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Physica B 403 (2008) 2143–2147

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Review

Third-order nonlinear optical response of Cu/Ag nanoclustersby ion implantation under 1064 nm laser excitation

Y.H. Wanga,�, C.Z. Jiangb, X.H. Xiaob, Y.G. Chengc

aDepartment of Applied Physics, Wuhan University of Science and Technology, Wuhan 430081, ChinabDepartment of Physics, Wuhan University, Wuhan 430072, ChinacDepartment of Physics, Henan University, Kaifeng 475001, China

Received 25 July 2007; received in revised form 18 October 2007; accepted 28 November 2007

Abstract

The evolution of nanoclusters in sequentially ion-implanted Cu/Ag into silica glasses has been studied. The doses for implantation

(� 1016 ions/cm2) were 5Cu/5Ag, 5Cu/10Ag and 5Cu/15Ag, respectively. The microstructural properties of the nanoclusters are

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

nonlinear optical absorption coefficients were measured at 1064 nm of wavelength using Z-scan technique. Results in this project indicate

that different optical nonlinearities could be selectively obtained at the near-infrared region of 1064 nm of wavelength by changing the

metal ingredient percentage in silica.

r 2007 Elsevier B.V. All rights reserved.

PACS: 61.46.+w; 61.72.Ww; 42.65.�k

Keywords: Ion implantation; Nanoclusters; Nonlinear optics; TEM

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2143

2. Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2144

3. Result and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2144

4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2146

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2147

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2147

1. Introduction

Metal nanoclusters possess linear and nonlinear opticalproperties. There has been an increasing interest in thethird-order nonlinear susceptibility and the photorefractiveeffect of noble-metal clusters embedded in dielectricmatrices [1–6]. Previous results indicate that the type and

e front matter r 2007 Elsevier B.V. All rights reserved.

ysb.2007.11.023

ng author.

ss: wyh61@163.com (Y.H. Wang).

size of the embedded metal clusters, the dielectric constant,thermal conductivity and heat capacity of the dielectricmatrices influence third-order nonlinearities of the metal/dielectric composite materials.Amongst the nanoclusters studied by earlier researchers,

copper and copper-containing nanomaterials have highnonlinear absorption and nonlinear refraction coefficients[2]. Also, it was observed that inputting dose and annealingtemperature influenced third-order nonlinear optical prop-erties of copper nanoclusters [5,6]. For silver, nonlinear

ARTICLE IN PRESSY.H. Wang et al. / Physica B 403 (2008) 2143–21472144

refractive index g change from positive to negative uponthe cluster’s growth [7]. Application aspects of the materialare most relevant to the optical properties change versusthe nanocluster structure.

Composite materials formed by nonometer-sized metalclusters embedded in dielectrics have attracted researches’interest because of the large values of fast optical Kerrsusceptibility. The most conspicuous manifestation ofconfinement in optical properties of metal nanoclustercomposite glasses (MNCGs) is the appearance of thesurface plasmon resonance (SPR) that strongly enhancestheir linear and nonlinear responses around SPR wave-length [2,3]. However, for optical applications it isimportant to have an enhanced nonlinearity of compositematerials at the specific wavelength for optical device. Theapplication aspects of the material should be most relevant.In this project, Cu and Ag mixtural nanoclusters areconstructed by sequential implanted Cu/Ag ions. Also, theinfluence of the dose ratio of Cu and Ag ions in silica isanalyzed by measuring the sample’s third-order opticalnonlinearities in the near-infrared region of 1064 nm.

2. Experiment

Three silica glass plates (10� 15� 1mm3) are implantedat room temperature. Three specimen are implanted withCu with a dose of 5� 1016 ions/cm2 and followed byimplanting of Ag. Implantation doses (� 1016 ions/cm2) are5Cu/5Ag, 5Cu/10Ag and 5Cu/15Ag, respectively. The Cuions flux density is 1.5 mA/cm2 and Ag ions flux densitiesare 2.5 mA/cm2. According to the TRIM calculation, theenergies of implantation (180 keV for Cu and 200 keV forAg) are chosen to obtain similar ion distribution withconsideration for surface sputtering effect. Optical absorp-tion spectra were recorded at room temperature using a

Fig. 1. Cross-sectional TEM bright-field images of the Cu/Ag mixtural nanocl

5Cu/15Ag (� 1016 ions/cm2).

