Qiu et al. Vol. 19, No. 1 /January 2002 /J. Opt. Soc. Am. B 37

Bulk and near-surface second-order nonlinearitiesgenerated in a

BK7 soft glass by thermal poling

Mingxin Qiu† and Toru Mizunami

Department of Electrical Engineering, Kyushu Institute of Technology, 1-1 Sensuicho, Tobata, Kitakyushu 804-8550,Japan

Ramon Vilaseca

Departament de Fı́sica y Enginyeria Nuclear, Universitat Politecnica de Catalunya, Colom 11, E-08222, Terrassa,Spain

Francesc Pi and Gaspar Orriols

Grup d’Optica, Departament de Fı́sica, Universitat Autonoma de Barcelona, Edifici Cc, E-08193, Bellaterra(Barcelona), Spain

Received January 2, 2001; revised manuscript received May 14, 2001

Bulk second-order nonlinearity was generated in BK7 glass at a higher temperature and with a longer polingtime than near-surface second-order nonlinearity. The temporal decay of the bulk second-order nonlinearitywas slower than that of the near-surface second-order nonlinearity. The thickness of the near-surface non-linear layer increased with poling time. Poled BK7 glass was also measured by x-ray photoelectron spectros-copy. Depletion of Na at the anodic surface and its accumulation at the cathodic surface was observed. Atthe cathodic surface, a higher-energy peak near O (1s) appeared, which shows peroxy-radical defects. At theanodic surface, a lower-energy peak near Si (2p) appeared, which may be attributed to E8 centers or to two-coordinated Si defects. The mechanisms of generation of these defects and of the second-order nonlinearitiesare discussed. © 2002 Optical Society of America

OCIS codes: 160.2750, 160.4330, 190.4160, 190.4350, 240.4350.

1. INTRODUCTIONElectric poling of polymer and glass has been extensivelystudied, as it can turn these centrosymmetric materialsinto asymmetric materials.1–5 Such poled materials havesecond-order nonlinearity (SON), so they are expected tobe useful as second-harmonic generators and electro-opticmodulators or switches. It has become interesting formany investigators to search for new effective materialsfor such applications. Poled glass is expected to be sucha low-cost material, and it is more heat resistant thanpolymers.

Although second-harmonic (SH) generation in poledglass has been studied mainly for silica glass, SH genera-tion is also possible in some other kinds of glass such asTeO-based glass, lead silica glass, Pyrex glass, and soda-lime silicate glass.6–8 It is expected that new materialswill offer some new properties that meet application re-quirements. However, SH-signal decay is a problem inpoled soft glass,8 perhaps because of the relatively largeelectrical conductivity of these kinds of glass at room tem-perature. The charges in the glass may disappear as aresult of this glass’s conductivity, as will the electric fieldand the SON.8 The decay may also be related to shallowtraps from which the charges escape and become mobile.It has been shown that the decay can be reduced at lowtemperatures.8

0740-3224/2002/010037-06$15.00 ©

Two kinds of SON are generated in poled silica. One isnear-surface SON in a thin layer below the anodic sur-face, and the other is bulk SON distributed through thewhole thickness of the silica sample. Most studies of SHgeneration in poled glass treated near-surface SON,2–5

and it has been relatively recently that bulk SON9–12 hasalso been studied. The general mechanism of near-surface SON is that the negative charges gather in theanodic surface to build an intense electric field during pol-ing by depletion,13 by accumulation,8,14 or by screening ofthe charges from the air4 to break the symmetry of theglass by the Kerr effect5 or by reorientation of themolecules.4 For bulk SON, Henry et al.10 gave a model ofreorientation of defects in the silica in an external electricfield to break the symmetry of the glass. Although bulkSON has been generated in silica glasses,9–12 there hasbeen no report of bulk SON in a soft glass. The latter, ifit can be produced, would be useful for fabrication of low-cost optoelectronic components such as optical switchesand modulators. Fabrication of BK7 glass opticalwaveguides has also been studied.15 It may also becomepossible to obtain SH generation from a BK7 lens or aBK7 plate itself, and this may bring some new applica-tions.

