Thin films made from a composite of the polymer polyvinyl-alcohol and cobalt oxide (Co3O4) nanoparticles were fabricated by spin coating. Linear and nonlinear optical properties of thin films with thicknesses of hundreds of nanometers were investigated. The refractive index and absorption coefficient were measured and two direct band gaps (Eg = 1.38 eV and 2.0 eV) were determined from the absorption spectrum. Reversed saturable absorption and saturable absorption were observed when the films were illuminated with the different fluences. Optical nonlinearities corresponding to reverse saturable absorption were measured by the z-scan technique. A nonlinear refractive index (n2) of ~10−10 cm2/W and nonlinear absorption (β) of ~103 cm/GW have been measured from 425 nm to 675 nm. The experimental results show that the Co3O4 nanoparticle/PVA composite is a promising material for nonlinear optical devices in the visible, since it takes advantages of the high optical nonlinearities of transition metal oxides and the superior mechanical properties and convenient fabrication properties of polymers.
©2011 Optical Society of America
Transition metal oxides, such as Co3O4, V2O5, CuO, Fe2O3, Mn3O4, and Cr2O3 , have been intensively investigated over the past few decades, because of their large optical nonlinearity (10−8 – 10−7 esu) and the advantages of good thermal and chemical stability in addition to mechanical strength. Nanostructures of transition metal oxides have attracted significant attention in recent years because the mechanical, electrical, optical, or magnetic properties of nano-structured materials often differ drastically from those of the corresponding bulk material and these properties can, in many cases, be tailored easily by controlling their structure, size, and environment. Among those transition metal oxides, cobalt oxide was found to have the largest figure of merit, that is, the ratio of the magnitude of the optical nonlinearity to the linear optical absorption. Cobalt oxide [Co2+(Co3+)2O4], as the most stable phase in the Co-O system, is a mixed valence compound with a normal spinel structure having Co2+ and Co3+ placed at tetrahedral and octahedral sites, respectively. Materials containing Co3O4 have been widely used for electrode materials , heterogeneous catalysis , solid-state sensors , energy storage , and magnetic materials [6,7]. Due to the rapid progress of nanotechnology in recent years, Co3O4 nanoparticles have been synthesized by various methods such as chemical vapor deposition [7,8], spray pyrolysis , sputtering , hydrothermal , thermal decomposition , sol-gel , electrochemical deposition , wet chemicals , microwave assisted , and ionic liquid assisted . On the other hand, composites of polymers and nanoparticles are attracting more and more interest because they open pathways for engineering versatile materials that exhibit advantageous electrical, optical, mechanical, or magnetic properties. Therefore, composites of Co3O4 nanoparticles and polymers are potentially promising materials for magnetic devices, optical devices, electric devices, and gas sensor devices.
We previously reported the nonlinear performance of a photonic band gap structure made from Co3O4 nanoparticles doped in poly(vinyl alcohol) (PVA) and poly(9-vinylcarbazole) (PVK) in . In that study, we found that, when a single-layer film made from a composite of Co3O4 nanoparticles and PVA was illuminated with a moderate fluence, it exhibited reversed saturable absorption that differed from the results presented in  and , where a Co3O4 thin film was found to exhibit saturable absorption. In order to explain the aforementioned discrepancy and fully understand the optical performance of the composite of Co3O4 nanoparticles and PVA, we have performed further investigation on the composite, which is critical for proper evaluation for future applications. In this paper, we present the linear and nonlinear optical properties of thin films made from a composite of Co3O4 nanoparticles and PVA.
2. Thin film fabrication and experiments
The composite of optical polymer PVA (Mw = 67,000) and Co3O4 nanoparticles was prepared by initially adding the nanoparticles to a 6% solids PVA solution. The Co3O4 nanoparticles were black particles with diameters in the range of 30-40 nanometers. The mixture was then ultrasonicated at 60°C for two hours. Aggregated particles were removed by using 0.2 micron syringe filters. A homogenous solution was finally obtained by centrifugation at 1000 rotations per minute (RPM). Uniform thin films were fabricated from the homogenous solution by the method of spin coating. The volume ratio of the PVA and the Co3O4 nanoparticles was about 9:1, as estimated from the weights and densities of the precursors.
