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Abstract
We experimentally determined the dispersion of third-order optical susceptibility χ(3) of graphene oxide (GO) in the visible region (450 - 750 nm) by combining spectroscopic ellipsometry and ultrafast pump and probe spectroscopy in the femtosecond regime. In order to mitigate the damage of wide-spectrum laser to photonic devices, GO has become a promising material for optical limiting (OL) devices. However, there is no report about the χ(3) dispersion of GO, which is a complex quantity that directly corresponds to nonlinear refraction and absorption and is a crucial parameter for the manipulation and application of its OL properties. Here, we identified that the linear optical response of GO shows a flat dispersion in the visible region. In contrast, its nonlinear optical response exhibits saturable absorption (SA) at the short wavelength and reverse saturable absorption (RSA) at the long wavelength. These results propel the application of GO in the broadband OL devices based on the RSA behavior. In addition, by controlling the fraction of sp2 and sp3 hybridizations, it also provides opportunities to tailor the NLO properties and OL performance of GO.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The utilization of laser technologies has greatly boosted the development of precise materials processing [1], medicine and biology [2], security detection [3] and other fields. Therefore, the use of high-intensity laser beams makes safety devices greatly necessary and challenging to avoid the potential damages from accidental exposures to laser radiation [4]. Consequently, optical limiters, which are used to protect sensitive photonic instruments from harmful effects induced by laser, have attracted enormous attention in the past few decades. The efficient optical limiting (OL) materials are based on high linear transmittance, large nonlinearities, and broadband spectral response [5]. For the sake of above, many OL materials have been explored considerably to meet these demands, such as fullerenes derivatives [6,7], metallic nanomaterials (zinc ferrites nanoparticles [8], gold clusters [9]). However, these materials exhibit OL behaviors only at certain wavelengths, which remains a serious obstacle to provide reliable protection over a wide spectral range in practical applications. To address these challenges, graphene oxide (GO), with broadband nonlinear optical (NLO) properties in the visible region, emerges to be a promising material for the OL devices [10]. In addition, GO is hydrophilic in nature with good dissolvent capability in the water, ethanol, and other solvents [11], leading to good workability in the manufacturing process. Moreover, due to the existence of hydroxyl and epoxy functional groups, GO can interact with various organic and inorganic substances and is highly compatible with a variety of additives [12]. These advantageous properties make GO a subject of intensive research in the context of third-order nonlinear optics.
In recent years, the NLO properties of GO have been extensively studied. For instance, the OL actions of few-layered GO dispersed in an organic solvent were observed by Z-scan at 532 nm [13]. GO nanosheets dispersed in the de-ionized water were also reported to show broadband NLO and OL properties at 1064 nm [14]. In addition, tunable OL properties of GO in ethanol solution were studied at 1550 nm [15]. However, these efforts have focused on the dispersions in certain solvents, and the OL properties of GO measured by the laser at 532, 1064, and 1550 nm. To broaden the applicability of GO in practical applications, it is necessary to investigate solid films and extend the operating wavelength to a wider range. In moving towards the application of GO in optical limiters in the visible region, the third-order nonlinear optical susceptibility ${\chi ^{(3 )}}$ is a crucial parameter that needs to be evaluated. Since the complex ${\chi ^{(3 )}}$ of GO has strong relationship with its nonlinear refraction and absorption, the imaginary part directly verbalizes the information about the saturable absorption (SA) and reversed saturable absorption (RSA), which are important bases for the OL applications. Meanwhile, owing to the electronic transitions, the NLO properties of GO show wavelength-dependent characteristics [16]. Unfortunately, to the best of our knowledge, previous studies on the ${\chi ^{(3 )}}$ of GO have been carried out only at some discrete wavelengths, such as at 532 nm reported by Khanzadeh et al. [17] and Biswas et al. [18], or at 1064 nm investigated by Liaros et al. [19]. In consequence, the dispersion of ${\chi ^{(3 )}}$ in GO is deserved to be determined experimentally in the continuous visible region.