UV–vis dual-beam spectrophotometer with wavelengthsfrom 1200 to 200 nm. Transmission electron microscopy(TEM) observations were carried out with a JEOL JEM2010 (HT) microscope operated at 200 kV. TEM bright-field images were used to determine the size distribution,and shape of nanoclusters.The measurements of third-order optical nonlinearities

of these samples were carried out using the standard Z-scanmethod. The excitation source is a mode-locked Nd:YAGlaser (PY61-10, Continnum), with a pulse duration of 38 psand a repetition frequency of 10Hz. Wavelength of1064 nm is used for excitation in the experiment. Thedetector is a dual-channel energy meter (EPM2000). With aconverging lens of f=260 mm, the radius of the Gaussianbeam spot at focal waist $0 is 44.7 mm. In the Z-scan test,the sample was moved step by step along the propagationdirection of the Gaussian beam under the control of a PC.Meanwhile, a detector monitored the transmitted laserpower and the signals were sent back to the computer andrecorded. Nonlinear refraction and nonlinear absorptionwere performed by both open- and closed-aperture Z-scansof a series of the samples at room temperature. Liquid CS2in a fused-silica cell, 1mm in thickness, was used as areference sample (gffi2� 10�14 cm2/W in 1064 nm).

3. Result and discussion

The cross-sectional TEM images of samples are shown inFig. 1. By sequential ion implantation of Cu and Ag ions,Ag, Cu mixtural nanoclusters are formed. It is interestingto observe nanoclusters with bright centers in somenanoclusters, as shown in Fig. 1 (a). With an increase ofAg ions dose, most of nanoclusters have the features of abright center and the sizes of these bright centers becomelarge, as shown in Fig. 1 (c) for the 5Cu/15Ag specimen.

usters: (a) 5Cu/5Ag (� 1016 ions/cm2); (b) 5Cu/10Ag (� 1016 ions/cm2); (c)

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0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Optical density (

a.u

.)

Wavelength (nm)

CuAg11

CuAg12

CuAg13

Fig. 3. Linear absorption spectra of the Cu/Ag mixtural nanoclusters.

Y.H. Wang et al. / Physica B 403 (2008) 2143–2147 2145

Our previous study showed that these nanoclusters withbright centers are hollow Cu nanoshells [8,9]. As can beseen from the image, the particle size distribution is notuniform. The size of nanoclusters varies from 2 to 20 nm,the larger nanoclusters being located around the projectedrange of the implanted atoms. The morphology of thenanoclusters is complex: the largest nanoclusters showhollow shell structure, while the smallest ones are single-phase. The comparative size distributions of three differentconcentrations are shown in Fig. 2. The average sizes ofnanoclusters in the three samples are 6.6, 8.2 and 8.1 nm.

Fig. 3 shows the optical absorption spectra of Cu/Agsequentially implanted samples. Three samples show strongplasmon resonant absorption peak near 420–500 nm. It isknown that the resonance peak grows, sharpens andexhibits a red-shift with increasing particle size [10,11],which is due to the multipolar excitation caused by thelarge interaction of Ag nanoclusters in the sample.

The third-order nonlinear absorption and refraction areinvestigated by Z-scan technique [12]. This technique is asimple and sensitive experimental technique for the studyof nonlinear optical properties and can determine the signsof the nonlinear refractive and absorption indices. Theopen- and closed-aperture Z-scan curves are theoreticallyfitted by [13]:

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 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 the

0 5 10 15 200

5

10

15

20

25

30

35

Num

ber

of nanoclu

ste

r

Diameter/nm

A

B

C

Fig. 2. Comparatively size distribution profiles of three different

concentration nanoclusters in silica samples: (a) 5Cu/5Ag (� 1016 ions/

cm2); (b) 5Cu/10Ag (� 1016 ions/cm2); (c) 5Cu/15Ag (� 1016 ions/cm2).

sample, which can be calculated from the real thickness L

and the linear absorption coefficient a0, in the form ofLeff ¼ [1�exp(�a0L)/a0]. The nonlinear refractive index iscalculated by DF0 ¼ (2p/l)gI0Leff, where 2p/l is the wavevector of the incident laser.In our experiments, Leff (nm) for samples A, B and C are

86, 78 and 71, respectively. a0 (mm�1) of the three samples

are 9.7 (A), 12.5 (B) and 13.6 (C). Third-order nonlinearoptical properties of these kinds of samples of 1064 nmwere shown in Fig. 4. It can be seen that the open-apertureZ-scan shows no nonlinear signal in the three samples,indicating that these samples have no nonlinear absorptionat 1064 nm. The peak-valley configuration shows thenegative signs of the nonlinear refractive index (n2o0) ofthe three samples. It is assumed that this comes from theoptical Kerr effect. Experimental determinations of non-linear refractive index g exist in the literature for pure silicawith typical values of 10�14 cm2W�1 [14]. The nonlinearproperty of the bare silica substrates was measured and nodetectable change of the transmitted intensity under thesame Z-scan conditions was observed. Also, it is noticedthat if the laser peak intensity was larger than 15GW/cm2,there was a probability for the high absorbing materials tobe damaged at the tested point due to accumulativeheating. This heating will produce an ablation hole. Theclosed-aperture Z-scan curve of the ablation hole issymmetric and has a peak-valley pattern, which is similarto materials with negative nonlinear refraction [15]. So thepeak intensity of 0.38GW/cm2 was selected for the samplestested at 1064 nm. In our experiment, the asymmetric curvedoes not show significant change when repeated at thesame point, suggesting no formation of the ablation hole inthese samples during the Z-scan.The absolute values of third-order nonlinear suscept-

