007) 7926–7929www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

Optical nonlinearities in Ag/BaTiO3 multi-layer nanocomposite films

Guang Yang a,⁎, Youhua Zhou a,b, Hua Long a, Yuhua Li a, Yifa Yang a

a Wuhan National Laboratory for Optoelectronics and School of Optoelectronics Science and Engineering,Huazhong University of Science and Technology, Wuhan 430074, People's Republic of China

b School of Physics and Information Engineering, Jianghan University, Wuhan 430056, People's Republic of China

Received 1 June 2006; received in revised form 5 February 2007; accepted 8 March 2007Available online 19 March 2007

Abstract

The multi-layer structure of barium titanate composite thin films containing Ag nanoparticles were grown on MgO (100) substrates usingpulsed laser deposition technique under the nitrogen pressure of 7.4 Pa. The X-ray photoelectron spectroscopy analysis indicated that the sampleswere composed of metal Ag embedded in the BaTiO3 matrices. The optical absorption properties were measured from 300 nm to 800 nm, and theabsorption peaks due to the surface plasmon resonance of Ag particles were observed. With the increasing of Ag concentration in composite films,the peak absorption increased and shifted to longer wavelength (red-shift). Furthermore, the third-order optical nonlinearities of the films weredetermined by z-scan method and the nonlinear refractive index, n2, and nonlinear absorption coefficient, β, were determined to be about−1.91×10−13 m2/W and −5.80×10−7 m/W, respectively.© 2007 Elsevier B.V. All rights reserved.

PACS: 77.84.-s; 81.15.Fg; 71.35.Cc; 42.65.-kKeywords: Ag/BaTiO3; Pulsed laser deposition; Surface plasmon resonance; Nonlinear optics

1. Introduction

In recent years, nanometer-sized metal particles embedded indielectric matrices (metal/dielectric composite materials) haveattracted much interest due to their possible applications for all-optical switching devices [1,2]. The nanosized metallic particlesgive rise not only to the change of color but also to the opticalenhancements in absorption, Raman scattering, luminescenceeffects, and especially in optical nonlinearity [3–5]. Theabsorption spectrum of such materials shows a strong resonancepeak due to the surface plasmon excitation of the metal clustersand at the frequencies close to the surface plasmon resonance(SPR) the third-order nonlinear optical effects are greatlyenhanced by the great increasing of the local electric field.Although there have been a lot of reports on the nanocompositethin films in which nanosized metal particles were embedded indielectric matrices, only a few studies have been reported onmetal particles embedded in BaTiO3, LiNbO3, PbTiO3, whichhas significant ferroelectricity and a very high dielectric

⁎ Corresponding author. Fax: +86 27 87543755.E-mail address: gyang@hust.edu.cn (G. Yang).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.03.027

constant [6,7]. It can be predicted that this kind of metalnanocomposite films should be interesting and significant forboth fundamental and application aspects.

During the past few years, a variety of growth methods,including magnetron sputtering, sol–gel, ion implantation, havebeen used to incorporate metal particles into dielectriccomposite films [8–10]. Most of these techniques are multi-step processes, in which a post-deposition treatment is oftenneeded to optimize the properties. Unlike these methods, pulsedlaser deposition (PLD) technique permits independent controlof synthesis of the metal nanocrystals and the embedded matrixand has shown its great advantage in stoichiometric thin filmdeposition of complex oxides. Recently, we have preparedseveral kinds of films, such as Ce-doped BaTiO3, and Rh-dopedBaTiO3 films [11,12], and observed large nonlinear opticalresponses determined by z-scan [13,14], a single beamtechnique based on the transformation of phase distortion toamplitude distortion as the laser beam traverses a nonlinearmaterial. The z-scan technique is an increasingly popularmethod for the measurement of the nonlinear absorptioncoefficient, β, and the nonlinear refractive index, n2. Comparedwith the degenerate four-wave mixing method, the great

Fig. 2. The optical absorption properties for samples A and B.