X-ray photoelectron spectroscopy (XPS) is also an effec-tive method for analyzing the surface state of materi-

2002 Optical Society of America

38 J. Opt. Soc. Am. B/Vol. 19, No. 1 /January 2002 Qiu et al.

als. XPS measurements of poled borophosphate glass16

and Na1-doped Nb2O5–TeO2 glass17 have been reported.However, similar measurements of poled BK7 glass havenot.

In this paper we deal with both near-surface and bulkSON. In what follows, we report on a large SH signalgenerated from poled BK7 glass and on its properties.We also report the XPS measurement of poled BK7 glass.

2. EXPERIMENTAL PROCEDUREThe size of the BK7 glass samples in our experiment was20 mm 3 20 mm, and the glass was polished on bothsides. Its thickness was 0.7 mm. The composition ofBK7 is listed in Table 1. The sample was sandwiched withtwo pieces of stainless-steel electrodes, and the whole con-figuration of sample and electrodes was blocked by twopieces of ceramic that served as electric insulators in anoven. The temperature in the oven could be automati-cally controlled.

After poling, a layer of chemicals was deposited ontothe cathodic surface. We assumed that the compositionof this film was a mixed compound, while the main mate-rials were Na and K, as in poled soda-lime glass.8 Thechemical composition of these surfaces was studied byXPS measurement.

The wavelength of the detecting fundamental light was1.06 mm, and the light came from a Q-switched Nd:YAGlaser with a pulse duration of 10 ns and a repetition rateof 10 Hz. The output of the laser was attenuated to 0.25mJ per pulse and focused with a 15-cm focal-length lensonto the center of the sample. The sample was fixedupon a rotation stage, and the angular dependence of SHsignals was measured. The detector was a photomulti-plier in the propagation direction of the fundamental la-ser light; at the entrance, two filters were attached to cutoff 1.06-mm light.

XPS measurement was made of three samples: of thecathodic and the anodic surfaces of poled BK7 glass and ofa surface of nonpoled BK7 glass. The poling voltage was3.5 kV, the poling temperature was 400 °C, and the polingtime was 60 min. Before the measurement, all sampleswere lightly sputtered with argon to remove surface con-tamination, which was mainly carbon.

3. EXPERIMENTAL RESULTSA. Second-Harmonic GenerationWe poled a BK7 sample at 250 °C and 3.5 kV for 30 min.Figure 1 shows the dependence of the SH signal on the

Table 1. Composition of BK7 Glass

Material

SiO2 B2O3 Al2O3 K2O Na2O As2O3 PbO

Amount ofmaterial(%) in BK7 glass

68.2 7.8 2.4 3.43 10.79 0.38 6.78

sample angle, or the Maker fringe pattern. As thesample has a smooth profile, it shows pure near-surfaceSON.

Figure 2 shows the decay of the SH signal with time atroom temperature. The poling conditions were the sameas for Fig. 1. The SH signal is shown with its maximumvalues for various sample angles. Just after poling, theSH signal was larger than that of Infrasil silica poled at300 °C and 3.5 kv for 30 min. The nonlinear coefficient,d33 , of the poled BK7 calibrated with a quartz plate was0.8–1 pm/V. However, 2 h later, its SH signal had be-come only half of the original value. After decaying for20 h at room temperature, the SH signal decreased to1/20 of that just after poling. The decay rate was tooquick in the first state; however, the saturation time was;45 h with a value of d33 of 0.2 pm/V. Although themechanism of the decay process is unclear, the decay pro-cess is normally connected to the loss rate of charges inthe layer by the electric conductivity of BK7 glass or withthe rate of escape of charges from traps.

Figure 3(a) shows the dependence of the SH signal onthe poling temperature. The poling voltage was 3.5 kV,and the poling time was 30 min. The SH signal can beexpressed in an Arrhenius plot as a function of polingtemperature as

S 5 ~A/Tb!exp~2eE/kT !, (1)

where T is the absolute temperature, e is the charge of anelectron, k is the Boltzmann constant, A 5 7 3 106, b5 0.781, and E 5 0.102 eV.

