Measurement of laser-induced refractive index change ofinverted ferroelectric domain LiNbO3

Yunlin Chen,1,* S. W. Liu,1 Dongdong Wang,2 Tianlin Chen,2 and Min Xiao1

1Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA2Department of Physical Science, Nankai University, Tianjin 300071, People’s Republic of China

*Corresponding author: ylchen@uark.edu

Received 24 July 2007; accepted 12 September 2007;posted 13 September 2007 (Doc. ID 85632); published 25 October 2007

Optical nonlinearities of periodically poled LiNbO3 crystals were investigated by the single beam Z-scantechnique with a continuous wave (cw) laser beam at 532 nm. The nonlinear optical absorption coefficientand refractive index change are determined to be 8.1 � 10�6 cm�W and 2.6 � 10�4 at 0.5 MW�cm2 lightintensity, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerablydifferent from the Z-scan results in congruent LiNbO3. The nonlinearities in the periodically poledLiNbO3 induced by 532 nm continuous waves are believed to be mainly due to the photorefractiveeffect. © 2007 Optical Society of America

OCIS codes: 190.5330, 160.2260.

1. Introduction

Ferroelectric material, lithium niobate �LiNbO3�, iswidely used for various applications, such as nonlin-ear frequency converters, photonic band-gap devices,electro-optic Bragg switches, and data storage [1–4].Especially, the periodically poled LiNbO3 (PPLN) hasbeen successfully exploited in quasi-phase-matched(QPM) frequency conversion processes. QPM fre-quency conversion has been the subject of numerousinvestigations. Most of the research efforts have beenfocused on the second-order optical nonlinearity ��2�

because of its applications in second harmonic gen-eration and optical parametric oscillation. However,less effort has been made to study the third-ordernonlinearity of ferroelectric domain inversion inLiNbO3. Congruent LiNbO3 (cLN) crystal possessesphotorefractive properties that may cause a changein optical characteristics upon exposure to laserradiation. It was demonstrated that the photore-fractive effect was strongly suppressed in periodicferroelectric domain inversion structure crystals[5,6]. Therefore, it is essential to measure the non-linear optical absorption and refraction of PPLNcrystals.

Several techniques [7,8] were used to measure thenonlinear optical absorption and refraction of LiNbO3

crystals, but they were performed at light intensitiesat least two orders of magnitude lower than thosetypically used for frequency conversion devices. Theinput intensity range of the measurements should bethe same as that used in the frequency conversionprocesses since the nonlinear optical absorption andrefraction depend on the intensity. The single-beamZ-scan is a well-established technique to measure thenonlinear absorption and refraction of solid and liq-uid samples [9]. Using the Z-scan technique, the non-linear optical absorption and refraction of undopedand doped LiNbO3 crystals have already been mea-sured [10–12]. However, reported studies using thismethod to investigate the nonlinear optical absorp-tion and refraction of PPLN crystals are scarce.

In this paper, we report an experimental investi-gation of nonlinear optical properties of PPLN crystalusing a single-beam Z-scan technique with continu-ous wave (cw) laser beam at 532 nm. Using this tech-nique we compared the nonlinear refractive indexof both LiNbO3 and ferroelectric domain invertedLiNbO3 crystal. It has been found that both the re-fractive index change and the nonlinear absorptioncoefficient of ferroelectric domain inverted LiNbO3

crystal are different from the results of the congruent0003-6935/07/317693-04$15.00/0© 2007 Optical Society of America

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LiNbO3 crystal. The transmitted beam spot shapeswere observed too.

2. Experiment

LiNbO3 crystals were grown by the Czochralski tech-nique with congruently melting composition. Thesamples were cut from c-cut optically polished, 1 mmthick wafers obtained from congruent LiNbO3 crys-tals. The inverted ferroelectric domain was fabricatedby electric field poling [13], and the inverted domainpattern was shown in Fig. 1. The dimensions are1 � 10 � 1 mm3, where the crystal thickness alongthe laser propagation direction is 1 mm and the crys-tal width normal to the crystal c-axis direction is10 mm.

The experiments were performed using a cw laserbeam at 532 nm. The schematic of our Z-scan exper-imental setup is shown in Fig. 2. The laser beam wasdivided by a beam splitter into two parts. The re-flected part was taken as the reference representingthe incident light and the transmitted beam wasfocused by a lens with a focal length of 80 mm toachieve the focal waist radius of �20 �m, as deter-mined from the measured beam divergence. The sam-ple was mounted on a translation stage that wascontrolled by the computer to move along the x axiswith respect to the focal point. The crystal’s c axis isoriented parallel to the gradient direction of theGaussian beam, i.e., the z axis. The c axis of thesample was always set parallel to the polarization ofthe laser beam in order to obtain the change of ex-traordinary refractive index. The open-aperture (OA)and close-aperture (CA) Z-scan measurements wereperformed on the crystal. The confocal parameterz0 � ��0

2��, where �0 is the Gaussian beam waistradius at the focus, and � is the wavelength of thelaser. The confocal parameter can be used to deter-mine the behavior of the beam inside the sample,

which was investigated by moving the LiNbO3 crystalalong the y axis, and we found that the confocal pa-rameter is not affected by varying positions. The CAZ-scan measurement was performed with a fixed lin-ear transmittance of S � 20%, defined as the fractionof the beam energy passing through the far-field ap-erture. The OA and CA Z-scan traces may be fitted bytwo corresponding numerical programs based on [9]to obtain the nonlinear optical absorption coefficient� and the refraction index change ne.

