ARTICLE IN PRESS

Physica B 404 (2009) 4295–4298

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

� Corr

E-m

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

Nonlinear optical properties of Cu nanocluster composite fabricated by180 keV ion implantation

Y.H. Wang a, Y.M. Wang b,�, J.D. Lu a, L.L. Ji a, R.G. Zang a, R.W. Wang a

a Hubei Province Key Laboratory of Systems Science in Metallurgical Process Wuhan University of Science and Technology, Wuhan 430081, Chinab Department of Nephrology, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China

a r t i c l e i n f o

Article history:

Received 7 October 2008

Received in revised form

30 July 2009

Accepted 3 August 2009

PACS:

61.46.Df

61.72.Ww

42.65.Hw

Keywords:

Ion implantation

Cu nanoclusters

Optical nonlinearity

Z-scan

26/$ - see front matter & 2009 Elsevier B.V. A

016/j.physb.2009.08.016

esponding author. Tel.: +86 27 61094568.

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

a b s t r a c t

Metal nanocluster composite glass prepared by 180 keV Cu ions into silica with dose of 5�1016 ions/cm2

has been studied. The microstructural properties of the nanoclusters has been verified by optical

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

properties of the nanoclusters were 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 were deduced. Results of the investigation of nonlinear refraction

by the off-axis Z-scan configuration were presented and the mechanisms responsible for the nonlinear

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

determined to be 8.7�10�8 esu at 532 nm and 6.0�10�8 esu at 1064 nm, respectively.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Metal nanoclusters possess linear and nonlinear opticalproperties. Increasingly attention has focused on 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 are influ-enced not only by the type and size of the embedded metalclusters, but also by the dielectric constant, thermal conductivityand heat capacity of the dielectric matrices [1–6]. The mostconspicuous manifestation of confinement in optical properties ofmetal nanocluster composite glasses (MNCGs) is the appearanceof the surface plasmon resonance (SPR) that strongly enhancestheir linear and nonlinear responses around SPR wavelength[7–9]. Amongst the nanoclusters studied by earlier papers, highnonlinear absorption and nonlinear refraction coefficients arefound in copper and copper containing nanomaterials [10–12].

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

ll rights reserved.

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

In this paper, MNCGs were prepared by Cu+ 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-length of 532 and 1064 nm.

2. Experiment

Silica slides were implanted at room temperature with copperions at 180 keV. The current density of ion implantation was1.5mA/cm2. Optical absorption spectra was recorded at roomtemperature using a UV–vis dual-beam spectrophotometer withwavelengths from 1200 to 300 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 the shape ofnanoclusters.

The measurements of third-order nonlinear optical of thesample were carried out by using the standard Z-scan method.

ARTICLE IN PRESS

0.1

0.2

0.3

0.4

0.5

perc

enta

ge

Y.H. Wang et al. / Physica B 404 (2009) 4295–42984296

The excitation source was 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) were used for excitation in the experiment. The detectorwas a dual-channels energy meter (EPM2000). With a converginglens of f ¼ 260 mm, the radiuses of the Gaussian beam spot atfocal waist $0 were about 45 and 26mm for 1064 and 532 nm,respectively. In the Z-scan test, the sample was moved step bystep along the propagation direction of the Gaussian beam underthe control of a PC. Meanwhile, a detector monitored thetransmitted laser power and the signals were sent back to thecomputer and recorded. Nonlinear refraction and nonlinearabsorption were performed by both open- and closed-apertureZ-scans of a series of the samples at room temperature.

00.0

Diameter/nm2 4 6 8 10 12

Fig. 2. Comparatively size distribution profiles of 5�1016 Cu+ ions/cm2 nanoclus-

ters in silica sample.

2000.0

0.5

1.0

1064 nm

532 nm

Opt

ical

den

sity

(a.u

.)

Wavelength (nm)400 600 800 1000 1200

Fig. 3. Optical absorption spectra of the Cu implanted sample to dose of

5�1016 ions/cm2.

