Second harmonic generation in poled tellurite glass

Journal of Non-Crystalline Solids 332 (2003) 207–218 www.elsevier.com/locate/jnoncrysol

Second harmonic generation in poled tellurite glass Brito Ferreira a, Evelyne Fargin a,*, Bertrand Guillaume a, Gilles Le Flem a, Vincent Rodriguez b, Michel Couzi b, Thierry Buffeteau b, Lionel Canioni c, Laurent Sarger c, Gilbert Martinelli d, Yves Quiquempois d, Hassina Zeghlache d, Laurent Carpentier e a

d

Institut de Chimie de la Mati
ere Condens ee de Bordeaux, UPR 9048, CNRS, Universit e Bordeaux I, Chat. Brivazac, Av.du dr. Schweitzer, 33608 Pessac cedex, France b Laboratoire de Physico-Chimie Mol eculaire, UMR 5803, CNRS, Universit e Bordeaux I, 33405 Talence cedex, France c Centre de Physique Mol eculaire et Hertzienne, Universit e de Bordeaux I, 33405 Talence cedex, France Laboratoire de Physique des Laser, Atomes et Mol ecules, UMR 8523, CNRS, Universit e des Sciences et Technologies de Lille I, 59655 Villeneuve d’Ascq, France e Laboratoire de Dynamique et Structures des Mat eriaux Mol eculaires, UPRESA 8024, Universit e des Sciences et Technologies de Lille I, 59655 Villeneuve d’Ascq, France Received 20 February 2003

Abstract New opportunities for photonic components result from the discovering of the second harmonic generation (SHG) in thermally poled glasses in the earliest 1990s. It has attracted a lot of interest because it offers new developments in the optical glassy material research. In this context a new tellurite glass with 70%TeO2 –25%Pb(PO3 )2 –5%Sb2 O3 composition has been developed and characterized. The structure of the unpoled glass has been studied by IR, Raman, and XAFS spectroscopy. Optical properties of transmission like the linear refractive index and third-order non-linear optical susceptibility vð3Þ as well as dielectric susceptibilities have also been investigated. A second harmonic signal was observed for the poled glass which is an order of magnitude better than the silica glass efficiency.
2003 Elsevier B.V. All rights reserved.

1. Introduction Non-linear optical properties of transparent oxide glasses are presently the focus of growing interest for elaboration of all-optical devices, such as photonic modulators, optical data storage and

*

Corresponding author. Tel.: +33-5 56 84 84 33; fax: +33-5 56 84 27 61. E-mail address: [email protected] (E. Fargin).

telecommunication, or for the spectral extension of laser sources. The recent discovery of second harmonic generation (SHG) in glasses after poling treatment (the glass being inserted between two electrodes and submitted to high dc field) and/or under high energy radiation opens new developments related to future functional integrated devices like electro-optic modulators. The choice of the glass composition is dictated by the necessity of increasing the non-linear optical efficiency but various other parameters must be considered e.g.

0022-3093/$ - see front matter
2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.09.015

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B. Ferreira et al. / Journal of Non-Crystalline Solids 332 (2003) 207–218

irradiation damage threshold, ability to device realization such as wave-guides or fiber etc. Basically, a polarization (~ P ) is induced in a medium by external oscillating electric fields ~ EðxÞ. This polarization can be expanded in a power series of the electric field: ~ P ¼ e0 vð1Þ~ EðxÞ þ e0 ðvð2Þ~ EðxÞ~ EðxÞ EðxÞ~ EðxÞ~ EðxÞ þ   Þ ¼ ~ P‘ þ ~ þ vð3Þ~ Pn‘ ; where e0 is the vacuum permittivity, vð1Þ is the linear susceptibility which accounts for the linear optical index, ~ P‘ the polarization term which is proportional to the applied field and ~ Pn‘ the nonlinear polarization. vð2Þ and vð3Þ correspond respectively to the second and third-order non-linear susceptibilities. For large intensities of an oscillating field ~ EðxÞ, the second term is responsible for the generation of a double frequency 2x oscillating field called second harmonic generation (SHG). However in centrosymmetric materials or amorphous materials with macroscopic inversion symmetry such as glass, all the components of the second-order susceptibility tensor vð2Þ are zero and second harmonic generation (SHG) is consequently forbidden. Nevertheless, SHG in bulk glasses has been induced by thermal or ultra-violet assisted poling treatment [1–4]. These techniques have been used to break the centrosymmetry. In particular, the thermal poling consists in applying a dc electric field below the transition glass temperature (Tg ) and cooling the glass before removing the dc bias. Thus, a poled glass is expected to exhibit SHG and is characterized by electro-optic coefficients which are involved in modulation or light switching. The mechanism underlying the formation of the non-linearity in bulk glass is not yet completely understood. According to Myers et al., a combination of two different mechanisms could be responsible for the induced second-order non-linear effect. They have proposed a model based on charge transport of mobile ions creating a depletion region in the anodic zone, followed by reorientation of dipoles. The permanent induced second-order linearity can be expressed as the sum of both effects [2]: vð2Þ / vð3Þ Edc þ ðNpb =5kT ÞEloc :

