The nonlinear optical properties of silver-nanoparticle-decorated functionalized (hydrogen-induced reduction, exfoliated) multilayered graphene (AgNP/fG) composite have been investigated in the picosecond time scale by using the Z-scan technique. Drastic changes in the Z-scan profiles are obtained in AgNP/fG at 1064 nm as the profile flips from the behavior of strong saturable absorption (SA) to reverse saturable absorption (RSA) on increasing the incident intensity. On the other hand, in graphene, the effect of weaker SA is observed at low intensities, but it flips to RSA at higher incident intensities. The strong SA in the case of the composite has been attributed to the electronic interaction between graphene and silver nanoparticles. The metallic plasmonic transition has a contribution to the optical nonlinearity in AgNP/fG at 532 nm.
© 2012 Optical Society of America
Graphene has attracted tremendous attention for its remarkable optical, electronic, and mechanical properties [1,2]. Graphene-based electronic and photonic devices have prompted a large number of investigations due to the high mobility of electrons [3,4]. It has been found that a single layer of graphene absorbs about 2.3% of light incident on it, and that multilayered graphene absorbs in the same proportion . Because of its zero bandgap nature, absorption of electromagnetic radiation of any wavelength is possible . Therefore, its optical properties can be exploited in a wide spectral range.
Pump–probe studies of epitaxial graphene have shown its saturable absorption (SA) behavior with ultrafast recovery time . Sun et al. , demonstrated graphene-based mode locker, followed by studies on ultrafast pulse generation in the 1–1.5 μm region [1,9]. On the other hand, silver nanoparticles (AgNPs) have been found be useful in various applications, such as plasmon-controlled fluorescence , multiphoton plasmon-resonance microscopy , femtosecond filamentation, and supercontinuum generation . Importantly, these features differ from those of the bulk silver and actually depend on the size and shape of the AgNPs .
The combination of the two classes of materials (graphene and nanoparticles) may lead to integration of the properties of the two components in new hybrid materials that possess important features for catalysis and nanotechnology . An economical alternate for the semiconductor saturable absorber mirror technology  can be nanoparticle-graphene-embedded solid-state saturable absorber devices in the visible and IR region. In addition, adsorption of various organic molecules or metal nanoparticles onto the graphene surface results in an increase in the number of either electrons or holes through molecular charge transfer . Such an induced charge transfer, in turn, can change the electronic and optical properties. This motivated us to study and compare the nonlinear properties of this hybrid material with the standard saturable absorber used in commercial lasers .
In contrast to planar graphene sheets, the wrinkled graphene has advantages in attaching the metal nanoparticles . Therefore, in this paper, we report our studies on the optical nonlinear properties of such functionalized multilayer graphene (fG) and silver-decorated graphene (AgNP/fG) by using the Z-scan technique as a function of the wavelength and the intensity of the incident radiation. It is observed that the effect of SA becomes stronger at 1064 nm with silver decoration. With increasing the intensity, the effect of reverse saturable absorption (RSA) is observed, indicating a promise of optical limiting (OL).
The experimental arrangement for the Z-scan is described in Fig. 1. The fundamental (1064 nm) and the second harmonic (532 nm) wavelengths of a picosecond Nd:YAG laser (Continuum Model YG601, 40 ps, 10 Hz) were used. The transmitted energy as a function of the sample position was measured at the far field with the help of a photodiode (Becker and Hickle, PDI-400), after focusing with a double convex lens of focal length 50 mm. A circular aperture of 1 mm diameter was used before the detector for the closed aperture (CA) Z-scan experiments. A large diameter double convex lens of focal length 200 mm replaced the circular aperture for the open aperture (OA) Z scan. The scattered light was measured by placing a detector at an angle of 25° to the beam axis. The beam was nearly Gaussian and we have considered the 86% beam criterion . The radius of the beam waist () was 24 μm with the corresponding Rayleigh range () of 2.0 mm. The sample thickness (1 mm) was less than the Rayleigh range to fulfill the thin sample approximation condition. 1 mg each of fG, AgNP/fG, and AgNPs in ethylene glycol were separately sonicated for 2 h for dispersion just before the experiment. The absorption spectra were recorded using a UV–visible spectrometer (JASCO, V-570) in a quartz cell of 1 mm thickness. Powder x-ray diffraction (XRD) studies were carried out using a PANalytical X’PERT Pro x-ray diffractometer with nickel-filtered Cu radiation as the x-ray source. The pattern was recorded in the range of 5° to 90° with a step size of 0.016°. The Raman spectra were obtained with a WITEC Alpha 300 Confocal Raman system equipped with a Nd:YAG laser (532 nm) as the excitation source. The intensity was kept at the minimum to avoid laser-induced heating. Transmission electron microscopy (TEM) was carried out using a JEOL JEM-2010F microscope. Atomic force microscope (AFM) measurement was carried out using Dimension 3100 Nanoscope IV digital instruments in tapping mode.
