Second harmonic generation of thermally poled tungsten tellurite glass

Optical Materials 31 (2009) 775–780

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Second harmonic generation of thermally poled tungsten tellurite glass C. Lasbrugnas a, P. Thomas a,*, O. Masson a, J.C. Champarnaud-Mesjard a, E. Fargin b, V. Rodriguez c, M. Lahaye b a

Science des Procédés Céramiques et Traitements de Surface, UMR 6638 CNRS, Université de Limoges, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France Institut de Chimie de la Matière Condensée de Bordeaux, UPR 9048 CNRS, 87 Avenue du Dr Albert Schweitzer, 33608 Pessac Cedex, France c Institut des Sciences Moléculaires, UMR 5255 CNRS, Université de Bordeaux I, 351 cours de la Libération, 33405 Talence Cedex, France b

a r t i c l e

i n f o

Article history: Received 22 October 2007 Received in revised form 9 July 2008 Accepted 11 August 2008 Available online 30 September 2008 PACS: 42.65.K 42.70.C Keywords: Tellurite glasses Nonlinear optics Second harmonic generation Thermal poling

a b s t r a c t Second harmonic signals have been successfully generated for thermally poled glasses with 85%TeO2– 15%WO3 molar composition. Thermal poling was undertaken at various temperatures and heating times (voltage: 4.5 kV, sample thickness: 500 lm). The second harmonic generation (SHG) of the poled glasses was analysed using the Maker fringes technique. After optimisation of the poling conditions (T = 280 °C, t = 1 h, 2 h), high v(2) values, up to 1.5 pm/V, were obtained. It was demonstrated that thermal poling has generated optically nonlinear zones at the anode side. The thickness of the SHG active layer was thin, lower than 20 lm. Two complementary hypothesis have been proposed to explain the origin of the second-order nonlinearity property of this tellurite glass: (i) a reorientation of the TeO4 glass structural entities under electric field and (ii) the formation of an anodic depletion region of sodium ions. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Since a few years, tellurium oxide-based glasses have been considered to be promising materials for a broad range of optical applications and so have attracted much attention due to their large glassy domains, their low melting temperatures, their transparency window from the visible to the infrared region out to 7 lm, their weak absorption coefficients and especially their high linear (n0) and nonlinear (n2, v3) indices [1]. The nonlinear optics consists in the study of phenomena induced by a modification of the optical properties of a dielectric material under an intense beam. Basically, the nonlinear optical properties of transparent materials result from the generation of a polarization P when an intense electromagnetic wave enters into these materials under an electric field E. This polarization P can be expressed as a Taylor’s series expansion of the electric field E

P ¼ P0 þ ðe0 vð1Þ  E þ vð2Þ  E  E þ vð3Þ  E  E  E þ   Þ

ð1Þ

(1)

where P0 is the permanent polarization, v is the linear susceptibility which accounts for the linear optical index. v(2) and v(3) correspond to the second and third-order nonlinear susceptibilities,

* Corresponding author. Tel.: +33 0 5 55 45 74 96; fax: +33 0 5 55 45 72 70. E-mail address: [email protected] (P. Thomas). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.08.002

respectively. For large intensities of an oscillating field E with frequency x, the second term is responsible for the generation of a double frequency 2x oscillating field, called second harmonic generation (SHG). The latest can appear only in a non-centrosymmetric material for which the second-order nonlinear susceptibility v(2) is different to zero. Amorphous materials, such as glass, with inversion symmetry in a macroscopic scale does not generate in principle second-order optical nonlinearity. Nevertheless, it is now well known that it is possible to break inversion symmetry in a microscopic scale through techniques such as thermal or all-optical poling and so to induce second-order nonlinearity. The thermal poling consists in applying a DC electric field under temperature, below the glass transition temperature Tg, and cooling the glass before removing the voltage [2]. All-optical poling is processed by exposition of the glass to intense laser-light [3]. Since about ten years, it has been demonstrated that thermal poling could generate second-order nonlinearity in various TeO2based glasses within the following systems: TeO2–ZnO [4–6], TeO2–WO3 [4,7], TeO2–B2O3 [8], TeO2–Nb2O5 [9,10], TeO2–Nb2O5– Li2O [11], TeO2–Na2O–Li2O [12], TeO2–ZnO–MgO [8,13], TeO2– ZnO–Na2O [14] and TeO2–Pb(PO3)2–Sb2O3 [15]. The second-order nonlinear susceptibilities (v(2)) have been measured in a range from 0.1 pm/V (TeO2–ZnO–MgO glasses) [8] to 0.9 pm/V (TeO2– ZnO glasses) [6], excepted for a 80TeO2–20WO3 (mol%) glassy sample for which a very high v(2) of 2.1 pm/V was obtained [7].

