In this work, we present a study of the nonlinear absorption properties from different gold nanorod (NR) systems in aqueous suspension. The NRs were obtained with the bottom-up protocol by the seed-mediated growth method (SMG), using Ag+ ions at different concentrations, and CTAB as surfactant. By using this method, aspect ratios between 2 and 5 were obtained. The transverse surface plasmons (TSP) are located between 514 – 535 nm, while the longitudinal surface plasmons (LSP) are between 639 – 921 nm, for the different samples studied. The Z-scan technique was implemented for open (OA) and closed (CA) aperture at 532 and 1064 nm, with laser pulses of 26 ps, for vertical and horizontal polarizations, with respect to the incidence plane (horizontal). At 532 nm all samples showed saturable absorption (SA), while for samples with LSP near 1064 nm, such effect was observed only at low-energy pulse experimental conditions. In the high-energy pulse regime, an apparent reverse-saturable absorption (RSA) was observed for both wavelengths. However for 532 nm, it was possible to determine that this effect results from structural changes in the samples, which are manifested through the behavior of nonlinear absorption and refraction curves. These results were used to determine the irradiances to which NRs can be modified by photodegradation.
© 2015 Optical Society of America
In recent years, metal nanoparticles (NPs) have been intensively studied due to their electrical, magnetic and optical properties. In particular, wide attention has been dedicated to NPs made of alkali and noble metals, such as copper, silver, and gold. These systems have a broad absorption band in the visible region of the electromagnetic spectrum, called surface plasmon resonance (SPR), which is responsible of confining resonant photons in such a manner as to induce coherent surface plasmon oscillation of their conduction band electrons . Scattering, strong absorption and local-field enhancement occurring at the SPR manifest in a large optical polarization associated with the collective electron oscillations. For this reason the nonlinear optical properties of metal nanorods or nanoparticles are being studied for their use in applications like plasmon waveguide, sensor protection, optical limiting device and medical therapies [2–5]. In general, optical response of metal NPs can be tuned by controlling their size, shape and environment . But, shape and crystallographic facets are also key factors to determine the surface and catalytic process of the NPs. For these reasons, much research is focused on finding synthesis techniques able to control the morphology of the NPs. While the chemical composition of NPs is important, their morphology as well as their colloidal colloidal characteristics are determinant for optical studies.
Nowadays, a significant number of research works is directed to one-dimensional NPs such as nanorods (NRs). Metal nanorods/wires have been synthesized by different approaches, using various methods such as templating [7,8], electrochemistry [6,9], photochemistry , and seed-mediated growth methods, using Ag+ ions and surfactants like hexadecyltrimethylammonium bromide (CTAB) . In comparison with nanoshells and nanospheres, colloidal NRs offer better tunability of surface plasmon resonance (SPR) bands in a wide range of optical frequencies . In the case of gold NRs, the excitation along the short axis induces an absorption band in the visible region, at wavelengths close to where spherical Au NPs usually absorb, while excitation along the long axis induces a stronger absorption band centered at longer wavelengths. Both modes of oscillation, namely longitudinal surface plasmon resonance (L-SPR), and transverse surface plasmon resonance (T-SPR), introduce additional variables upon which NRs optical response is sensitive to, like incident frequency and polarization state of light. The transverse band is not very sensitive to the size of NRs, while the longitudinal band is red-shifted largely from the visible to the near-infrared region, accordingly to increments on the NRs aspect ratio. This behavior is explained by Gans theory, which describes the optical properties of ellipsoidal particles using a dipole approximation. This has been related to the fact that, in aqueous solution, the L-SPR absorption maximum λmax, is linearly proportional to the aspect ratio R by the empiric relationship [13,14].
