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Physica E 33 (2006) 244–248

www.elsevier.com/locate/physe

Effect of thermal treatments on third-order nonlinear opticalproperties of hollow Cu nanoclusters

Y.H. Wanga, C.Z. Jianga,b,�, F. Rena,b, Q.Q. Wanga,c, D.J. Chena, D.J. Fua

aDepartment of Physics, Wuhan University, Wuhan 430072, ChinabCenter for Electron Microscopy, Wuhan University, Wuhan 430072, China

cCenter of Nanoscience and Nanotechnology Research, Wuhan University, Wuhan 430072, China

Received 24 January 2006; received in revised form 22 February 2006; accepted 25 February 2006

Available online 21 April 2006

Abstract

Metal nanocluster composites prepared by Cu ion implantation have been studied. The formation of nanoclusters has been evidenced

by optical absorption spectra and transmission electron microscopy (TEM). Fast nonlinear optical refraction and nonlinear optical

absorption coefficients were measured at 790 nm for Cu nanocluster composites by the Z-scan technique. With the increase of annealing

temperature, the size of nanoclusters increased significantly, and optical nonlinearities was enhanced. It is suggested that by changing the

ingredient configuration of metal nanoclusters in silica, different optical nonlinear properties could be selectively obtained.

r 2006 Elsevier B.V. All rights reserved.

PACS: 61.46.+W; 61.72.Ww; 42.65.�K

Keywords: Ion implantation; Nanoclusters; Nonlinear optics

1. Introduction

Recently, there has been an increasing interest in thethird-order nonlinear susceptibility and the photorefractiveeffect of noble-metal clusters embedded in dielectricmatrices [1–5]. The type and size of the embeddedmetal clusters, the dielectric constant, thermal conductivityand heat capacity of the dielectric matrices influencedthird-order nonlinearities of metal/dielectric compositematerials. Amongst the nanoclusters studied by earlierresearchers, nonlinear absorption and nonlinear refractionwere found to be higher in copper and copper containingnanomaterials [1,2,5,6]. Recently, many workers haveobserved the core/shell nanoclusters formed by single- ordouble-element ion implantation [7–12]. Therefore, appli-cation aspects of this kind of nanomaterial are mostrelevant to the optical properties change versus thenanocluster structure. It is well known that thermal

e front matter r 2006 Elsevier B.V. All rights reserved.

yse.2006.02.006

ing author. Department of Physics, Wuhan University,

, China. Tel.: +86 27 68752567; fax: +8627 68753587.

ess: czjiang@whu.edu.cn (C.Z. Jiang).

annealing may promote the growth of nanocluster [13]and optical properties are strongly affected by the interfaceand the microstructure of the nanoclusters. In this paper,metal nanocluster composite glasses (MNCGs) wereprepared by Cu implantation into silica. We focused ourinterest on analysis of the influence of thermal annealingtreatments on the microcrystallite structure, linear andnonlinear optical properties of the Cu nanoclusters insilica. The third-order nonlinear optical properties of Cunanoclusters were measured by femtosecond laser systemsat the NIR near infrared wavelength of 790 nm. The pulsewidth and peak fluence were 150 fs and 5.8GW/cm2,respectively.

2. Experiment

Silica slides were implanted at room temperature bycopper ions at 180 keV to 2� 1017 ions/cm2. The currentdensity of ion implantation was lower than 1.5 mA/cm2.Samples Cu500 and Cu900 are named for the copperMNCG samples thermal annealed at 500 and 900 1C for 1 hin an Ar+H2 mixture, respectively. Optical absorption

ARTICLE IN PRESS

Fig. 1. TEM bright-field images of the Cu sample implanted by 2� 1017

Cu+ ions/cm2. (a) Cu; (b) annealed at 900 1C; Arrows indicate core-shell

nanoclusters.

Y.H. Wang et al. / Physica E 33 (2006) 244–248 245

spectra were recorded at room temperature using aUV–VIS dual-beam spectrophotometer with wavelengthsfrom 1000 to 400 nm. Transmission electron microscopy(TEM) observations were carried out on a JEOL JEM 2010(HT) microscope operated at 200 kV. Selected area electrondiffraction (SAED) was used to determine the crystalstructure, size distribution, and shape of nanoclusters.

The measurements of the third-order optical nonlinea-rities of these samples were carried out using the standardZ-scan method [14]. There were 150-fs-laser pulses at76MHz repetition rate employed. A mode-locked Ti:sapphire laser generated the 790 nm femtosecond laser.With a converging lens of f ¼ 150mm, the radius of theGaussian beam spot at focal waist $0 is calculated to beabout 5.5 mm. In the Z-scan test, the sample was movedstep by step along the propagation direction of theGaussian beam under the control of a PC. Meanwhile, adetector monitored the transmitted laser power and thesignals were sent back to the computer and recorded.

