Optical limiting properties of natural carbon nanoparticles (Shungite carbon) and fullerene aggregates in aqueous dispersions were for the first time studied and juxtaposed. For the picosecond laser pulses both dispersions exhibited comparable optical limiting property, apparently due to nonlinear absorption (mainly TPA). In the domain of the nanosecond laser pulses, nonlinear scattering NLS is thought to be main optical limiting mechanism in the dispersions under study. Shungite carbon dispersion showed stronger optical limiting (being similar to CBS) in visible and IR, which we ascribe to its structure more favorable to the thermal relaxation processes responsible for the NLS mechanism.
© 2014 Optical Society of America
Development of new nonlinear optical limiting media is an urgent task to produce the protective appliances for the eyes and optoelectronic devices. Among materials for optical limiting applications dispersions of carbon nanoparticles are rated quite high and intensively studied to date [1–7]. Indeed, they are either colorless or slightly colored and have both a wide working spectral range (from visible to near infrared light) and a considerable limiting efficiency (102-103 times). Aqueous dispersions of shungite carbon (ShC) nanoparticles and fullerene C60/C70 aggregates stabilized without any surfactant, are interesting carbon-based media to study their optical limiting properties. To the best of our knowledge, optical limiting in such systems has not been studied yet. Meanwhile, the carbon black suspension (CBS) is a well-known benchmark for optical limiting , so we chose it as a comparative sample in our studies.
The term “shungite” is used to denote both carbon-rich rocks (with carbon content more than 30 wt.%) and the specific structural form of carbon, which is a part of these rocks wide spread in the Karelia region (Russia). The shungite rocks consist of carbon, the mineral constituent and a minor amount (0.11-0.25 wt. %) of organic substances (mainly saturated aliphatic ketones and esters). Their composition is strongly dependent on the geological prehistory of the shungite deposits. Nevertheless, carbon of all the shungite rocks is structurally identical regardless on the different genesis of the rocks, sedimentary and volcanic .
The ShC is characterized by multi-level structural organization that is provided with aggregates of globular clusters of 5-10 nm in size. In its turn, the globules are arranged from fragments of ≤1 nm in size. These basic flakes are believed to be chemically reduced graphene oxide .The globular clusters are assembled into 100 nm globules of the second level. This structural hierarchy at the nanolevel is observed in aqueous dispersions and in the films, prepared of the condensed aqueous dispersion. A high ability of ShC to be dispersed up to individual aggregates in aqueous solution convincingly proves such vision of its structure [10,11]. The aqueous dispersions exhibit a large number of peculiar properties that, on one hand, are connected with the unique properties of ShC, while on the other, are similar to those characteristic for aqueous dispersions of quantum dots . An important and special ShC feature is a capability to form stable dispersions in both polar and nonpolar solvents. The method we used for preparation of the aqueous dispersion of ShC nanoparticles is described in . The production of the ShC dispersion is an innovative technology, which is currently undergoing the patenting routine for the future commercialization. The raw material for the production of the ShC dispersion is abundant in Karelia region of Russia and sufficient for long-term exploitation.
The availability of natural resources, specific features of ShC structural organization together with its ability to form stable aqueous dispersion of carbon nanoparticles, makes shungite materials interesting for optical limiting investigation.
As to fullerene molecules and their aggregates they are also known to form stable aqueous dispersions . There are several reasons to study nonlinear optical limiting of the fullerene dispersions. On the one hand, the dispersions contain relatively large particles which can provide thermal mechanism of nonlinear scattering (NLS). On the other hand it is proved that fullerene molecules included in the aggregate can be driven into the lowest excited triplet state being irradiated with a visible light  and at a sufficiently high light intensity can manifest a reverse saturable absorption (RSA) from this state.
The triplet state quantum yield (and therefore the contribution of RSA in total optical limiting) can be increased by amorphization i.e. increasing the degree of defectiveness of the fullerene aggregate structure . Attempting to have a large RSA effect we produced our fullerene dispersion by method  which provides a high amorphization of fullerene aggregate structure (70% according to X-ray diffraction analysis of the freeze-dried dispersion ). So the contribution of RSA effect in total nonlinear optical limiting for visible light is expected to be considerable.
Stable aqueous dispersions of shungite with carbon concentration of 0.1 mg/ml are produced by ultrasonic treatment of the shungite powder (98 wt.% carbon, from Shun’ga deposit) at a frequency of 22 kHz and an operating power of 300 W according to the protocols described in [10,18,19]. The dispersions were filtrated and ultracentrifuged in RotinaR centrifuge (30 min, 700 rpm).
