Physics Letters A 357 (2006) 364–368

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Effect of ingredient concentration on structure and optical propertiesof Cu nanoclusters

Y.H. Wang a,∗, F. Ren a,b, Q.Q. Wang a,c, D.J. Chen a, D.J. Fu a, C.Z. Jiang a,b,∗

a Department of Physics, Wuhan University, Wuhan 430072, Chinab Center for Electron Microscopy, Wuhan University, Wuhan 430072, China

c Center of Nanoscience and Nanotechnology Research, Wuhan University, Wuhan 430072, China

Received 5 January 2006; received in revised form 21 April 2006; accepted 21 April 2006

Available online 3 May 2006

Communicated by R. Wu

Abstract

Metal nanocluster composite glass prepared by Cu ion implantation have been studied. The formation of nanocluster has been evidenced byoptical absorption spectra and transmission electron microscopy (TEM). Fast nonlinear optical refraction and nonlinear optical absorption coeffi-cients were measured at 790 nm of wavelength for Cu nanocluster composites by the Z-scan technique. With an increase in the implanted dose,nanoshells were formed and the optical nonlinearity shows a significant change from positive to negative values. The reason for the enhancementand sign reversal of the optical nonlinearities of Cu nanocluster in silica are discussed. It is suggested that by changing the ingredient percentageof metals in silica, different optical nonlinearities could be selectively obtained.© 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

Third-order nonlinearities of metal/dielectric composite ma-terials were influenced by the type and size of the embeddedmetal clusters, by the dielectric constant, thermal conductivityand heat capacity of the dielectric matrices [1–5]. The most con-spicuous manifestation of confinement in optical properties ofmetal nanocluster composite glasses (MNCGs) is the appear-ance of the surface plasmon resonance (SPR) that strongly en-hances their linear and nonlinear responses around SPR wave-length [6–8].

Amongst the nanoclusters studied by earlier researchers,nonlinear absorption and nonlinear refraction were found to behigher in copper and copper containing nanomaterials [1,2,5,9].Experimental determinations of nonlinear refractive index γ

* Corresponding authors. Tel.: +86 27 68752567; fax: +86 27 68753587.E-mail addresses: wyh61@163.com (Y.H. Wang), czjiang@whu.edu.cn

(C.Z. Jiang).

0375-9601/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.physleta.2006.04.073

exist in the literature for pure silica in the fs pulse width, withtypical values of 10−14 cm2 W−1 [10]. Recently, many work-ers have observed the core/shell nanoclusters formed by singleor double-element ion implantation [11–16]. Therefore, appli-cation aspects of the material are most relevant to the opticalproperties change versus the nanocluster structure. In this Let-ter, we focused our interest on comparing the change of con-figuration, linear and nonlinear optical properties of MNCGsimplanted by different doses of Cu+. MNCGs were preparedby Cu+ implantation into silica. With the increase of implanteddoses, hollow Cu nanoshells were formed in MNCGs. Nonlin-ear optical properties were measured at the NIR wavelength of790 nm.

2. Experiment

Silica slides were implanted at room temperature by cop-per ions at 180 keV. Samples A, B and C are the copper MNCGsamples with Cu doses of 5×1016 ions/cm2, 1×1017 ions/cm2

Y.H. Wang et al. / Physics Letters A 357 (2006) 364–368 365

and 2 × 1017 ions/cm2, respectively. The current density ofion implantation was lower than 1.5 µA/cm2. Optical absorp-tion spectra were recorded at room temperature using a UV–vis dual-beam spectrophotometer with wavelengths from 1000to 300 nm. Transmission electron microscopy (TEM) observa-tions were carried out with a JEOL JEM 2010 (HT) microscopeoperated at 200 kV. TEM bright field images were used to de-termine the size distribution, and shape of nanoclusters.

The measurements of third-order optical nonlinearities ofthese samples were carried out using the standard Z-scanmethod. We employed 150 fs laser pulses at 76 MHz repetitionrate. A mode-locked Ti:sapphire laser generated the 790 nmfemtosecond laser. With a converging lens of f = 150 mm, theradius of the Gaussian beam spot at focal waist �0 is calculatedto be about 5.5 µm. In the Z-scan test, the sample was movedstep by step along the propagation direction of the Gaussianbeam under the control of a PC. Meanwhile, a detector mon-itored the transmitted laser power and the signals were sentback to the computer and recorded. Nonlinear refraction andnonlinear absorption were performed by both open- and closed-aperture Z-scans of a series of the samples at room temperature.