In general, the refractive index n and the opticalabsorption a are described as function of the intensity I

of the incident laser beam according to n ¼ n0 þ gI ,a ¼ a0 þ bI , where, n0 and g are linear and nonlinearrefractive index, a0 and b are linear and nonlinearabsorption coefficient. g and b are related to the real andimaginary part of the third-order electrical susceptibilityw(3). To determine g and b, both open- and closed- apertureZ-scans of a series of the samples were performed. Theclosed-aperture Z-scan experimental is shown in thefollowing. Using a single Gaussian laser beam in a tightfocus geometry, the transmittance of a nonlinear mediumwas measured through a finite aperture in the far field as afunction of the sample position z. The thicknesses of thesamples were narrower than the beam depth of focus,which can be regarded as a thin negative lens (n2o0) ofvariable focal length. The scan started from a distance faraway from the focus, the beam irradiance was low andnegligible nonlinear refraction occurs. When the sampleswere moved toward the focus, the increased irradiance ledto a negative lensing effect that tended to collimate thebeam, leading to the narrowing of beam at the aperture,and increasing the transmittance. As the scan in z

continued and the samples passed the focal plane to theright, the self-defocusing increased the beam diffusion,leading to the broadening of beam at the aperture, whichresulted in a decrease of transmittance. If we take out theaperture, the detector may record all the transmittancepassed though samples, which is open-aperture technique[14,15]. All of which were performed under the same laserpower at room temperature.

3. Result and discussion

The cross-sectional TEM images of as-implanted andannealed samples are shown in Fig. 1. Spherical clusters areformed during the ion implantation process. In Fig. 1(a),Cu nanoclusters are dispersed in the matrix. At the same

time, it is interesting to observe core/shell nanoclusters withbright centers as shown in nanoclusters. As can be seen, thesize distribution is not uniform. The cluster size differsfrom 2 to 15 nm, the larger nanoclusters being placed in theprojected range. These core/shell nanocluster have beenproved to be hollow Cu nanoclusters in our previous work[16,17]. To promote core/shell nanoclusters growth, weannealed the sample at the temperatures of 500 and 900 1C.The TEM of Sample Cu900 is shown in Fig. 1(b), the largernanoclusters have bigger sizes than those in the un-annealed sample and the nanoclusters distribution is alsocomplex: the largest nanoclusters show core-shell structure,which are indicated with arrows, while the smallest onesare single-phase. The comparative size distributions of un-annealed sample and annealed sample at 900 1C are shownin Fig. 2. Thus, it can be said that the metal nanoclustersaggregate together and grow into bigger nanoclusters.Fig. 3 shows the optical absorption spectra of Cu

implanted samples. A strong plasmon resonant absorptionpeak near 570 nm is observed for three samples. It is knownthat the resonance peak grows, sharpens and exhibits red-shift with increasing particle size [18,19], which is due to themultipolar excitation caused by the large interaction of Cunanoclusters in the sample. Therefore, the above absorp-tion spectra can be regarded as the indication of increaseorder of Cu particle size. In our experiment, the size ofnanoclusters in Sample Cu500 has a little increased andthat of Sample Cu900 increased visible. This is consistentwith particle sizes presented by TEM images as shown inFig. 1.The nonlinear absorption in the sample can be calculated

from b, which includes saturated absorption (SA) and

ARTICLE IN PRESSY.H. Wang et al. / Physica E 33 (2006) 244–248246

reversed saturated absorption (RSA) [20]. The third-ordernonlinear absorption and refraction are investigated byZ-scan techniques, which are simple and sensitive experi-mental technique for the study of nonlinear optical

2 4 6 8 10 12 14 16 18 20 220

5

10

15

20

25

30

Num

ber

of n

anoc

lust

er

Diameter/nm

un-annealed

900°C annealed

Fig. 2. Comparatively size distribution profiles of nanoclusters in silica

samples.

400 600 800 1000 12000.0

0.5

1.0

1.5

2.0

2.5

3.0

Opt

ical

den

sity

(a.

u.)

Wavelength (nm)

CuCu500Cu900

Fig. 3. Linear absorption spectra of some of the Cu MNCGs. (A) Cu;

(B)Cu500; (C) Cu900.

-20 -10 0 10 20

1.0

1.2

1.4

1.6

1.8

2.0Open Aperture

ExperimentTheoritical fit

Nor

mal

ized

Tra

nsm

ittan

ce

Z (mm)(a)

Fig. 4. Normalized open-aperture (a) and the divided Z-scan result (b)

properties and allow determining the sign of the nonlinearrefractive and absorption indices. The open- and closed-aperture Z-scan curves are theoretically fitted by [21]

