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We report the observation of optical limiting in Fe3O4-Ag nanocomposites in solution. With these nanocomposites, we demonstrate that broad temporal optical limiting can be accomplished with low limiting threshold. Due to the presence of Ag nanoparticles, nonlinear scattering gives rise to enhanced optical limiting responses to 532-nm nanosecond laser pulses, with a limiting threshold comparable to carbon nanotubes. As exposed to 780-nm femtosecond laser pulses, the largest value (~10−44 cm4s photon−1 or 106 GM) for two-photon absorption cross-sections reported to date results in superior limiting responses with a limiting threshold as low as 0.04 J/cm2 or 100 GW/cm2 for Fe3O4–Ag (7nm) solution in 1 cm quartz cell. The limiting threshold can be further reduced by increasing Ag particle size through plasmon enhancement or taking advantage of self-defocusing.
©2010 Optical Society of America
1. Introduction
As more and more powerful pulsed lasers become widely available, there is a growing need for the protection of optical sensors or human eyes from intense laser irradiation [1,2]. There have been significant research efforts and advances in the field of nonlinear optics, whereby the light absorption and/or light scattering of a material increase with the energy of incident laser pulses. The nonlinear processes in organic molecules, polymers and nanomaterials have been extensively investigated, including two-photon absorption, excited-state absorption, and light-energy-dependent scattering (or nonlinear scattering) [1–8]. However, the lack of strong performers in conventional nonlinear materials has hindered practical applications. Herein we present Fe3O4-Ag semiconductor-metal nanocomposites as a new type broad temporal optical limiter. With these nanocomposites, we demonstrate that broad temporal optical limiting can be accomplished with low limiting threshold. Due to the presence of Ag nanoparticles, nonlinear scattering gives rise to enhanced optical limiting responses to 532-nm nanosecond laser pulses, with a limiting threshold comparable to carbon nanotubes. As exposed to 780-nm femtosecond laser pulses, the largest value (~10−44 cm4s photon−1 or 106 GM) for two-photon absorption cross-sections reported to date results in superior limiting responses with a limiting threshold as low as 0.04 J/cm2 or 100 GW/cm2. The limiting threshold can be further reduced by increasing Ag particle size through plasmon enhancement or taking advantage of self-defocusing.
A desirable optical limiter should be highly transparent at low light irradiance, but opaque under high laser irradiation, and should exhibit fast switching between these two states. It also needs to be functional in broad temporal and spectral ranges with a high damage threshold. Effective optical limiting of nanosecond laser pulses is achievable by utilizing the excited-state absorption in fullerenes [1] and phthalocyanine complexes [2], and the nonlinear scattering in carbon nanotubes [3–5], nanoparticles [6,7] and metallic nanowires [8]. Excited-state absorbers have been proven as one of the best limiters, but they perform poorly in response to femtosecond laser pulses. On the other hand, optical limiters based on nonlinear scattering provide broadband limiting because the origin of the creation of new scattering centers is thermal in nature: self thermal expansion due to absorption of intense laser energy and formation of ionized microplasmas and/or solvent microbubbles. Such limiters, however, are known to be less effective towards femtosecond laser pulses.
To regulate intense femtosecond laser pulses, two-photon absorption (TPA) in semiconductor quantum dots [9] and organic molecules [10] has been proposed and investigated. The best TPA cross-sections are found to be 103–106 Goeppert-Mayer units (1 GM = 10−50 cm4 s per photon). Such two-photon absorbers typically exhibit poor performance towards nanosecond laser pulses because the intensities of nanosecond laser pulses are inadequate to induce significant TPA prior to laser-induced damage in the material.
There have been many attempts to enhance nanosecond optical limiting by blending two materials that are two-photon absorbers, excited-state absorbers or nonlinear scatters [11–13]. No study has been reported on enhancing femtosecond limiting performance. Herein we propose and demonstrate how to engineer nanohybrid composites for superior optical limiters on both femtosecond and nanosecond timescales. The nanohybrid composites consist of two components; one component is selected for its giant TPA, and the other is chosen for its metallic properties to reinforce nonlinear scattering induced by nanosecond laser pulses and to greatly enhance TPA through its surface plasmon. With this new approach, we have achieved a limiting threshold as low as ~0.01 J/cm2 (or less than 102 GW/cm2) for femtosecond laser pulses, and a limiting response as good as carbon nanotubes on the nanosecond time scale.