The first term is related to the interaction between the residual electric field Edc inside the material after removing the applied dc field and the thirdorder non-linearity. The second term is the resulting macroscopic second-order non-linearity induced by reorientations of polar bonds during the poling treatment, each of them with a permanent dipole moment p, N is the number of permanent dipole per volume, with a microscopic hyperpolarizability b corrected from a local field Eloc which is effectively seen by each dipole. The implementation of the field Edc has been related to the migration of mobile ions (in particular Naþ in the case of silica glasses) towards the cathode face of the poled material. An expended model including ion-exchange between a high mobility ion and a much lower mobility ion (related to Hþ ) driven by the high electric field has also been proposed [5]. The creation of a depletion zone is induced near the anodic face with a few microns depth as it has been experimentally proved mainly in silica glass but also in more exotic oxide glasses [6–8]. Poling of fused silica glass has been already largely reported [1,2,4,5,8–12]. In order to explore the occurrence of the previously given mechanisms in non-silica glass an investigation of the optical properties of a new tellurite glass with molar composition 70%TeO2 –25%Pb(PO3 )2 – 5%Sb2 O3 is reported. The choice of the composition took into account the expected high susceptibility vð3Þ for a lead-tellurite glasses, and the previously reported ability of terminal phosphate entities to reorientation under poling treatment [7,8].

2. Experimental 2.1. Preparation The investigated glasses are prepared from commercial reagent grade powders of a-TeO2 paratellurite (99.99%), PbO (99.99%), Sb2 O3 (99.999%), (NH4 )2 HPO4 (99.99%). Antimony oxide was introduced to stabilize the oxidation states of tellurium(IV) and lead(II). The finely ground appropriate mixture is introduced in alumina crucible to undergo a pre-heating treatment

B. Ferreira et al. / Journal of Non-Crystalline Solids 332 (2003) 207–218 2000 1800 1600 1400

T = 250°C t1/2 =6s

1200

I (nA )

at 400 C for water leaving and to form the lead phosphate. The mixture is then melted at 900 C in alumina crucible for about 30 min and the molten mixture is quickly quenched and annealed over night at 30 C below the glass transition temperature. The glass has been controlled to be homogeneous by electronic microscopy and the final composition obtained by X-microprobe in EDS analysis mode is given in Table 1. The obtained glass is colorless and transparent as was expected. All samples about 1mm thick are polished on both sides to eliminate light scattering in optical measurements. Samples can then be thermally poled by the use of a poling apparatus working under high vacuum (108 bar). The 1 mm thick glass is sandwiched between a Si wafer at the anode and a thin borosilicate plate at the cathode, then pressed on both sides by the stainless steel plate electrodes, the poling voltage being 4 kV and the temperature adjusted between 200 and 275 C. Once the field is applied for 1 h, the sample is cooled down to room temperature and the dc bias is then removed. The non-linearity of a poled sample can be erased by reheating it to 300 C without bias. The reproducibility of the poling treatment was simultaneously controlled by the measurement of the poling current which is transmitted through the sample (Fig. 1). For each poling temperature, the quality of the electric contacts can then be controlled to ensure the minimum current intensity is reached after 1 h of poling treatment. It was not possible to perform the thermal poling at 275 C because the glass became conductive at this temperature.

209

T = 225°C t1/2 = 10s

1000 800

T = 200°C

600

t1/2 = 12s

400 200 0 0

5

10

15

20

25

30

35

40

45

50

55

60

t (min)

Fig. 1. Transmitted current during poling time for different poling temperatures. The curves have been fitted with exponential decreasing functions and the half-time t1=2 has been estimated.

2.2. Characterization 2.2.1. Thermal properties and densities The glass transition (Tg1 ) and (Tg2 ) temperatures have been accurately measured (deviation ±5 C) using a DSC apparatus and are reported in Table 1. The heat rate was 600 C h1 in the 25–550 C range. Two glass transition temperatures are observed respectively at 380 and 450 C but the glass crystallization is detectable by annealing the glass during 1–2 h at temperatures higher than 450 C. The X-ray spectra were rather complex which excluded a complete identification of the crystalline phases. The density is measured by immersing the samples in calibrated diethylorthophthalate with accuracy better than 0.3% (Table 1).

