We present the study of optical limiting in iron oxide nanoparticles of diameters 31, 44, and 61 nm dispersed in toluene under exposure to nanosecond laser pulses at 532 nm. In the low fluence region smaller size nanoparticles show better optical limiting compared to larger size nanoparticles while in the high fluence region all the three samples show same limiting performance. Experimental results were compared with the well reported limiter fullerene C60 dissolved in toluene. Iron oxide nanoparticles show better optical limiting compared to C60 in the intermediate fluence region and comparable performance in the high fluence region. The pico-second Z-scan studies indicate that the contribution of electronic nonlinear refractive index and the two-photon absorption to the optical limiting is negligible. Our observations further indicate that the dominant mechanism for the optical limiting in iron oxide nanoparticles is nonlinear scattering.
©2008 Optical Society of America
There are considerable research efforts to study the mechanisms of optical power limiting, synthesize new materials and develop devices with advanced properties . An optical limiter is a device that strongly attenuates optical beam to a threshold level at high intensity while exhibits linear transmittance at low intensity. Such devices are used for protecting human eyes and optical sensors from damage due to exposure to intense radiation. The search for efficient optical limiters has lead to the study of various materials [1,2]. On the material front, synthesis of nanoparticles of various materials has received much attention due to the possibility of enhancement in the nonlinear optical properties for use in photonic devices [3–5]. Optical limiting performance of various nanoparticles including silver [6–9], gold [10–12], platinum , CdS , PbS, carbon [15,16], Si/SiO2 , CdxAg1-xS  and carbon nanotubes [16,18–20], has been reported. Nanoparticles exhibit a range of unique optical, physical, chemical, electronic and magnetic properties due to finite size effect [3,4,21]. Recently iron oxide nano-particles have received great attention due to its variety of applications such as environmental and energy applications , material nano-composites , pigment for paint industry , drug delivery , magnetic resonance imaging , and various other biomedical applications [25,27]. However, there are only a few reports on the nonlinear optical properties of iron oxide nanoparticles [28–30]. Optical nonlinearities (nonlinear refractive index and two-photon absorption coefficient) of iron oxide nanoparticles were studied using single beam Z-scan in the transparency region at laser wavelength of 1064 nm using nanosecond laser pulses . The Z-scan studies have also been performed in the absorption regime (visible wavelength region) of iron nanoparticles using cw lasers at wavelength of 632.8 nm, 514 nm and 458 nm [29,30]. The mechanism of nonlinearity in these studies was attributed to thermal effects. Recently, optical limiting properties of micro emulsions composed of normal (oil-in-water) and inverse (water-in-oil) micellar solutions were presented along with aqueous suspensions of silica coated and uncoated iron oxide nanoparticles and scattering measurements were carried out in the forward direction (transmitted beam) using nano-second pulses at laser wavelength of 532 nm . Both the samples, silica coated and uncoated iron oxide nanoparticles exhibited similar optical limiting behavior.
In this paper we present the optical limiting studies on iron oxide(Fe2O3) nanoparticles capped with oleic acid in toluene using nanosecond laser pulses at 532 nm wavelength. The effect of nanoparticle size on the optical limiting is also investigated. In the low fluence region smaller size iron oxide nanoparticles show relatively better optical limiting compared to larger size nanoparticles. The pico-second Z-scan studies indicated that the contribution of electronic nonlinear refractive index and the two-photon absorption to the optical limiting is negligible. The sign of electronic nonlinearity measured with pico-second pulses and the thermal nonlinearity with the cw pulses are opposite to each other. The distribution of nonlinear scattering process is probed both in the forward and backward direction. We compared the nonlinear light scattering by iron oxide nanoparticles and a well known optical limiter -fullerene C60. Iron oxide nanoparticles show better optical limiting performance compared to C60. It was observed that the nonlinear light scattering is significantly larger in iron oxide nano-particles compared to C60.
