ARTICLE IN PRESS

Physica B 403 (2008) 3399– 3402

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Optical nonlinearities of Au nanocluster composite fabricatedby 300 keV ion implantation

Y.H. Wang a,�, J.D. Lu a, R.W. Wang a, S.J. Peng a, Y.L. Mao b, Y.G. Cheng b

a Department of Applied Physics, Wuhan University of Science and Technology, Wuhan 430081, Chinab Department of Physics, Henan University, Kaifeng 475001, China

a r t i c l e i n f o

Article history:

Received 26 January 2008

Received in revised form

23 April 2008

Accepted 28 April 2008

PACS:

61.46.Df

61.72.Ww

42.65.Hw

Keywords:

Ion implantation

Gold nanoclusters

Optical nonlinearity

Z-scan

26/$ - see front matter & 2008 Elsevier B.V. A

016/j.physb.2008.04.040

esponding author. Tel.: +86 27 61094568.

ail address: wyh61@163.com (Y.H. Wang).

a b s t r a c t

Metal nanocluster-glass composite prepared by implantation of 300 keV Au ions into silica at a dose of

1�1017 ions/cm2 has been studied. The microstructural properties of the nanoclusters are characterized

by optical absorption spectra and transmission electron microscopy (TEM). Third-order nonlinear

optical properties of the nanoclusters are measured at 1064 and 532 nm excitations using Z-scan

technique. The nonlinear refraction index, nonlinear absorption coefficient, and the real and imaginary

parts of the third-order nonlinear susceptibility are deduced. Results of the investigation of nonlinear

refraction using the off-axis Z-scan configuration are presented and the mechanisms responsible for the

nonlinear response are discussed. Third-order nonlinear susceptibility w(3) of this kind of sample was

determined to be 1.3�10�7 esu at 532 nm and 4.3�10�8 esu at 1064 nm, respectively.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Metal nanoclusters possess linear and nonlinear optical proper-ties. There has been an increasing interest in the third-ordernonlinear susceptibility and the photorefractive effect of noble-metal clusters embedded in dielectric matrices [1–3]. Third-ordernonlinearities of metal/dielectric composite materials were influ-enced by the type and size of the embedded metal clusters, by thedielectric constant, thermal conductivity and heat capacity of thedielectric matrices [1–6]. The most conspicuous manifestation ofconfinement in optical properties of metal nanocluster compositeglasses (MNCGs) is the appearance of the surface plasmonresonance (SPR) that strongly enhances their linear and nonlinearresponses around SPR wavelength [7–9]. Amongst the nanoclustersstudied by earlier researchers, gold and gold-containing nanoma-terials have high nonlinear absorption and nonlinear refractioncoefficients [10–12].

Ion implantation has been shown to produce high-densitymetal colloids in glasses. The high-precipitate volume fraction andthe small size of nanoclusters in glasses lead to the generationof third-order susceptibility much greater than those for metal-

ll rights reserved.

doped solid. The third-order optical nonlinear responses ofthe metal nanocluster–glass composites can be understood inthe framework of dielectric and quantum confinement effects.Application aspects of the material are most relevant to thechange in optical properties versus the nanocluster structure.

In this paper, MNCGs were prepared by Au+ implantation intosilica. We focused our interest on studying the nonlinear opticalproperties of this kind of metal nanoclusters. Nonlinear opticalproperties were measured by Z-scan method under the wave-lengths of 532 and 1064 nm.

2. Experiment

Silica slides were implanted at room temperature with goldions at 300 keV. The current density of ion implantation was2.5mA/cm2. Optical absorption spectra were recorded at roomtemperature using a UV–VIS dual-beam spectrophotometer withwavelengths from 1200 to 200 nm. Transmission electron micro-scopy (TEM) observations were carried out with a JEOL JEM 2010(HT) microscope operated at 200 kV. TEM bright field images wereused to determine the size distribution and shape of nanoclusters.

The measurements of third-order optical nonlinearities ofthe sample were carried out using the standard Z-scan method.

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The excitation source is a mode-locked Nd:YAG laser (PY61-10,Continnum), with a pulse duration of 38 ps and a repetitionfrequency of 10 Hz. 1064 nm wavelength and doubled frequency(532 nm) are used for excitation in the experiment. The detector isa dual-channels energy meter (EPM2000). With a converging lensof f ¼ 260 mm, the radius of the Gaussian beam spot at focal waist$0 were about 45 and 26 mm for 1064 and 532 nm, respectively. Inthe Z-scan test, the sample was moved step by step along thepropagation direction of the Gaussian beam under the control of aPC. Meanwhile, a detector monitored the transmitted laser powerand the signals were sent back to the computer and recorded.Nonlinear refraction and nonlinear absorption were performed byboth open- and closed-aperture Z-scans of a series of the samplesat room temperature.

0 3 6 9 12 15 18 21 24 27 30

0.0

Diameter/nm

Fig. 2. Comparatively size distribution profiles of 1�1017 Au+ ions/cm2 nanoclus-

ters in silica sample.