In the present work, femtosecond pump and probe spectroscopy was used to investigate the nonlinear transmission changes of the GO/PVA composite film in a continuous wavelength range from 450 to 750 nm, and the ${\chi ^{(3 )}}$ dispersion of GO was experimentally determined by combining with the spectroscopic ellipsometry (SE). We identified that the linear optical properties of GO show a flat dispersion in the visible region. In contrast, the nonlinear optical response of GO exhibits saturable absorption (SA) at the short wavelength and reverse saturable absorption (RSA) at the long wavelength. These results propel the application of GO in the broadband OL devices based on the RSA. Furthermore, it allows us to make a comparison with the graphene fabricated by the chemical vapor deposition (CVD) method, where the electronic structure is mainly established by sp2 hybridizations. Physical identification of carrier dynamics that origin from sp2 domains and sp3 carbons provides us opportunities to tailor the NLO properties and OL performance of GO.
2. Experimental section
2.1 Fabrication of the GO/PVA thin film
GO was synthesized according to the modified Hummers method as previously reported [20]. All chemicals were purchased and used without further purification. Briefly speaking, graphite flake powders (500 mg) and NaNO3 (250 mg) were mixed firstly in a cold and concentrated H2SO4 (50 mL) under magnetic stirring. Then, 1500 mg KMnO4 was slowly added into the mixture in an ice-water bath under stirring. After stirring for 48 h, de-ionized (DI) water (100 mL) and H2O2 (30 mL) solution were gradually added into the beaker until a yellow suspension was obtained. Finally, the solution was washed several times through centrifugation using 1 M HCl solution and DI water respectively until the PH reached 7. To examine the linear and NLO properties, the prepared GO was mixed with polyvinyl alcohol (PVA) solution under magnetic stirring at 353 K to form a homogenous suspension. Then, the GO/PVA thin film was deposited on a transparent SiO2 substrate through spin coating. Typically, the suspension of the GO/PVA composite on the substrate was treated at a spin speed of 200 rpm for the 30s firstly. Then the film was formed at a spin speed of 700 rpm for 5 min. The schematic diagram of the fabrication process of the GO/PVA thin film was shown in Fig. 1. Note that the SiO2 substrate was precleaned with aqua regia and rinsed several times with DI water prior to use. In order that the hydrophobic substances on the substrate were removed clearly, and the surface of substrate was modified to hydrophilic, which is a key point to fabricate good film through spin coating with DI water as the solvent.
2.2 Structural and optical characterizations
The crystalline phases and compositions were characterized by using the powder X-ray diffraction (XRD) Rigaku Mini Flex with the Cu-Kα radiation (λ = 1.5418 Å) in the range of 5 - 50°. Fourier transform infrared spectroscopy (FTIR) was carried out to detect the chemical bonding of the as-prepared samples. The transmission electron microscopy (TEM) images were recorded on a JEOL, JEM-2100 F1 instrument operating at 200 kV. The linear transmission, complex dielectric function (refractive index, absorption coefficient) of GO were determined by a variable angle spectroscopic ellipsometer (J. A. Woollam, VASE). Ellipsometric spectra were measured at incidence angles of 70°, 75°, and 80°, respectively. The thickness of the GO/PVA thin film was evaluated according to the SE results. The nonlinear transmission changes (ΔT/T), which represent the difference between transmitted light in the ground state and excited state, were collected by the pump and probe spectroscopy in the femtosecond regime. The schematic drawing of pump and probe measurement was presented in Fig. S1 (Supplementary materials). In the measurement, the fundamental laser source was supplied by a regenerative amplifier (Spectra Physics, Spitfire) using Ti: sapphire (Mai Tai, Spectra-physics) and Nd: YLF (Empower, Spectra-physics). The laser source has an output pulse of 130 fs at 800 nm and 1 kHz repetition. Then the fundamental laser beam was divided into two portions: pump and probe beams. The wavelength of pump beam is 400 nm, which was converted by a BBO crystal through second harmonic generation. In addition, the repetition of pump beam was converted to 0.5 kHz by an optical chopper. The pump peak power density was 1.36 GW/cm2 and the focal size was 168.2 μm. Probe beam is white light continuum with a wavelength range from 450 to 750 nm, which was generated using a sapphire crystal. The chirping effect was corrected by the Kerr gate method as reported previously [21].