2. Experimental details
Iron oxide nanoparticles were synthesized by thermal decomposition of ferrocene in solvent-surfactant mixture. Varying concentration of precursor and its proportion with the capping agent were used as parameters to control the diameter of nanoparitcles for details please see ref . Nanoparticles were magnetically separated from the solution and washed several times with toluene and ethanol mixture. Purified nanoparitcles were dispersed in toluene with oleic acid as capping agent and agitation by ultrasonic agitation. The use of capping agent makes the nanoparticle dispersion stable against particle agglomeration and subsequent sedimentation. Samples for scanning electron microscopy (SEM) characterization were made by dip coating a silicon wafer in the iron oxide solution. SEM pictures show that the diameter of nanoparticles is uniform and the average sizes of nanoparticles measured from the SEM images were 31 nm, 44 nm and 61 nm with standard deviation of 4.8, 6.2 and 6.3nm respectively. SEM image and absorption spectra of 31 nm iron oxide nanoparticles is shown in Fig. 1 (a) and (b). As the absorption spectra of 31, 44 and 61 nm iron oxide nanoparticles are almost identical therefore only one is shown in Fig. 1. The iron nanoparticles show absorption over a broader wavelength region compared to C60. Further pure toluene shows negligible absorption in the visible region.
Optical limiting experiments were carried out with frequency doubled Q-switched Nd:YAG laser delivering 30 nanosecond (FWHM) pulses at 532 nm wavelength. Input laser beam was focused using a 20 cm focal length lens on a 5 mm path length cell containing the sample. Incident and transmitted energies were measured with calibrated photodiodes in two geometries. Geometry (i) for optical limiting measurements, in which, transmitted beam was collected through a 90% transmission aperture placed in front of the photodiode. In this geometry decrease in the transmission would be due to absorption, defocusing and scattering originating from absorption induced thermal effects. Geometry (ii), in which, transmitted light was measured simultaneously by putting a beam splitter near the sample to exclude defocusing and small angle scattering in the forward direction. The input fluence was varied with neutral density filters. Linear transmission of all the samples dissolved in toluene was kept 70% at excitation wavelength.
To understand the mechanism responsible for optical limiting on axis temporal profiles were measured. Further the amplitude of the scattered light around the sample was measured for both the nanoparticle and C60 samples as a function of the input fluence. Significant differences in the scattered signal for iron oxide and C60 samples were observed. Z-scan experiments were performed with nano and pico-second pulses (30 pico-second FWHM) at laser wavelength of 532 nm to distinguish between the electronic and thermal nonlinearity. Closed and open aperture Z-scan profiles were significantly different for iron oxide nanoparticles measured with nano and picosecond pulses.
3. Results and discussion
The observed variation of transmission with input fluence for iron oxide nano-particles of 31, 44 and 61 nanometer sizes for geometries (i) and (ii) is shown in Figs. 2 and 3. With increase in input fluence the transmission through the sample decreases for both the geometries.
Decrease in the transmission is larger for smaller size nanoparticles in the intermediate fluence region of 0.5 to 3.0 J/cm2. For example in the geometry (i) (transmission measurement through an aperture) for input fluence of 0.5 J/cm2 transmission is 0.35, 0.3 and 0.2 for nano-particle sizes of 61, 44 and 31 nm, respectively. In case of geometry (ii) for the same fluence of 0.5 J/cm2 the transmission is 0.4, 0.35 and 0.3 for nano-particle sizes of 61, 44 and 31 nm, respectively. Since the linear transmission of all the three samples is same, the number of nanoparticles per unit volume would be more for smaller size nanoparticles. Hence, the number of nanoparticles interacting with the laser beam would be more for smaller size nanoparticles resulting in more effective optical limiting. In addition, in case of smaller particle size the large surface to volume ratio provides larger surface area for interaction with the laser beam. With further increase in the input fluence the transmission saturates at the same value for all the nanoparticles. At the highest used input fluence of 4 J/cm2 in the experiments, the transmission reaches 0.1 for geometry (i) while for geometry (ii) it reaches 0.15 in case of all the three nanoparticle samples. In the entire measured fluence region transmission is lower in geometry (i) compared to (ii) for all the samples. This clearly indicates that nonlinear refraction and/or nonlinear scattering also play an important role in optical limiting in these materials. For example, for 31 nm size nanoparticles at input fluence of 0.5 J/cm2 the transmission is 0.2 and 0.3 for geometry (i) and (ii), respectively and for 2 J/cm2 input fluence the transmission is 0.1 for geometry (i) and increases by two times to 0.2 for geometry (ii), respectively. The optical limiting performance of iron oxide nanoparticles and C60 were overall comparable in both the geometries except in the intermediate fluence region where iron oxide nanoparticles show better optical limiting performance than C60. For example, in Fig. 2 at 0.5 J/cm2 the input fluence transmission is 0.2 for 31 nm iron oxide nanoparticles and 0.3 for C60 while in Fig. 3 the transmission is 0.3 for 31 nm iron oxide naoparticles and 0.4 for C60 at 0.5 J/cm2 input fluence. At high fluence values of ~4 J/cm2 in both the Figs. (2) and (3) corresponding transmission saturates at almost the same value for iron oxide nanoparticle as well as for C60.