200 400 600 800 1000 1200

0.0

0.5

1.0

1.5

2.0

1064 nm

532 nm

Opt

ical

den

sity

(a.u

.)

Wavelength (nm)

Fig. 3. Optical absorption spectra of the Au-implanted sample to a dose of

1�1017 ions/cm2.

3. Results and discussion

The TEM micrograph for the sample implanted at1�1017 Au+ ions/cm2 are shown in Fig. 1. As can be seen from theimage, spherical gold clusters are formed during the implant-ation process; the particle size distribution is not uniform.The size of nanoclusters varies from 3 to 25 nm. Interestingly,the comparative size distributions of Au nanoclusters areshown in Fig. 2. The average size of nanoclusters in this sampleis 6.9 nm.

The linear optical absorption spectra of the sample investi-gated are shown in Fig. 3. The spectra range from 200 to 1200 nm.The SPR absorption peaks of Au nanoclusters around 535 nm areobserved, superimposed on the monotonous absorption stem-ming from interband transitions in gold nanoclusters. Thisselective absorption band is due to the SPR that is extra evidenceof Au nanoclusters formation. The various multipoles excitationsmay compensate each other and lead to large apparent widths ofthe resonances.

The nonlinear absorption in the sample can be described by b,which includes saturated absorption (SA) and reversed saturatedabsorption (RSA) [13]. The nonlinear absorption is expressed bya ¼ a0+bI, where a0 is the linear absorption coefficient of thesample and I is the intensity of the laser. The third-order nonlinearabsorption and refraction are investigated by Z-scan techniques[14]. This technique is a simple and sensitive experimentaltechnique for the study of nonlinear optical properties and allowsdetermining the sign of the nonlinear refractive and absorptionindices. The open- and closed-aperture Z-scan curves are

Fig. 1. Cross-sectional TEM image for the sample implanted by 300 keV,

1�1017 Au+ ions/cm2.

theoretically fitted by [14]

TðzÞ ¼X1

m¼0

½�q0ðzÞ�m

ð1þ x2Þmðmþ 1Þ3=2

ðmX0Þ (1)

TðzÞ ¼ 1þ4DF0x

ðx2 þ 9Þðx2 þ 1Þ(2)

where x ¼ z/z0, T is the normalized transmittance and z is thedistance along the lens axis in the far field. The nonlinearabsorption coefficient b can be obtained by q0 ¼ bI0Leff, where I0

is the intensity of the laser beam at the focus (z ¼ 0), Leff is theeffective thickness of the sample, which can be calculated fromthe real thickness L and the linear absorption coefficient a0, in theform of Leff ¼ [1�exp(�a0L)]/a0. The nonlinear refractive index iscalculated by DF0 ¼ (2p/l)gI0Leff, where 2p/l is the wave vector ofthe incident laser.

A normalized open-aperture Z-scan of the sample is displayedin Fig. 4(a). The open-aperture measurement shows an obvious

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Fig. 4. Z-scan experiment results for 532 nm normalized open-aperture (a) and

the divided result (b). Solid line: theoretical curve.

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Fig. 5. Z-scan experiment results for 1064 nm normalized close-aperture. Solid

line: theoretical curve.

Y.H. Wang et al. / Physica B 403 (2008) 3399–3402 3401

enhanced transmittance near the focus, occurring due to thesaturation of absorption. This reveals negative nonlinear absorp-tion coefficient. For visible light and for particle diameters dpl,where l is the vacuum wavelength of the optical wave, only theelectric-dipole contribution to the re-radiated optical field needsto be taken into account. Metal valence electrons move freelyinside the sphere. The particles form a strong dipole in appliedelectric field, enhancing the local field and the polarization ofdielectric [12]. The excitation wavelength of 532 nm was close tothe SPR peaks of this sample, in which Au nanoclusters display thestronger saturable absorption performance.

The nonlinear refraction was obtained by dividing the closed-aperture data by the open-aperture data [14,15]. In Fig. 4(b), thepeak–valley configuration indicates the negative sign of thenonlinear refractive index (n2o0). Third-order nonlinear opticalproperty of this kind of sample of 1064 nm is shown in Fig. 5; theopen-aperture Z-scan shows no nonlinear signal, which indicatesthat the sample has no nonlinear absorption at 1064 nm. A self-defocusing refraction also was found from the peak–valley curveof closed-aperture data. The nonlinear property of the bare silicasubstrate was measured and shows no detectable change of thetransmitted intensity under the same Z-scan conditions. Also,it is noticed that, if the laser peak intensity was larger than15 GW/cm2, there was a probability for the high-absorbingmaterials to be damaged at the tested point due to accumulativeheating. This heating will produce an ablation hole. The closed-

aperture Z-scan curve of the ablation hole is symmetric and has apeak–valley pattern, which is similar to materials with negativenonlinear refraction [16]. So the peak intensity of 0.9 GW/cm2

was selected for the sample at 532 nm and 0.38 GW/cm2 at1064 nm. In our experiment, the asymmetric curve does not showsignificant change when repeated at the same point, suggestingno formation of the ablation hole in this sample during the Z-scan.