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Recently, by all-optical poling, high v(2) have also been measured in TeO2–ZnO, TeO2–Tl2O [3] and TeO2–GeO2–PbO [16] systems, but the corresponding values were not comparable with those obtained by thermal poling. Up to now, the most reliable models to explain the origin of SHG in tellurite glasses have been based on either the migration of charge impurities, e.g. Na+, towards the cathode side face of poled sample [6,9,10,17] or the orientation, under the electric field, of the asymmetric structural units [TeOn] (n varying from n = 3 or 4 with respect to the composition of glasses) of glassy samples which possess electric dipoles [4,5,8,13,14]. In this paper we report on the SHG induced by thermal poling in a 85TeO2–15WO3 (mol%) glass. The choice of this composition takes the following points into account: it presents a good thermal stability, a high mechanical strength and has previously exhibited relatively high third-order nonlinear susceptibility v(3) [18,19]. Moreover, this composition is of interest since it allows to obtain by heating the crystallisation of the non-centrosymmetric metastable c-TeO2 [19,20] phase. This compound presents a second harmonic signal 70 times higher than that of a-quartz which corresponds to 6% of the signal of the LiNbO3 frequency doubling crystal [21]. This c-TeO2 crystallisation process can allow the elaboration of glass-ceramics with SHG properties. The purpose of the present study was investigations on SHG of 85TeO2–15WO3 (mol%) glass, via an optimisation of the thermal poling conditions. In order to elucidate the mechanism at the origin of the second harmonic generation in such tellurite glass, structural and chemical study of the unpoled and poled glasses was performed using Raman scattering and energy dispersive X-ray spectroscopy (EDS) coupled with a scanning electron microscope (SEM).

2. Experimental 2.1. Glass preparation Glasses with 85TeO2–15WO3 (mol%) composition were obtained from appropriate quantities of reagent grade TeO2 and WO3 (commercial product Interchim, +99%) as raw materials. TeO2 was prepared in the laboratory by decomposition at 550 °C of commercial H6TeO6 (Aldrich, 99.9%). A very weak amount of Sb2O3 (2 mol%) was added in order to lighten glasses. Glassy pellets were prepared by first melting the powder mixtures in platinum crucibles, at 800 °C for 30 min. The melts were then quickly quenched by flattening between two preheated brass blocks separated by a brass ring to obtain cylindrical samples 10 mm wide and 1.5 mm thick. Samples were then annealed at 300 °C for 10 h (about 40 °C below their glass transition temperature: Tg = 344 °C) to release the thermal stresses in the as-quenched glasses. The pellets were then mechanically polished to obtain plane disks (500 lm thick) with parallel faces suitable for optical measurements. The obtained glasses were pale yellow and transparent as well expected. Their homogeneity was controlled by optical microscopy. 2.2. Thermal poling Glassy disks were thermally poled by using a home-made apparatus. The 500 lm thick glass was sandwiched between two thin commercial borosilicate plates and kept in contact with stainless steel electrodes equipped with a heating stage. This equipment was placed inside a vacuum chamber and the operating pressure was about 105 Torr. Different poling conditions were investigated. The glassy samples were heated up to temperatures ranging from 250 °C to 310 °C, about 40–90 °C lower than the glass transition

temperature Tg (344 °C). After stabilization of the temperature, a DC poling voltage was applied under different conditions: voltage from 3 kV to 7 kV and poling time from 20 min to 8 h. The glasses were then cooled slowly to room temperature keeping the voltage on under vacuum. The voltage was switch off once the samples reached the room temperature.