3. Result and discussion

The cross-sectional TEM images of as-implanted samplesare shown in Fig. 1. Spherical Cu nanoclusters are formed for5 × 1016 Cu+ ions/cm2 implantation. For further ion implanta-tion, it is interesting to observe nanoclusters with bright centersas shown in Fig. 1(b) and (c). Our previous study showed thatthese nanoclusters are hollow Cu nanoshells [17,18]. As can beseen from the image, the particle size distribution is not uni-form. The size of nanoclusters varies from 2 nm to 15 nm, thelarger nanoclusters being located around the projected rangeof the implanted atoms. The morphology of the nanoclustersis complex: the largest nanoclusters show hollow shell struc-ture, which are indicated with arrows, while the smallest onesare single-phase. Therefore, the MNCGs are composed of Cu

Fig. 1. Cross-sectional TEM bright-field images of the Cu+ as-implanted sam-ples. (a) 5 × 1016 Cu ions/cm2; (b) 1 × 1017 Cu ions/cm2; (c) 2 × 1017 Cuions/cm2. Arrows indicate hollow Cu nanoshells.

Fig. 2. Comparatively size distribution profiles of three different concentrationnanoclusters in silica samples. (A: 5 × 1016 Cu+ ions/cm2; B: 1 × 1017 Cu+ions/cm2; C: 2 × 1017 Cu+ ions/cm2).

Fig. 3. Linear absorption spectra of some of the copper MNCGs implanted byA: 5 × 1016 Cu+ ions/cm2; B: 1 × 1017 Cu+ ions/cm2; C: 2 × 1017 Cu+ions/cm2.

nanoshells and Cu nanoclusters. The comparative size distri-butions of three different concentrations are shown in Fig. 2.The average sizes of nanoclusters in three samples are 3.2, 5.6and 6.9 nm. Thus, it can be said that the metal nanoclusters ag-gregated together and grow into bigger nanoclusters with inputdoses increased.

Fig. 3 shows the optical absorption spectra of Cu implantedsamples. Only increasing shoulders for samples A and B areobserved in the range from 500 to 650 nm. The strong plasmonresonant absorption peak near 570 nm is observed for the Csample. It is known that the resonance peak grows, sharpens andexhibits red-shift with increasing particle size [19,20], which isdue to the multipolar excitation caused by the large interactionof Cu nanoclusters in the sample.

The nonlinear absorption in the sample can be describedby β , which includes saturated absorption (SA) and reversedsaturated absorption (RSA) [21]. The nonlinear absorption isexpressed by α = α0 + βI , where α0 is the linear absorptioncoefficient of the sample and I is the intensity of the laser.The third-order nonlinear absorption and refraction are investi-

366 Y.H. Wang et al. / Physics Letters A 357 (2006) 364–368

Fig. 4. Normalized open-aperture (a) and the divided Z-scan result (b). Solid line: theoretical curve (5 × 1016 Cu ions/cm2).

Fig. 5. Normalized open-aperture (a) and the divided Z-scan result (b). Solid line: theoretical curve (1 × 1017 Cu ions/cm2).

gated by Z-scan techniques [22], which are simple and sensitiveexperimental technique for the study of nonlinear optical prop-erties and allow determining the sign of the nonlinear refractiveand absorption indices. The open- and closed-aperture Z-scancurves are theoretically fitted by [23]:

(1)T (z) =∞∑

m=0

[−q0(z)]m(1 + x2)m(m + 1)3/2

(m � 0),

(2)T (z) = 1 + 4�Φ0x

(x2 + 9)(x2 + 1).

Where x = z/z0, T is the normalized transmittance and z isthe distance along the lens axis in the far field. The nonlinear ab-sorption coefficient β can be obtained by q0 = βI0Leff, whereI0 is the intensity of the laser beam at the focus (z = 0), Leff isthe effective thickness of the sample, which can be calculatedfrom the real thickness L and the linear absorption coefficientα0, in the form of Leff = [1 − exp(−α0L)]/α0. The nonlinearrefractive index is calculated by �Φ0 = (2π/λ)γ I0Leff, where2π/λ is the wave vector of the incident laser.

If thermo-optical effects are important in the experiment, thenonlinear refractive index of the sample should be expressed asnT

2 , which is the thermo-optical nonlinear refraction coefficient.In general, nT

2 is not a constant value during the scanning, andis not constant inside a finite length sample. It depends on thepower of the Gaussian light beam and the thermal conductiv-ity κ of sample. The Kerr and the thermo-optical effects areindependent of each other; the general nonlinear refraction co-

efficient n∗2 will be the sum of two effects [24]:

(3)n∗2 = nT

2 + n2 = 1

4κ

dn

dT

{α

2ω2

i + β[(1/2)Iiω2i π]

π

}+ n2.

As can be seen from this equation, nonlinear refraction isrelated to absorption both from linear and nonlinear parts.