A. Preparation of Functionalized Graphene
Graphene oxide was prepared by a modified Hummers’ method  and it was reduced to graphene by exfoliation in the presence of hydrogen gas . Thus, synthesized multilayered graphene was functionalized in a acid medium. 100 mg of (fG) was dispersed in deionized (DI) water first by ultrasonication and then by magnetic stirring for 5 h.
B. Preparation of Silver Nanoparticles
The AgNPs were prepared as follows. 150 mg of was dispersed in water by ultrasonication, followed by stirring. After 12 h, 20 ml of 0.1 M and 20 ml of 1 M NaOH were added to the above solution under dropwise stirring. The final solution was centrifuged and the supernatant was washed several times with water. Finally, the sample was dried in a vacuum oven at 80 °C for 10 h .
C. Preparation of Graphene Decorated with Silver Nanoparticles
To decorate the graphene with AgNPs, a known quantity of silver nitrate solution was added to the fG solution while stirring. After 24 h, 40 ml of reducing solution (mixture of and NaOH) was added drop by drop. The solution was washed with a copious amount of DI water and filtered to obtain the AgNP/fG. The filtrate was dried at 70 °C under vacuum .
The transmission is calculated following a well-documented procedure for the Z-scan technique [24,25]. The input intensity (assumed to be Gaussian in nature with a beam waist and the pulse width ) is given as26] 3) is integrated numerically by using the Runge–Kutta method over from 0 to to obtain the output intensity at each . This is followed by its integration over from 0 to and over from 0 to (40 ps) to get the transmitted energy . The normalized transmission is obtained by dividing by the linearly transmitted energy.
4. RESULTS AND DISCUSSION
A. Characterization of AgNP/fG
The absorption spectrum of fG (Fig. 2) shows the characteristic band at 5.4 eV, which corresponds to the transition of the aromatic bond. The absorption spectrum of AgNP/fG exhibits a broad peak at 2.9 eV, corresponding to the localized surface plasmon (LSP). The LSP peak of AgNP has been reported at 3.1 eV . The small variation is attributed to the change in the dielectric environment and the electron density of AgNPs induced by the graphene sheets . In AgNP/fG, the transition of the aromatic bond shifts to 4.7 eV, which has been attributed to the restoration of electronic conjugation within the graphene sheets .
Figure 3(a) shows the x-ray spectrum for AgNP/fG. The characteristic face centered peaks of AgNPs are present in the spectrum. The peaks at , 44.3°, 64.4°, 77.5°, and 81.6° correspond, respectively, to the , , , , and planes of AgNP, indicating its presence. A small peak around 26.6° represents the hexagonal structure of graphene. The particle size of AgNPs, calculated using Scherer’s equation for the 38.2° XRD peak, is .
The Raman spectra can reveal the presence of electronic interaction between the graphene and AgNPs . The Raman spectrum of graphene shows characteristic D () and G () bands. While the G band corresponds to the vibration of the bonded carbon atoms, the D band is a defect-induced feature that is absent in defect-free samples. The 2D band (second order of the D band) at [shown in the inset of Fig. 3(b)] is weak and smeared along with the band. This is due to the fact that the 2D band is very sensitive to the stacking order along the axis, as well as on the number of layers. Thus it indicates toward the randomly arranged and disordered multilayered graphene sheets in the present case. In the case of AgNP/fG [Fig. 3(b)], the positions of the D band (), the G band (), and the 2D band () show small () shifts, which confirms the silver decoration of graphene [16,31]. The intensity ratio of the D band to the G band, () is a measure of the disorder in the sample . The ratios obtained for fG and AgNP/fG are 0.99 and 1.07, respectively, indicating more disorder in the latter. The intensity ratio () of the 2D band to the G band for AgNP/fG (0.73) is smaller than that for fG (0.91), confirming the doping .
The TEM image of the AgNP/fG was recorded to determine the morphology and presence of AgNPs. Figure 4(a) shows the uniformly distributed AgNPs on graphene sheets. From a magnified image of AgNPs [Fig. 4(b)], the average size of the AgNPs can be seen as . The wrinkled morphology of exfoliated graphene using AFM has been reported earlier .