3. Results and discussion
3.1 Structural characterizations of GO
The XRD patterns of GO and graphite are given in Fig. 2(a). The typical peak (002) at 26.6° of graphite disappeared after the liquid oxidation reaction, while a new peak (001) at 10.7° can be observed in GO, implying that GO was successfully obtained. It should be noted that the interlayer spacing of GO (0.84 nm) is much larger than that of original graphite (0.34 nm), indicating that the oxygen-containing functional groups were introduced into the GO. In addition, the distinctive degree of oxidization of GO was verified by the FTIR spectroscopy as detailed in Fig. 2(b). The peaks of O-H, C = O, C-OH, C-O, and C-O-C can be identified in the FTIR spectrum of GO, respectively. Thus, the strong oxidation of GO has been achieved in the liquid reaction process. Fig. S1 (Supplementary materials) provides the TEM images of GO that the thin sheet structures with wrinkled texture were observed. Moreover, due to the interaction of oxygen-containing functional groups in the basal plane, GO tends to form a multilayer structure and low crystallinity.
3.2 Linear optical properties of GO
The experimental and fitted linear extinction spectrum of the GO/PVA composite are presented in Fig. 3(a). The extinction was extracted according to -ln(T/T0), in which T and T0 are the linear transmissions of the substrate with and without samples, respectively [22]. The experimental and SE model fitted T spectra were given in Fig. S2 (Supporting Information). Note that this extinction mainly stems from the absorption and scattering of the GO nanosheets owing to the transparent characteristics of PVA in the visible region [23]. The results show that the linear extinction strength of GO at 450 - 750 nm exhibits a flat dispersion characteristic. The results were consistent with the absorption spectrum measured in the dimethylformamide (DMF) solution [24], indicating that GO nanosheets dispersed well in the PVA matrix as illustrated in the inset photograph in Fig. 3(a). In the fitting process, the GO/PVA composite film was described by an effective medium approximation (EMA) with a linear dispersion of a concentration about 72.7%, and a thickness of 1895 nm. The experimental and fitted psi and delta patterns of the GO/PVA composite can be seen in Fig. S3 (Supporting Information). In the SE model, the band-gap structure of GO was characterized by using Lorentz oscillators, with parameters listed in Table S1 (Supporting Information). GO has a larger energy gap compared to graphene, which is ascribed to the introduction of σ states in the sp3-bonded carbons due to the formation of oxygen-containing functional groups in the synthesis process [25]. It has been demonstrated by theoretical studies that the energy gap varies with the different coverage of oxygen-containing functional groups in GO [26]. The mean square error (MSE) between the experimental results and the SE model is used to evaluate the fitting quality [27]. Its accuracy is sufficient to provide good fitting qualities with experimental data when the value of MSE is less than 10 [28]; the MSE of the GO/PVA composite was 9.386.
Fig. 3. (a) Experimental and fitted extinction spectra of the GO/PVA composite film. (b) Linear dielectric function ε of GO nanosheets. Inset: photograph of the GO/PVA film on a silica substrate (scale bar, 5 mm).
As shown in Fig. 3(b), the linear complex dielectric function ε of GO was evaluated through the SE analyses, where the blue and red lines show the real (Re[ε]) and imaginary (Im[ε]) components, respectively. The dot, dash, and dash-dot lines represent the individual contributions of the intra-band transition, excitonic effect, and π* plasmon peak to the imaginary component, respectively. In contrast to the complex ε of graphene, Im[ε] of GO is much lower due to the large band-gap value resulted from the high oxygen content [29]. At the same time. the linear refractive index n of GO, as well as absorption coefficient k, were also obtained by the SE analyses as in Fig. S4 (Supporting Information), which is in good agreement with previous literature [30]. Thus, the obtained ε in the above experiment can be used to analyze the NLO properties through the combination with pump and probe measurement.
