ARTICLE IN PRESS

Physica B 405 (2010) 2664–2667

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

n Corr

E-m

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

Experimental analysis of optical limiting properties of Cu nanoclusters

Y.H. Wang a, Y.M. Wang b,n, C.J. Han a, J.D. Lu a, L.L. Ji 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 20 January 2010

Received in revised form

19 March 2010

Accepted 19 March 2010

Keywords:

Ion implantation

Cu nanoclusters

Optical limiting

26/$ - see front matter & 2010 Elsevier B.V. A

016/j.physb.2010.03.046

esponding author. Tel.: +86 27 61094568.

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

a b s t r a c t

Optical nonlinearities of Cu nanoclusters prepared by implanting Cu ions into silica at 180 keV

with dose of 1�1017 ions/cm2 have been studied. The third-order nonlinear optical properties of

the nanoclusters were measured at 1064 nm excitations using Z-scan technique. 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. Innovative point of this paper

is that we carried out curve fitting analysis, which is based on MATLAB features, for Cu optical

limiting experiment. Our results showed that Cu nanoclusters have refractive optical limiting effect at

1064 nm.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Potential applications of optical limiters in the protection ofsensors from intense laser pulses have motivated many efforts todesign new nonlinear optical systems [1]. Recently, increasingattention has been focused on the third-order nonlinear suscept-ibility and the photorefractive effect of noble-metal clustersembedded in dielectric matrices [2,3]. Third-order nonlinearitiesof metal/dielectric composite materials are influenced not only bythe type and size of the embedded metal clusters but also by thedielectric constant, thermal conductivity, and heat capacity of thedielectric matrices [2–6]. Amongst the nanoclusters studied byearlier papers, high nonlinear absorption and nonlinear refractioncoefficients are found in copper and copper containing nanoma-terials [7,8].

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 a greater third-order susceptibility than those of metal doped solid. The third-order nonlinear optical responses of the metal nanocluster–glasscomposites can be understood in the framework of dielectric andquantum confinement effects. Optical nonlinearities and opticallimiting effects of the nanocomposites with metal nanoparticlescan be significantly enhanced by increasing the number densityand the size of metal particles [9]. Application aspects of thematerial are closely related to the change of optical propertiesversus the nanocluster structure.

ll rights reserved.

At present, optical limiting results obtained by the analysis ofexperimental practice including the completion of the researchtasks, engineers and technicians draw, are usually analyzed byauxiliary tools, such as Microcal Origin, Microsoft Excel, and so on.Although these analysis tools provide great help to variousexperimental results, all of these tools are direct applicationsoftware, which is not comprehensive enough to fitting curve, andsome limitations still remain on analysis of experimental data.

MATLAB incorporates engineering calculation, visual functionof figure into an organic whole, and offers Windows interfacedesign method of a figure. MATLAB has stronger operation ability,powerful and intelligent mapping, and higher programmingefficiency, in particular, it can be used as the applicationdevelopment in this field. In this study, MNCGs were preparedby Cu+ implantation into silica. We focused our interest onstudying the nonlinear optical properties and optical limitingproperties of this kind of metal nanoclusters herein.

2. Experiment

Silica slides were implanted with copper ions at 180 keV atroom temperature. The current density of ion implantation was1.5 mA/cm2. Optical absorption spectra were recorded at roomtemperature using a UV–vis dual-beam spectrophotometer withwavelengths from 1200 to 300 nm. The third-order nonlinearoptical measurements of the sample were carried out by thestandard Z-scan method. The excitation source was a mode-locked Nd:YAG laser (PY61-10, Continnum), with a pulse durationof 38 ps and a repetition frequency of 10 Hz. 1064 nm wavelengthwas used for excitation in this experiment. The detector was adual-channel energy meter (EPM2000). With a converging lens of

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200 400 600 800 1000 12000.0

0.5

1.0

1.5

2.0

570 nm

1064 nm

Opt

ical

den

sity

(a.u

.)

Wavelength (nm)

Fig. 2. Optical absorption spectra of the Cu implanted sample.