Fig. 1. Measured profile of a Maker fringe of BK7 glass poled at250 °C and 3.5 kV for 30 min. The Maker fringe shows pure near-surface second-order nonlinearity.

Fig. 2. Decay of the SH signal with time at room temperature.The sample was poled at 250 °C and 3.5 kV for 30 min.

Qiu et al. Vol. 19, No. 1 /January 2002 /J. Opt. Soc. Am. B 39

Figure 3(b) shows the dependence of the SH signal onthe poling time at 350 °C and 3.5 kV. The SH signal in-creases with the poling time and decreases for poling oflonger than 1 h. The reason for this decrease is dis-cussed below.

Figure 3(c) shows the dependence of the SH signal onthe poling voltage at 350 °C for 30 min. It can be ex-pressed by a power function as

S 5 BVd, (2)

Fig. 3. Dependence of SH signal on (a) poling temperature at3.5 kV for 30 min, (b) poling time at 350 °C and 3.5 kV, and (c)poling voltage at 350 °C for 30 min.

Fig. 4. Evolution of the thickness of the second-order nonlinearlayer with poling time.

where B 5 1.2 and d 5 6. Hence the SH signal in poledBK7 glass increases with the increase of the poling volt-age to the sixth power, not to the second power, as wasnormally observed in poled fused silica.

The thickness of the near-surface SON layer is shownin Fig. 4. The poling temperature was 300 °C, and thepoling voltage was 3.5 kV. We determined the thicknessby fitting the Maker fringe, and the method is the samewith the double fitting described below, except that thebulk component is assumed to be zero. The thickness in-creases with poling time up to 270 min. For BK7, coher-ence length Lc is 21 mm. When the thickness exceedsLc , the SH signal will saturate,18 as is illustrated in Fig.3(b). The decrease in signal in Fig. 3(b) for a poling timeof 90 min may be due to the electric breakdown betweenthe positive and negative charges in the layer.

For poling of BK7 glass at 400 °C and 3.5 kV for 3.5 h,the Maker fringe was changed into a mixture of fastripple and slow variation with incident angle, as shown inFig. 5(a). The mixed profile of the Maker fringe can beexplained by two kinds of charge distribution: One is thecharge distributed near the anodic surface, and the otheris the charge distributed through the entire glass sample.They correspond to near-surface SON and bulk SON, re-spectively. The Maker fringe of the mixture of near-surface and bulk SON can be separated by doublefitting.12 The double-fitted Maker fringe is shown in Fig.5(b). From the fitting we get a thickness of the near-surface SON layer of 100 mm and a thickness of the bulkSON distribution of 0.73 mm. The latter is similar to thethickness of BK7 glass (0.7 mm).

When we introduced a parameter r, which is the ratioof the susceptibility of bulk to that of near-surface SON,as in Ref. 12, we found that the r value increased with

Fig. 5. (a) Maker fringe by BK7 glass poled at 400 °C and 3.5 kVfor 3.5 h, showing the mixture of bulk SON and near-surfaceSON. (b) Corresponding Maker fringe from the double fittingmethod.

12

40 J. Opt. Soc. Am. B/Vol. 19, No. 1 /January 2002 Qiu et al.

time at room temperature. This means that the decayrate of the bulk SON is less than that of the near-surfaceSON.

B. XPS MeasurementXPS measurement was performed for a photoelectron en-ergy range of 0–1100 eV. The charge shift was compen-sated for by a C (1s) signal at 285 eV.19 The observed spec-tra and their corrected binding energies were Si (2p),103.47 eV; Si (2s), 155.eV; Na KLL Auger transition, 498eV; O (1s), 532.93 eV; and Na (1s), 1073 eV; etc. Theatomic composition was automatically calculated; B, Pb,K, Al, and As were not included in the calculation. Wealso obtained high-resolution curves of O (1s), Si (2p), andC (1s).