Table 1 Physico-chemical characterizations of the studied glass Theoretical composition (at.%) ± 1% Measured composition (at.%) ± 2%

Te 15 14

Pb 5 4

P 11 7

Sb 2 3

Glass transition temperatures (C) ± 5 C Volumic weight (g/cm3 ) ± 0.02 g/cm3 Refractive index n0 0:001 (k ¼ 0:532 nm) Refractive index n0 0:03 (k ¼ 0:800 nm) Refractive index n0 0:001 (k ¼ 1:064 nm) Third-order susceptibility vð3Þ (m2 /V2 ) ± 10%

Tg1 ¼ 380 C, Tg2 ¼ 450 C q ¼ 5:28 n0 ¼ 1:986 n0 ¼ 1:97 n0 ¼ 1:957 ð3Þ vð3Þ ¼ 2:44 1021 (vsilice infrasil ¼ 0:11 1021 m2 /V2 )

Al 0.5

FT Amplitude (a.u.)

B. Ferreira et al. / Journal of Non-Crystalline Solids 332 (2003) 207–218

FT Amplitude (a.u.)

210

Te K-edge

0

1

2

3

4

5

6

Pb LIII-edge

0

1

2

R (Å)

3

4

5

6

R (Å)

Fig. 2. Uncorrected for phase shift Fourier transforms. The first peak due to oxygen atoms first neighbors has been filtered from 1 to  for Te K-edge, and from 1.25 to 2 A  for Pb LIII-edge. 1.7 A

2.2.2. EXAFS Extended X-ray absorption fine structure (EXAFS) measurements were carried out at the K-edge absorption of tellurium and LIII-edge of lead. The spectra were registered at LURE (Orsay, France) on the beam station D44. The DCI storage ring was operating at 1.85 GeV, with 290 mA positron injection current. An Ge[4 0 0] (resp. Si[1 1 1]) double crystal monochromator was used to collect Te K-edge (resp. Pb LIII-edge) data in transmission detection mode. Energy calibration was carried out by the use of a stain (resp. Ti) foil, ensuring an experimental accuracy in energy <2 eV. EXAFS modulations were analyzed by using standard methods [13,14]. Their Fourier transforms (FT) yielding radial structure functions (uncorrected for phase shift) consisting of peaks representing the different coordination shells are given in Fig. 2. The first peak corresponding to oxygen neighbors for both tellurium and lead has been filtered and Fourier backtransformed (FT1 ), giving the corresponding first oxygen shell EXAFS

signal. It can be simulated by using the calculated Mac-Kale parameters, the number Ni of oxygen atoms at a distance Ri from the absorbing atom, the r2i Debye–Waller coefficient related to thermal and static disorder, remaining free to improve the fitting. The results of the best simulations are given in Table 2. 2.2.3. IR and Raman measurements Reflectance IR spectra were recorded on a spectrophotometer equipped with a DTGS detector and a germanium-coated KBr beamsplitter. A total of 100 scans were averaged at 4 cm1 resolution. The spectrometer was purged with dry air to minimize atmospheric CO2 and water vapor. Reflectance experiments were performed using an external reflection attachment at an angle of incidence of 12. A computer program has been used to calculate the optical constants (real and imaginary parts of the dielectric constant e) from the reflectance spectra by the Kramers–Kr€ onig analysis. Then, the maxima of the mImðeÞ spectra give

Table 2 EXAFS best simulations results for the first coordination shell around tellurium and lead Debye–Waller factors, r2 2 ) (±104 A 2 ) (103 A

Glass of composition 70%TeO2 – 25%Pb(PO3 )2 –5%Sb2 O3

Number of oxygen neighbors, N

Distances, ) R (A

Edge energy (eV) (±1 eV)

Te K-edge

2 (fixed) 1 (±50%)

1.88 2.01

33 848

3.6 2.5

Pb LIII-edge

2 (fixed) 1 (±50%)

2.30 2.44

13 044

10 8.1

B. Ferreira et al. / Journal of Non-Crystalline Solids 332 (2003) 207–218 80

70

60

Raman Intensity (a.u.)

directly the frequency of the absorption bands. The both unpoled glass sample surfaces were analyzed before a poling treatment (Fig. 3(a)). Then, both surfaces were immediately analyzed again after control of SHG signal in the poled glass. Structural modifications after poling have been examined by comparing the poled anode or cathode face reflectance signal to the corresponding unpoled face signal (Fig. 3(b)). The Raman spectra were recorded with a Labram confocal micro-Raman instrument (typical resolution of 2 cm1 ), in backscattering geometry at room temperature. The spectrophotometer includes a holographic Notch filter for Rayleigh rejection, a microscope equipped with 100 objectives, and a CDD detector. The 514.5 nm emission line of an argon ion laser was used for excitation. As for FTIR measurements both unpoled and poled surfaces of the same glass sample were analyzed. Fig. 4 shows the Raman spectrum which corresponds to the direct Raman spectrum corrected from the Bose–Einstein population factor [15].