2.3. Optical measurements The optical transmission spectra of the unpoled and poled glasses were recorded at room temperature using a double beam spectrophotometer (CARY 5000 UV–Vis–Near IR) in the wavelength range from 300 nm to 1600 nm. The SHG intensity of poled glasses was measured using the Maker fringes method. The whole Maker fringe signal is recorded for two polarization configurations: pp and sp polarizations, where pp and sp mean linearly polarized incident beam with a polarization parallel to the incidence plane (p) or with a polarization perpendicular to the incidence plane (s). More particularly, this method consists of varying the angle between the x incident laser beam and the glass sample containing at least one nonlinear optical planar layer. The intensity of the transmitted and generated 2x optical signal is then recorded by oscillating in a periodic fashion, according to the simplified formula

Ið2xÞ ¼

 2 128p2 L ð2Þ 2 sinðDkL=2Þ 2 ð x Þ½d  I eff nð2xÞn2 ðxÞCk ðDkL=2Þ

ð2Þ

n(x) and n(2x) correspond to the refractive indexes for the wavelength k = 2pc/x and k/2; C is the speed of light in vacuum; L represents the thickness of the generated nonlinear zone within the sample; I(x) is the intensity of the fundamental wave; deff symbolizes the second-order nonlinear coefficient depending on pp or sp polarization configuration; Dk = |k(2x)2k(x)| corresponds to the difference between the fundamental and the harmonic wave vectors and is connected to the coherence length of the material; sin(DkL/2)/(DkL/2) represents the oscillating term. The Maker fringes patterns have been fitted using a generalized SHG transfer matrix optical model for planar multilayered system [22]. Only two nonlinear parameters d33 and d31 are non-zero in the poled layer. The d31 has been constrained to d33/3. Two remaining parameter have then been fitted to simulate the Maker fringes pattern: the second-order nonlinear coefficients d33 and the thickness L of the nonlinear zone. It is worth noting that the nonlinear coefficient d33 is directly linked to the second-order susceptibility v(2) by the following relation: d33 = ½ v(2) [23]. The experimental set-up for SHG measurements is shown in Fig. 1. The light source was a Q-switched Nd/YAG laser operating at 1064 nm wavelength (repetition rate of 30 Hz, pulse width of 20 ns). A beam attenuator placed just after the laser source allowed adjustment of the pulse energy at the sample. The polarized source beam was split into two rays by a beam splitter: one rays recorded the fundamental (x) intensity with a near-infrared diode detector; the other, which was firstly polarized by a Glan polarizer, passed through a half-wave (1064 nm) rotation plate and a set of appropriate filters and then was focused on the sample with a spot of 80 lm diameter. The harmonic (2x) transmitted signal was collected in the 80° to +80° h range, onto a first detection unit consisting of a 1064 nm selective reflector, a bandpass filter (532 nm), a rotation analyser and finally a photomultiplier tube. The reflected fundamental and harmonic waves were simultaneously collected by a second detection unit mounted on a h2h rotating stage. To overcome errors due to fluctuations between laser pulses, the collected intensities were averaged over 100 pulses. The absolute cal-

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PM2

Nd:YAG LASER BA

150 mm θ λ/2 F focal lens 2θ

Glan BS polarizer

PM1

sample

Fig. 2. Optical transmission spectra of the 85TeO2–15WO3 (mol%) glass: (a) unpoled glass; (b) poled glass; and (c) poled glass polished on cathode side.