B. Z-Scan Profiles at 1064 nm
The OA Z-scan profiles were recorded at 1064 nm for fG, AgNP, and AgNP/fG samples for a concentration of . The Z-dependent normalized transmission through AgNP/fG fG, and AgNPs for various intensity values at focus are shown in Figs. 5(a)–(c), respectively. The normalized transmission for AgNP/fG at lower intensities ( and ) shows a peak at the focus, indicating the effect of SA. With an increase in the intensity, the Z-scan profiles show a dip at the center, indicating a flip from the behavior of SA to RSA. The profiles at higher intensities (, , and ) are composed of three distinctive regions, namely, (i) linear absorption far from the focus (low intensity), (ii) SA near the focus (medium intensity), and (iii) RSA at the focus (high intensity). The OA Z-scan measurement for the fG [Fig. 5(b)], however, shows weak SA at . This is consistent with the earlier report at 1040 nm, in which SA was observed beyond under femtosecond pumping . The weaker SA in the present case is attributed to the wrinkled nature of the multilayer graphene. With an increase in the intensity to and , it exhibits the effect of RSA. Similar OA measurements carried out for AgNPs [Fig. 5(c)] did not contain any signature of SA. The observed stronger behavior of SA for the composite, therefore, is due to the interaction between AgNPs and fG. The measurement for the pure solvent was also carried out under identical experimental conditions to ensure that the observed nonlinearity is due only to the samples. The scattered light measurements show that, at lower pump intensities, there was no scattering, as compared to that at higher pump intensities, where nonlinear scattering ( times weaker than the signal) was observed when the samples passed through the focus. This indicates that, at higher incident flux, the thermal contribution of the nonlinear scattering is present, similar to that reported in the case of nanosecond pumping .
The observed Z-scan profiles are fitted using the method described in Section 3, by keeping and as free parameters. The presence of contributions of both SA and RSA necessitates the data of fG and AgNP/fG to be fitted with Eq. (3), while Eq. (4) is used for AgNPs in the absence of SA. The obtained values of and for these samples are given in Table 1. The low-energy Z-scan profile of AgNP/fG is similar to the standard SA IR26, as shown in Fig. 5(d).
The CA Z-scan measurement at the intensity of was carried out for the AgNP/fG composite. The inset of Fig. 5(a) shows the normalized transmission as a function of the distance from the focus. The profile clearly shows a valley–peak structure, indicating a positive nonlinear refractive index. The real part of was obtained as from this profile. The pure refractive contribution was obtained by taking the ratio of CA and OA profiles at . It can be seen that the data exhibit large error at the peak. The asymmetric nature of the profile is due to the dominant contribution of RSA along with that of weak SA at higher incident flux.
C. Z-Scan Profiles at 532 nm
At 532 nm, the absorption spectrum of AgNP/fG (Fig. 2) shows considerable absorption as compared to that at 1064 nm. The normalized transmission for AgNP/fG in Fig. 6(a) at lower intensity of shows the behavior of SA. With increase in the intensity to , the profile shifts to RSA. The OA Z-scan measurements for AgNPs and fG were also performed at similar pump energies. The Z-scan profile of fG [Fig. 6(b)] shows linear absorption up to the intensity of . With increase in the intensity to , the effect of SA is observed. With further increase in the intensity to , the profile exhibits RSA, similar to that observed in AgNP/fG. On the other hand, the AgNPs [Fig. 6(c)] shows very weak SA at lower intensity (). This corresponds to the bleaching of the plasmonic band. With increase in the intensity the contribution of RSA is observed. The obtained values and are also given in Table 1, along with the available values in literature . It can be seen that the value of AgNP/fG is lower by 1 order as compared to graphene. The interaction between fG and AgNPs causes the to be lower than that of fG and also allows us to achieve SA at lower intensities. In the present case, the obtained value of for graphene matches well with that reported in the literature .
D. Mechanism of Nonlinear Absorption at 1064 and 532 nm
Pure graphene has zero bandgap (Fig. 7) and the Fermi level is at the intersection of V-shaped conduction band (CB) and the valence band (VB) at the K point of the Brillouin zone. Absorption of light of any wavelength is thus possible. The high-intensity excitation creates large transient populations of carriers in the VB and the CB. Following this, the nonequilibrium carrier distributions in the CB and VB undergo ultrafast interband relaxation through nondissipative carrier–carrier scattering and carrier–phonon coupling within 150 fs . This is followed by the electron-hole recombination process to attain the equilibrium electron and hole distribution within . The relaxation times are shorter than the pulse duration (40 ps) used in the present experiments. Therefore, during the pulse at lower intensities, the carrier populations thermalize in each band. With increasing the intensity, more electron–hole pairs are generated and cause the states near the edge of the CB to fill. This blocks further absorption, which results in the SA behavior at both the wavelengths at lower intensity. With further increase in the intensity with the availability of the large photon flux, 2PA takes place, resulting in the effect of RSA, as indicated in Fig. 7. This observation is similar to that observed in graphene oxide . At high intensities, the scattering measurements were similar to that described in Subsection 4.B, indicating minor additional contribution of nonlinear scattering to RSA. Studies on carbon black  and carbon nanotubes (CNTs)  have revealed the mechanisms of nonlinear scattering as the formation of the bubble clouds or microplasmas.