211

50

40

30

20

10

0 200

400

600

800

1000

1200

1400

Wavenumber (cm-1) Fig. 4. Raman spectrum for the unpoled glass. The main components are referenced in Table 4.

Fig. 3. Infra-red reflectance spectra in the 500–1600 cm1 region: (a) for the unpoled glass where the phosphate bands have been deconvoluted in accordance with A.M. Efimov et al. (see Table 4); (b) differential spectra between the unpoled and then poled cathodic and anodic faces compared to the original spectrum on both unpoled faces (straight line).

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Fig. 5. Real and imaginary parts of the measured dielectric constants for different temperatures.

2.2.4. Dielectric susceptibility measurements When a continuous electric field is applied to a material, in case of blocking not injecting electrodes, the free carriers built up on the opposite sign electrode. They constitute a space charge and leave, on other side, not compensated donors forming also a space charge. After a sufficient polarization duration, a balance is reached, and it appears two charge densities of opposite signs but of the same value on the electrodes–sample interfaces. The sample is then comparable to a macroscopic dipole. If the electric field direction is reversed, one observe a space charge reorganization, which tends towards the symmetrical distribution to the preceding case. If, instead of applying a continuous electric field, one applies alternate electric field, all internal sample characteristics such as electric field Eðx; tÞ, potential V ðx; tÞ and carrier density nðx; tÞ will oscillate with the applied field pulsation. It is then possible to deduce the dielectric permittivity versus the applied electric field pulsation corresponding to the macroscopic dipole relaxation [16,17]. The ac dielectric susceptibilities were measured by the use of a dielectric analyzer whose frequencies range from 103 to 105 Hz. with a peak to peak amplitude of 1 V. The sample was electroded using two ceramic plates sensors, with gold circular electrodes. During the experiments a constant compressive force was applied to ensure good contact between the sample and the sensors and a dried airflow pro-

tected the dielectric cell against room atmosphere. The real e0 and imaginary e00 parts of the dielectric susceptibilities versus frequency for different temperatures are reported on Fig. 5(a) and (b). 2.3. Optical measurements 2.3.1. Absorption The UV–VIS–NIR absorption spectrum was recorded between 300 and 2000 nm at room temperature using a spectrophotometer and was normalized to a 1 mm thickness (Fig. 6). 2.3.2. Third-order non-linearity The measurements of third-order non-linear optical susceptibility vð3Þ were obtained using a

Fig. 6. Transmission spectrum of the studied glass.

B. Ferreira et al. / Journal of Non-Crystalline Solids 332 (2003) 207–218

transient absorption experiment described elsewhere [18] (Table 1). The laser source was a mode locked titanium sapphire which delivered linearly polarized pulses of 100 fs around 800 nm with a peak power of 100 kW. In this pump probe experiment, the two beams which are orthogonally polarized are focused on the sample. The probe intensity is affected by the pump induced non-linearity of the material. Analysis of the signal intensity as a function of the delay between the pump and the probe allows one to characterize completely the third-order non-linear optical susceptibility. The accuracy of all laser parameters limits the absolute precision of our measurements of about 10%. 2.3.3. Second-order non-linearity and refractive indices The source was a Q-switched Nd/YAG laser operating at wavelength 1064 nm. The pulse width and the repetition rate of output pulses from the laser were 200 ns and 100 Hz, respectively. The polarized source beam was split into two branches by a beam splitter. One branch recorded the fundamental intensity with a first photomultiplier; the other, which passed through a polarizer to adjust its polarization, was focused on the sample with a spot of 100 lm diameter. The 2x transmitted signal was detected by a second photomultiplier and averaged over 25 pulses. The pulse energies at the sample were no more than 200 lJ for the infrared beam. The absolute calibration of the SHG intensities is obtained using a quartz z-cut taking v11 ¼ 0:6 pm/V as reference [19]. This versatile experimental setup allows linear refractive index to be estimated at x or 2x with a third photomultiplier by collecting the corresponding 2-theta reflected wave around the Brewster angle over the 15–80 theta range. A LiNbO3 crystal in phase matching condition was used as a 532 nm source to record reflection signal. The SHG maker fringes were analyzed using a new general ellipsometrical analysis for planar multilayered strata [20]. The whole Maker fringe signal (in pp and sp polarization configurations, where pp (resp. sp) means p-polarized (resp. spolarized) incident pump beam and p-polarized transmitted second harmonic beam) was recorded