7927G. Yang et al. / Thin Solid Films 515 (2007) 7926–7929

advantages of the z-scan technique are its ability to separate theimaginary and real parts of χ(3), and to further determine thesigns of the two components.

In this paper, we report the fabrication and properties ofmulti-layer structure of barium titanate composite thin filmscontaining Ag nanoparticles using PLD technique. The linearand nonlinear optical properties of the films were discussed.

2. Experimental details

A sintered target of BaTiO3 (40 mm in diameter) wasprepared at 1340 °C for 2 h using the conventional ceramictechnique. The X-ray diffraction results showed that the targetwas composed of perovskite structure. In order to deposit themulti-layer structure of Ag/BaTiO3 films, the 0.5 mm-thicksilver chip (99.999%, 1/2 area of BaTiO3 target) was put ontothe BaTiO3 target. After finishing the deposition of BaTiO3,then the laser was focused onto the surface of Ag chip byrotating the Ag/BaTiO3 target. The Ag concentrations incomposite films can be easily controlled by changing thenumber of laser pulses on the targets of BaTiO3 and silver chip,respectively. In the PLD processing, a XeCl excimer laser beam(308 nm, 20 ns, 4 Hz, 2 J/cm2) was used as the laser source. Thetargets were mounted onto a rotating holder, 40 mm from theMgO substrates, 0.5 mm in thickness and polished on bothsides. Before the sputtering, the deposition chamber wasvacuumed to a base pressure of 5×10− 4 Pa, and thenpressurized by introducing nitrogen to 7.4 Pa. In the experi-ments, two samples, A and B, were fabricated. For sample A(B), the time for the deposition of Ag and BaTiO3 targets was0.5 min (1 min) and 4.5 min (4 min), respectively. This processwas repeated four times and the total thickness of (Ag/BaTiO3)4multi-layer films was measured to be about 150 nm. In order todetermine the Ag concentration in multi-layer films, other twosamples, corresponding to samples A and B, have been grownwith the same number of laser pulses on the Ag and BaTiO3

targets, respectively [15]. Then the Ag concentration wasmeasured by A VGESCALab-5 X-ray photoelectron spectros-copy (XPS) with MgKα (1253.6 eV) exciting radiation and theresults showed that the concentrations (Ag/Ag+Ba+Ti, at.%)

Fig. 1. The X-ray photoelectron core-level spectra of Ag3d.

of Ag particles in samples A and B were about 5.3 at.% and12.5 at.%, respectively. As a reference, the undoped BaTiO3

thin films were also grown on MgO substrates under the samecondition.

The chemical nature of Ag in the samples were detected byXPS under the vacuum at 1.3×10−8 Pa. The binding energieswere corrected with reference to the assumed value of 284.6 eVfor the resulting C1s line from the adsorbed hydrocarboncontaminant. The optical absorption properties of the films wereinvestigated in the range from 300 nm to 800 nm using a HitachiU-3410 spectrophotometer.

The signs and values of nonlinear absorption and nonlinearrefractive index were determined by z-scan using the open-aperture and small-aperture, respectively. In our measurements,a Q-switched Nd: YAG laser with frequency doubled at 532 nmand characterized by a pulse duration of 10 ns was employed asthe laser source. The samples were placed on a track near thefocus of the laser beam. The focal length of the lens was120 mm. The radius of the beam waist (ω0) was 30 μm, whichwas calculated from the following equation,

xðzÞ2 ¼ x20ð1þ z2=z2RÞ ð1Þ

where zR=πω02/λ is the Rayleigh length. Hence, the value of zR

was calculated to be 5.3 mm, much larger than the thickness ofeither substrates or films. In order to reduce the influence of thelaser power fluctuation, the transmitted beam energy, thereference beam energy and their ratio were measured by a dual-channel energy ratiometer (Rm6600, Laser Probe Corp.)simultaneously. For the small-aperture measurement, the lineartransmittance of the far-field aperture, S, defined as the ratio ofthe pulse energy passing the aperture to the total energy, wasmeasured to be 0.1. In addition, In order to reduce the possiblethermal accumulative effect we set the repetition rate to 1 Hz.