3.3 Nonlinear optical properties of GO
To investigate the NLO properties of GO, the transmission changes ΔT/T of the GO/PVA composite film were measured by the femtosecond pump and probe spectroscopy as illustrated in Fig. 4(a). As demonstrated by Zhang et al. [31], PVA has no contribution to the nonlinearity of the composite. Because the long-term exposure of the GO/PVA composite in the laser beam with a wavelength of 400 nm leads to an unexpected reduction of GO in the pump and probe measurement process. In consideration of that, the laser position on the sample was moved every 20 s to avoid the laser-induced reduction effects. The schematic illumination of the operating procedures was shown in Fig. S5 (Supporting Information). Identical measured results were obtained by multiple scans of the sample, which confirmed that no laser-induced damage occurred in the GO/PVA composite film during the measurement. The ΔT/T pattern of the GO/PVA composite was plotted in Fig. 4(b), with a wavelength range from 450 to 750 nm. In contrast, previous reports have focused on measurements at discrete wavelengths [32–34], where the fragmental results have led to some conflicting identification of the NLO properties of GO. Some of them may be caused by partial reduction of GO induced by the high-intensity laser beams during the measurement. Because of different characteristics could be observed in the ΔT/T pattern of GO if the sample was exposed in the laser without changing the interaction position with the beam (implying that the sample was partially damaged by the laser), which can be seen in Fig. S6 (Supporting Information). Our measurement was performed at a continuous spectral region and in a special operating procedure as mentioned above. The coherent spectral transient response enable us to fully understand the photoinduced absorption and transparency of GO. Note that the ΔT/T spectrum of GO in Fig. 4(b) includes regions of positive and negative signs. A positive region of ΔT/T was observed from 450 to 513 nm, while a negative region of ΔT/T was in the range of 513 to 750 nm. The negative ΔT/T is a clear indication of the OL ability of GO, which corresponds to the RSA behavior that absorption across the section in the excited state is higher than that in the ground state. In contrast, graphene exhibits positive and monotonic increasing ΔT/T dispersion in the visible region, which represents the strong SA [35,36]. These results can be explained that GO has unique heterogeneous chemical and electronic properties due to co-existence of the sp2 hybridizations and sp3 bonded carbons [25,37]. The sp2 hybridizations yield a positive contribution to the transmission changes ΔT/T, while the negative photoinduced sign is ascribable to the sp3 domains [38,39]. The result implies that the confined sp2 hybridization dominates the nonlinear response in GO at the short wavelength, whereas the contribution of sp2 is overwhelmed by the sp3 terms in the long wavelength. By contrast, a much larger fraction of sp2 domains dominates the structure in graphene.
Fig. 4. Spectral dependence of nonlinear transmission changes of the GO/PVA composite. Wavelength range: 450 to 750 nm.
To further identify the origin of the ultrafast carrier dynamics of GO, the time delay was measured using the GO/PVA composite film at 735 nm with a femtosecond pulse laser, compared it with the graphene fabricated by CVD method. The differential transmittance of GO and CVD graphene are presented in Fig. 5(a). It was observed that the carrier dynamics of CVD graphene here are consistent with that in previous reports [40,41]. By comparison with graphene, a longer time delay was observed in GO, which is fundamentally different from that of graphene. The longer time delay in GO is composed of two parts, which respectively reflect the carrier intraband relaxation process and interband carrier recombination [42]. The difference in the time delay between GO and graphene verified that the sp2 and sp3 structures also have an important impact on the relaxation process. Figure 5(b) shows the structural illustrations of GO and CVD graphene, respectively. The longer time delay could be ascribed to the confined sp2 hybridizations, where the π electrons were confined because of the presence of a large fraction of sp3 bonded carbons. The energy gap of sp3 bonded carbons is larger than that of sp2 carbons, resulting in a larger carrier transport gap in GO.
Fig. 5. (a) Carrier dynamics of GO and CVD graphene at 735 nm excited by a laser pulse with a wavelength of 400 nm. (b) Structural illustrations of GO and CVD graphene.