3. Result and discussion

The TEM micrograph for the sample implanted by 5�1016 Cu+

ions/cm2 is shown in Fig. 1. As can be seen from the image,spherical copper clusters are formed during the implantationprocess; the particle size distribution is not uniform. The size ofnanoclusters varies from 1 to 5 nm. Then the comparative sizedistribution of Cu nanoclusters is shown in Fig. 2. The average sizeof nanoclusters in this sample is 3.2 nm.

The linear optical absorption spectra of the sample investi-gated is shown in Fig. 3. The spectra ranges between 200 and1200 nm. Only increasing shoulders of the sample was observed inthe range of 500–650 nm. The dependence of this absorption bandon the mean cluster diameter has been reported, and it has beenshown that the band becomes noticeable and sharpens only whenthe diameter is about larger than 5 nm [13,14]. The Cu cluster sizesestimated from the absorption spectra are thus consistent withthe values obtained from Fig. 2. This selective absorption band isdue to the surface plasmon resonance (SPR) that extra evidence ofCu 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 as b,which includes saturated absorption (SA) and reversed saturatedabsorption (RSA) [15]. The nonlinear absorption is expressed asa 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[16]; and the techniques are simple and sensitive experimentaltechniques for studying nonlinear optical properties and deter-mining the sign of the nonlinear refractive and absorption indices.

Fig. 1. Cross-sectional TEM image for the sample implanted by 180 keV, 5�1016

Cu+ ions/cm2.

The open- and closed-aperture Z-scan curves are theoreticallyfitted by [16]:

TðzÞ ¼X1

m¼0

½�q0ðzÞ�m

ð1þ x2Þmðmþ 1Þ3=2

ðmZ0Þ ð1Þ

TðzÞ ¼ 1þ4DF0x

ðx2 þ 9Þðx2 þ 1Þð2Þ

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 , whereI0 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 ½1� expð�a0LÞ�=a0. The nonlinear refractive index iscalculated by DF0 ¼ ð2p=lÞgI0Leff , where 2p/l is the wave vectorof the incident laser.

Normalized open-aperture Z-scan of sample is displayed inFig. 4(a). The open-aperture measurement shows an obviousenhanced transmittance near the focus, occurring due tothe saturation of absorption. This reveals negative nonlinear

ARTICLE IN PRESS

-200.8

1.0

1.2

1.4

1.6

Nor

mal

ized

Tra

nsm

ittan

ce

Z (mm)

Experiment Theoritical fit

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Nor

mal

ized

Tra

nsm

ittan

ce

Experiment Theoritical fit

-10 0 10 20

-20Z (mm)

-10 0 10 20

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

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

-300.6

0.8

1.0

1.2

1.4

Nor

mal

ized

Tra

nsm

ittan

ce

Z (mm)

Experiment Theoritical fit

-25 -20 -15 -10 -5 0 5 10 15 20 25 30

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

line: theoretical curve.

Y.H. Wang et al. / Physica B 404 (2009) 4295–4298 4297

absorption coefficient. For visible light and for particle diametersdrl, 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 [17]. The excitation wavelength of 532 nm is close to theSPR peaks of this sample, in which Cu nanoclusters display thestronger saturable absorption performance.

The nonlinear refraction is obtained by dividing the closed-aperture data by the open-aperture data [16,18]. In Fig. 4(b),the peak-valley configuration indicates the negative sign of thenonlinear refractive index (n2o0). Third-order nonlinear opticalproperties 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 is found from the peak-valley curve ofclosed-aperture data. The nonlinear property of the bare silicasubstrate is measured and gets no detectable change of thetransmitted intensity under same Z-scan conditions. Also, it isnoticed that if the laser peak intensity is larger than 15 GW/cm2,there is a probability for the high absorbing materials to bedamaged at the tested point due to accumulative heating. This

heating will produce an ablation hole. The closed-aperture Z-scancurve of the ablation hole is symmetric and has a peak-valleypattern, which is similar to materials with negative nonlinearrefraction [19]. So the peak intensity of 0.9 GW/cm2 is selected forthe sample at 532 nm and 0.38 GW/cm2 at 1064 nm. In ourexperiment, the asymmetric curve does not show significantchange when repeated at the same point, suggesting no formationof the ablation hole in this sample during the Z-scan.