In our experiment, three PVA:Co3O4 thin films with thicknesses of 500 nm, 570 nm, and 720 nm were then fabricated from the mixture by the method of spin coating. A PVA thin film with a thickness of 500 nm was also fabricated as a reference. Linear transmission of these thin films was measured with a Cary 5000 UV-Vis-IR (Varian, Inc.). In Fig. 1 , the transmission spectra of the four thin films are plotted in a wavelength range of 350 – 800 nm. Obviously, compared to the PVA film, PVA:Co3O4 films have two absorption bands centered at 420 nm and 740 nm. The absorption coefficient of the composite (shown by the magenta short dashed curve) is obtained by averaging the absorption coefficients of the three PVA:Co3O4 films. Note that, the absorption coefficient of this composite is much smaller than that of the pure Co3O4 thin films reported in  and , since the volume percentage of Co3O4 is much less. Based on the absorption coefficient of the PVA:Co3O4 film, the band gap energy can be obtained from the plot of (αhν)2 versus photon energy (hν) shown in Fig. 2 by using the relation (αhν)2 = c(hν - Eg), where α is the absorption coefficient, hν is the photon energy, c is a constant, and Eg is the band gap energy. Direct band gaps of 1.38 eV and 2.0 eV were obtained by fitting the linear regions of the curve shown in Fig. 2. The first band gap is assigned to the charge transfer process Co3+(πt2) → Co2+(σ*t2) and the second one is O(π*Γ)→ Co2+(σ*t2) . The two values are very close to the results (1.44 eV and 2.06 eV) of Co3O4 thin films prepared by spray pyrolysis , indicating that the polymer matrix has only a minor effect on the energy levels of Co3O4 nanoparticles.
For the measurement of the refractive index of the composite, a thin film with a thickness of about 60 nm was prepared on silicon substrate. The refractive indices of the composite and the PVA were measured by spectroscopic ellipsometer (SopraLab) and are shown in Fig. 3 . The refractive index of thicker films of the composite measured by the technique of prism coupling is also included in Fig. 3. The refractive index of the composite is much larger than that of the PVA in the wavelength range of 400 nm – 800 nm due to the large index of Co3O4 .
The nonlinear transmission characteristics of the three PVA:Co3O4 films were measured with the experimental setup shown in Fig. 4 . The laser source was a frequency-doubled Q-switched Nd:YLF laser (TFR 523Q, Spectra-Physics) that has a wavelength of 523.5 nm and a pulse width of about 10 ns. Two polarizers were used to adjust the input energy. The laser beam was focused on the sample with a 5 cm focal length lens, while the transmitted beam and a reference beam were measured with high-speed silicon detectors (DET10A, Thorlabs). The nonlinear transmissions of the three samples as a function of the illuminating fluence are plotted in Fig. 5 . Contrary to the negative nonlinear extinction coefficient reported in , the transmission of the PVA:Co3O4 films decreases with the increasing laser intensity and reaches a minimum value at an illuminating fluence of about 600 mJ/cm2. The transmission then increases with illuminating fluence at higher light intensities due to saturable absorption of Co3O4 caused by the band filling effect . The reverse-saturable absorption of the single-layer PVA: Co3O4 film at moderate light intensities may be due to excited state absorption, two-photon absorption or charge transfer processes . In , reverse-saturable absorption was also observed in a Co3O4 film when a 532 nm picosecond pulsed laser was used. It should be noted that, saturable absorption and reverse saturable absorption of the Co3O4 film were separately reported in  and , respectively, but that both nonlinear responses were observed in our experiment. The main difference in the three experiments is the pulse duration of the laser sources. The pulse duration is 100 ns in , 35 ps in , and 10 ns in our experiment. The distinct performances may be ascribed to the different response times and nonlinearities of the two nonlinear processes occurring in the Co3O4 material. The saturable absorption is a slow process and the reversed-saturable absorption is a fast process. When the pulse duration is 100 ns, saturable absorption is dominant, while reverse saturable absorption is more dominant when the pulse duration is 35 ps. In our experiment, reverse saturable absorption occurs first because the threshold for reverse saturable absorption is smaller than that of the saturable absorption when the pulse duration is 10 ns. However, the magnitude of the saturable absorption is much larger than that of the reverse saturable absorption. Therefore, the absorption decreases with increasing fluence for fluences larger than 600 mJ/cm2.