We selected Fe3O4 nanocrystals as a key component of the nanohybrid composite since Fe3O4 is well known for its nanosecond and femtosecond optical limiting performance [14] and large TPA [14–16]. Furthermore, a Fe3O4 nanocube is chosen because TPA theories on CdS nanocrystals conclude that the cubic shape should provide the largest the TPA cross-section, compared to spherical or cylinder shapes for a given same volume [17]. In addition, the recent finding shows that Fe3O4 nanocrystals are also an effective nonlinear scatterer [18]. Silver nanoparticles were chosen as the metallic component for the nanohybrid composite due to its ease of synthesis with Fe3O4 nanocrystals [19–21], high chemical and thermal stability, as well as excellent limiting property under nanosecond laser excitation [6,7]. The Fe3O4–Ag heterodimer exhibited unique synergistic features in its optical properties. In particular, the surface plasmon of Ag particles considerably enhanced the effective TPA on the femtosecond time scale, as elaborated below.
2. Material synthesis and characterizations
The Fe3O4 nanocubes were synthesized by thermal decomposition of iron oleate complex following the published procedures [22–24]. The Fe3O4–Ag heterodimers were prepared by seeded growth using pre-synthesized Fe3O4 nanocubes as seeds [25]. High-resolution transmission electron microscopy (TEM) was used to characterize the structure of the heterodimers. The Fe3O4 nanocubes and Ag nanoparticles were nearly monodispersed with size dispersion of 20% or less, and with an average length and an average diameter of 13 nm and 7 nm, respectively, see Figs. 1(A) , 1(B) and 2(a) . The heterodimers were capped with an organic layer of oleic acid and oleylamine molecules, respectively, as shown in Fig. 1(C). The X-ray diffraction (XRD) pattern in Fig. 2(b) clearly illustrates the crystalline peaks associated with Fe3O4 phase [26] and Ag phase.
Fig. 1 (A) Low-resolution and (B) high-resolution TEM images of Fe3O4–Ag. (C) Schematic of a Fe3O4–Ag nanohybrid composite with a capping layer.
Fig. 2 (a) Absorption spectra of Fe3O4–Ag heterodimers (red) and Fe3O4 nanocubes (green) in toluene. The difference between the two spectra is shown by the black line. The inset displays the size distributions of Fe3O4 nanocubes and Ag nanoparticles. (b) Powder XRD pattern of Fe3O4–Ag heterodimers.
Figure 2(a) shows the comparison between the linear (or small-signal) absorption spectra of Fe3O4–Ag heterodimers and Fe3O4 nanocubes in toluene. In the spectral range from 200 to 1000 nm, the similarity between the two spectra suggests that the absorption was predominated by the Fe3O4 nanocubes. Shown by the black line in Fig. 2(a), the slight difference between the two spectra reflects the presence of the Ag nanoparticles, which manifest themselves by a surface plasmon resonance (SPR) at ~410 nm, in agreement with the Mie’s theory for 7-nm Ag nanoparticles in toluene [27,28]. The fundamental absorption band edge of the pure Fe3O4 nanocubes was uncertain due to the fact that a number of narrow d-d bands lie within the gap, some of which even extended to the infrared with a definite probability [21,29].
3. Optical limiting performance in nanosecond time regime
The nanosecond experiment was conducted with 7-ns, 532-nm and 10-Hz laser pulses, which were produced by a Q-switched and frequency-doubled Nd:YAG laser. The laser pulses were divided into two parts with a beam splitter; one part was taken as the reference, and the other part was focused onto the sample with a concave mirror (f = 25 cm). The beam radius at the focus was 35 ± 4 μm. The incident and transmitted pulse energies were measured with two energy detectors (Laser Probe RjP-735) simultaneously. The single-walled carbon nanotubes suspended in water were measured under the same conditions as a reference. Figure 3(a) shows that the limiting performance of the Fe3O4–Ag (7nm) solution was marginally better than that of the carbon nanotube suspension, which has been known as a benchmark optical limiter. The Fe3O4–Ag (7nm) heterodimer outperforms the Fe3O4 nanocube in the context of limiting threshold, defined as the incident fluence at which the transmittance falls to 50% of the normalized linear transmittance. The limiting threshold of the heterodimer solution in 1 cm quartz cell was ~2.5 J/cm2, which was half of that of Fe3O4 solution, indicating that the optical limiting of nanosecond laser pulses benefits from the presence of Ag nanoparticles.