Y.H. Wang et al. / Physica B 405 (2010) 2664–2667 2665

f¼260 mm, the radius of the Gaussian beam spot at focal waist$0 was 45 mm. 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.

We carried out curve fitting analysis of Cu optical limitingexperimental results based on MATLAB features. According to theCu experimental data obtained, we applied the polynomial curvefitting method to get the fitting procedure, which is as follows:

%program Cu.mx10¼[0:0.4:3.2];x11¼[4:0.4:21.8];x12¼[22.4:0.4:34.8];x¼[x10,x11,x12]0;y¼[], here [] is the output data of Z-scan p1¼polyfit(x,y,3);yy¼polyval(p1,x);plot(x,yy,x,y,0+ 0),grid on Fig. 2.X¼[ones(size(x)) exp(-x) x.nexp(-x)];a¼X\y;Y¼[ones(size(x)) exp(-x) x.nexp(-x)]na;plot(x,Y,0-0,x,y,0.0),grid onThe valuable data of the experiments were declare in the

program, while the non-value ones were removed. Furthermore,the distribution of valuable data points were analyzed by indexregression equation.

3. Result and discussion

The comparative size distribution [4] of Cu nanoclusters isshown in Fig. 1. The average size of nanoclusters in this sample is5.6 nm. The linear optical absorption spectra of the sampleinvestigated is shown in Fig. 2. The spectra range between 200and 1200 nm. The plasmon resonant absorption peak near 570 nmwas observed for the sample. The dependence of this absorptionband on the mean cluster diameter has been reported, and it hasbeen shown that the band becomes noticeable and sharpensonly when the diameter is about larger than 5 nm [10,11]. Thisselective absorption band is due to the surface plasmon resonance

0 2 4 6 8 10 12 140.0

0.1

0.2

0.3

perc

enta

ge

Diameter/nm

Fig. 1. Comparative size distribution profiles of 1�1017 Cu+ ions/cm2 nanoclus-

ters in silica sample.

(SPR). The various multipoles excitations may compensate eachother and lead to large apparent widths of the resonances.

The third-order nonlinear absorption and refraction areinvestigated by Z-scan techniques [12]. This technique is simpleand sensitive for studying nonlinear optical properties anddetermining the sign of the nonlinear refractive and absorptionindices. The open and closed- aperture Z-scan curves aretheoretically fitted by [12]

TðzÞ ¼X1

m ¼ 0

½�q0ðzÞ�m

ð1þx2Þmðmþ1Þ3=2

ðmZ0Þ ð1Þ

TðzÞ ¼ 1þ4DF0x

ðx2þ9Þðx2þ1Þð2Þ

where x¼z/z0, T is the normalized transmittance and z is thedistance along the lens axis in the far field. The nonlinearrefractive index is calculated by DF0 ¼ ð2p=lÞgI0Leff , where 2p/lis the wave vector of the incident laser, I0 is the intensity of thelaser beam at the focus (z¼0), Leff is the effective thickness of thesample, which can be calculated from the real thickness L andthe linear absorption coefficient a0, in the form of Leff ¼

½1�expð�a0LÞ�=a0.The third-order nonlinear optical property of the sample at

1064 nm is shown in Fig. 3; the open-aperture Z-scan shows nononlinear signal, which indicates that the sample has no nonlinearabsorption at 1064 nm. In Fig. 3, the peak-valley configurationindicates the negative sign of the nonlinear refractive index(n2o0). A self-defocusing refraction is verified from the peak-valley curve of closed-aperture data. The nonlinear property of thebare silica substrate is measured and has no detectable change ofthe transmitted intensity under same Z-scan conditions.

In our experiments, Leff (nm) for sample is 65 nm. The peakintensity of 0.38 GW/cm2 is selected for the sample. Fitting theZ-scan data of the closed-aperture with Eq. (2), we get valueof gE�1.1�10�10 cm2/W for 1064 nm. The absolute valueof third-order nonlinear susceptibility w(3) for Cu+ implantedsample is calculated using the following equations [12,13]:

DTp�v ¼ 0:406ð1�SÞ0:259Df09 ð3Þ

Rewð3Þ ¼ 2n20e0cg ð4Þ

Thus, we obtain the absolute value of w(3) as 1.2�10�7 esu for1064 nm.

