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Abstract
We have obtained reduced graphene oxide (rGO) by reducing the graphene oxide (GO) polymer using a femtosecond (fs) pulse laser. A method for the fabrication and preparation of GO-polymer composite is first described. We then discuss the reduction of GO-polymer to rGO using direct laser writing technology. We have characterized a fabricated rGO binary phase grating and shown that the degree of the photoreduction of GO-polymer to rGO-polymer can be controlled by the laser power. Phase modulation has been found to be more than 2π/3. Finally, we have designed a computer-generated hologram to realize an rGO fork grating. Upon illumination of the grating, a vortex beam is generated through diffraction.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The discovery of two-dimensional carbon nanomaterial graphene can be traced back to 2004, when Novoselov et al. first reported a single layer graphene [1]. This newly available material has attracted enormous interest among the scientific community because of its exceptional mechanical, electrical, thermal and optical properties. The ideal graphene has a completely symmetrical electronic band structure. The valence band and the conduction band are symmetrically distributed above and below the Fermi level, which makes the single-layer graphene possible to have optical absorption of about 2.3% in a wide spectral range [2]. Graphene also has found applications in laser switches [3]. On the other hand, graphene oxide (GO) has emerged as a graphene-based material having the potential for cost-effective and large-scale production. Graphene oxide has a lot of oxygen-containing functional groups and structural defects on the basal plane or at the edges [2], which is different from graphene. The electrical and optical properties of GOs can be modified by reduction of the oxygen-containing groups [4,5] through chemical and thermal method, thereby creating the so-called reduced graphene oxide(rGO). The obtained rGO has similar properties and morphological characteristics with pristine graphene. In particular, manipulation of the residual oxygen and structural defects provides a viable method for modulating the optical properties [6–8] in graphene-based [9–13] materials and graphene-polymer composites [14,15]. So far, controlling the extent of the localized reduction of GOs has been investigated by a number of photo-thermal [16] and photo-catalytic [17] approaches including near-field scanning hot tips [18,19] and continuous-wave- or quasi-continuous pulsed laser irradiations [20–22]. Meanwhile, devices such as super-capacitors [20–23], flexible circuits [10], sensors [24,25], nanomechanical resonators [26,27], solar cells [28,29], ultra-thin planar lenses [30] and nonbleaching optical probes [31] based on rGO have also been reported.
In optics, refractive index modulation is one of the most important properties of GOs. As such, focusing a fs pulse laser beam to reduce the GO introduces a localized refractive-index change and has enabled tremendous progresses on developing nanophotonic devices such as nanofabrication and ultra-high density optical data storage [32,33]. The two-photon reduction of the GO-polymer exhibits a giant refractive-index modulation on the order of 10−2 to 10−1[34] by precisely controlling the processing parameters, which is two or three orders of magnitude larger than the fs laser-induced refractive-index modulation in many other optical media such as Lithium Niobate with refractive-index modulation ∼$2.0 \times {10^{ - 5}}$[35]. The giant refractive index changes can be used for multimode recording by combining bit-by-bit three-dimensional optical data storage with holographic data storage on the same material platform [34]. The rGO composite has a flat absorption spectrum in the visible range, which makes it possible to produce colored holograms using rGO-polymers [7]. Moreover, graphene oxide is highly hydrophilic and is stable in water solutions; therefore, it can be incorporated into water-soluble polymers, such as adding an aqueous solution of graphene oxide to a polyvinyl alcohol (PVA) solution. The refractive index has been modulated by controlling the fs pulse laser power to reduce the GO-polymer, and the viewing angle can be increased up to 52 degrees [36] when the pixel size is reduced to 0.55 µm, which is one order of magnitude larger than that of metamaterials [37–39] or carbon nanotube [40] holograms with a similar image size.
The organization of the paper is as follows, In Section 2, we discuss the preparation/fabrication of GO-polymers, and the reduction of the GO-polymer to obtain an rGO grating. In Section 3, we first characterize the fabricated rGO grating. We then generate a computer-generated hologram for vortex beam visualization. Finally, in Section 4, we give some conclusions.