To control the optical properties and design colloidal NRs with specific applications, a systematic and quantitative description of interactions between NRs and light is necessary. Optical properties of gold NRs at low intensity have been widely investigated, including the dependence of light scattering, absorption in function of size, aspect ratio, among others [12,15–17]. Third-order nonlinear optical response of NRs, saturable absorption and reverse-saturable absorption as function of intensity, has been previously reported [4,18–23]. However, only the particular experimental condition of high-energy pulses has been widely studied, while the behavior of NRs under short-energy pulses has not been well determined, yet. Therefore, a systematic study of the nonlinear optical response in colloidal NRs is transcendental so that a proper design can be fulfilled in order to exploit a specific application such as optical limiting devices [3,4].
In this work, we present a systematic study of the third-order nonlinear optical properties of four different colloidal Au NRs systems. The NRs were prepared with the bottom-up method by seed growth , using Ag+ ions and CTAB as surfactant, obtaining an aspect ratio between 3 and 5. The third-order nonlinear optical properties were measured by implementing the Z-scan technique, using both experimental configurations, that is, open (OA) and closed aperture (CA) . Excitation of NRs was performed with pulsed light of 26 ps, with a repetition rate of 10 Hz, at wavelengths of 532 and 1064 nm. The obtained results show a saturable absorption effect for all samples at both excitation wavelengths, and a small nonlinear refraction for some of them. Small changes in the nonlinear absorption coefficients were obtained modifying the incident polarization of light. The samples were studied for several irradiances; in the case of 1064 nm, increasing the irradiance gives place to a decreasing effect regarding the nonlinear absorption Z-scan characteristic curve, which in turn, appoints for a possible presence of the reverse-saturable absorption process. For 532 nm, the increment of the irradiance causes structural changes in the samples, as shown by electron micrographs and given that nonlinear absorption and refraction Z-scan characteristic curves are significatively modified.
2. Experimental methods
2.1 Sample preparation
In this work, we use the well-established aspect control and silver-assisted seeded growth protocol that provide monodisperse gold nanorods in high yield relative to other shapes. However, the overall yields are relatively low as nearly 15%. In the preparation of Au NRs by the seed-mediated growth procedure, we use small NRs seeds (1.5 nm) stabilized with CTAB surfactant, which grow with the reduction of Au(III) salt to Au(0) induced mainly by ascorbic acid, with or without the presence of silver ions. However, a small amount of silver ions could be used to control the aspect ratio of NRs ranging from 1.5 to 5. The growth mechanism of silver-assisted Au NRs is still a matter of debate .
Seed solution: Colloidal Au NRs were prepared as follows: 2.5 mL of HAuCl4 solution (9 × 10−4 M), 2.5 ml of deionized water, and 5 mL of CTAB (hexadecyltrimethylammonium bromide) solution (0.2 M) were stirred together and heated to constant temperature of 45°C. 600μl of ice-cold NaBH4 solution (0.01 M) was added and stirred for 2 min at 25°C, obtaining a brown solution. Growth solution: 150 mL of CTAB solution and 150 mL of HAuCl4 solution were mixed, and 890 μL of HCl solution (0.10 M) was added. To gain a better growth control on NRs size, four different samples were produced to be analyzed in this work by addition to the growth solution of 1, 2, 3 and 4 mL of AgNO3 solution (4 × 10−3 M), respectively. Then, to each of one, 1250 μL of ascorbic acid in aqueous solution (7.8 × 10−2 M) was aggregated, obtaining colorless solutions. Finally, 200 μL of the seed solution were incorporated to all and each of them. All this mix was maintained at temperature of 40°C with gentle and constant stirring overnight. Finally, for all colloidal systems, separation of NRs from spherical nanoparticles was carried out by centrifugation (at least three times at 6 000 rpm, 1 hour). The morphologies of the Au NRs were determined by means of transmission electron microscopy (TEM) using a JEOL 2010 FE-TEM. Figure 1 shows the TEM images corresponding to the four samples analyzed after centrifugation. Typically, by using this synthesis method, approximately 1% of spherical NPs remain into the colloid system. On the other hand, samples with high content of silver ion in the growth solution not always increase the Au NRs aspect ratio due to interaction with the bromide counter ion of the surfactant monomers. Consequently, a larger fraction of nanospheres may be present, even up to 5% . Samples 1 to 3 mL have approximately the same concentration of Au NRs (98-99%) with high monodispersity. On the contrary, sample 4 mL, due to the Ag ion effect, was unable to be satisfactorily purified by the same method, and the Au NRs concentration reduced to 95%, with the polydispersity being slightly increased. The micrographs were analyzed by Digital Micrograph software. Figure 2 shows the aspect ratio histograms corresponding to the statistical analysis. According to them, the length of the NRs is between 25 and 60 nm, while the diameter goes between 11 and 12 nm, approximately. The proper information obtained from these two figures is presented in Table 1.