The fullerene aqueous dispersion was produced by the methods described in  using fullerene powder contained C60/C70 = 83/16 and 1 wt% of higher fullerenes (research company «Intellekt», Saint-Petersburg).
The aqueous CBS was made on the base of a surfactant solution in water (0.03 wt%). The surfactant was hexadecyltrimethylammonium bromide (Fluka). The carbon nano-powder (8.5 mg) was added to 100 ml surfactant solution and sonicated at a frequency of 22 kHz and an operating power of 300 W following the scheme: (10 min processing, 10 min cooling) × 3 times. Afterwards the prepared dispersion was centrifuged in RotinaR centrifuge (15 min, 400 rpm). Supernatant portion (3/4 of the total volume) was used for the further experiments.
All the dispersions under the study maintained sedimentation stability spanning at least during one year. Average diameters of the dispersions’ particles obtained by dynamic light scattering using nanoparticle size analyzer ZetasizerNano ZS (Malvern Instruments) were approximately 100 nm, 70 nm and 50 nm for ShC, fullerene aqueous dispersions and CBS respectively. Absorption spectra of the dispersions are shown on Fig. 1.
For nonlinear optical transmission experiments the dispersions under study were poured into 10 mm spectrophotometric cuvettes and placed in an intermediate focal plane of a single Keplerian telescope with a relative aperture of the objective and the eyepiece 1/5. The objective and the eyepiece were the singlet lenses with the diameters of 8 mm and the focal lengths of 40 mm. The spot diameter in the focal plane was 30 ± 2 µm measured by Videoscan-205/P/K-USB CCD camera at 1/e2 level. The Q-switched, multimode Nd:YAG laser with pulse duration of 7 ns, wavelengths 1064 and 532 nm and divergency of 0.8 mrad was used for nanosecond measurements. For picosecond measurements Q-switched, TEM00 Nd:YAG laser with a longitudinal modes selector, pulse duration of 30 ps and wavelength of 560 nm was used. All nonlinear transmittance measurements were carried out in a single pulse mode. The output signal was collected in the field of view equal to 1.5 mrad. The experimental setup was similar to that shown in . The prepared dispersions were diluted to provide initial transmission of each sample of ~50% at 532 nm. So that at 1064 nm the dispersions’ transmissions were ~70% for CBS and ShC and ~90% for fullerene aggregates.
3. Results and discussion
3.1 ShC dispersion vs. CBS
First of all a comparison of nonlinear optical responses of ShC aqueous dispersion and CBS was carried out. The registration of nonlinear optical transmission was made using a laser with wavelength of 532 nm and pulse duration of 7 ns. It was shown (see Fig. 2) that nonlinear optical responses of these dispersions practically coincide within the measurement accuracy. This coincidence is most probably due to the same mechanism of nonlinearity in ShC dispersion as in CBS which is known for the nanosecond range to be the NLS . The additional mechanism making a minor contribution to the nonlinear optical response is nonlinear absorption [1,15]. Therefore, we can assume that the ShC dispersion also exhibits NLS (as a main mechanism) and nonlinear absorption (as an additional mechanism). A slightly lower activation threshold of ShC than CBS may be related to the ShC’s larger particle size. The comparative numerical modelling of the nonlinear optical processes in different nanocarbon dispersions is supposed to be carried out in the future.
The presence of nonlinear absorption contribution in the optical limiting of the ShC dispersion was proved by experiments carried out with picosecond pulses (30 ps). Indeed nonlinear optical response of the ShC dispersion was obtained not only in nanosecond range but also in picoseconds range (see Fig. 3), in which the NLS contribution must be negligible because the thermal processes leading to the NLS occurred in a time of the order of nanoseconds . The observed nonlinear absorption in the ShC dispersions can be explained by the presence of the reduced graphene oxide in the ShC structure. This hypothesis can be indirectly confirmed by the maximum at 265 nm in the absorption spectra of the ShC dispersion (see Fig. 1) and the existence of the same maximum in the absorption spectra of the reduced graphene oxide dispersion .
There are several studies on the nonlinear optical properties of both graphene oxide [4,22,23] and reduced graphene oxide [4,23]. For the dispersions of reduced graphene oxide it was shown that, the main mechanism of optical limiting was an NLS, and an additional mechanism might be a two-photon absorption . Therefore, we believe that the nonlinear absorption in the ShC dispersion at a wavelength of 532 nm can also be caused by two-photon absorption.