2. Method for fabrication and preparation sample
2.1 Fabrication of the GO-polymer composite
Sample fabrication: We dispersed GOs in N-methyl-2-pyrrolidone (NMP) [41] to obtain GOs organic solution. Specifically, GOs of 20 mg were first added to the NMP solvent of 10 ml by electronic balances. In the ultrasonic cleaning machine, GOs were oscillated for 5∼6 hours and fully dispersed in the NMP solvent, and then the solution was put into the centrifuge. After centrifugation at a speed of 1000 rpm for 1 hour, a brown-yellow graphene oxide solution with a concentration of 2 mg/ml was prepared. Methyl methacrylate (MMA), 2,2’-Azobis (2-methlpropionitrile) (AIBN) and 9,10-phenanthrenequinone (9,10-PQ) with the optimized weight ratio of 100:1:0.8 were ultrasonically vibrated for 2 hours to make PQ and AIBN dissolved in MMA, and the undissolved PQ was filtered by filter paper with diameter circles of 0.22 µm. Finally, the obtained polymer solution and the graphene oxide solution were mixed at a volume ratio of 5 to 1 to prepare a graphene oxide polymer solution, and the obtained solution was placed in a constant temperature water bath with magnetic stirring. The temperature was heated to 40 ° C and the stirring was about 20 hours until the mixed solution became viscous. The viscous liquid was then poured into a glass mold and baked at 40 °C for 30 hours until the material was completely thermally polymerized into a solid block material.
2.2 Reduction of GO-polymer to obtain rGO grating
The GO-polymer composite was fabricated according to the procedures described in the previous Section. The composite was reduced by a fs pulse laser. A fs pulse laser beam at the wavelength of 800 nm with repetition rate of 1 KHz was focused by an objective lens with a numerical aperture NA of 0.75, giving the calculated diffraction limited focal spot size about 0.61λ/NA≈0.65 µm. The reduction process induced by the focused fs pulse laser beam in the GO-polymer is shown in Fig. 1(a). The degree of photoreduction of the GO-polymer is controlled by the laser power to achieve the modulation phase change. Holographic phase information of a coded image can be recorded in the polymer by laser direct writing method. In the experiment, a reduced polymer grating with a period of 4 µm and a duty cycle of 1/2 was formed by strictly controlling the laser processing parameters. Figure 1(b) is a schematic view of the grating obtained by processing the GO-polymer. After reduction, GO becomes rGO with a dark brown color. To obtain the pattern in Fig. 1(b), we have used line scanning during the reduction process and the step size is considered to be the size of the focused spot. The scan speed of 100µm/s has been used. The dwell time per pixel is calculated to be
Fig. 1. (a) Schematic illustration of photo-reduced GO-polymer through a direct laser writing technique. (b) Optical microscopic image of the rGO grating by fs pulses.
3. Optical experiments
3.1 Characterization of the rGO grating
A GO-polymer sample with thickness of 2.5 µm was first prepared and the rGO-polymer grating was recorded as shown in the last Section. The resultant grating was observed using the atomic force microscope (AFM) of NT-MDT model NT1503210 at a temperature of 21 ° C ∼ 23 ° C, relative humidity of about 50%. Figure 2(a) shows the AFM image of the rGO grating at a laser power of 1.6 mW. The decrease of the height of the rGO grating is from the process of laser induced photoreduction of the GO [39], which is due to the volatile gases such as CO or CO2 produced by pyrolysis of oxygen-containing functional groups on the basal plane or at the edges of the GO at high temperature [42,43]. In order to clearly show the depth information of the grating, Fig. 2(b) shows a line trace of the grating structure, illustrating the depth of about 0.4 µm with a duty cycle of 1/2. The period of the grating is 4 µm. In Fig. 2(c), we show Raman spectra of the GO-polymer and the rGO polymer. The spectra display two broad peaks at 1357 and 1590 cm−1 corresponding to the D and G bands, respectively. The peaks of both D and G bands of the rGO-polymer have become distinct and the bandwidths are slightly narrower as compared with those of the GO-polymer. From the figure, indeed, the D/G intensity ratio of rGO (ID/IG = 0.84) is slightly larger that of GO (ID/IG = 0.75), illustrating effective reduction of GO. In obtaining the result shown in Fig. 2(a), we have also used line scanning with the same dwell time as discussed in the previous section.