Fig. 3 Fluence-dependent transmittance measured for toluene solution of (▪) Fe3O4, nanocubes, (●) Fe3O4–Ag (7 nm) heterodimers, (▲) Fe3O4–Ag (10 nm) heterodimers, and (•) carbon nanotubes in water. The data (▼) were measured from Fe3O4–Ag (10 nm) heterodimers with an aperture of 90% transmittance placing in front of the transmission detector.
To elucidate the mechanism responsible for the observed optical limiting responses to 532-nm, 7-ns laser pulses, the scattered light energy was measured as a function of the input energy, from the Fe3O4-Ag (7 nm) solution and the Fe3O4 solution. The solution samples were contained in 1 cm quartz cells with the 70% linear transmittance. The experimental set-up was the same as the one for the optical limiting measurement except for the transmitted detector located at various angles from the optics axis. The single-walled carbon nanotubes suspension was used a standard sample. As shown in Fig. 4(a) and 4(b), similar behavior for the three samples indicates that the OL responses originate from nonlinear scattering processes. The scattered signal from the Fe3O4–Ag (7 nm) solution is stronger than from the Fe3O4 solution. This unambiguously demonstrates that the nonlinear scattering is enhanced by the presence of Ag nanoparticles.
Fig. 4 Scattered light measured with 532-nm, 7-ns laser pulses (a) at an angle of 45۫ (b) at various angles with an input energy of 1 mJ.
4. Optical limiting performance in femtosecond time regime
The femtosecond experiments were conducted with 330-fs (FWHM), 780-nm and 1-kHz laser pulses, which were generated by a Ti:Sapphire regenerative amplifier (Titan, Quantronix). The enhanced optical limiting of femtosecond laser pulses is illustrated in Fig. 3(b). The limiting threshold of Fe3O4–Ag (7 nm) solution in 1 cm quartz cell was ~0.04 J/cm2 (or ~102 GW/cm2), which is about one order of magnitude lower than that of Fe3O4 nanocubes. We would like to clarify that we haven’t found any obvious changing in the limiting threshold when they were exposed to high intensity laser pulses (≥ 300 GW/cm2) 2 to 3 hours. However, parts of the Fe3O4–Ag nanocomposites tend to aggregate within one day. They can be re-suspended in toluene with few minutes ultrasonication. The TPA and optical Kerr nonlinearity (n2) could be determined unambiguously by conducting open-aperture and close-aperture Z-scans, respectively [30]. Typical Z-scans on Fe3O4–Ag (7nm) solution in 1 mm quartz cell are depicted in Fig. 5(a) . By assuming spatially and temporally Gaussian profiles for the input laser pulses, Z-scans can be fit by solving the propagation equation of electric field envelope E:
where k is wave vector, describes both one-photon and two-photon absorption, and I is the light irradiance.Fig. 5 (a) Open-aperture Z-scans (open circles) with theoretical fits (solid lines). The inset shows the dependence of effective TPA coefficients on input irradiance. (b) Transient absorption measurements with two-exponential fittings (black solid lines). The measurements were conducted on the 1-mm-thick toluene solution of Fe3O4–Ag (7 nm) heterodimers with 780-nm, 330-fs laser pulses.
From the best fits, we extracted the effective TPA coefficients ( ) and plotted them as a function of the laser irradiance, as displayed in the inset of Fig. 5(a). From the intercept, the intrinsic TPA coefficient was determined to α2 = 0.040 ± 0.005 cm/GW. The TPA cross-section (σ2) was then derived as 1.3 × 107 GM from , where N was the heterodimer density (8 × 1013 cm−3). This value was one order of magnitude greater than the largest value published for organic molecules [10] and at least two orders of magnitude higher than the values reported for semiconductor quantum dots [31–33]. The slope of the line was related to both excited-state absorption (ESA) cross-section and lifetime [34]. At higher laser excitation, TPA and ESA could be effectively treated as a two-step three-photon absorption process, consistent with the previous findings [14].