213

for the studied glass alone, and then sandwiched between two hemicylindrical lenses to increase the complete scanning angle range inside the material [21]. The analysis has been performed for each configuration. The previous determination of the linear refractive indices with precision (Table 1) allows one to estimate the thickness of the nonlinear zone with very good accuracy. Fig. 7 shows experimental and simulated Maker fringes for different poling temperatures. The only non-linear coefficients which are non-zero are d33 ¼ v33 =2 and d31 ¼ v31 =2. The estimated d31 and d31 =d33 and optically active zone depths deduced from the best simulations are given in Table 3 for the different poling temperatures. The value of the ratio d31 =d33 when close to 1/3 supposes both migration and reorientation mechanisms without discrimination. But according to Singer et al., the generation of a smaller ratio is due to the nonnegligible reorientation mechanism involvement [22].

3. Results 3.1. Structural characterization of the glass Several structural investigations by IR or Raman spectroscopy of tellurite glasses have been reported, in particular, for the glasses of the TeO2 – P2 O5 and TeO2 –PbO systems [23–26]. IR reflectance spectra (Fig. 3(a)) shows two large bands. Between 500 and 850 cm1 , the observed bands are due to Te–O bonds vibrations in TeO4 sites around 655 cm1 and TeO3þ1 around 755 cm1 [25,26]. It was possible to decompose the phosphate bands with respect to previous work on zinc phosphate glasses [27] (Fig. 3(a)). The absorption bands due to P–O bond vibrations in [PO4 ]3 , [PO3 ]2 , [PO2 ] , and [P–O–P] sites are observed between 850 and 1300 cm1 . The relatively low absorption component around 1200 cm1 which is usually attributed to the asymmetric stretch of (PO2 ) group allows one to suppose reduced phosphate polymeric chains or metaphosphate rings in this glass [28]. The Raman spectrum (Fig. 4) shows two broad bands in the same energy ranges than IR spectra

214

B. Ferreira et al. / Journal of Non-Crystalline Solids 332 (2003) 207–218 . . . . . . . . . . . . .

Fig. 7. pp and respectively sp polarization spectra (dashed lines) and simulations (continuous lines) for the glass alone (a respectively b); pp and respectively sp polarization spectra (dashed lines) and simulations (continuous lines) with the use of lenses (c respectively d), for the 250 C poling temperature.

Table 3 Second harmonic generation results Poling temperature 200 C

225 C

250 C

Duration (min) Voltage DV (kV)

60 4

60 4

60 4

L (lm) (±10%) d33 (pm/V) (±0.01) d33 L (106 pm2 /V) d31 =d33 (±10%)

5 0.20 1 0.33

7 0.29 2. 0.32

27 0.33 0.9 0.35

DV =L (108 V/m) ð3Þ 2d33 =3v3333 (108 V/m)

8 0.53

5.7 0.8

1.5 0.9

ferent components deduced from IR and Raman spectra are summarized in Table 4. EXAFS oscillations simulations (Table 2) evidence a first coordination shell around tellurium  disconstituted of two oxygen atoms at a 1.88 A  tance and one oxygen atom at 2.01 A. This is in accordance with the usual local structure observed in tellurite glasses, therefore mixed TeO4 and TeO3þ1 oxygenated sites [29]. The first coordination shell of lead atoms consists of oxygen atoms . at distances equal to 2.30 and 2.44 A 3.2. Dielectric characterization of the glass

corresponding to phosphate and tellurite groups vibrations, respectively. An additional strong band around 450 cm1 can be attributed to the symmetric stretching vibrations of Te–O–Te linkages between TeO3þ1 or TeO4 polyhedra [25]. The dif-

The real e0 and imaginary e00 parts of the dielectric susceptibilities versus frequencies for different temperatures are reported on log–log scale in Fig. 5(a) and (b). The main feature is a maximum of e which shifts to higher temperatures as

B. Ferreira et al. / Journal of Non-Crystalline Solids 332 (2003) 207–218

215

Table 4 Main frequency vibration modes (in cm1 ) in the glass of composition 70%TeO2 –25%Pb(PO3 )2 –5%Sb2 O3 deduced from: a simulation of IR spectrum with gaussian functions, b qualitative examination of Raman spectrum (for information: c IR band assignments published by Efimov et al. [14]) Structural entities

Frequency range (cm1 ) Asymmetric stretch mas

3

[PO4 ] [PO3 ]2 [PO2 ] P–O–P Te–Oeq (in TeO4 and TeO3þ1 groups) Te–O (in TeO3þ1 groups) Te–O (in continuous network TeOn ) d(O–Te–O) (in TeO3 groups)

a

1004 ± 77 1080 ± 112a 1160 ± 100a 901 ± 40a 936 ± 50a 756 ± 71a

the frequency is increased and a flattening of eðT Þ for the highest frequencies. This is typical of a thermally activated Debye relaxation in the frequency space. The subsequent monotonous increase of e at higher temperatures is due to the conductivity increase of the sample. It has been possible to simulate the relaxation time s and the conductivity r by simple mathematical laws of Arrhenius type (Fig. 8). For both the activation energies are identical which is generally the signature of space–charge relaxation model.

