WO3)35 has caused low variation of the glass refractive indices anddensity, which are closely related properties. This was expected due toits chemical nature similar to Na2O present into the matrix. These re-sults are consistent with those of literature for NaF–Na2O–B2O3 glasses[37] and for Na2O–B2O3–WO3 glasses at similar WO3 concentration[29]. Finally, one can observe a blue-shift of the glass cut-offwavelengthafter addition of sodium fluoride due to the high electronegativity offluorine anions, resulting in a widening of the glass optical band gap.

The blue-shift of the UV cut-off wavelength may also be related tothe decrease of non-bridging oxygen (NBO) content in the network.The transmission of oxide glasses in the UV-region is indeed dependenton its concentration of NBOs in the sense that electrons belonging toNBOs possess weaker binding forces than those belonging to bridgingoxygens, due to their lower Madelung potential [38]. A lower energy,i.e. a longer wavelength, is thus required to excite electrons of NBOsthan those of bridging oxygens; in other words, the presence of NBOsin a glass will red-shift its cut-off wavelength. For the borate glasses,the addition of alkali to vitreous B2O3 which is built on planar BO3 trian-gles (forming the so-called boroxyl rings) results in the formation of BO4

tetrahedra close to the alkali cations, and few if any NBOs are formed[39]. When the alkali concentration further increases within thenetwork (e.g. above 15 mol% of Na2O in Na2O–B2O3), the change from3-fold coordinated boron to 4-fold boron atoms saturates and the net-work begins to break up, creating NBOs located on the BO4 tetrahedra.The alkali cations are then located close to the NBO anions to ensurethe charge neutrality. In brief, increasing the alkali content in binary al-kali borate glasses leads to the successive formation of BO4 tetrahedraand NBOs [39]. Then, as reported by Shelby and Ortolano [37], the addi-tion of NaF leads to the replacement of the BO4 tetrahedra (containingone or more NBOs) by BO3F and BO2F2 (at high NaF content) tetrahedra

containing one and two non-bridgingfluorines, respectively. The chargeof Na+ cations is therefore compensated by the presence of single bond-ed fluorine anions. This has two related consequences: the reduced needof NBOs into the matrix and the increased need of bridging oxygens toensure the network connectivity. Hence, the NaF addition results in a de-crease of the concentration of NBOs in the (NaF)15–(Na2B4O7)50–(WO3)35 glass compared to the (Na2B4O7)65–(WO3)35 glass.

The structure of the alkali borate glasses such as the Na2B4O7 glasshas beenwell investigated by Raman and FTIR spectroscopies by severalauthors [30,31], permitting to establish some model to describe thiscomplex glass network. The latter is composed of different structuralring arrangements based on planar pyramidal BO3 and tetrahedral BO4

subunits [30,31]. On the other hand, WO3-containing glasses have alsobeen extensively studied for different types of vitreous systems, suchas tellurite [40], phosphate [41] and lithium borate [32], own to theunique electrochromic, photochromic or even ferroelectric propertiesexhibited by the WO6 octahedra similar to those of tungsten bronzes[42]. Nonetheless, although fewworks have been reported on the struc-ture of lithium tungsten borate glasses [32], a limited attentionwas paidon sodium borate counterpart [43]. Recently, the glass-formation rangeand some optical characteristics of sodium tungstate borate glasseshave been studied by Shelby and Brickwedel [29], howeverwithout em-phasizing the glass structure.

To help interpreting the Raman spectra presented in Fig. 2, theRaman spectra of commercial crystalline compounds γ-WO3 andNa2WO4 have also been recorded. First, crystalline WO3, known for itsseveral allotropic modifications with temperature, is, at room tempera-ture, in a monoclinic phase made of corner-sharing distorted and tiltedWO6 octahedra, building up a tridimensional networkwithout terminalW\O bonds. From its Raman spectrum, the bands centered at 808 and728 cm−1 were attributed to asymmetric and symmetric stretching vi-brations ofW\O\W linkages, respectively, while the bands at 328 and273 cm−1 were assigned to bending modes of WO6 octahedra and the135 cm−1 band to lattice modes [44].

158 Y. Ledemi et al. / Journal of Non-Crystalline Solids 385 (2014) 153–159

Then, in crystalline Na2WO4 which has a spinel structure, the W6+

cations are in tetrahedral environment, forming WO42− isolated

tetrahedral where all the W\O bonds are terminal, i.e. W\O− orW_O [45]. Regarding its Raman spectrum, the bands centered at 930and 812 cm−1 have been usually assigned to symmetric and asymmet-ric W\O stretching modes in WO4 tetrahedra, respectively, while thebands at 312 and 374 cm−1 have been attributed to O\W\O bendingmodes [46]. However, according to Poirier et al. [47] and Sekiya et al.[40], the position of the Raman bands of tungstate compounds is inde-pendent of the tungsten coordination number, and the 930 and812 cm−1 bands for Na2WO4 cannot be assigned to stretching vibra-tions of WO4 units. They assume that both bridging W\O\W and ter-minal W\O bonds exist in Na2WO4 and all tungsten atoms are six-foldcoordinated. Consequently, the 930 and 812 cm−1 Raman bands can bethus ascribed to asymmetric and symmetric stretching modes of termi-nal W\O bonds, respectively [47].