2.2 Optical measurements
Linear optical absorption measurements were performed with an Ocean Optics USB2000 + UV–visible spectrophotometer.
The absorption spectra of Au NRs are characterized by a dominant L-SPR band, corresponding to longitudinal resonance, and a weaker transverse resonance T-SPR, corresponding to diametric resonance. Figure 3 shows the absorption spectra for all the samples. From this figure, it can be observed the presence of two SPR peaks in all cases. The T-SPR is located around of 514-535 nm, while the L-SPR goes from 626 to 900 nm. The red-shift of longitudinal resonance with concentration makes evident its evolution with aspect ratio.
The linear absorption coefficients were estimated by the expression , with Pt and P0 the transmitted and incident powers respectively, α0 the linear absorption coefficient and l the thickness of the sample.
Z-scan technique at 532 nm and 1064 nm was used to study the third-order NLO properties of these colloids, which were scanned around the waist beam ω0 along the optical axis (z direction). As light source was used a PL2143A laser system by EKSPLA, featuring 26 ps pulses with a 1-10 Hz repetition rate. The experiments reported were realized at a frequency of 10 Hz, with vertical and horizontal polarization, with respect to the incidence plane (horizontal), for several irradiances. A quartz cell of 1 mm thickness was used in the experiments. The reproducibility of results was verified, however damage was found at 532 nm for irradiances approximately larger than I0 = 2.551 GW/cm2, for 1 and 2 ml samples, and 3.811 GW/cm2 for 3 and 4 ml samples..
The laser beam was focused by a lens with a focal length of 400 mm, where the radius of the waist beam for each wavelength was determined by the knife-edge method . The obtained values were ω0 = 35.20 ± 0.97 μm, and ω0 = 60.24 ± 0.70 μm for 532 and 1064 nm, respectively. The corresponding calculated Rayleigh lengths were Z0 = 0.732 cm, and Z0 = 1.071 cm approximately, so that the thin medium condition is fulfilled. The transmitted beams for CA and OA for Z-scan technique were measured with Thorlabs DET 10A fast photodiodes, where an aperture of 25 mm in diameter was used for the CA measurements.
3. Results and discussion
3.1 Low-energy pulse
The characteristic OA and CA Z-scan curves are presented in Figs. 4 and 5. Figure 4 corresponds to results obtained using an incident wavelength of 532 nm, for a peak irradiance at the focus of I0 = 0.400 GW/cm2. a) and c) correspond to OA experiments for horizontal and vertical polarizations, respectively. b) and d) show CA results, again, for horizontal and vertical polarizations, respectively. It is clear from Fig. 4(a), first, that a negative, saturable absorption (increased transmittance) is present for all the samples; second, that nonlinear absorption increases with AgNO3 concentration, that is, with aspect ratio (or, more importantly, with linear optical absorption, since changes in aspect ratio are mostly reflected in longitudinal surface plasmon resonance shifts); however this behavior is not preserved for vertical polarization, see Fig. 4(c). In this case, sample with 2 ml of concentration of AgNO3 presents a higher response than the other samples. Regarding the nonlinear refraction response, it does not exhibit a clear trend with AgNO3 concentration. The curves show positive or negative responses depending on the sample, for both polarizations. This erratic response is clearly due partially to the proximity of 532 nm to the respective plasmon resonance. For this reason it was not possible to fit the CA curves by using the Sheik-Bahae theory ; instead, only some curves could be fitted using the phenomenological model proposed by García-Ramírez et al. .