In case of C60 it is well known that the optical limiting is primarily due to the reverse saturable absorption with some contribution due to nonlinear scattering [33,34]. To understand the mechanism of optical limiting in iron oxide nanoparticles we measured the scattering of transmitted beam through the sample in the far field. This experiment was performed with 44 nm iron oxide nanoparticles as well as with C60 at an angle of about 10 degrees from the transmitted beam. The magnitude of the scattered signal is comparable for iron oxide nanoparticles and C60. With the visual observation of the transmitted light in the far field on a screen we found that the spatial patterns are significantly different for iron oxide nanoparticles and C60 . In case of iron oxide nanoparticles transmitted light is uniformly scattered around the central portion of the beam while in case of C60 there is a plume localized around the beam . In addition bubble formation was observed at much lower fluence in iron oxide nanoparticle sample than in C60. These observations gave us a clue to understand the mechanism. In case of C60 the light is primarily scattered in the forward direction at a small angle while in case of iron oxide nanoparticles light may be scattered in all the directions including the backward direction. We therefore measured the light scattered by iron oxide nanoparticles, C60 and pure solvent toluene (to estimate the background scattered signal) in the backward direction as a function of input fluence. Scattered light signal was normalized with respect to the input intensity. Figure 4 shows normalized scattered light signal of 44 nm size iron oxide nanoparticles, C60 and pure toluene in the backward direction at 30 degree angle from the incident beam with variation in input energy. This experimental data indicated that the scattering is comparable in C60 and toluene while in case of iron oxide nanopartilces it is significantly larger. Variation of scattering is nonlinear in iron oxide nanoparticles sample. Normalised scattering first increases with increase in input energy and then shows saturation with further increase in input energy. Normalized scattering in iron oxide nanoparticles is very large in the backward direction that results in decrease in transmission in Figs. 2 and 3. From Fig. 4 we observed that for pure toluene normalized scattering signal is constant, this implies that scattering increases linearly with input energy. For C60 the normalized scattering signal is constant up to input energy of ~400 µJ and then increases marginally for higher energies as shown by triangles in Fig. 4. At these large input energies the normalized scattering amplitude of C60 and toluene are approximately equal. This happens only after prolonged exposure to the laser pulses at the same position of the sample cell. For input energies greater than 400 µJ normalised scattering amplitude in C60 does not increase if every laser pulse hits the sample at a fresh position as shown by crosses in Fig. 4.
The normalized scattered signal is slightly larger for toluene than for C60 dissolved in toluene. The scattered signal in both pure toluene and C60 solution is linear with input intensity. It is expected that the scattered signal would be proportional to the input energy. The reduction of input energy in C60 solution due to larger linear absorption well explains the lower amplitude of scattered signal for C60 compared to toluene. The scattering of light in iron oxide solution was nonlinear as a function of input fluence in contrast to the linear dependence observed in C60 and toluene.