In order to further evaluate the reverse saturable absorption, a z-scan measurement was conducted on the setup shown in Fig. 4 with a fluence of 500 mJ/cm2. Open aperture and closed aperture z-scan measurement results are shown in Figs. 6(a) and 6(b), respectively. Using the method of Gaussian decomposition , we can determine the nonlinear absorption coefficient (β) and the nonlinear refractive index (n2) by fitting the experimental results shown in Fig. 6. A nonlinear absorption coefficient β = 6.2 × 103 cm/GW and nonlinear refractive index n2 = −1.08 × 10−9 cm2/W are obtained. The nonlinearity of the composite is much lower than that of the pure Co3O4 film (β = 9.6 × 104 cm/GW at 532 nm in Reference 23 and n2 = 1.0 × 10−6 cm2/W at 405 nm and n2 = −5.5 × 10−7 cm2/W at 650 nm in ), but consistent with the relative incorporation of cobalt oxide in these films. Clearly the optical and nonlinear optical properties of the composite can be tailored by simply changing the volume fraction of cobalt oxide, providing a pathway to fabricate materials with desired linear and nonlinear properties. Such control of optical properties is essential for nanophotonics and nanoelectronics applications.
Nonlinear absorption coefficients of the composite at different wavelengths in the visible were measured using the similar setup shown in Fig. 4 with an optical parametric oscillator (OPO) laser source. The OPO laser source was pumped by a frequency-tripled Continuum Surelite III laser at 355 nm. Tunable radiation from 425 nm to 675 nm was focused on the 500 nm thick thin film with a 10 cm focal length lens. Some open aperture Z-scan measurement results are shown in Fig. 7(a) . Clearly, the nonlinear absorption is stronger at short wavelengths. Nonlinear absorption coefficients (β) can be determined by fitting the experimental results with the method of Gaussian decomposition. Nonlinear absorption coefficients for wavelengths in the visible are shown in Fig. 7(b). Although the nonlinear absorption coefficient decreases with the increasing wavelength, the β value is ~103 cm/GW over the entire visible range
3. Conclusion and discussion
Linear and nonlinear optical properties of a PVA:Co3O4 composite have been investigated. Refractive index and linear absorption of the composite have been measured and two direct band gaps (Eg = 1.38 eV and 2.0 eV) were determined from the absorption spectrum. Nonlinear performance of the thin film made of the composite PVA:Co3O4 has been measured. When the pulse duration is 10 ns, the nonlinear response of PVA:Co3O4 exhibits two processes: when the fluence is moderate, reverse saturable absorption is dominant, and saturable absorption becomes more prominent as the fluence exceeds 600 mJ/cm2. Using the technique of Z-scan measurement, nonlinear refractive index (n2) of ~10−10 cm2/W and nonlinear absorption (β) of ~103 cm/GW have been determined in the visible region from 425 nm to 675 nm. Our experiments demonstrate that the composite PVA:Co3O4 possesses a high nonlinearity and has tailorable linear and nonlinear optical properties. This material approach is promising for applications where the nonlinear and linear optical properties need to achieve a targeted value and other properties such as mechanical, chemical, and magnetic properties are also important.
Besides the changes to the linear and nonlinear optical properties that can be effected by changing the volume fraction of Co3O4, these properties can also be tailored by using different polymers since the refractive indices, the absorption coefficients, and solubilities of potential polymer hosts vary substantially. The polymer host is not expected effect the time scale for the saturable or reverse-saturable absorption since this is taken to be intrinsic to the Co3O4 particles. However, the choice of polymer can certainly affect the size of the nonlinear absorption simply due to the fact that the concentration of the Co3O4 nanoparticle that can be incorporated in the composite free of aggregation depends on the composition and structure of the polymer.
We need to note that, due to the large refractive index contrast between the Co3O4 nanoparticles (nCo3O4 = 2.4-2.5) and the PVA polymer (nPVA = 1.5-1.6), there are some observed scattering losses in the thin films. However, the scattering loss has been minimized by avoiding particle agglomeration and ensuring even distribution in the PVA, resulting in homogeneous thin films with good uniformity. Moreover, the scattering loss is de facto quite small according to the 80% transmission of these thin films throughout the visible. While nonlinear scattering due to the index change of the Co3O4 nanoparticles may influence the nonlinear measurement, this can be expected to be a very small effect based on the known index change and refractive indices of the respective materials.
This work was supported by the US Army Research Office MURI through the University of Central Florida under contract/grant 50372-CH-MU.
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