The OA Z-scan curves were fitted using the relationships given by Sheik-Bahae et. al. . The OA experimental curves were fitted by using the equation:
The β coefficients corresponding to 532 nm, for several irradiances, for horizontal and vertical polarizations, are reported in Table 2. It is evident how, for each AgNO3 concentration, this coefficient decreases in magnitude as irradiance increases. On the other hand, as mentioned above, this coefficients increases in magnitude with AgNO3 concentration, at least clearly for horizontal polarization and for the smallest irradiance. This is clear consequence of the increment in resonance, that is, of the increment with AgNO3 concentration of the linear absorption coefficient, as indicated by α0 values in Table 1. Experiments realized at larger irradiances are reported in section 3.2.
The samples were also analyzed at 1064 nm. Figure 5 shows absorption and refraction nonlinear curves at I0 = 2.9657 GW/cm2, for vertical and horizontal polarization. In this case, samples 3 and 4 ml exhibit a better absorptive response compared to 1 and 2 ml samples, while their refractive response shows a wide peak and valley for vertical case. However, it can be observed in Fig. 5(b) that the response does not reach the linear regimen at the end of z-scanning. Several experiments were realized under the same conditions, obtaining similar results. Figure 5(d) shows the refractive response for horizontal polarization. It can be seen valley suppression for samples 3 and 4 ml, and the null response for samples 1 and 2 ml. It is important to mention that the nonlinear absorption response is considerably wide.
Nonlinear absorption coefficients obtained at several irradiances are reported in Table 3. Sample 1 ml did not present any response for all the irradiances when using horizontal polarization.
Regarding the results for different polarizations, it can be observed how they are quite different for the smallest irradiance and the largest nanorods, for both wavelengths. On one hand, this indicates that these differences are not related to a nonlinear optical phenomenon since, when increasing the irradiance, the results become similar. On the other hand, preliminary birefringence measurements performed on these colloids  have shown quite small values for all the samples, but also the presence of a Fano-like resonance in the birefringence for the longitudinal plasmon resonance of the 4 ml sample [30,31]. This last result is an indication that, possibly because of the size of the NRs, there is a preferential orientation of the NRs into the colloid, although quite small, which will be supported by the different β values shown in Tables 2 and 3 at low irradiances, for that sample.
3.2 High-energy pulse
These samples were also studied for high irradiances for both wavelengths. Because of the similarity in the behavior of the absorptive response, Fig. 6 shows only the results, at 532 nm, for samples 1 ml and 2 ml. The irradiances values went from I0 = 2.551 GW/cm2 to I0 = 5.215 GW/cm2. As it can be observed for both samples, the NLO response shows a peak and a valley. At first, observing Fig. 6, one could think that, when increasing I0, there could be a competition between SA and RSA such that RSA would dominate from I0>5.215 GW/cm2. However, if that was the case, there should be some symmetry in the curves, the change for one response to the other should have been gradual, and the transmittance minimum should be located at the focal position. None of these three facts happened for these curves. The response corresponding to SA did not decrease when increasing I0, actually, what happened is that, when increasing energy, the valley broadened while the peak moved further to negative values of the z position. The ensemble of these observations raised the importance of guaranteeing the reproducibility of the results as follows.
To analyze the reproducibility or not of the results, the experiments were repeated several times under the same illumination conditions; this can be observed in Fig. 7, where curves corresponding to four scans made immediately one after each other for sample 2 ml are shown. For the smallest I0, I0 = 2.551 GW/cm2, the curves were reproducible after several scans. But, for larger I0s the response varied with each scan, as can be seen in Fig. 7(a). As said before, the curves show very wide valleys with a poor defined transmittance minimum, which cleary differs from a typical RSA response. Also, the fact that the response changed for every scan is a clear indication of a damage being produced on the sample, increasing the asymmetry of the measured curve. Finally, a clear proof of this damage is shown in Fig. 7(b), where it can be seen a curve obtained at a much lower irradiance, I0 = 0.896 GW/cm2, after the fourth scan at I0 = 4.991 GW/cm2. It can be clearly observed that this last curve is very different from the curves obtained at the low-energy pulses regime, Fig. 4.