2. Silver Nanoparticles
The localized surface plasmon band of AgNPs at 420 nm (2.9 eV) is situated below the interband transition (d to s-p band, with 4 eV). To understand the mechanism at 532 nm, Gaussian analysis of the absorption spectrum of the composite (Fig. 2) was done. It can be seen that there is a direct excitation to the localized surface plasmonic band near 532 nm (2.3 eV). The observed SA at low intensity results from the intraband electron excitation within the CB, leading to the ground-state bleaching of the plasmon band. With increase in the intensity, the transient absorption from free carriers becomes significant, resulting in RSA. In addition, the photoejection of the electrons can occur at higher laser intensities due to multiphoton absorption , leading to RSA. However, the interband transition (d to s-p band) due to 2PA can also take place at 532 nm . At 1064 nm, however, the RSA is observed only after pumping with high intensity, which can only be attributed to 2PA to the edge of the intraband following the aforementioned mechanism.
3. Silver-Nanoparticle-Decorated Graphene
As suggested by Rao and co-workers , when in physical contact, electron transfer takes place from graphene to the AgNPs to achieve a common Fermi level (Fig. 7). Under illumination, electrons from the VB are excited to the CB. The first principles calculations  show the possibility of intermediate AgNP metal states that are flat and extend into graphene. However, the excited electrons have higher probability of transferring to the metal states across the surface than to the graphene VBs owing to considerably larger density of states in the metal. The excited carriers from graphene can transfer to those metal states before returning to the VB. Besides providing long lifetimes, the transferred electrons eventually return to graphene, but the time scale is much larger than the pulse width of the laser. Since the carriers are excited faster than their return to the original state, the bleaching of the ground state takes place, resulting in strong SA behavior. With increase in the intensity, the RSA dominates over the SA, as shown in Figs. 5(a) and 6(a). The observed RSA at 1064 nm can be attributed due to the similar mechanism of nonlinear scattering and 2PA, as in the case of fG. Besides the above mentioned mechanism, at the wavelength of 532 nm, the observed behavior can be understood due to the LSP of the AgNP.
E. Optical Limiting
To study the OL behavior of the system, we used the Z-scan curves from Fig. 5 and plotted the normalized transmittance as a function of the input intensity. The so-obtained OL curves for AgNP/fG, fG, and AgNPs are shown in Fig. 8 at 1064 nm. For fG (lower curve), the value of normalized transmittance falls near unity at lower intensity, indicating the linear absorption. With the increase in the intensity, the transmittance shows a weak SA effect, followed by a decrease, indicating the OL behavior. In the AgNP/fG system, the normalized transmittance is more than unity for the incident intensity range of , indicating the dominant effect of SA. With increase in the incident intensity, the transmittance decreases sharply, indicating strong OL. The three regions of the absorption discussed earlier for the AgNP/fG system are also seen in Fig. 5(a). The AgNPs [Fig. 5(c)] do not show any significant limiting. It is to be noted that carbon-based materials, such as CNTs and carbon black, are strong optical limiters due to the nonlinear scattering in the nanosecond regime [36,37].
The nonlinear optical properties of silver-decorated multilayered wrinkled graphene have been studied by using the Z-scan technique at 1064 and 532 nm at the picosecond time scale. The intensity-dependent studies on fG at 1064 nm exhibit a weak SA, followed by RSA at higher intensities. However, the AgNP/fG has strong SA and RSA effects at low and high intensities, respectively. This indicates that the saturable intensity in multilayer graphene can be modified by doping with metal nanoparticles. The appearance of stronger SA in AgNP/fG composite is attributed to the electronic interaction between the AgNP and fG. The OL studies show that the composite is stable for applications with higher threshold intensities of . At the wavelength of 532 nm, the observed behavior can be understood due to the combined effects of graphene and LSP of the silver nanoparticles.
We thank the Council of Scientific and Industrial Research (CSIR) New Delhi for financial assistance. We thank Dr. Srini Krishnamurthy for the critical reading of the manuscript.
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