3. Results and discussion

Fig. 1 exhibits typical X-ray photoelectron core-level spectraof Ag3d lines from the surface of the samples. For samples Aand B, at least two spots were measured and the results is almost

Fig. 3. The open-aperture z-scan data of sample A. The solid line is thetheoretical fit.

Fig. 4. The small-aperture z-scan data of sample A. The solid line is thetheoretical fit.

7928 G. Yang et al. / Thin Solid Films 515 (2007) 7926–7929

the same. The dashed line in Fig. 1 is fitted using Lorenzequation. It is can be seen that the peaks of Ag3d5/2 and Ag3d3/2are located at 368.18 eV and 374.21 eV, respectively, which arecorresponding to the normal XPS spectra of Ag metal. Theatomic ratio of Ba/Ti, which could be derived from the XPS data(not shown here), was calculated to be 1:0.997, indicating thatthe stoichiometric Ag/BaTiO3 composite films were preparedusing the PLD method.

The linear optical properties of the composite films weremeasured at room temperature in air from 300 nm to 800 nm.Fig. 2 shows optical absorption spectra of samples A and B asthe function of the photon energy. The data were automaticallycorrected by the spectrophotometer to account for theabsorbance from the MgO substrates. As a reference, theoptical absorption spectrum of undoped BaTiO3 films was alsorecorded. Without Ag in the BaTiO3 films, no absorption peakwas observed. When the BaTiO3 films were doped with Ag, thestrong absorption peaks due to the surface plasmon resonance ofAg particles were clearly found (see Fig. 2 curves a and b).

The surface plasmon resonance of metal nanoparticles havebeen studied widely before, and the absorption properties of thecomposite films can be given by [3],

a0 ¼ 18pe3=2m

kpe2

ðe1 þ 2emÞ2 þ e22ð2Þ

where p is the volume fraction occupied by the metal particles,ε1 and ε2 are the real and the imaginary parts of the dielectricconstant of the metal, εm is the dielectric constant of the matrix.The absorption peak due to the SPR of metal particles occurs atthe frequency where ε1+2εm=0.

From Fig. 2, it can be seen that for sample A, with Agconcentration being 5.3 at.%, the absorption peak is located atabout 433 nm. For sample B, with Ag concentration being muchhigher, the peak is found to be about 469 nm, which is largerthan that observed in sample A. The results indicate the positionof absorption peak changes to longer wavelength, called red-shift, with the increasing of Ag concentration. This phenom-enon was in good agreement with that reported by others [16].

Fig. 3 shows the z-scan results without an aperture (open-aperture) for the Ag/BaTiO3 thin films (sample A). The curveshown in Fig. 3 comprises a normalized transmittance peak,indicating the presence of nonlinear saturation in the compositefilms. The shape of the z-scan results for the MgO substrate isalmost flat, which suggests that the substrates have a very smallnonlinear optical effect that can be neglected, so the largeoptical nonlinearity observed here results from the Ag/BaTiO3

films. The nonlinear absorption coefficient β (m/W), defined asα=α0+βI, can be calculated using the equation [14],

Tðz; S ¼ 1Þ ¼Xl

m¼0

½1−bI0Leff=ð1þ z2=z2RÞ�mðmþ 1Þ3=2

ð3Þ

where I0=E0/πω02τ is the laser intensity at the focal point,

E0 =82.5 μJ is the pulse energy at the focal spot, T(S=1) thenormalized transmittance for the open-aperture, Leff = [1−exp(−αL)] /α the effective thickness of the films, L the filmsthickness, α the linear absorption coefficient of the films at532 nm where the z-scan measurements were made.