In our experiments, Leff (nm) for sample is 77 nm. The solidcurve in Fig. 4(a) is fitted by using Eq. (1) with the experimentparameters and the nonlinear absorption coefficient obtained areb ¼ �151 cm/GW for 532 nm. Fitting the Z-scan data of the closed-aperture with Eq. (2), we get values of gE�6.1�10�11 cm2/W for532 nm and gE�5.6�10�11 cm2/W for 1064 nm. The absolutevalue of third-order nonlinear susceptibility w(3) for Cu+ implantedsample is calculated using the following equations [16,18]:

DTp�v ¼ 0:406ð1� SÞ0:25jDf0j ð3Þ

Rewð3Þ ¼ 2n20e0cg ð4Þ

Imwð3Þ ¼ ðl=2pÞn20e0cb ð5Þ

wð3Þ ¼ ½ðwð3ÞRe Þ2þ ðwð3ÞImÞ

2�1=2 ð6Þ

Thus, we obtain the absolute value of w(3) are 8.7�10�8 esu for532 nm and 6.0�10�8 esu for 1064 nm.

Tanahashi et al. [8] detected negative value for g in silicacontaining copper clusters. We find that our data is twice as largeas theirs. Probably due to the low pulse duration they used.(150 fs). Ganeev et al. [20] 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. [21] considered the local-fieldenhancement factor strongly depended on the particle size andcomposition. The third-order nonlinear response of the presentcomposite material thus mainly originates from electronic effectsin copper 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 thelevels 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 by

ARTICLE IN PRESS

Y.H. Wang et al. / Physica B 404 (2009) 4295–42984298

the 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) [22].

Refractive index changes due to thermal nonlinearitiesarise due to density changes in the materials propagating withacoustic wave speed caused by heating. In this paper, becauseabsorption at 532 nm is very high, intra-pulse thermal effectscould still be present even for ps pulses. A refractive index changesin accordance to relation Dntherm ¼ ðdn=dtÞDT . dn/dT derivativeof MNCGs prevailing negative sign, thus leading to the self-defocusing of a laser beam propagated through medium [16].An order of magnitude estimate of the thermal contribution maybe presented. Our studies have shown the negative sign ofrefractive index in 532 nm is about �3.4�10�8 esu due to thermaleffect. Thermal heating induced by a single laser pulse persistsover some characteristic time tc. As a result, when the timeinterval between consecutive laser pulses is shorter than tc, thethermal effect increases. It is a common assumption that Z-scanmeasurements should be made with repetition rate of few ofHertz in order to extract a nonlinear refractive index influenced byonly electronic 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 speaking, the value of D rangesbetween 1�10�7 and 6�10�7 m2/s. The magnitude of thecalculated tc is within 10�3 s, which is much smaller than thetime interval between consecutive laser pulses 0.1 s used in ourexperiment. [23]

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, Cu+ implanted nanostructures samplesdiffer in size and shape as well as surface structure films, so do inthe nonlinear optical behaviors. In this work, the formations ofCu nanoclusters are incorporated into silica will give a newcircumstance in the study of the nonlinear optical response.

4. Conclusion

In summary, Cu nanoclusters in silica has been formed by theion implantation of Cu+ ions. The nonlinear optical property ofthis sample was investigated by the Z-scan technique. Thew(3) measured at 532 nm manifests a real part of �6.1�10�8 esuand an imaginary part of �5.6�10�8 esu. During the 1064 nm

excitation, the sample had no nonlinear absorption and w(3)

is �6.0�10�8 esu which all come from nonlinear refractioncontribution. The results show that this new structure metalnanoclusters will give a new circumstance in the study of themetal nonlinear optical response. The further studies are inprogress.