D

Fig. 1. Scheme of the experimental setup for SHG acquisitions: BA, beam attenuator; BS, beam splitter; D, near-infrared diode; F, 1064 nm filter or LiNbO3 crystal and 532 nm filter; PM1, 532 nm filter, analyser and photomultiplier tube; PM2, 532 nm or 1064 nm filter, analyser and photomultiplier tube. A first unit detection PM1 permits the transmitted SHG signal to be detected, and a coupled rotation (2h) of the second detection unit PM2, with respect to the sample angle (h), allows reflection measurements.

ibration of the SH signal intensities was obtained using a z-cut quartz (presenting a second nonlinear coefficient v11 = 0.6 pm/V) as reference [24]. This set-up has also allowed measuring linear refractive index at x and 2x by collecting the 2h reflected wave around the Brewster angle over the [15–80°] h range. A LiNbO3 crystal in phase matching condition was used as a 532 nm source to record reflection signal. Its very important to determine these two linear refractive indices at 1064 nm and 532 nm, respectively, because it allows to evidence the indices dispersion and to estimate the coherence length of the nonlinear material. 2.4. Structural and chemical characterizations A structural approach of the unpoled and poled glasses was realized using Raman scattering. The Raman spectra were recorded in the 100–1200 cm1 range using a Dilor spectrometer (XY model), equipped with a CCD detector and an Ar+ laser (514.5 nm exciting line) operating in a backscattering geometry. The diameter of the laser spot focused on the sample was about 1 lm. Measurements were made at low power (<100 mW) of the exciting line, in order to avoid any damage of the samples. The spectral resolution was about 4.5 cm1 at the exciting line. A chemical composition’s analysis was carried out with quantitative EDS analysis (energy dispersion spectroscopy) coupled with a scanning electron microscope (SEM). Acquisitions were collected for an electron beam of 40 nA and an acceleration tension of 20 kV, corresponding to a detection limit of the sodium of 340 ppm (0.0340 wt% or 0.085 at%). The diameter of the electron beam was 10 lm. Measurements were performed on both surfaces of the unpoled and poled glasses.

1000 nm (Fig. 2a). The glass surface on contact with the anode during the poling treatment keeps its intact transparency. Nevertheless, the thermal poling process, especially for high poling temperature and/or long poling time, blackens the surface on contact with the cathode which increases the UV–Visible absorption of the glass due to especially losing reflection or losing diffusion (Fig. 2b). After a slight polishing of this face, on about 10 lm deep, the glass finds again its properties (Fig. 2c) and the second-order nonlinearity is conserved. Moreover, the maximum of transmission stabilized at 80%, is due to many reflections on the glass faces. This maximum of transmission indicates that this glass should possess high linear refractive indices (n0 > 2) as classically observed with TeO2-based glasses. This is confirmed by measurement of these indices using the Brewster angle reflection method: n0 = 2.193 (±0.005) and n0 = 2.087 (±0.005) for the 532 nm and 1064 nm wavelengths, respectively. 3.2. Second harmonic generation As SHG of a 85%TeO2–15%WO3 glass has never been studied, different thermal poling conditions have been investigated to optimise the second harmonic signal. After preliminary experiments, the voltage was fixed to 4.5 kV to avoid damaging of the glass and the thickness of the glass-pellets was adjusted to 500 lm. From these conditions, only two parameters were varied: temperature and poling time. Second-order nonlinear coefficients (d33) and nonlinear zone depths (L) deduced from Maker fringes simulations are reported in Table 1. Significant second harmonic signals are observed for all poling conditions. An active SHG layer of about 11–17 lm thick is obtained at the anode face. It is worth noting that for high poling temperatures (T > 300 °C) and/or long poling

Table 1 Second-order nonlinear coefficient (d33:d33 = ½ v(2)) and SH layer thickness (L), for different thermal poling conditions, of glasses with 85TeO2–15WO3 (mol%) composition (voltage: 4.5 kV and thickness of samples: 500 lm) Poling condition Temperature (°C)

