The fractal dimension Df and the critical parameter v of the sputtered percolation Ag island films are measured to be about 1.57 and 1.15, respectively. A modified Z-scan method including both transmittance Z-scan and reflection Z-scan is proposed to measure the pure nonlinear absorption of the semi-continuous Ag films near the percolation threshold. As the microstructure of the sputtered Ag films varies from the discontinuous to the continuous, the linear absorbance decreases from 0.97 to 0.58 while the nonlinear absorption coefficient changes from -1.4×10-5m/W to -4.6×10-6m/W at the wavelength of 800nm. The largest nonlinear absorption coefficient β of the sputtered Ag films is obtained in the discontinuous film near the percolation threshold, which is mainly contributed by the large enhancement of local field of dis-continuous Ag particles.
© 2006 Optical Society of America
Nanosized films are interesting materials because of their novel chemical, mechanical and optical properties comparing to the bulk materials. Recently, noble metal/dielectric matrix composite films have attracted much attention due to their potential applications in all-optical switch, optical correlators, and optical computing [1–4]. The optical properties of nanocomposite films depend not only on the matrix but also on the metal . Silver is a good choice for the metal in nanocomposite films. Ag nanoparticles have the potential to obtain the largest enhancement factor of the local field for their strong surface plasmon resonance [6–8] and large third-order optical nonlinearity [8–10]. Therefore, series of researches were concentrated on Ag and Ag alloy deposited on Si, SiO2 and other matrices [11–13]. It is reported that the second harmonic generation (SHG) of the Ag island films reached the maximum near the percolation threshold [14–15], while the optical percolation and the third-order optical nonlinearity of sputtered Ag island films were seldom reported. The optical nonlinearity can be measured using different methods, such as optical Kerr effect (OKE), degenerate four wave mixing (DFWM) and Z-scan technique [16–18].
In this Letter we investigate the optical percolation properties of sputtered Ag island films from the linear transmittance, propose a reflection and transmittance Z-scan technique to measure the pure nonlinear absorption. The linear and nonlinear optical absorption of Ag films near the percolation were investigated.
Ag films were deposited onto the glass substrates in Ar atmosphere by radio frequency (R.F.) sputtering technique. The background pressure is about 9 × 10-5 Torr. The working pressure is about 3.0 × 10-2 Torr and the sputtering power is about 60W. The transmittance spectra of the films were recorded with a UV-VIS-NIR spectrophotometer (Varian Cary 5000). The surface morphology was examined by an atomic force microscopy (AFM, Form Talysurf Series2).
The nonlinear absorption of samples was measured using a modified Z-scan setup, as depicted in Fig. 1. The sample S and the detector D1 were mounted on a PC controlled translation stage moving along the axis of the incident laser beam (from -z to +z). The Z-scan measurements were performed at 800 nm using a Ti: sapphire laser (Coherent, Mira 900) with a pulse duration of 150 fs and a repetition rate of 76 MHz. Two detectors were used to record the reflected and transmitted intensity, respectively.
3. Results and discussion
A series of Ag films were prepared with the deposition time tD controlled in the range of 10s to 300s. The electrical conductivity σ is too small to be measured for the films with the deposition time tD < 90 s. And σ of the films changes little when deposition time tD > 300s. So we considered that the film was continuous and the normalized conductivity equals to 1. Therefore, as shown in Fig. 2, a curve of normalized conductivity versus tN was obtained through measurement of the sample resistances with different deposition time, where tN is defined as the normalized deposition time and tN=tD/300. One can see that σ increases rapidly when tN > 0.3, revealing the variation of the structure of the film from the dis-continuous to the continuous. So tc=0.3 is defined as the critical point.
Figure 3(a) shows the transmittance spectrum of the Ag films with different tN. Ag films with tN<tC show strong absorption peak near 600nm, and the transmittance increase as the increasing of the wavelength in the regime of 1000~2600 nm, indicating that the films are discontinuous. While for the ones with tN>tC the absorption peak in visible regime disappeared, and the transmittance decrease as the increasing of the wavelength when λ>1000 nm, indicating that the films have changed to the continuous ones. The result of spectra is consistent with that shown in Fig. 2. When tN=tC, the transmittance varies a little in the range of 1000~2600nm, which is the behavior of optical percolation.
Figure 3(b) is the relationship between ln|T - Tc| and lnλ, where Tc ≈ 0.40 is the theoretical value at critical point [19–21]. We get the slope tgθ=0.87 by linear fitting, the critical parameter v=1/tgθ=1.15 , and the fractal dimension Df=2 - (2v)-1 =1.57 . The measured and the theoretical value of the percolation parameters (Dy and v) is shown in Table 1. The measured value of critical parameter v is between the predictions from 2-D Ising model and 2-D percolation model. The measured fractal dimension Df is less than the theoretical prediction, which means that numerous holes exist in the percolation films.
Figure 4 shows normalized transmittance Z-scans of the Ag film at tc. The open-circles are the Z-scans, the solid curve is the fitting result. The transmittance Z-scans indicate that the Ag film shows saturable nonlinear absorption. In order to eliminate the influence of the surface feature and the reflection due to the nonlinear refraction near the threshold, the pure absorption of the sample is obtained by subtracting the reflected and transmitted Z-scan signal. The modified nonlinear absorption coefficient is calculated from the formula , where q 0 = β I 0 Leff , and I0 = 0.44GW/cm2. The largest one was measured to be -1.4 × 10-5m/W for the percolation films. It is little smaller than the result by G. Yang , which may be caused by the elimination of the reflection.
The linear and nonlinear absorption at the wavelength of 800 nm as a function of deposition time tN are shown in Fig.5. In the dis-continuous regime (t<tC), both linear and nonlinear absorption increases as the deposition time increases; the nonlinear absorption reaches its maximum right before percolation threshold. When the microstructure of the films changes from the dis-continue to the continue at the percolation threshold, the linear absorption decrease from 0.97 to 0.58, while the module of nonlinear absorption coefficient decreases from 1.4×10-5m/W to 4.6×10-6m/W, which means that both abnormal linear absorption and large enhanced nonlinear absorption are caused by the surface plasmon absorption of the Ag nanoparticles, the microstructure of Ag island films with a deposition time of 90s (tN =0.3) is clearly shown in Fig. 6.
The linear and nonlinear optical absorption of sputtered Ag island films near the percolation threshold were measured. The fractal dimension Df and the critical parameter v of the percolation Ag island film are measured to be about 1.57 and 1.15, respectively. The enhancement of linear and nonlinear absorption was found near the percolation threshold. Using the reflection and transmittance Z-scan technique, the largest nonlinear absorption β was measured to be -1.4 × 10-5m/W at the percolation threshold. This enhancement is originated from the surface plasmon absorption of the Ag nanoparticles.
Thank to Prof. Zheng-Guo Zhou for the sample preparation, and Prof. Zhu Wang and Sheng Xu for the performance of AFM.
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