The third-order nonlinear response of the present compositematerial thus mainly originates from electronic effects in copper

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-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Nor

mal

ized

Tra

nsm

ittan

ce

Z (mm)

Experiment Theoritical fit

Fig. 3. Z-scan experiment results for 1064 nm normalized close-aperture. Solid

line: theoretical curve.

0 5 10 15 20 25 30 35 400

2

4

6

8

10

12

14

16

18

20

Input (mJ/cm2)

Out

put (

mJ/

cm2 )

Fig. 4. Cu nanoclusters three times polynomial fitting curve.

0 5 10 15 20 25 30 35 40-5

0

5

10

15

20O

utpu

t (m

J/cm

2 )

Input (mJ/cm2)

Fig. 5. Cu nanoclusters exponential regression analysis curve.

Y.H. Wang et al. / Physica B 405 (2010) 2664–26672666

nanoclusters. These electronic contributions are due to bothintraband and interband transitions. The first one corresponds totransitions within the conduction band and the second oneto transitions from the upper levels of the filled d band tothe levels above the Fermi level in the conduction band. Whereasonly intraband contribution to the nanoclusters intrinsic third-order susceptibility is thought to be size-dependent, it isdominated by the size-independent interband contribution inthe SPR spectral domain. In the case where the clusters are excitedby ultra-short laser pulses, a hot electron phenomenon maysuperimpose on the pure electronic nonlinear contributions to Rew(3) [14].

Refractive index changes due to thermal nonlinearities arisedue to density changes in the materials propagating with acousticwave speed caused by heating. Thermal heating induced by asingle laser pulse persists over some characteristic time tc. As aresult, when the time interval between consecutive laser pulses isshorter than tc, the thermal effect increases. It is a commonassumption that Z-scan measurements should be made with arepetition rate of few Hertz in order to extract a nonlinearrefractive index influenced only by electronic effects. The timescale of this cumulative process is given by tc ¼ $0

2/4D, where D isthe thermal diffusion coefficient of the materials. Generally, thevalue of D ranges from 1�10�7 to 6�10�7 m2/s. The magnitudeof the calculated tc is within 10�3 s, which is much smaller thanthe time interval between consecutive laser pulses 0.1 s used inour experiment [15].

We carried out optical limiting measurement for the samplehaving no nonlinear absorption at 1064 nm. The results ofrunning the MATLAB program Cu.m obtained fitting curve areshown in Fig. 4 and Fig. 5.

As shown in Fig. 4, we found that the fitting Cu image outputvalues increase along with the increase in x. However, theincreasing rate of the output value gradually reduced to zero,there is a drop in the curve tail section. Nevertheless, fitting curveis still a very good line to reflect the optical limiting character-istics of the curve, indicating the Cu with the optical limitingeffect. Moreover, the index of regression analysis, which is shownin Fig. 5, is a typical optical limiting characteristic curve. Itappeared in a certain turning point only in the initial segmentbecause of the experimental error.

From the figures above, it is clear that Cu nanoclustersexhibited a certain degree of optical limiting properties. MATLAB,

as a fitting tool, has successfully accomplished the completion offit tasks and is a good helper to study the materials opticallimiting properties.

4. Conclusion

In conclusion, we report the experimental observations of thenonlinear optical responses of Cu nanoclusters using pecosecondlaser pulses. During the 1064 nm excitation, the sample had nononlinear absorption and w(3) is �1.2�10�7 esu, which all comefrom nonlinear refraction contribution. Moreover, the opticallimiting effect in the 1064 nm was also observed in this study. Weapplied the polynomial curve fitting method and make indexregression analysis by using MATLAB software. Basically, thecurve fitting reflects that the sample has optical limiting propertyat near-infrared field. MATLAB in the study materials aboutoptical limiting performance is effective. Further studies about Cunanoclusters are in progress.

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Acknowledgment

This work was supported by the National Natural ScienceFoundation of China (No. 10805035).

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