Time

250

4h 8h 2h 4h 8h 20 min 1h 2h 30 min 1h 2h 1h

265

3. Results and discussion 3.1. UV–Visible absorption and linear refraction indices

280

The optical transmittance of the glass is an important parameter to be determined before beginning the simulation of the measured SH signal since it gives us precious information such as the dispersion of the material. The 85TeO2–15WO3 (mol%) glass exhibits a large domain of transparency which spreads from 500 nm to

295

300 *

Lc, coherence length.

d33 (pm/V) (±0.01)

L (lm) (±0.01) ±2 Lc* (5 lm)

0.57 0.44 0.48 0.43 0.48 0.42 0.74 0.75 0.59 0.61 0.47 0.72

17.06 11.91 7.70 16.59 11.52 12.00 11.59 12.00 7.60 11.95 16.56 16.73

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times (t > 4 h), glasses were damaged by breaking and/or blackening. No apparent coherence is observed between the different values of the d33 coefficient and the depths of the nonlinear zone. These results do not evidence a clear evolution with respect to the poling temperature or the poling time. Nevertheless, some main points can be underlined. The d33 coefficients are about 0.5–0.6 pm/V, which represent high values compared to those classically obtained with tellurite glasses or other oxide glasses. The highest value of the d33 coefficient is obtained under the following optimised poling conditions: 280 °C for 1 h or 2 h and 300 C for 1 h poling duration. Fig. 3 shows experimental and simulated Maker fringes for these poling conditions. The corresponding second-order nonlinear susceptibility v2 is of 1.5 pm/V, which corresponds to one of the highest values never observed in thermally poled tellurite glasses. This value can be reasonably related to that of the third-order nonlinear susceptibility v3 which is very high for this glassy composition (v3  350 1023 m2/V2, measured at k = 0.8 lm) [25]. 3.3. Discussion about the origin of the second harmonic generation The mechanism underlying the formation of the nonlinearity in bulk glass is not yet completely understood. According to Mukherjee et al. [26], two different mechanisms can account for the induced second-order nonlinear susceptibility: the first one is a charge migration and the second one is a reorientation of dipoles. The model based on these two mechanisms and applied on poled silica glasses, can be expressed as the following equation [2,26]

vð2Þ ð2xÞ ¼ 3vð2Þ ð2x; x; x; 0ÞEdc þ

Nb l Eint 5e0 kT

ð3Þ

The first term is related to the interaction between the third-order susceptibility v(3) and the residual electrostatic field Edc inside the material after removing the applied dc field. It represents the charge transport of mobile cations, as Na+, through the glass toward the cathode. This migration involves the formation of an anodic space-charge region, within which the nonlinearity can be measured. The second term is the resulting macroscopic second-order nonlinearity induced by reorientation of polar bonds during poling treatment, each of them with a permanent dipole moment (N is the number of permanent dipole) and an hyperpolarizability b* being then submitted to a local field Eint. As far as we know, the second mechanism has never been clearly correlated to SHG efficiency, whereas the first one has been successfully applied to different oxide glasses [27–29]. According to these previous works we postulated (v(2) = 3v(3)Eint), justifying the relation used in the Maker fringes simulations: d33 = 3d31. In order to evidence any structural and/or chemical changes in our glassy samples before and after poling process, we have characterised all of them and moreover for all the poling conditions reported in Table 1, using Raman scattering and electron microprobe.

sp polarization

1.0 1.

0.02 0.01

-80 -60 -40 -20

pp polarization

0.8

0.03

0.00

Fig. 4. Raman spectra of the 85TeO2–15WO3 (mol%) glass: (a) before poling; (b) after poling on anode face; (c) after poling on cathode face (poling conditions: T = 280 °C, U = 4.5 kV, t = 1 h).

I2w(a.u.)

I2ω(a.u.)