Table 2 lists the atomic composition ratios of Na/O andSi/O for signals of Na (1s), Si (2p), and O (1s). The signalof Na at the anodic surface is below the detection limit.Although Na is one of the components of BK7 glass, it es-caped from the anodic surface almost completely. In con-trast, at the cathodic surface, the composition ratio of NaTable 2. Atomic Composition Ratio of Poled BK7

Glass by XPS Measurement

CompositionRatio

Surface

Non-poled BK7 Anodic Surface Cathodic Surface

Na/O 0.041 ,0.001 0.838Si/O 0.449 0.470 0.070

Fig. 6. O (1s) peaks for (a) non-poled BK7 glass, (b) the anodicsurface of poled BK7 glass, and (c) the cathodic surface of poledBK7 glass. The deconvolution curves are also shown in (c). Forcorrection for a charge shift, see text.

became 20 times that of nonpoled glass. The compositionratio of Si at the cathodic surface is only 1/6 of that of thenonpoled glass, and that at the anodic surface is a littlelarger than that of nonpoled glass.

In Fig. 6, the measured XPS spectra of the O (1s) peakare shown for nonpoled BK7 glass, the anodic surface ofpoled BK7 glass, and the cathodic surface of poled BK7glass. To compensate for the charge shift, one should add0.07, 0.15, and 0.17 eV to the binding energies in Figs.6(a), 6(b), and 6(c), respectively. The measured O (1s)peak of nonpoled BK7 was at 532.93 eV. This is almostthe same for anodic surface (533.05 eV). However, at thecathodic surface, the energy (531.3 eV) shifted downwardby 1.63 eV, and a new peak at 536.04 eV appeared. Thisnew peak was located at 3.11 eV above O (1s) of nonpoledglass, or 4.74 eV above that of the cathodic surface.

For Si (2p), the measured XPS spectra are shown inFig. 7 for nonpoled BK7 glass, the anodic surface, and thecathodic surface. The correction for the charge shift isthe same as for Fig. 6: 0.07, 0.15, and 0.17 eV should beadded to the binding energies in Figs. 7(a), 7(b), and 7(c),respectively. The measured Si (2p) peak of nonpoled BK7was at 103.47 eV. For the anodic surface, the peak wasalmost the same (103.9 eV); however, a new peak ap-peared at 101.98 eV, which is 1.92 eV downward. For thecathodic surface, the peak energy of Si (2p) was 102.65 eV.

4. DISCUSSIONThere are thresholds for both near-surface and the bulkSONs. For BK7 glass, near-surface SON always takes

Fig. 7. Si (2p) XPS peaks for (a) nonpoled BK7 glass, (b) the an-odic surface of poled BK7 glass, and (c) the cathodic surface ofpoled BK7 glass. The deconvolution curves are also shown in (a)and (b). For correction for a charge shift, see text.

Qiu et al. Vol. 19, No. 1 /January 2002 /J. Opt. Soc. Am. B 41

place if the poling temperature is higher than 170 °C.The threshold for bulk SON is 400 °C for the poling tem-perature and 2 h for the poling time. Thus, bulk SON isalways accompanied by near-surface SON.

In Ref. 5 it was found for the near-surface SON thatthere are dipolar layers constructed in the anodic regionin a poled silica glass. We can assume that near-surfaceand bulk SON are caused by a similar mechanism inwhich the electric field of the dipolar layers breaks thesymmetry of the glass by means of the Kerr effect. Wehave proposed an ionic wave model for the near-surfaceSON in poled silica glass as such a mechanism.20 Thismodel assumes that positive ions injected by the electricdischarge at the anode travel toward the cathode and thatthe distribution of the ions is frozen when the glass iscooled.20 This result is consistent with the observation ofpositive–negative–positive structure reported by Pruneriet al.5 If there are two kinds of ions, which have widelydifferent coefficients of recombination when charges takeopposite signs, this model may also be applicable to bulkSON. As the glass material is different from silica, ap-plication of our model to poled BK7 glass is still a hypoth-esis. However, application to the near-surface SON ismore plausible. The construction of dipolar layers inbulk glass requires a higher poling temperature and alonger poling time than that at the surface.

