Saturable absorption and reverse saturable absorption in platinum nanoparticles
Introduction
Nanostructured materials have unique physical and chemical properties as a result of their small size. These properties differ largely from those of the corresponding single molecules and bulk materials [1]. In recent years, nanomaterials have attracted substantial attention, due to their possible applications in optoelectronics and anticipated versatility in providing a deeper understanding of size-sealed structure–function relationships [2]. For example, intensive studies are currently being pursued in the nonlinear optical properties of noble metal nanoparticles [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], owing to their potential use in optical and nonlinear optical devices [3].
In this context, gold and silver nanoparticles have received special attention [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] because they both show a broad absorption band in the visible region of the electromagnetic spectrum. This is substantially different from the flat absorption spectra of the corresponding bulk metals in the same region. The broad absorption band is due to the electromagnetic field-induced collective oscillation of the free conduction electrons occupying states near the Fermi level in the conduction band, which is referred to as surface plasmon resonance (SPR) absorption [4]. Ya-Ping Sun et al. [5] investigated silver-containing metal–halide clusters and nanosols. The optical limiting in the material for nanosecond laser pulses was found to be even stronger than that in the benchmark materials fullerenes and metallophthalocyanine. It has also been demonstrated by Ispasoiu et al. [2] and Zhang et al. [6], respectively, that silver-dendrimer nanocomposite and nanosized polyicosahedral gold–silver cluster could exhibit large optical limiting. On the other hand, we have recently studied the optical nonlinearities and optical limiting behavior in gold and silver nanoparticles based on C60[7], [8], [9], [10], and found stronger optical limiting in them than that in C60. Philip et al. [4] studied the picosecond optical nonlinearity in monolayer-protected gold, silver and gold–silver alloy nanoclusters at 532 nm. They observed the changing from saturable absorption to reverse saturable absorption in gold nanoparticles using Z-scan technique. Because the wavelength of SPR in the gold nanoparticles is 520 nm, and very near the excitation wavelength of 532 nm used in their experiments, they interpreted the flip of saturable absorption to reverse saturable absorption in terms of SPR. Specifically, they attributed the origin of the saturable absorption to the bleach of the ground-state plasmon band occurring at moderate intensities, while attributed reverse saturable absorption to the transient absorption (caused by free carriers) which is significant at higher pump intensity. To our knowledge, however, the research concerning optical nonlinearity in platinum nanoparticles is still rarely reported [11], [13]. Moreover, only reverse saturable absorption was observed in [11], [13]. In general compounds [14], [15], [16], [17], the simultaneous appearance of both saturable absorption and reverse saturable absorption was found and discussed. But the novel phenomenon in platinum nanoparticles has not been reported anywhere.
In this paper, we conduct the investigation on platinum nanoparticles in aqueous solution using 532 nm nanosecond laser pulses. Even though platinum’s SPR at 215 nm [18] lies away from excitation wavelength of 532 nm, we still observed saturable absorption at lower fluences which is usually linked to SPR in gold nanoparticles [4]. Furthermore, we found the changeover from saturable absorption to reverse saturable absorption at higher input bump intensities.
Section snippets
Samples and experiments
The sample used in our experiment is platinum nanoparticles stabilized by poly (N-vinyl-2-pyrrolidone), denoted as Pt–PVP, which is synthesized by microwave irradiation. Details of sample synthesis and characterization have been discussed in [19]. The average size of Pt–PVP was estimated by transmission electron microscopy (TEM) to be of the order of 2 nm. PVP is a macromolecular polymer with an average molecular weight of 40,000, with which Pt nanoparticles in small size and with narrow size
Results and discussions
To determine the irradiance dependence of nonlinear absorption, our measurements were performed at different laser energies. In Fig. 2(a)–(d), we present some of the obtained experimental curves. In particular, we noted that the increase of laser intensity induces a switch from saturable absorption to reverse saturable absorption. Fig. 2(a) shows the results obtained at a relatively low intensity of 1.1 × 1011 W/m2 at the beam waist. When the sample is far from the beam waist, the laser radiation
Conclusions
In summary, the changeover from saturable absorption to reverse saturable absorption in Pt nanoparticles was investigated at the wavelength of 532 nm. It was found that the sample shows saturable absorption at lower intensities, while reverse saturable absorption at higher intensities. The shift was analyzed phenomenologically in terms of an intensity-dependent saturable absorption and reverse saturable absorption. The saturation intensity and nonlinear absorption coefficient was obtained to be
Acknowledgments
This work was supported by National Natural Science Foundation of China (Nos. 50172013, 20131040) and the Multidiscipline Scientific Research Foundation of Harbin Institute of Technology (No. HIT.MD2001.25).
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