The nonlinear scattering in the iron oxide nanoparticles could be attributed to the bubble formation. On careful observation of the samples it was observed that the bubble formation started in iron oxide nanoparticles at pulse energy of about ~80 µJ while in C60 it was observed at energies greater than 450 µJ and with prolonged exposure, at a fixed position in the sample. The bubble formation would result in increase of light scattering form iron nanoparticles and in case of C60 at higher input energy with prolonged exposure as shown in Fig. 4 (triangles). Crosses in Fig. 4 represent the scattered light signal for C60 solution when each laser pulse falls on the sample at different position. Thus when fresh sample is exposed to the laser beam no change in the scattered signal is observed at large energy for C60 solution. Starting of bubble formation was spotted visibly in the energy range of 70 to 90 µJ for all the three iron oxide nanoparticle samples. This may be due to larger thermal conductivity of the iron oxide nanoparticles compared to C60 causing localised heating in the medium at low energy which gets transferred to the surrounding solvent resulting in bubble formation. Once the bubble formation starts scattering from bubbles contributes to the optical limiting signal. Bubble formation depends on the total heat deposited, rise in temperature and transfer of heat to solvent. Formation of bubbles at low energy indicates that these factors are more efficient in iron oxide nanoaprticles than C60. The size of the bubble increases with increase in the incident energy ensuing increase in the scattering. To estimate the nonlinear absorption in iron oxide nanoparticles one needs to quantify the total scattered light along with transmitted light.
To understand the light scattering by iron oxide nanoparticles in detail we measured the scattered signal at different angles (i.e 20, 30, 40, 45, 50, 130, 135, 140, 150 and 160 degrees) with respect to the incident beam, in samples of iron oxide nanoparticle as well as in C60 solution for comparison. Results are shown in Fig. 5 (a). For all the three samples the scattered signal amplitude is significantly larger in the forward direction compared to that in the backward direction. Further the scattering of light by iron oxide nanoparticles is significantly larger compared to pure toluene and C60. Scattered signal is maximum at about 10–20 degrees angle (forward direction). For scattering angles in the range of 20 to 50 degrees the amplitude of scattered signal is much larger in iron oxide nanoparticles compared to C60. At 20 degrees the amplitude of scattered signal is almost double in iron oxide nanoparticles compared to C60 and toluene. However, at smaller angles (about 10 degrees and less) C60 also shows large scattering in the forward direction. Fig. 5 (b) shows light scattering data in the 90 to 180 degrees range on an enlarged scale. It is evident from Fig. 5 (b) that in the backward direction the scattered signal is maximum at ~140 degrees and is more than three times large in iron oxide nanoaprticles compared to C60 and toluene. The light scattering pattern observed on the screen for the transmitted beam in iron oxide nano-particles is similar to that reported for carbon black nano-particles which follows the Mie scattering [35–38].
For iron oxide nanoparticles the difference in transmission in geometry (i) and (ii) is small. This implies that either absorption and/or scattering is the dominant mechanism for the loss of the transmitted beam. Initially nanoparticles absorb light energy, and then the absorbed energy is transferred from particles to surrounding solvent and generates solvent bubbles that work as scattering centers in optical limiting. The bubble formation in iron oxide nanoparticles is observed visibly at much lower energy of ~80 µJ. At this energy the strong contribution due to light scattering was experimentally observed both in the forward and the backward direction. Thus it is evident that the nonlinear light scattering is contributing significantly to the optical limiting process in iron oxide nanoparticles.
Temporal profile of the input and transmitted output pulse was measured in iron oxide nanoparticle sample and was compared with C60 at two positions, one in the transmitted beam (on axis) and other in the off axis position. At on axis position both nanoparticle and C60 show almost similar temporal profiles with some pulse narrowing as shown in Figs. 6 (a) and (b), respectively. The temporal pulse profile becomes asymmetric with the peak of the transmitted pulse shifting towards the leading edge of the pulse as the trailing part of the pulse is absorbed more compared to the rising part of the pulse. This type of behavior indicates that the nonlinear absorption and/or scattering increases as the pulse propagates through the sample i.e. the process depends on the fluence of the laser pulse rather than its intensity. For C60, this is well known and is attributed to reverse saturable absorption which is a fluence dependent process . The temporal profile observed for iron oxide nanoparticles is similar to that observed in C60. This indicates that the phenomenon responsible for optical limiting in iron oxide nanoparticles is also fluence dependent. The off axis temporal profiles observed at small angle for C60 show peak of transmitted pulse shifted towards its trailing edge [33,34]. For iron oxide nanoparticles the transmitted pulse followed the input pulse. This is probably due to the absence of formation of plume in case of iron oxide nanoparticles.