Symmetric stretch ms c

1002 1098c 1164c 1202c

900–950b 1000–1100b 1000–1200b 690–790b

900–940c 650b 655 ± 92a 400–500b 200–300b

fringes method [20]. The second-order susceptibility is localized within a thin glass layer located close to the anodic surface. The value of d33 ¼ 0:2– 0.33 pm/V depending on the poling temperature, is about one order of magnitude higher than that obtained for fused Herasil silica glass with the same poling conditions. For a constant duration fixed to 1 h, the depth of the optically active zone is continuously increasing with the temperature. Nevertheless, the SH coefficient d33 is at least one order of magnitude smaller than those recently announced for other tellurite glass systems [7].

3.3. Second harmonic generation 3.4. Structural study of poled glasses Optically induced SHG has been observed after poling treatment and the second harmonic (SH) coefficient d33 was measured using the Maker

The evolution of the IR reflectance spectra obtained before and after poling treatment can be

Fig. 8. Arrhenius laws deduced from dielectric measurements for the conductivity and the relaxation time of the studied glass.

216

B. Ferreira et al. / Journal of Non-Crystalline Solids 332 (2003) 207–218

correlated to an evolution of the glass structure near the anodic surface. In Fig. 3(b) we can observe noticeable differences between the spectra on the anodic surface, which is not the case on the cathodic surface. These differences are observed and significative within the phosphate bands, the signal versus noise ratio being defavorable within tellurite bands.

4. Discussion The quantitative EXAFS results joined to qualitative deductions from IR and Raman spectra will be worth to be compared with some reference crystals structural data (Table 5) [29–33]. At first view the glass structure can be regarded as a tellurite more or less ramified network including isolated phosphates mono [PO4 ]3 or dimer [P2 O7 ]4 groups. The tellurite network would be constituted of distorted bipyramidal TeO4 entities connecting together by one common oxygen atom to form chains. The presence of TeO3þ1 where one of the two axial Te–O bonds is elongating while the other one is shortening, and TeO3 pyramidal entities are the signature of breaking off in the

network. Such arrangement is consistent with the structure of the lead Pb2 Te3 O8 and PbTe5 O11 tellurite crystal [31,32]. Both phases contain the three basic oxygenated tellurium sites: the trigonal bipyramide TeO4 , the pyramidal TeO3 , and the intermediate TeO3þ1 polyhedron. The Te–O 2.01  distances deduced from the EXAFS and 1.88 A data are close to those found in these crystals . Such which are included between 1.85 and 2.17 A structural entities witness of a strong stereoactivity of the free 5s2 tellurium doublet which exhibits a large third-order hyperpolarizability. Some additional information about the local structure around lead atoms can be complemented by EXAFS. From a general point of view the crystal chemistry of Pb2þ in oxides is dominated by two main behaviors: a possible stereoactivity of the 6s2 free doublet in addition to a versatile coordination involving a large range of Pb–O distances, or the lone pair is engaged in bonds which are formed with oxygen atoms and is then inactive. For example, in Na2 PbP2 O7 , the structure of which consists of a tridimensional framework of [Pb2 P4 O14 ]4 entities obtained by the association of corner-shared PbO5 and diphosphate [P2 O7 ]4 groups, the lone pair is active. The Pb–O distances

Table 5 Structural elements from different crystals containing tellurium oxides or lead or phosphates Crystal

Variety

Space group

Crystalline parameters

Shortest distances Pb–O or Te–O

TeO2

a

P41 21 2

 a ¼ b ¼ 4:810 A  c ¼ 7:612 A

 (·2) d(Te–Oeq ) ¼ 1.88 A  (·2) d(Te–Oax ) ¼ 2.12 A

PbTeO3

b

P41

 a ¼ b ¼ 5:304 A  c ¼ 11:900 A

 d(Pb–O) ¼ 2.27, 2.45, 2.49, 2.69, 2.60 (·2) A  d(Te–O) ¼ 1.84, 1.85, 1.89 A

Pb(PO3 )2

P21 /c

 a ¼ 7:29 A  b ¼ 7:95 A  c ¼ 17:28 A

 d(Pb–O) ¼ 2.33, 2.38, 2.43, 2.52, 2.69, 2.76 A

Na2 PbP2 O7 PbTe5 O11

) d(Pb–O) ¼ 7 · (2.43–2.79 A

C2/c

 a ¼ 18:959 A  b ¼ 4:414 A  c ¼ 25:798 A b ¼ 98:12

, 2.51 A , d(Pb–O) ¼ 2.45 A –2.88 A ), and 3.08 A  5 · (2.63 A  Diphenoidal TeO4 :2d(Te–O) from 1.88 to 1.94 A  and 2 d(Te–O) from 2.08 to 2.44 A  Polyhedral TeO3þ1 : 3 d (Te–O) from 1.82 to 1.96 A  and 1 d(Te–O) from 2.41 to 2.44 A