Definite understanding of the two-photon transitions would require quantum chemical computations for excited states in the Fe3O4 nanocube. Such theoretical studies are difficult for transition metal species with an unfilled d electron shell [35]. The enhancement in the measured TPA of Fe3O4–Ag heterodimers, however, was a consequence of the presence of surface plasmon resonance (SPR) in the Ag particle. The nonlinear absorption contribution from the same concentration Ag nanoparticles in toluene is negligible within our experiment uncertainly. Recently, a 157-fold enhancement has been reported [36] for the effective TPA cross-section of organic dye through an electric-field augmentation via the SPR of Au nanospheres. The SPR enhancement is related to the metal dielectric constant, solvent dielectric constant, excitation wavelength and metal particle size. Here we demonstrate that the σ2 of Fe3O4 (13 nm) nanocube can be tailored through altering the size of Ag nanosphere. As the Ag diameter is increased from 0 to 10 nm, the σ2 is improved from 0.3 × 107 GM to 1.8 × 107 GM.
The enhanced σ2 can greatly reduce the limiting threshold. Figure 3(b) clearly demonstrates that the greater the Ag particle size is, the lower the limiting threshold is. To further reduce the limiting threshold, one may take advantage of self-defocusing, originating from the enhanced optical-Kerr nonlinearity in the heterodimer. Our close-aperture Z-scans (not shown here) indicate that the optical Kerr nonlinearity is negative. Similarly, the measured nonlinear refractive cross-sections are found to increase with the Ag particle size, see Table 1 , which is indicative of an enhancement by the presence of SPR in Ag particles. With an aperture of 90%-transmittance placed in front of the transmission detector, the limiting threshold reaches as low as 0.01 J/cm2 or 30 GW/cm2, see Fig. 3(b).
Table 1. Effective nonlinear refractive and TPA cross-sections
Above we have demonstrated that the heterodimers exhibit ultrafast response to femtosecond laser pulses. However, the capability of fast recovery is another crucial indicator for optical limiting application. To elucidate it, we have performed the femtosecond time-resolved degenerate transient absorption experiments with the same laser system as used for the above-said femtosecond measurements. After a spatial filter, the laser pulses were also divided into two parts; one part was used as the pump, and the other part was employed as the probe. The linear polarization of the probe pulse was adjusted to be perpendicular to that of the pump pulse with a polarizer and a quarter waveplate. The probe pulse energy was regulated to be less than 5% of the pump pulse energy by utilizing neutral density filters. The experimental results are shown in Fig. 5(b), the overall signals are dominated by multiphoton absorption induced excited states absorption. The decays were best fit with two exponentials. The fast time component was ~1 ps, while the slow time constant was in the range of 18 to 30 ps, depending on the pump pulse energy, which was different from the electron dynamics reported for iron oxide nanoparticles whereby the relaxation was independent of the pump intensity [26,29]. With the Ag nanoparticle attachment, the difference should be anticipated as there existed an extra relaxation channel whereby some energy of the photoexcited carriers in the Fe3O4 nanocube was energy resonantly transferred to the metallic component, and subsequently dissipated through electron-electron, electron-phonon and phonon-phonon interactions [27].
5. Conclusion
We have demonstrated that broad temporal optical limiting can be accomplished with low limiting threshold by utilizing Fe3O4-Ag nanocomposites in solution. Due to the presence of Ag nanoparticles, nonlinear scattering gives rise to enhanced optical limiting responses to 532-nm nanosecond laser pulses, with a limiting threshold comparable to carbon nanotubes. As exposed to 780-nm femtosecond laser pulses, the largest value (~10−44 cm4s photon−1 or 106 GM) for two-photon absorption cross-sections reported to date results in superior limiting responses with a limiting threshold as low as 0.04 J/cm2 or 100 GW/cm2 for Fe3O4–Ag (7nm) solution in 1 cm quartz cell. The limiting threshold can be further reduced by increasing Ag particle size through plasmon enhancement or taking advantage of self-defocusing. Taking into account the ~30 ps recovery time, Fe3O4-Ag nanocomposites are highly promising for optical limiting applications.
Acknowledgements
This work is supported by the National University of Singapore (NUS) (Grant #R-144-000-213-112), and the Institute of Bioengineering and Nanotechnology (Biomedical Research Council, Agency for Science, Technology and Research, Singapore).
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