Finally, the dispersion of ${\chi ^{(3 )}}$ of GO was extracted by combining the SE and pump and probe spectroscopy. In the beginning, the linear ε and T has already been evaluated from the SE results. Subsequently, the $\varepsilon + \Delta \varepsilon $ (the dielectric function at the excited state) was obtained by fitting the $T + \Delta T$ and the SE model used in the linear optical analysis; then the dielectric function changes $\Delta \varepsilon $ can be calculated [43]. In the fitting process, only the parameters of oscillators were regarded as the variable values; the oscillators’ parameters in the excited state were given in Table S2 (Supporting Information). Eventually, the ${\chi ^{(3 )}}$ was extracted according to the Eq. (1):
Where $\frac{3}{4}$ is the intensity-dependent refractive index, and I is the intensity of the pump laser beam [44]. Figure 6 present the real and imaginary components of ${\chi ^{(3 )}}$ of GO, respectively. The dispersion of Re[${\chi ^{(3 )}}$] and Im[${\chi ^{(3 )}}$] show wavelength-dependent characteristics in the visible region. As mentioned in the introduction, they cannot be understood well using the single wavelength measurements. In particular, the Im[${\chi ^{(3 )}}$] of GO shows distinct dispersions with regions of positive and negative values in the different wavelength range, which is consistent with the aforementioned ΔT/T. The Im[${\chi ^{(3 )}}$] value of GO reported by M. Ebrahimi et al. at 532 nm was 2.17 ${\times} $10−14 m2/V2 [45]. Liaros et al. reported that the Im[${\chi ^{(3 )}}$] of GO dispersed in different solvents at 532 nm varied from 1.0 ${\times} $10−19 to 5.1 ${\times} $10−19 m2/V2 [13]. In addition, the Re[${\chi ^{(3 )}}$] and Im[${\chi ^{(3 )}}$] of GO were reported by Khanzadeh et al. to be -3.2 ${\times} $10−16 and 4.0 ${\times} $10−16 at 532 nm, respectively [17]. At the same wavelength, the Re[${\chi ^{(3 )}}$] and Im[${\chi ^{(3 )}}$] obtained in our results are -8.97 ${\times} $10−18 and 1.5 ${\times} $10−19 m2/V2, respectively. The different magnitude of ${\chi ^{(3 )}}$ probably stems from the different oxygen content in the sample or the different test methods. Noticeably, unlike the previous reports measured at a single wavelength, where the ${\chi ^{(3 )}}$ of GO has huge variations. Our results obtained the dispersion of ${\chi ^{(3 )}}$ in a continuous wavelength range from 450 to 750 nm. The positive sign of Im[${\chi ^{(3 )}}$] was observed from 513 to 750 nm, which indicates that the imaginary component of dielectric function increases in the excited state. In consequence, the excited state absorption is higher than that in the ground state, and GO exhibits RSA behavior [46], which can be utilized in the optical limiters. The nonlinear response with SA behavior was observed in the short wavelength range.Fig. 6. Real, and imaginary components of the third-order optical susceptibility ${\chi ^{(3 )}}$ of GO, wavelength range: 450 to 750 nm.
For graphene, it mainly consists of large number of sp2 hybridized carbon atoms. The energy gaps between valence band and conduction band in the π states of sp2 hybrid carbons are reported to be approximately 0.5 eV [47]. As a result, electrons in sp2 hybridizations exhibited saturated absorption due to the valence depletion and conduction band filling [48]. These sp2 hybridizations dominate the nonlinear optical response of graphene under the laser excitation. From our measurements, GO shows SA and RSA behaviors at different wavelength range. Most probably the wavelength-dependent characteristics could be explained by the heterogeneous electronic structure in GO, which is determined by the δ states of sp3 hybridizations in addition to the π states of sp2 hybrid carbons due to the existence of oxygen functional groups [49]. Bound electrons and free carriers were involved in the δ states of sp3 bonded carbons [50], which brought on reverse saturable absorption. Furthermore, the cross point can be tuned by controlling the fraction of sp2 and sp3 hybridizations in GO. This provides us opportunities to tailor the NLO properties and OL performance of GO by tuning its oxidization degree.
4. Conclusion
In summary, we have experimentally determined the third-order optical susceptibility of graphene oxide by combining spectroscopic ellipsometry and pump and probe spectroscopy. We identified that the linear optical response of GO presents a flat dispersion in the visible region. While the nonlinear response of GO exhibits saturable absorption (SA) at the short wavelength and reverse saturable absorption (RSA) at the long wavelength. Compared to the graphene that the structure is dominated by sp2 hybridized carbons, GO can be utilized in the optical limiter based on RSA behavior in the wavelength range of 513 to 750 nm. These results propel the application of GO in the broadband OL devices. In addition, the physical identification of carrier dynamics that origin from sp2 domains and sp3 carbons provides us opportunities to tailor the NLO properties and OL performance of GO by controlling the oxidization degree.
Acknowledgments
A part of this work was supported by the NIMS microstructural characterization platform as a program of “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors are grateful to the International Center for Materials and Nanoarchitectonics (MANA) foundry in the National Institute for Materials Science (NIMS).
Disclosures
The authors declare no conflicts of interest.
See Supplement 1 for supporting content.
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