From the above description, one can reliably assign the intense bandat 935–958 cm−1 on the Raman spectra of the (Na2B4O7)100 − x–

(WO3)x glasses (Fig. 2) to the vibration of W\O terminal bonds(W_O orW\O−) even if concluding about the 4- and/or 6-fold coordi-nation remains hazardous from these results. In the reported literatureonWO3-containing alkali borate based glasses, all the authors have pro-posed the coexistence of both tetrahedral and hexagonal sites for thetungsten atoms [32,48,49]. From our point of view, further structuralstudies are required to overcome this uncertainty, e.g. through XANESspectroscopy, as achieved by Poirier et al. [47]. Then, the large shoulderextending from about 600 up to 900 cm−1 for the sample containing35 mol% of WO3 (x = 35) can be ascribed unambiguously to the clus-tering of WO6 octahedra, owing to the higher content of tungsten thanalkali within the network, already reported [43].

Furthermore, one can observe on the Raman spectrum of the(NaF)15–(Na2B4O7)50–(WO3)35 glass a slight shift toward lowerwavenumbers of the 358 and 950 cm−1 bands accompanied by de-creasing of the intensity of the shoulder at 600–900 cm−1, if comparedto the spectrum of the (Na2B4O7)65–(WO3)35 sample. This may be ex-plained by the addition of fluorine anions F− into the network whichreduces its phonon energy and thus redshifts the Raman bands.Then, formally, the oxygen anions O2− are replaced by two fluorineanions F− of comparable ionic radius, resulting in a break of theformer network and thus a lowering of intensity of the large shoulderat 600–900 cm−1.

Finally, it is worth noting that contrary to previous studies reportedon blue- and brown-colored Li2O–B2O3–WO3 glasses [32,50], all theglasses under study here are colorless, even for the (Na2B4O7)65–(WO3)35 composition, suggesting thus the absence in the glass of W5+

species, known to be an intense blue color center, in contrast to theW6+ ions which leave the glass colorless.

4.2. Femtosecond laser exposition

Results presented above highlight two distinct responses of the ma-terial when irradiated with femtosecond pulses. At low laser fluence,the inscription process is athermal and is primarily governed by non-linear processes. In this case, modifications are approximately the sizeor slightly larger than the estimated beamwaist at the focal and the re-fractive index decreases at the irradiated zone. Such decrease of the re-fractive index has been related to volume expansion and local densitydecrease of the glass network for multicomponent glasses like fluorideglasses [51] whose thermal properties are rather similar than thoseof the (NaF)15–(Na2B4O7)50–(WO3) 35 glass: low glass transitiontemperature (Tg = 387 °C, see Table 1), low thermal diffusivity(~2.0 × 10−3 cm2/s reported in similar borate glass [52]) and highthermal expansion (α = 16.0 × 10−6 K−1, see Table 1). At highfluence, thermal effects dominate the inscription process and exposureto fs pulses leads to an increase of the refractive index. Although theheat accumulation effect is usually observed for exposition made with

repetition rates in the MHz range [11], the large channels observedand the low transition temperature (Tg = 387 °C) of the (NaF)15–(Na2B4O7)50–(WO3)35 glass suggest that 100 kHz is sufficient to driveheat accumulation. For particular exposure conditions in this heat accu-mulation regime, we observed that a positive index change can be in-duced in (NaF)15–(Na2B4O7)50–(WO3)35 glass, which is in agreementwith previous works on glasses prone to high thermal expansion[34,51,53].

Photo-response of borate glasses to fs-laser pulses has been rarelyreported in the literature [22,25]. The most relevant is the workby Yang et al. where they reported inscription of single mode wave-guides with a 800 nm fs-laser in a zinc bismuth borate glass witha maximum value of Δn = +4.5 · 10−3 for a 280 nJ pulse energy[25]. Liu et al. have exposed sodium borate glasses to 800 nm fs-laser(repetition rate of 250 kHz, pulse width of 150 fs, and laser intensityof 1014 W/cm2) for a long time, attaining thus a very high energy depos-it rate and heat accumulation effect [54]. They evidenced the migrationof sodium and oxygen ions from the focal spot due to its very hightemperature reached during irradiation,which can be far above thema-terial melting temperature. After an even longer fs-laser exposure of al-uminiumbariumborate glasses, Yu et al. have observed the formation ofβ-BaB2O4 crystals at the focal point [55]. It is clear from the exposureconditions used in both latterworks that the observed changes originat-ed from a thermal regimemuchmore intense than that observed in thepresent work.

5. Conclusions

New NaF–Na2B4O7–WO3 glasses have been prepared and character-ized from an optical and structural point of view. Addition of NaF to theNa2B4O7–WO3 glass network has allowed to improve the glass formingability and thus to obtain large volume glassy bulk containing largeamount of WO3 (35 mol%) of excellent optical quality and withoutstriae. The (NaF)15–(Na2B4O7)50–(WO3)35 glass has been irradiated by800 nm fs-laser pulses within its volume at a repetition rate of100 kHz for different input pulse energies and scanning speeds. The pa-rameters of laser exposure were determined to inscribe permanentstructures with a local change of the refractive index. A maximumnegative refractive index change of Δn = −8.5 × 10−2 wasmeasured.Potentially waveguiding structures with Δn = +5 × 10−3 havebeen inscribed with a pulse energy of 0.5 μJ and a scanning speed of5 mm/s. Further examination of local structure within the irradiatedmaterial and optical loss measurements of the inscribed waveguidesare required in order to reach a better understanding of the behaviorof borate glass matrix under fs-laser pulse exposition.

Acknowledgments

The authors are grateful to the Canadian Excellence Research Chairprogram (CERC) on Enabling Photonic Innovations for Informationand Communication, to the Natural Sciences and Engineering ResearchCouncil of Canada (NSERC), the Fonds Québecois de la Recherche sur laNature et les Technologies (FQRNT) and the Canada Foundation forInnovation (CFI) for the financial support. I. Chermiti is also acknowl-edged for her help in the glass sample preparation.

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