Acknowledgments

This work was supported by the Natural Science Foundation ofChina (no.10805035).

References

[1] G. Battaglin, P. Calvelli, E. Cattaruzza, F. Gonella, R. Polloni, G. Mattei,P. Mazzoldi, Appl. Phys. Lett. 78 (2001) 3953.

[2] Y.H. Wang, J.D. Lu, R.W. Wang, Y.L. Mao, Y.G. Cheng, Vacuum 82 (2008) 1220.[3] J. Olivares, J. Requejo-Isidro, R. del Coso, R. de Nalda, J. Solis, et al., J. Appl.

Phys. 90 (2001) 1064.[4] Y.H. Wang, F. Ren, Q.Q. Wang, D.J. Chen, D.J. Fu, C.Z. Jiang, Phys.Lett. A 357

(2006) 364.[5] R.F. Haglund Jr., L. Yang, R.H. Magruder, J.E. Wittig, K. Becker, R.A. Zuhr, Opt.

Lett. 18 (1993) 373.[6] Y.H. Wang, C.Z. Jiang, F. Ren, Q.Q. Wang, D.J. Chen, D.J. Fu, J. Mater. Sci. 42

(2007) 7294.[7] A.I. Ryasnyanskiy, B. Palpant, S. Debrus, U. Pal, A.L. Stepanov, Opt. Commun.

273 (2007) 538.[8] I. Tanahashi, H. Inouye, K. Tanaka, A. Mito, Jpn. J. Appl. Phys. 38 (1999) 5079.[9] P. Wang, Y. Lu, L. Tang, J. Zhang, H. Ming, J. Xie, F.H. Ho, H.H. Chang, H.Y. Lin,

D.P. Tsai, Opt. Commun. 229 (2004) 425.[10] Y.H. Wang, C.Z. Jiang, F. Ren, Q.Q. Wang, D.J. Chen, D.J. Fu, Phys. E 33 (2006)

244.[11] Y.H. Wang, C.Z. Jiang, X.H. Xiao, Y.G. Cheng, Phys. B 403 (2008) 2143.[12] Y.H. Wang, S.J. Peng, J.D. Lu, R.W. Wang, Y.G. Cheng, Y.L. Mao, Vacuum 83

(2009) 408.[13] R.H. Magruder III, R.F. Haglund Jr., L. Yang, J.E. Wittig, R.A. Zuhr, J. Appl. Phys.

76 (1994) 708.[14] Y. Takeda, V.T. Gritsyna, N. Umeda, C.G. Lee, N. Kishimoto, Nucl. Instrum.

Methods Phys. Res. B 148 (1999) 1029.[15] S. Couris, E. Koudoumas, A.A. Rutht, S. Leach, J. Phys. B: At. Mol. Opt. Phys. 28

(1995) 4537.[16] M. Sheik-Bahae, A.A. Said, T.H. Wei, et al., IEEE J. Quant. Electron. 26 (1990)

760.[17] Shiliang Qu, Yawen Zhang, Huajun Li, Jianrong Qiu, Congshan Zhu, Opt. Mater.

28 (2006) 259.[18] M. Sheik-Bahae, D.J. Hagan, E.W. Ban Stryland, Phys. Rev. Lett. 65 (1990) 96.[19] G. Battaglin, P. Calvelli, E. Cattaruzza, R. Polloni, E. Borsella, T. Cesca,

F. Gonella, P. Mazzoldi, J. Opt. Soc. Am. B 17 (2000) 213.[20] R.A. Ganeev, A.I. Ryasnyansky, S.R. Kamalov, et al., J. Phys. D: Appl. Phys. 34

(2001) 1602.[21] E. Cattaruzza, G. Battaglin, P. Calvelli, F. Gonella, et al., Compos. Sci. Tech. 63

(2003) 1203.[22] F. Hache, D. Ricard, C. Flytzanis, U. Kreibig, Appl. Phys. A 47 (1988) 347.[23] A. Gnoli, L. Razzari, M. Righini, Opt. Express 13 (2005) 7976.