B. Ferreira et al. / Journal of Non-Crystalline Solids 332 (2003) 207–218

 for a coordiare ranged between 2.32 and 2.76 A nation number of 5 [34]. The structure of Pb(PO3 )2 exhibits two different lead sites with coordination numbers of 7 and Pb–O distances included between  for [Pb(1)] and 2.43 and 2.79 A  for 2.51 and 2.71 A [Pb(2)] respectively [35]. In both sites no effect of the lone pair can be detected. In PbTe5 O11 , the lead coordination number is 8 and the Pb–O distances  with no noare included between 2.47 and 3.03 A ticeable lone pair effect [31,32]. Intermediate situations with the lone pair more or less engaged are observed in crystals PbTeO3 and Pb2 Te3 O8 where the coordination number is 6 and distances ranged  and 2.37 and 2.93 A  rebetween 2.27 and 2.69 A spectively [32,33]. EXAFS simulation allows only the estimation of the shortest Pb–O distances and does not give any information on the coordination number. From the previous examples the lead oxygen distances which have been evidenced by EXAFS are in agreement with the general range of distances observed in crystals. To conclude, lead atoms are localized in more or less distorted oxygenated sites like in crystallized lead–tellurium oxides previously described or Na2 PbP2 O7 . They are supposed to connect the tellurite network to the phosphate groups, though IR and Raman spectra cannot confirm this hypothesis within the studied energy ranges. The magnitude of the non-linear coefficient vð3Þ is due to the hyperpolarizable tellurium or lead oxygenated groups. The relatively small value of this coefficient when compared to other tellurite glasses with similar proportion of tellurium oxide seems to be an indication of a mean small availability and then hyperpolarizability of the 6s2 doublet on lead atoms. Concerning the induced mechanisms during the poling treatment, dielectric measurements allow one to suppose a charge movement of a macroscopic nature in the material under the applied field, so the overall migration of sodium ions existing as impurities in the glass. On the other side, IR and Raman spectroscopies show differences between anodic poled and unpoled regions signals. This effect can be explained by a globally decreasing oscillators strength – related to the phosphate groups – due to the remaining internal field in the anodic region, and/or the reorientation of the dipolar moments during the poling treatment.

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Complementary polarization modulation-infrared reflection absorption spectroscopy (PMIRAS) measurements would be able to remove the ambiguity. Finally, the depth L of the active zone is continuously increasing with temperature. Assuming a mean field Edc ¼ DV =L in the non-linear anodic zone for full charge screening, the comparison of the experimental data with the calculated values ð3Þ vð2Þ =vð3Þ which is 2d33 =3v3333 for our experimental configuration can be made. They would be identical if a unique charge migration model is involved. But we observe for all the temperatures, ð3Þ DV =L is larger than 2d33 =3v3333 (Table 3). We can consequently suppose that the bias has not been totally screened during poling. This could be improved by the modification of the poling procedure, for example alternate poling [36]. On the other side it is not possible to conclude to the involvement of structural reorientations in the nonlinear response. 5. Conclusions A new composition tellurite glass has been poled and shows better SHG performances than silica glass. The poling treatment under high vacuum is supposed to prevent the anodic injection of charged species like Hþ . The transmitted current control allows one to check the electric contacts quality and the efficiency of the poling time duration. Dielectric measurements demonstrate a space charge relaxation due to the macroscopic migration of charged impurities like Naþ . Such a migration is implied in the creation of a few microns depth non-linear depletion zone, but the complete screening of the field is not still obtained. On the other side structural modifications have been evidenced in the poled region localized near the anode, though such reorganizations cannot be clearly involved in the generated non-linear response of the poled glasses. Acknowledgements To Corning France Society and the EEC via the GLAMOROUS contract (IST-2000-28366) for

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financial support, LURE for EXAFS measurements.