TðzÞ ¼X1m¼0

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 isthe distance along the lens axis in the far field. Thenonlinear absorption coefficient b can be obtained byq0 ¼ bI0Leff , where I0 is the intensity of the laser beam atthe focus (z ¼ 0), Leff is the effective thickness of thesample, which can be calculated from the real thickness L

and the linear absorption coefficient a0, by the formulaLeff ¼ ½1� expð�a0LÞ�=a0. The nonlinear refractive indexis calculated by DF0 ¼ ð2p=lÞgI0Leff ;, where 2p=l is thewave vector of the incident laser.If thermo-optical influence plays a role in the experi-

ment, the nonlinear refraction of sample could be expressedas nT

2 . In general, nT2 is not a constant value during the

scanning, and is not constant inside a finite-length sample.It depends on the power of the Gaussian light beam andthe thermal conductivity k of the sample. The Kerr and thethermo-optical effects are independent of each other; thegeneral nonlinear refraction coefficient n�2 will be the sumof two effects [22]:

n�2 ¼ nT2 þ n2 ¼

1

4kdn

dT

a2o2

i þb ð1=2ÞI io2

i p� �

p

� �þ n2. (3)

In our experiments, Leff for the three samples is 150 nm.Normalized open-aperture Z-scans of sample Cu aredisplayed in Fig. 4(a); the open-aperture measurementshows an obvious enhanced transmittance near the focus,occurring due to the saturation of absorption, whichreveals a negative nonlinear absorption coefficient. Non-linear refraction indices were obtained by dividing closed-aperture data by the open-aperture data. In Fig. 4(b), thepeak-valley configuration indicates self-defocusing refrac-tion and a negative sign of the nonlinear refractive indexðn�2o0Þ. Z-scans curves of Sample Cu900 are shown in

-20 -15 -10 -5 0 5 10 15 20

2

ExperimentTheoritical fit

Nor

mal

ized

Tra

nsm

ittan

ce

(b) Z (mm)

. Solid line: theoretical curve (2� 1017 Cu ions/cm2, un-annealed).

ARTICLE IN PRESS

-20 -10 0 10 20

1.0

1.2

1.4

1.6

1.8

2.0Open Aperture

ExperimentTheoritical fit

Nor

mal

ized

Tra

nsm

ittan

ce

Z (mm)

-20 -15 -10 -5 0 5 10 15 200.20.40.60.81.01.21.41.61.82.0

Experiment

Theoritical fit

Nor

mal

ized

Tra

nsm

ittan

ce

Z (mm)(a) (b)

Fig. 5. Normalized open-aperture (a) and the divided Z-scan result (b). Solid line: theoretical curve (Sample Cu900).

0 200 400 600 800 10000

1

2

3

10-7

(es

u)

T (°C)

abc

Fig. 6. Absolute values of wð3Þ, wð3ÞRe and wð3ÞIm as functions of annealing

temperature. a is wð3ÞIm, b is wð3ÞRe and c is wð3Þ.

Y.H. Wang et al. / Physica E 33 (2006) 244–248 247

Fig. 5, which have the similar shapes compared to Fig. 4,but nonlinear absorption and refraction have enhanced.

The third-order nonlinear susceptibility wð3Þ, including the

real part wð3ÞRe and the imaginary part wð3ÞIm, can be written

as wð3Þ ¼ wð3ÞRe

� �2þ wð3ÞIm� �2� �1=2

, whose real part is related

to the nonlinear refractive index coefficient n�2, and the

imaginary part is related to the nonlinear absorptioncoefficient b. The absolute values of wð3Þ, wð3ÞRe and wð3ÞIm as afunction of annealing temperature are shown in Fig. 6.With increasing annealing temperature, both wð3ÞRe and wð3ÞImare increased. As a result, the absolute values of third-ordernonlinear susceptibility wð3Þ are increased. We think this isdue to the aggregation of nanoclusters. With the increase ofannealing temperature, metal nanoclusters aggregate to-gether and grow into bigger nanoclusters for the Oswaldripening effect. The absolute values of nonlinear absorp-tion b and nonlinear refraction n�2 increased as nanoclustersgrow. Therefore the change of the nonlinear opticalproperty may come from a change of Cu nanoclusterconfiguration in the samples. With annealing temperatureincreased, the radius of Cu nanoclusters increased in silica

and nanoshells increased significantly. As a result, non-linear absorption and nonlinear fraction change withdifferent configuration of nanoclusters even at the sameinput power for the same MNCGs but different nanoclus-ter configuration.

4. Conclusion

In summary, hollow Cu nanoclusters in silica have beenformed by ion implantation. The nonlinear optical proper-ties were investigated by the Z-scan technique. Hollow Cunanoclusters were grown with the increase of annealingtemperature. The nanoclusters exhibited strong nonlinearoptical characters and the absolute values of third-ordernonlinear susceptibility wð3Þ are increased with annealingtemperature. This is useful in fabrication of optical devicesby control of annealing temperature to form differentmetal nanoclusters in silica.

Acknowledgments

This work was supported by the National NaturalScience Foundation of China (Nos.10005005, 10375044,10435060) and the Key Project of Chinese Ministry ofEducation (No. 104122).

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