Fig. 4 shows the z-scan experimental data with a small aper-ture for the films. In order to obtain nonlinear refraction in-formation, a simple and approximate method was used that thesmall-aperture transmittance was divided by the correspondingopen-aperture data. The shape of the z-scan exhibits a negativevalue for the nonlinear refractive index. From Fig. 4 we can alsofind that the distance between the peak and the valley (ΔTp–v) isabout 9 mm as compared to 1.71 zR, which indicates that thenonlinear effect is the third-order response [14]. The nonlinearoptical refractive index n2 (m2/W), defined as n=n0+n2I, iscalculated using the following equation,

n2ðm2=W Þ ¼ 1:232ksn20DTp�v=ð1−SÞ0:25LeffE ð4Þ

where n0 is the linear refractive index of the samples, λ the lightwavelength, τ the laser pulse width, ΔTp–v=0.39, c the velocityof the light.

7929G. Yang et al. / Thin Solid Films 515 (2007) 7926–7929

The calculated values of n2 and β of sample A were−1.91×10−13 m2/W and −5.80×10−7 m/W, respectively. Thereal and imaginary parts of the χ(3) of the films can be obtainedby the following equations [17],

Revð3ÞðesuÞ ¼ cn20n2ðm2=W Þ=120k2 ð5Þ

Imvð3ÞðesuÞ ¼ c2n20bðm=W Þ=240k2x ð6Þ

where ω=2πc/λ is the angular frequency of the light field.From Eqs. (5) and (6), the values of Reχ(3) and Imχ(3) were

calculated to be −2.56×10−7 esu and −1.45×10−8 esu, re-spectively, indicating the large third-order nonlinear opticalproperties in Ag/BaTiO3 multi-layer films. The above measure-ments were repeated on different spots of the same sample inorder to check the uniformity of the films. The z-scan results forsample B were similar to that of sample A, but the signal wastoo weak to get precise data due to the thicker color for higherAg concentration.

For practical applications, the more useful parameter is thefigure of merit, χ(3)/α0. At the wavelength of 532 nm, the valuesof α0 and χ(3)/α0 of sample A were calculated to be1.07×104 cm−1 and 2.40×10−11 esu cm−1, respectively. It isworth noting that the values of χ(3) and χ(3)/α0 are much largerthan those obtained for other metal-doped composite films. Thehigh optical nonlinearities for such a kind of metallicnanocluster-composite films were known to stem from thegreat enhancement of the local electric fields near and inside themetal particles. Meanwhile, as we know, the nonlinear opticalproperties of metal nanocomposites are related not only to themetal itself, but also to the matrix in which the metal wasembedded. In this work the embedding matrix BaTiO3 is aferroelectric material with very high dielectric constant, whichalso contributes to the enhancement of the nonlinearities.Recently, Yang and Wang also reported the optical nonlinea-rities in Ag:BaTiO3 and Au:BaTiO3 nanocomposite films [6,7],suggesting the great potential optical applications of metal-ferroelectric composite system.

For our results, the large third-order nonlinear opticalsusceptibility ofAg/BaTiO3multi-layer films compares favorablywith the large nonlinearities found in some representativenonlinear optical materials, such as some transition-metal oxidethin films (3.10×10−8 esu) [18], Cu-doped Al2O3 thin films(2.06×10−8 esu) [19], Ce:BaTiO3 quantumdots (2.21×10−8 esu)[11], and organic polymer (1.07×10−7 esu) [20].

4. Conclusion

In summary, the multi-layer structures of Ag/BaTiO3

composite films were prepared on MgO substrates by PLDtechnique. The films, 150 nm in thickness, were grown under

the nitrogen pressure of 7.4 Pa. The XPS data indicated that thesamples were composed of metal Ag embedded in the BaTiO3

matrices. The absorption peaks due to the surface plasmonresonance of Ag particles were observed at 433 nm and 469 nmfor the different Ag concentration. With the increasing of Agconcentration in composite films, the peak absorption increasedand shifted to longer wavelength (red-shift). Furthermore, thethird-order nonlinear optical properties of the films weredetermined by z-scan method and the signs and values ofnonlinear refractive index and nonlinear absorption coefficientwere obtained, respectively.

Acknowledgements

The authors acknowledge the support of the National NaturalScience Foundation of China (10604018) and the ScientificResearch Foundation for the Returned Overseas ChineseScholars, State Education Ministry.

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