References [1] R.A. Myers, N. Mukherjee, S.R.J. Brueck, Opt. Lett. 16 (1991) 1732. [2] N. Mukherjee, R.A. Myers, S.R.J. Brueck, JOSA B 11 (1994) 665. [3] T. Sekiya, N. Mochida, A. Ohtsuka, M. Tonokawa, J. Non-Cryst. Solids 144 (1992) 128. [4] T. Fujiwara, M. Takahashi, A.J. Ikushima, Appl. Phys. Lett. 71 (8) (1997) 1032. [5] T.G. Alley, S.R.J. Brueck, R.A. Myers, J. Non-Cryst. Solids 242 (1998) 165. [6] V. Nazabal, E. Fargin, J.J. Videau, G. Le Flem, A. Le Calvez, S. Montant, E. Freysz, A. Ducasse, M. Couzi, J. Solid State Chem. 133 (1997) 529. [7] K. Tanaka et al., Opt. Lett. 25 (2000) 251. [8] V. Nazabal, B. Ferreira, E. Fargin, G. Le Flem, V. Rodriguez, M. Couzi, T. Buffeteau, B. Desbats, J. NonCryst. Solids 290 (2001) 73. [9] H. Takebe, P.J. Kazanski, St.J. Russell, K. Morinaga, Opt. Lett. 21 (1995) 468. [10] I. Hiroaki, S. Horinouchi, N. Asakuma, K. Fukao, J. Appl. Phys. 84 (1998) 5415. [11] V. Pruneri, F. Samoggia, G. Bonfrate, P.G. Kazanski, G.M. Yang, Appl. Phys. Lett. 74 (1999) 2423. [12] Y. Quiquempois, G. Martinelli, P. Dutherage, P. Bernage, P. Niay, M. Douay, Opt. Commun. 176 (2000) 479. [13] We used the EXAFS data analysis set of programs implemented by A. Michalowicz, EXAFS pour le Mac, in: Societe francßaise de chimie (Eds.), Logiciels pour la Chimie, Paris, 1991, p. 102. [14] G. McKale, B.W. Veal, A.P. Paulikas, S.-K. Chan, G.S. Knapp, J. Am. Chem. Soc. 110 (1998) 3763.

[15] F.L. Galeener, A.J. Leadbetter, M.W. Stringfellow, Phys. Rev. B 27 (1983) 1052. [16] D.W. Shin, M. Tomozawa, J. Non-Cryst. Solids 211 (1997) 237. [17] D.W. Shin, M. Tomozawa, Phys. Chem. Glasses 42 (3) (2001) 199. [18] M.O. Martin, L. Canioni, L. Sarger, Opt. Lett. 23 (24) (1998). [19] V.G. Dmitriev, G.G. Gurzadyan, D.N. Nikogosyan (Eds.), Handbook of Nonlinear Optical Crystals, Springer, Berlin, 1997. [20] V. Rodriguez, C. Sourisseau, J. Opt. Soc. Am. B 19 (2002) 2650. [21] D. Pureur, A.C. Liu, M.J.F. Digonnet, G.S. Kino, Opt. Lett. 23 (1998) 588. [22] K.D. Singer, M.G. Kuzyck, J.E. John, J. Opt. Soc. Am. B: Opt. Phys. 4 (1987) 968. [23] J.C. McLaughlin, S.L. Tagg, J.W. Zwanziger, D.R. Haeffner, J. Non-Cryst. Solids 274 (2000) 1. [24] A. Berthereau, E. Fargin, Villesuzanne, J. Solid State Chem. 126 (1996) 143. [25] N. Mochida, T. Sekya, A. Ohtsuka, M. Tonokawa, Nippon Seram. Kyo. Gak. Ronbushi 96 (1988) 973. [26] M.A.P. Silva, Y. Messaddeq, S.J.L. Ribeiro, M. Poulain, J. Phys. Chem. Solids 62 (2001) 1055. [27] A.M. Efimov, J. Non-Cryst. Solids 209 (1997) 209. [28] A.M. Efimov, J. Non-Cryst. Solids 232–234 (1998) 433. [29] A. Bystroem, Mineral Geol. 20 (1945) 1. [30] P. Sciau, J. Lapasset, J. Moret, Acta Crystallogr. C 17 (1964) 1539. [31] J.C. Champarnaud-Mesjard, P. Thomas, M. Colas-Dutreilh, A. Oufkir, Z. Kristallogr. 216 (2001) 185. [32] Maggy Dutreilh-Colas, PhD, Limoges University, France, November 2001. [33] P.A. Thomas, J. Phys. C 21 (1988) 4611. [34] N. Dridi, Solid State Ionics 127 (2000) 141. [35] K.H. Jost, Acta Crystallogr. 17 (1964) 1539. [36] G. Martinelli, Y. Quiquempois, A. Kudliski, H. Zeghlache, Electron. Lett. 38 (2002) 570.