The Z-scan experiments performed with nano-second and pico-second pulses at a given wavelength can provide information about the fluence or intensity dependence of the nonlinearity. The closed and open aperture Z-scan data for the nano-second pulses is shown in Fig.7. The shape of the closed and open aperture Z-scan profiles are similar. The absence of peak in the closed aperture profile and presence of strong valley in both closed as well as open aperture signifies that nonlinear refraction does not contribute significantly to the optical limiting. The dominant contribution is due to nonlinear absorption/scattering. For picosecond pulses, the damage threshold of material is higher compared to the nano-second pulses. Furthermore, for pico-second pulses higher intensities are obtained with small fluence compared to nano-second pulses. Use of lower fluence reduce thermal effects. The closed aperture Z-scan signal with pico-second pulses are thus attributed to the electronic nonlinearity.
Figure 8 shows the closed aperture Z-scan signal for iron oxide nanoparticle solution and that for pure toluene at the input intensity of 3.2 GW/cm2. The Z-scan profiles are in complete contrast with that obtained for nano-second pulses. The pico-second Z-scan signal for iron oxide nanoparticle solution is about double compared to that of pure toluene. The nonlinear refractive index is estimated by theoretically fitting the experimental data, using nonlinear refractive index as a parameter. The estimated values of nonlinear refractive index for iron oxide nanoparticle solution and that of pure toluene are 1.8×10-18 m2/W and 0.88×10-18 m2/W, respectively. The estimated value of nonlinear refractive index for pure toluene matches well with the earlier reported value . It may be noted that the maximum intensity used in the optical limiting experiments with nanosecond pulses is ~0.1 GW/cm2. This intensity is more than an order of magnitude smaller than the intensity used in picosecond Z-scan experiments. Thus for nanosecond experiments the contribution to the optical limiting due to electronic nonlinearity could be neglected. The open aperture Z-scan data is not shown as it does not show any appreciable dip in the normalized signal. This indicated that the two-photon absorption in iron oxide nanoparticles is insignificant at these intensities. Thus the contribution of two-photon absorption to the optical limiting with nanosecond pulses could also be neglected.
Recently optical limiting in gold nano-particles of diameters 30, 50, 100 and 140 nm has been reported under nanosecond laser pulse excitation . It was found that the optical limiting initially increases with increase in nano-particles size upto 50 nm size and then decreases with further increase in nano-particle size. There seems to be an optimum value for the size of nano-particles for which maximum optical limiting could be observed. In our case optical limiting is decreasing with increase in nanoparticle size. Hence it is necessary to study the optical limiting for the smaller iron nanoparticles to estimate the optimum size from which the maximum optical limiting can be observed. The absorption spectra of the iron oxide nanoparticles does not show any sharp feature hence, these nanoparticles could be used as an optical limiter over a wide visible wavelength region.
In conclusion we have studied optical limiting in iron oxide nanoparticles of various diameters dispersed in toluene under exposure to nanosecond laser pulses at 532 nm. In the low fluence region smaller size nanoparticles show better optical limiting while in the high fluence region all the three samples show same limiting performance. Iron oxide nanoparticles show better optical limiting compared to C60 in the intermediate fluence region and comparable performance in the high fluence region. Our observations indicate that the dominant mechanism in the optical limiting in iron oxide nanoparticles is nonlinear scattering.
The authors are thankful to Dr. S.C. Mehendale for stimulating discussions and for critical reading of the manuscript. Mr. G.M. Bhalerao acknowledges RRCAT, Indore for providing the fellowship.
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