3.2 Fullerene vs. ShC
In further experiments, we compared the nonlinear optical responses of the fullerene and the ShC aqueous dispersions. The comparison was made at a wavelength of 560 nm when exposed to picosecond laser pulses (see Fig. 3), as well as at wavelengths of 532 nm and 1064 nm when exposed to nanosecond laser pulses (see Fig. 4).
As can be seen from Fig. 3, the fullerene dispersion and the ShC dispersion both have a significant nonlinear optical response (nonlinear absorption) in the picosecond regime. However, the mechanism of nonlinear absorption in fullerene dispersion can be more complicated. It is well known that the RSA plays a key role in the optical limiting of molecular fullerene solutions in the spectral range 440–560 nm, whereas at longer wavelengths two-photon absorption begin dominating over RSA . Also it was obtained  that some important parameters responsible for RSA effect such as the triplet and singlet state quantum yields downgrade when fullerene molecules consolidate into aggregates. From the other hand, the intensity of the dipole-allowed transition (in F-zone), responsible for two-photon absorption in fullerene in visible range, weakly changes upon aggregation . Taking into account all this arguments, we accept that the two-photon absorption dominates over RSA in aggregated fullerene nanoparticles especially under conditions of picosecond pulse irradiation when the light intensity is much greater than that in case of nanosecond pulse.
Figure 3 also shows that the effect of nonlinear absorption of the fullerene dispersion exceeds the one of the ShC dispersion in picosecond regime. As for nanosecond regime, the RSA effect, more typical for fullerene than for ShC, should become important, and this gave prerequisites to expect the increasing of nonlinear-optical response for the fullerene dispersion compared to that of the ShC dispersion, at least for the visible range. However, the results obtained for the nanosecond range (Fig. 4) showed that these expectations were not met. These results confirm the interesting findings made in  where the influence of a degree of fullerene nanoparticles structure amorphization on their nonlinear optical properties was studied. For this reason aqueous colloidal solutions contained fullerene particles of identical average size but different amorphization degree were prepared. It was shown that particles with more amorphous structure has a greater RSA effect. In case of particles having less degree of amorphization where RS is less significant the optical limiting effect observed was greater due to more pronounced NLS mechanism.
We can see (see Fig. 4) that the ShC dispersion proves itself as a medium with higher optical limiting properties than the fullerene dispersion in both visible and near infrared regions. The optical limiting parameters of the ShC dispersion are about 2 times better than that of the fullerene dispersion for 532 nm, and about 5 times better for 1064 nm. Table 1 presents optical limiting parameters of dispersions under study obtained for nanosecond regime.
It is worth noting that nonlinear optical responses of both the ShC dispersion and the fullerene dispersion deteriorate with the transition from 532 to 1064 nm. However, the optical limiting parameters of the samples deteriorate differently, viz., limiting threshold grows in 25 times for the fullerene dispersion and in 10 times for the ShC dispersion; limiting efficiency decreases in 8 times for the fullerene dispersion and in 3 times for the ShC dispersion. The RSA effect in C60 media completely vanishes in the IR region , so the optical limiting effect obtained at 1064 nm in fullerene dispersion is determined by the NLS. Thus, the noted more significant diminution of the optical limiting of the fullerene dispersion is thought to be due to full exclusion of possible RSA effect and of course, due to weaker IR absorbance of fullerene dispersion (see Fig. 1), which negatively affects thermodynamics of the NLS mechanism.
The new type of nanocarbon material – shungite carbon (ShC) – is studied with respect to the optical limiting effect in its aqueous dispersions. The nonlinear optical response of the ShC dispersion coincides with that of the CBS. Therefore, it may probably mean, that a prevailing mechanism of the optical limiting of ShC dispersion is the same as for CBS, which is known to be NLS.
The presence of nonlinear absorption, which presumably caused by two-photon absorption, in the nonlinear optical response of both ShC and fullerene dispersions is evidenced by experiments in picosecond regime of pulse duration.
Regarding optical limiting of fullerene aqueous dispersion in nanosecond time scale, we obtained significant contributions of both nonlinear scattering and RSA. However, as shown by the results of experiments, an additional contribution of the RSA gives the fullerene dispersion no advantage over the ShC dispersion considering an overall magnitude of optical limiting effect. Thus, we conclude that the ShC dispersion is very promising nonlinear optical substance protecting from visible and near-IR pulsed radiation of nanosecond pulse duration.
The work was supported by the Russian Foundation for Basic Research (grant # 14-02-00851, 13-03-01111 and 13-03-00422), Earth Sciences Section-5 (RNN and RSS), and by the Government of Russian Federation (grant no. 074-U01).
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