In our experiments, Leff (nm) for samples A, B and C are77, 65 and 61, respectively. α0 (µm−1) of the three samples are5.4(A), 9.0(B) and 10.7(C). The solid curve in Fig. 4(a) andFig. 5(a) are fitted by using Eq. (1) with the experiment pa-rameters and the nonlinear absorption coefficient obtained areβB = −1.6 × 103 cm/GW and βC = −9.0 × 103 cm/GW.

Normalized open-aperture Z-scans of sample A are dis-played in Fig. 4(a). A sign alternation of nonlinear absorptionhas been shown here. It is reported that there is an intensitythreshold Ic in the four-energy level model. Once the intensityis over Ic, the RSA-to-SA conversion occurs [25]. The real in-cident intensity I ′(z) in the sample depends on two conditions:(1) the sample position z in Z-scan. Since the beam is a fo-cused one, the closer the sample is to the focal point, the largerthe value of I ′(z) is. I ′(z) comes to a climax I ′

0 while z = 0.(2) The numbers of electrons occupying the excited states ofthe energy levels are different for nanoclusters with differentmorphology and size. Here Ic is different for the samples withdifferent Cu contents. With Cu dose increase in silica, spheroidradius increased, hollow nanoshells were observed as shown inFig. 1(b) and (c). Compare Fig. 5(a) and Fig. 6(a) with Fig. 4(a),the open-aperture measurement shows an obvious enhanced

Y.H. Wang et al. / Physics Letters A 357 (2006) 364–368 367

Fig. 6. Normalized open-aperture (a) and the divided Z-scan result (b). Solid line: theoretical curve (2 × 1017 Cu ions/cm2).

Fig. 7. Values of α0 and n∗2 as a function of Cu dose.

transmittance near the focus, occurring due to the saturation ofabsorption. This reveals negative nonlinear absorption coeffi-cient and indicates that the local field and collective oscillationof free electrons on the surface of nanocluster decreased withthe increase of nanocluster sizes.

The closed-aperture Z-scans are sensitive to both the non-linear absorption and nonlinear refraction. The nonlinear re-fraction was obtained by dividing the closed-aperture data bythe open-aperture data [22,23]. In Fig. 4(b) and Fig. 5(b), thevalley-peak configuration indicates the positive sign of the non-linear refractive index (n∗

2 > 0), which is consistent with Fal-conieri and Battaglin result [1,26]. However, n∗

2 decreases whenthe Cu concentration increased. We think this comes from adecreased nonlinear absorption. Absorption has a double ef-fect. On one hand, it causes the decrease of the intensity duringthe propagation of the light beam. On the other hand, it is re-sponsible for the creation of the nonlinear refraction [24]. Thevalues of n∗

2 and α0 as a function of nanocluster dose are shownin Fig. 7. Though α increases in high dose sample, n∗

2 de-creases more rapidly with the number of nanoclusters in silica.In Fig. 6(b), a self-defocussing refraction also is found from thepeak-valley curve, which means that with Cu doses increasedin silica, nonlinear refraction changes from positive to nega-tive. The nonlinear properties of the bare silica substrates wasmeasured and get no detectable change of the transmitted in-tensity under same z-scan conditions. We notice that when thelaser peak intensity is larger than 15 GW/cm2, there is a prob-

ability for the high absorbing materials to be damaged at thetested point due to accumulative heating, which will producean ablation hole. The closed-aperture Z-scan curve of the abla-tion hole is symmetric and has a peak-valley pattern similar tothat of some materials with negative nonlinear refraction [3]. Sothe peak intensity of 14.5 GW/cm2 was selected for the threesamples. In our experiment, the asymmetric curve changes lit-tle when repeated at the same point. Thus we suggest therewas no formation of the ablation hole in these samples duringthe Z-scan. So the change of nonlinear refraction may comefrom a change of Cu nanocluster configuration in the samples,as shown in Eq. (3). With Cu dose increases in silica, hollowCu nanoclusters were fabricated and α and β may change withdifferent configuration of nanoclusters. As a result, n∗

2 were dif-ferent even at the same input power for one same MNCGs butdifferent ingredient fraction.

4. Conclusion

In summary, Cu nanoclusters and hollow Cu nanoshells insilica have been formed by ion implantation. The nonlinearoptical properties were investigated by the Z-scan technique.The nonlinear absorption and nonlinear refractive indexes havedifferent characters for samples with varied nanocluster sizes.By changing the ingredient percentage of metal in silica, dif-ferent optical nonlinearities could be selectively obtained. Thisis useful in fabrication of optical devices by control ingredientconcentration of metal in silica.

Acknowledgements

This work was supported by the Natural Science Foundationof China (Nos. 10005005, 10375044, 10435060) and the KeyProject of Chinese Ministry of Education (No. 104122).

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