ibility w(3) for Cu+ implanted samples are calculated usingthe following equations [12,13]:

DTp�v ¼ 0:406ð1� SÞ0:25jDf0j (3)

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Theoritical fit

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Theoritical fit

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0.4

0.6

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1.6

1.8

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Theoritical fitExperiment

Fig. 4. Z-scan experiment results for 1064 nm normalized close aperture.

Solid line: theoretical curve: (a) 5Cu/5Ag (� 1016 ions/cm2); (b) 5Cu/10Ag

(� 1016 ions/cm2); (c) 5Cu/15Ag (� 1016 ions/cm2).

Table 1

Measured values of nonlinear susceptibilities of Cu/Ag mixtural

nanoclusters for picosecond pulses at 1064 nm

Implant dose (� 1016 ions/cm2) 5Cu/5Ag 5Cu/10Ag 5Cu/15Ag

g (cm2/GW) 0.16 0.18 0.30

w(3) (� 10�7 esu) 1.1 1.2 2.1

Y.H. Wang et al. / Physica B 403 (2008) 2143–21472146

Re wð3Þ ¼ 2n20�0cg (4)

wð3Þ ¼ ½ðwð3ÞReÞ2þ ðwð3ÞImÞ

2�1=2, (5)

where Df0 is the on-axis phase shift at the focus, DTp�v isthe difference of transmittance between the normalizedpeak and valley. The linear transmittance of the far-fieldaperture, S, is defined as the ratio of the pulse energypassing through the aperture to the total energy. S is 0.3 inthis experiment. The absolute values of g and w(3) arepresented in Table 1.The third-order nonlinearity observed is not induced by

the thermal effect within the pulse temporal width. Theelectronic nonlinearities arise very rapidly (within the 38 pspulse duration). Refractive index changes due to thermalnonlinearities arose. This happened because of densitychanges in the materials propagating with acoustic wavespeed, which was caused by heating. If we estimate acousticwave speed to be on the order of 3� 10�3m/s, the time topropagate a distance equal to the beam radius at this focusis about 15 ns at 1064 nm excitation, about 380 times longerthan the pulse width. Also, thermal heating induced by asingle laser pulse persists over some characteristic time tc.As a result, when the time interval between consecutivelaser pulses is shorter than tc, the thermal effect increases.It is a common assumption that Z-scan measurementsshould be made with the repetition rate of a few of Hertz inorder to extract a nonlinear refractive index influenced byonly electronic effects. The time scale of this cumulativeprocess is given by tc ¼ $0

2/4D, where D is the thermaldiffusion coefficient of the materials. Generally, the valueof D ranges from 1� 10�7m2/s to 6� 10�7m2/s. Themagnitude of the calculated tc is within 10�3 s, which ismuch smaller than the time interval between consecutivelaser pulses 0.1 s used in our experiment [16,17].The nonlinear refraction may come from the optical

Kerr susceptibility. Hollow metal mixtural nanoclusterswere fabricated with Ag dose increases in silica and w(3)

may change with different configurations of nanoclusters.As a result, w(3) was different even at the same input powerfor the same MNCGs but different ingredient fraction.

4. Conclusion

In this project, Cu/Ag mixtural nanoclusters are formedby the sequential ion implantation of Cu and Ag ions, asdemonstrated in TEM image. The nonlinear opticalproperties of these nanoclusters were investigated by the

ARTICLE IN PRESSY.H. Wang et al. / Physica B 403 (2008) 2143–2147 2147

Z-scan technique. The nonlinear refractive indexes for thesamples have different characters with varied nanoclustersizes at near-infrared region of 1064 nm wavelengthexcitation. By changing the ingredient percentage of metalin silica, different optical nonlinearities could be selectivelyobtained. This is useful in the fabrication of optical devicesby controlling ingredient concentration of metals in silica.

Acknowledgments

This work was supported by the Natural ScienceFoundation of China (Nos. 10005005, 10375044, 10435060)and the Key Project of Chinese Ministry of Education(No. 104122).

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