To distinguish between the different types of lightinduced effects, during the sample scanning the beamshape was also recorded in the direction of the opticalaxis by a charge coupled device (CCD). Built-up pho-torefraction needs time and depends on the materialinvestigated and the light intensity used. When thetransmitted beam shows strong fanning, we can as-sume that dominantly the photorefraction is present.In this case the Z-scan method cannot be used to givea quantitative result for the light-induced change inrefraction [14].

3. Results and Discussion

When a photorefractive material is illuminated inho-mogeneously, an internal space-charge field E buildsup owing to a redistribution of photoexcitable free-charge carriers. This space-charge field produces aspatial variation of the refractive index through theelectro-optic effect. For the extraordinary polariza-tion, the nonlinear variation of the refractive index,ne, is due to the z component of the space chargefield, Ez, and can be expressed

ne � 0.5 r33ne3Ez. (1)

The change in refractive index was determined byperforming CA Z-scan measurement. Due to self-focusing or self-defocusing effects, a Z-scan trace witha peak and valley was thus observed, and the sign ofthe nonlinear refractive index can be determined di-rectly from the shape of the Z-scan trace. Figure 3(a)shows a Z-scan trace of congruent LN crystal withintensity of 0.5 MW�cm2 in the focal plane. The shapeof the Z-scan trace is similar to other congruent LNsamples, showing a negative change in the refractiveindex. To the contrary, the Z-scan trace of PPLNrecorded at the same laser intensity and shown inFig. 3(b) exhibits a nearly symmetric valley with re-spect to the focal point, indicating that the inverteddomain LiNbO3 crystal possesses a positive nonlinearoptical refraction, which indicates that the laser

Fig. 1. (Color online) SEM image of the etched � c surface withinverted ferroelectric domain LiNbO3 crystal.

Fig. 2. Scheme of the experimental setup for Z-scan measure-ments.

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beam propagating in the sample undergoes a self-focusing process. The traces measured are fitted(solid lines) using formulas (28) of Hermann et al.[15], which are applicable for thick samples too.From the fit we obtained ne � �1.3 � 10�2 and2.6 � 10�4 refractive index changes for the cLN andPPLN crystals, respectively.

The Z-scan technique can also provide informationon the damage sensitivity, which are defined as [12]

ne* � ne�I, (2)

where I is the light intensity, and ne is the steadystate refractive index change caused by illumination.The damage sensitivities of the congruent LN andPPLN crystals investigated are ne

* � �2.6 � 10�2 and5.2 � 10�4 cm2�MW, respectively. It is worth notingthat it is 20 times smaller for PPLN than for cLN.From the low damage sensitivity of the PPLN crystalit is adaptable to be used in nonlinear frequency con-version.

The OA Z-scan technique gives a better demonstra-tion of the nonlinear absorption. The normalizedtransmission for the OA condition is given by [10]

T�z� � 1 �I0�1 � e��L�

2�2�1 � z2�z02��

, (3)

where � is the absorption coefficient, L is the thick-ness of the sample, and I0 is the intensity of thelaser beam at the focus �z � 0�. From the best fit ofEq. (3) to the experimental data in Fig. 4, the non-linear absorption coefficients were obtained to be2.7 � 10�3 cm�W and 8.1 � 10�6 cm�W for cLNand PPLN with the incident irradiance being0.5 MW�cm2, respectively. The � value of the PPLNis lower than that of the cLN. This difference maybe attributed to the inverted domain structures ofLiNbO3 crystal, since the photorefractive effect wasstrongly suppressed in such domain inversion struc-ture crystals [5,6]. The contribution of two-photonabsorption is negligible in the nonlinear absorptionwe measured, since the magnitude of two-photon ab-sorption is three orders of magnitude smaller [11]than our measurement.

The fanning effect was observed in the directionof the optical axis. The obvious light-induced beampattern fanning occurs with increasing irradianceintensity during the sample scanning. Figure 5 de-picted CCD recordings taken from the spot of the0.5 MW�cm2 and 2 MW�cm2 laser beam intensity be-hind the sample when it was at focus. The transmit-ted beam shape lost its circular symmetry when theirradiance intensity was 2 MW�cm2, and the fanningeffect was observed. This beam spot shape makes itpossible to determine the refractive index change andthe photorefractive sensitivity from Z-scan traces.

Fig. 3. (Color online) Close-aperture Z-scan traces of the (a) con-gruent LiNbO3 crystal and (b) periodically poled LiNbO3 crystal,measured at incident laser intensity 0.5 MW�cm2.

Fig. 4. (Color online) Open-aperture Z-scan traces of the congru-ent LiNbO3 and periodically poled LiNbO3 crystals, respectively,measured at incident laser intensity 0.5 MW�cm2.

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4. Conclusion

Nonlinear optical properties of inverted ferroelectricdomain LiNbO3 crystal was investigated by thesingle-beam Z-scan technique using a cw laser beamat 532 nm wavelength. The nonlinear optical absorp-tion coefficient � and refractive index change ne ofthe PPLN were determined to be 8.1 � 10�6 cm�Wand 2.6 � 10�4, respectively. The fanning effect wasobserved using a CCD in the far field, and the trans-mitted beam spot fanning occurs with increasingirradiance intensity during the sample scanning. Us-ing this technique we compared the nonlinear opticalabsorption coefficient � and refractive index changene of both congruent LiNbO3 and ferroelectric do-main inverted LiNbO3. Both � and ne are smallerthan that of the congruent LiNbO3 crystal. Theseresults indicate that inverted ferroelectric domainLiNbO3 crystal is a promising material for nonlinearoptical applications.

This work is partly supported by grants from theArmy Research Laboratory (Award DAAD19-03-2-0017) and National Science Foundation�MaterialsResearch Science and Engineering Center (NSF�MRSEC), the National Natural Science Foundationof China under Grant 60544004, and the “863”Project of China 2006AA03Z423.

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