After performing these measurements for all the samples, a limit of reproducibility was obtained for each of them. For samples 1 and 2 ml, the limit was I0 = 2.551 GW/cm2, while for samples 3 and 4 ml, this limit was 3.811 GW/cm2. To understand what happened to the samples after irradiating them above these irradiances, sample structure was studied using electron microscopy. Sample 2 ml was studied in Central Laboratory of Microscopy-IFUNAM with JEOL JSM-7800F, Field Emission Scanning Electron Microscope. Micrographs were obtained for this sample before z-scan, and after 4th z-scan, at I0 = 4.991 GW/cm2. Figure 8 shows the structural modifications induced in the sample by large irradiances. Besides of aggregation effects, there is also a clear change in the shape of the NPs, passing from being NRs to show spherical shapes, with diameters around the original large axis of the NRs. It was not possible to obtain higher resolution images since the electron beam started to evidently affect the samples. However TEM microscopy was still feasible; Fig. 9 shows several micrographs taken before and after z-scan. In Figs. 9(d)-9(f), it can be observed the trend of the particles to form clusters after de 4th z-scan. Moreover, it is possible to observe the different morphologies present in the sample. It is important to mention that the analyzed sample’s amount and the preparation method for microscopy of the samples were the same before and after the z-scan measurements. In Fig. 10, it is shown the absorption spectra corresponding to sample 2 ml before and after z-scan, the spectrum after the 4th z-scan shows a decrease relative to the spectrum of sample before experiments.
Similar experiments were made at 1064 nm. Results shown in Fig. 11 correspond to sample 4 ml for several irradiances. In this case, it can be observed the decrease of NLA with irradiance increment. Actually, for I0 = 20.39 GW/cm2, it starts to be evident a possible RSA, which is a typical behavior for optical limiting phenomena. The origin of the RSA is attributed to the formation of strong ligth scattering centers due to the vaporization of the initial particles induced by the laser pulse . The transformation from saturable to reverse absorption is an interesting effect that can be used for optical pulse compressor, optical switching and laser pulse narrowing . It is worth remarking how, for 1064 nm, the change from SA to RSA is totally different from what happened at 532 nm, and was discussed above. In this case, the curves are all symmetrical and there is a clear decrease of the transmittance maximum when increasing the irradiance.
On the other hand, an ideal nonlinear material for an optical switching device should fulfill the following conditions: 1) the effect of linear absorption must be weak compared to the effect of nonlinearity, and 2) the effect of two-photon absorption must be weak compared to the nonlinear effect. These conditions are quantified in terms of the Stegeman figures of merit W (one photon), and T (two photon), respectively :
Third-order nonlinear optical response was studied for different colloidal Au NR systems, for wavelengths close to transverse and longitudinal SPRs. Saturable absorption was obtained for low irradiances, increasing with AgNO3 concentration. For high irradiances, at 532 nm, an apparent reverse-saturable absorption was obtained. However, further analysis of this effect showed that photodegradation of the NRs is responsible of this change of sign. The irradiances, from which photodegradation of the NRs was observed, were I0 = 2.551 GW/cm2 for samples 1 and 2 ml, and 3.811 GW/cm2 for samples 3 and 4 ml. In the case of 1064 nm, saturable absorption decreases when increasing irradiance, and for irradiances larger than 20.39 GW/cm2, reverse-saturable absorption starts to show.
The authors wish to acknowledge the technical assistance of Diego Quiterio Vargas and Samuel Tehuacanero Núñez. We also acknowledge financial support from CONACyT through grants CB2010-156529 and PROMEP-102491; and from DGAPA through postdoctoral fellowship for E.V.G.R. and grant PAPIIT IT102013.
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