0.04

Whatever the investigated poling conditions, the Raman spectra of thermally poled samples recorded on both anode and cathode sides were strictly the same and as an example those related to the following poling conditions, T = 295 °C, U = 4.5 kV, t = 2 h, are illustrated in Fig. 4. The spectrum of the original glass is quite similar to that of pure TeO2 glass [30]. It presents tow main bands located near 450 and 650 cm1, which correspond to the symmetric vibrations of the Te–O–Te bridges and to the symmetric vibrations of the Te–O terminal bonds, respectively, both coexisting in TeO2 glass. This indicates that this glassy network is mainly constituted of TeO4 units. The band at 930 cm1 corresponds to the W = O stretching vibrations. No significant differences are observed between the spectrum of the unpoled glass and those of the poled sample. It is worth noting that the spectra recorded on both faces of the poled glass are quite identical. The only weak difference between spectra of the unpoled and poled glasses comes from the broad band situated in the range 700–800 cm1, which is attributed to the asymmetric vibrations of the Te–O terminal bonds. The relative intensity of this band seems to have slightly increased after poling. This could mean that no significant structural modification is induced by thermal poling: TeO4 units are conserved which means that no depolymerization of the tellurite glass network occurs. Only the Te–O terminal bonds are involved, which could correspond to a possible implication in the SHG phenomenon of the reorientation of the glass structural TeO4 entities occurring under electric field. The presence of the 5s2 electronic lone pair on Te (IV) atoms certainly enhances this reorientation. The chemical composition of an unpoled glass and of two glasses poled under extreme conditions (heating at 250 °C for 8 h and heating at 295 °C for 30 min) has been checked at both sides of the poled glass plates. The values reported in Table 2 are average ones and it is important to underline that the percentage of each element has not shown any variation all along the anode or the

0.6 0.4 0.2 0.0

0 20 θ (˚)

40 -60 -80

-80 -60 -40 -20

0

20

40 -60 -80

θ (˚)

Fig. 3. Experimental (lines with circles) and simulated (continuous lines) Maker fringes for the 85TeO2–15WO3 (mol%) glass poled at 280 °C for 1 h (voltage of 4.5 kV, 500 lm thick).

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which were not obviously correlated to poling parameters like temperature or duration could be explained by the complex internal electric field which is built in the glass. The Maker fringes patterns simulations lead to a mean constant nonlinear coefficient but at least two different coefficients would be involved in the nonlinear zone. Indeed, we must remind the reader that here, since the dispersion of the tellurite samples is strong, the coherence length is quite low (circa 2.5 lm). As a consequence, we expect a constant amplitude of the SHG signal modulo 2Lc from the p–p and s–p signals. However, in that case a slight modification of the s–p profile is expected that we have taken into account. As a conclusion, we would like to point out that any etching method here is somewhat useless, in contrast with poled silica materials [32,33] that exhibit a very weak dispersion with a coherence length of about 20 lm which is very efficient to solve properly the question of the thickness of the NLO layer.

Table 2 Percentages of the different atoms constituting the 85%TeO2–15%WO3 poled glass (esd are given in parentheses)

*

27.2 68.2 4.6 0 0 ppm

Poling conditions 250 °C, 8 h, 4.5 kV

295 °C, 30 min, 4.5 kV

Anode (at%)

Cathode (at%)

Anode (at%)

Cathode (at%)

27.0 (2) 67.9 (6) 4.5 (2) 0.52 (5) 2130 ppm

27.2 (2) 68.2 (6) 4.6 (2) 0.01 (5) 19 ppm

27.1 (2) 68.0 (6) 4.5 (2) 0.44 (5) 1820 ppm

27.3 (2) 68.2 (6) 4.5 (2) 0.02 (5) 92 ppm

Detection limit of sodium: 340 ppm.