In our experiments, Leff (nm) for the sample is 110 nm.The solid curve in Fig. 4(a) is fitted by using Eq. (1) with theexperiment parameters and the nonlinear absorption coefficientobtained is b ¼ �97 cm/GW for 532 nm. Fitting the Z-scan data ofthe closed-aperture with Eq. (2), we get values ofgE�1.2�10�10 cm2/W for 532 nm and gE�0.4�10�10 cm2/Wfor 1064 nm. The absolute value of third-order nonlinear suscept-ibility w(3) for Au+-implanted sample is calculated using thefollowing equations [14,15]:

DTp�v ¼ 0:406ð1� SÞ0:25jDf0j; (3)

Re wð3Þ ¼ 2n20�0cg (4)

Im wð3Þ ¼ ðl=2pÞn20�0cb (5)

wð3Þ ¼ ½ðwð3ÞRe Þ2þ ðwð3ÞImÞ

2�1=2 (6)

Thus, we obtain the absolute values of w(3) as 1.3�10�7 esu for532 nm and 4.3�10�8 esu for 1064 nm.

Tanahashi et al. [8] detected negative value for g in silica-containing copper clusters. We find our data three times largerthan theirs, probably due to the low pulse duration they used(150 fs). Ganeev et al. [17] studied the optical nonlinearities ofdifferent metal colloidal solutions by Z-scan, finding negative gvalues for the gold and copper cases at the initial stages ofaggregation. Cattaruzza et al. [7] think the local-field enhance-ment factor strongly dependent on the particle size andcomposition. The third-order nonlinear response of the presentcomposite material thus mainly originates from electronic effectsin gold nanoclusters. These electronic contributions are due toboth intraband and interband transitions. The first one corre-sponds to transitions within the conduction band and the secondone to transitions from the upper levels of the filled d-band to the

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levels above the Fermi level in the conduction band. Whereas onlyintraband contribution to the nanoclusters intrinsic third-ordersusceptibility is thought to be size-dependent, it is dominated bythe size-independent interband contribution in the SPR spectraldomain. In the case where the clusters are excited by ultra-shortlaser pulses, a hot electron phenomenon may superimpose on thepure electronic nonlinear contributions to w(3) [18].

Refractive index changes due to thermal nonlinearities arisedue to density changes in the materials propagating with acousticwave speed caused by heating. In this paper, because absorptionat 532 nm is very high, intra-pulse thermal effects could stillbe present even for ps pulses. A refective index changes inaccordance to the relation Dntherm ¼ (dn/dT)DT. The dn/dT deriva-tive of MNCGs shows negative sign, thus indicating to the self-defocusing of a laser beam propagated through medium [14]. Anorder of magnitude estimate of the thermal contribution may bepresented. Our studies have shown the negative sign of refrectiveindex in 532 nm is about �3.4�10�8 esu due to thermal effect.Thermal heating induced by a single laser pulse persists oversome characteristic time tc. As a result, when the time intervalbetween consecutive laser pulses is shorter than tc, the thermaleffect increases. It is a common assumption that Z-scan measure-ments should be made with a repetition rate of a few Hertz inorder to extract a nonlinear refractive index influenced by onlyelectronic effects. The time scale of this cumulative process isgiven by tc ¼ $0

2/4D, where D is the thermal diffusion coefficientof the materials. Generally, the value of D ranges from 1�10�7 to6�10�7 m2/s. The magnitude of the calculated tc is within 10�3 s,which is much smaller than the time interval between con-secutive laser pulses (0.1 s) used in our experiment [19].

Longer pulse duration of the excitation wavelength would leadto combined mechanisms including both optical processes ofquick response time and slower ones. Excitation frequency isanother cause of the variance. Various wavelengths may inducedifferent nonlinear optical transitions. Moreover, for diversepreparation techniques, Au+ implanted nanostructure samplesdiffer in size and shape as well as surface structure films, and sodo in the nonlinear optical behaviors. In this work, the formationsof Au nanoclusters are incorporated into silica, which gives a newcircumstance in the study of the nonlinear optical response.

4. Conclusion

In summary, Au nanoclusters in silica are formed by the ionimplantation of Au+ ions. The nonlinear optical property of thissample was investigated by the Z-scan technique. The w(3)

measured at 532 nm manifests a real part of �1.2�10�7 esu andan imaginary part of �3.6�10�8 esu. During the 1064 nmexcitation, the sample has no nonlinear absorption and w(3) is�4.3�10�8 esu, all of which come from nonlinear refractioncontribution. The results show that this new structure metalnanocluster gives a new circumstance in the study of the metalnonlinear optical response. Further studies are in progress.

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