Bulk SON has a property of decay too; however, its de-cay rate is slower than that near-surface SON. Thecharges for near-surface and bulk SONs may be different,as mentioned above, and the difference in the decay char-acteristics may be due to a difference in the kinds of ionsin the two types of SON. Although the decay of near-surface SON in too fast for practical applications, the de-cay of bulk SON is relatively slow. It has been shownthat the decay of near-surface SON can be reduced at asuitably low temperature.8 It was found that H2 loadingwas highly effective in improving the decay rate for UV-poled GeO2–SiO2 glass.21 Development of such a tech-nique is expected to improve the rate of decay of BK7glass at room temperature. The increase in the thick-ness of the near-surface nonlinear layer with poling timewill be due to the same mechanism as that measured forfused silica18: The separation of positive and negativecharge layers will increase the poling time.18

From XPS measurement it has been shown that Na es-capes from the anodic surface and accumulates at the ca-thodic surface. This is so because Na1 ions have positivecharge and they migrate toward the cathode under the in-fluence of the poling electric field. This is consistent withthe behavior of the depletion model. Although such mi-gration of Na has already been confirmed in silica glass bysecondary ion mass spectroscopy measurement,22 this isthe first time to our knowledge that it has been measuredin BK7 glass or by XPS. In contrast, the Si (2p) signal atthe cathodic surface became weak. This is so mainly be-cause the migrated Na masked the cathodic surface. Itmay also be related to the facts that negative radicalssuch as Si–O2 have a formalized motion by exchange ofelectrons and O atoms and that they accumulate at theanodic surface.8,14 An increase in the Si/O ratio at the an-odic surface also suggests such an accumulation, and theobserved layer of silica oxide at the anodic surface after

poling will correspond to the accumulation. Thereforethese experimental results are considered to support theaccumulation model based on the motion of such negativeradicals.

In the XPS measurement, at the cathodic surface the O(1s) main peak shifted to the lower energies by 1.63 eV.This is considered to be a chemical shift accompanied by achange in chemical composition, because Na is accumu-lated and Si is depleted at the cathodic surface and, forhigher composition of Na2O, the O (1s) energy shiftsdownward.23 The new peak at 536.04 eV, which is 3.11 eVhigher for O (1s) of nonpoled BK7, is considered to be dueto peroxy-radical defects,24,25 as discussed below. Asmonovalent Na increases and quadrivalent Si decreases,the cathodic surface is considered to have an oxygen ex-cess. It has been known that, when there is an oxygenexcess, peroxy radical defects are generated. Accordingto first-principle calculation, a peroxy-radical defect has abinding energy that is higher by 2.8 eV,25 which agreeswith the experiment. At the anodic surface, a new en-ergy peak appeared that was 1.92 eV lower than Si (2p).This peak is considered to be related to Si defects, e.g., E8center26 and divalent (two-coordinated) Si (5Si:)defects.25 Si defects with unbonded electrons should havea binding energy that is lower than that of normallybonded Si. The calculated energy shift of 5Si: was 21.4eV,25 which agrees with the experiment. The origin ofthese defects is considered to be O2 deficiency, and thereason for O2 deficiency is opposite that for the cathodicsurface: depletion of Na and perhaps accumulation of Si.

5. CONCLUSIONSGeneration of bulk SON in a soft glass by thermal polingwas reported for the first time to our knowledge, and itsrelation to near-surface SON was studied. At a rela-tively low poling temperature and a short poling time,only the near-surface SON appears. At a higher polingtemperature for a longer poling time, both bulk and near-surface SONs appear. The value of the near-surfaceSON just after poling is higher than or equal to that inpoled Infrasil silica. The decay of the bulk SON wasmuch slower than that of the near-surface SON. XPSmeasurements showed migration of Na1 ions from the an-odic to the cathodic surfaces. XPS measurements alsoshowed the change in the O (1s) and Si (2p) spectra thatresulted from poling. At the cathodic surface a new peakthat exhibited peroxy-radical defects was observed, and atthe anodic surface a new peak with O2 deficiency defectswas observed. Generation of both defects can be ex-plained by the presence of O2 deficiency and O2 excess atthe anodic and cathodic surfaces, respectively, induced bymigration of Na1 ions.

M.-X. Qiu’s e-mail is mingxinqiu@netscape.net; that ofT. Mizunami is mizunami@elcs.kyutech.ac.jp.

†Permanent address, P. O. Box 40-004, Shanghai200040, China.

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