cathode surfaces. These analyses clearly show that no noticeable modification of glass composition occurs after poling. It is just observed, for all poled samples, the appearance of a significant change in sodium profile at the anode side. The level of the mean atomic percentage of sodium in the bulk is around 100 ppm which is at the detection limit of this quantitative analysis. But around 2000 ppm of sodium is measured just under the anode surface which is clearly higher than the detection limit of sodium. This concentration of sodium at the anode side is certainly resulting from the migration of sodium through the face on contact of the borosilicate plate used as the anode during the poling process. Moreover, a cross-section analysis of sodium (Fig. 5) has been realized for these samples poled under extreme conditions (heating at 295 °C for 30 min: Fig. 5a and heating at 250 °C for 8 h: Fig. 5b). For each poling condition, it is evidenced that the cross-section of the atomic percentage of sodium through the glass thickness, follows the same tendency as that observed previously by Alley et al. [31] on silica poled glasses. According to the model of anodic ions injection proposed by Alley, we suppose that sodium ions have been progressively injected through the anode surface, while sodium ions initially in the glass are depleting during the poling process. When the depletion is complete, sodium ions are still injected leading to internal electric field profile changing with poling duration. As the profile of sodium injected from the anodic surface inside the glass increases from 2 lm deep for a short poling time of 30 min to 5 lm deep for a long poling time of 8 h, the sodium depleted region front slightly moves from 8 lm to 10 lm deep for 30 min to 10–15 lm deep for a long poling time of 8 h. This induces a complex internal electric field profile embedded in the nonlinear zone. These results, obtained using electron microprobe, quantitatively nicely agree with SHG Maker fringes simulations reported in Table 1, indicating that the nonlinear zone thickness is all the more extended as the poling time is long: in the range 7–12 lm and 12–17 lm deep for short poling times (t < 1 h) and long poling times (t P 2 h), respectively. The simulated nonlinear coefficients

Thermally poled glasses with 85TeO2–15WO3 (mol%) composition have shown improved SHG performances with respect to silica glasses (generally <1 pm/V) and most of tellurium oxide-based glasses. After optimisation of the thermal poling conditions, a high second harmonic signal (v(2) = 1.5 pm/V) has been obtained. The SH active layer was thin, between 12 lm and 17 lm, and obtained at the anode side. Two complementary hypothesis can be proposed to explain the origin of the SHG. Firstly, no significant structural modification has been observed on both surfaces of the poled glasses and only a reorientation of the TeO4 glass structural entities could be envisaged under electric field. Secondly, the diffusion of sodium ions from the borosilicate plate onto the anodic surface of the sample, and then the migration of these cations through the glass thickness towards cathodic face, were systematically observed. This generated the formation of an anodic depletion region of sodium ions, which can be considered as the main origin of the second-order nonlinearity property of this tellurite glass. Acknowledgements One of us, C. Lasbrugnas, is grateful to the European Community (European Social Funds) and the Conseil Régional du Limousin for financial support. We acknowledge Région Aquitaine and the Agence Nationale de la Recherche ANR (Grant ANR-05-BLAN0212-01) for financial support. References [1] R.A.F. El-Mallawany, Tellurite Glasses Handbook: Properties and Data, CRC Press, Boka Raton, FL, 2002. [2] R.A. Myers, N. Mukherjee, S.R.J. Brueck, Opt. Lett. 16 (22) (1991) 1732. [3] G. Vrillet, P. Thomas, V. Couderc, A. Barthelemy, J.C. Champarnaud-Mesjard, J. Non-Cryst. Solids 345–346 (2004) 417.

t = 30 min

0.3 0.2

depletion

0.1

Detection line of Na

Anode surface

0 2 4 6 8 10

110

210

Thickness (µm)

310

410

Na (atomic %)

b 0.6

0.4

Cathode surface

Na (atomic %)

a 0.5

4. Conclusion

0.5

t=8h

0.4 0.3 0.2 0.1

depletion

0 2 4 6 8 10

Detection line of Na

110

210

Thickness (µm)

310

410 Cathode surface

Te O W Na*

Before poling

Anode surface

Atom

Fig. 5. Electron microprobe analysis of the atomic percentage of sodium ions through the thickness of the 85TeO2–15WO3 (mol%) poled glass under the following thermal poling conditions: (a) T = 295 °C, U = 4.5 kV, t = 30 min; and (b) T = 250 °C, U = 4.5 kV, t = 8 h.

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