Fig. 2. (a) AFM image of rGO grating at the power of 1.6 mW. (b) Line trace along the blue line in (a). (c) Raman spectra of the GO-polymer and rGO-polymer.
3.2 Characterization of the phase modulation strength of the rGO grating
In this subsection, we show how phase modulation is obtained experimentally with the grating we have fabricated. Diffraction intensities of the rGO-polymer grating were measured by illumination of a collimated He-Ne laser beam at wavelength of 632 nm. We define diffraction intensity ratio R as the intensity ratio between the 1st order and the 0th order diffraction beams, or between the -1st order and the 0th order diffraction beams. Indeed, the diffraction intensity ratio has been employed to characterize the phase modulation strength, which can be expressed as follows [34],
where Id and It are the first-order diffracted light intensity (either the + 1st order or the -1st order) and the 0th-order diffracted light intensity through the grating, respectively. The inset in Fig. 3. shows the diffracted images of ± 1st and 0th orders from the rGO-polymer grating. The intensity ratio has been measured as a function of the direct writing fs pulsed laser power of 800 nm, and the result is shown in Fig. 3. The figure shows that the diffraction intensity ratio is controlled by the incident laser power. The ratio R is almost constant when the laser reduction power reaches 1.6 mW.Fig. 3. Intensity ratio R varies with the fs laser power. The inset shows a typical diffracted images of $ \pm 1$st and 0th orders from the grating.
From the experiments, the phase modulation from the rGO can be extracted by using R. The calculation formula is expressed as follows [44]:
Fig. 4. (a) Phase modulation as a function of the fs laser power. (b) Laser-induced height change as a function of the fs laser power
3.3 Generation of the vortex beam from a fork rGO phase grating
To illustrate the use of phase modulation capabilities, we have fabricated a fork grating (also called a vortex grating) to generate a vortex beam with the rGO-polymer at the laser power of 1.6 mW. A vortex beam is a kind of hollow beam, which carries spiral phase. The intensity of the vortex beam at the center is zero, illustrating the influence of the spiral phase on the beam [45,46]. Vortex beams have been used in many applications recently such as in optical image processing as well as in digital holography [47,48]. We obtain a binary computer-generated hologram by simulating the interference of optical beams carrying a spiral phase and a tilted plane wave. The interference pattern has been directly written and recorded onto an rGO-polymer by fs laser pulses. The vortex grating contains partially approximately parallel lines and has a bifurcated structure (a folk-like pattern) at the center of the pattern, as shown in Fig. 5(a). In obtaining the pattern of Fig. 5(a), we have used point by point scanning and the step size used is 1µm with the rest of the parameters the same as those in the line scanning mentioned in an earlier Section. The dwell time is 50 ms and the writing time is about 250 seconds for the 150 µm x 150 µm pattern. The intensity pattern of the first order diffraction is shown in the Fig. 5(b) when the fork grating is illuminated by a Gaussian beam at wavelength of 632nm. The intensity is distributed in a doughnut shape, and the center intensity is zero, illustrating that indeed the optical vortex is of one topological charge [47,48]. For comparison, in Figs. 5(c) and 5(d), we show a computer-generated hologram and its optical reconstruction, corresponding to those shown in Fig. 5(a) and 5(b), respectively. It is interesting to point out that the vortex grating shown in Fig. 5(a) experimentally shows some irregularities from the center to the right edge of the pattern. This is because the optical vortex has a phase discontinuity at the origin, which leads to an abrupt shift of the scan line position during reduction. Interestingly, this does not show a detrimental effect on the reconstruction of the vortex beam.
Fig. 5. (a) Reduced GO-polymer vortex grating. (b) Vortex beam produced by vortex grating using rGO-polymer. The diffraction distance is 25 cm, and the size of the vortex beam is 1440 µm*1080 µm. (c) Computer-generated hologram used to generate the vortex beam. (d) Generated vortex beam in simulation.
4. Conclusions
By controlling the power of the fs pulse laser, phase modulation exceeding 2π/3 has been achieved with an rGO phase grating. The interference between a tilted plane wave and a beam with spiral phase has been calculated in a computer to construct a computer-generated hologram known as the fork grating (vortex grating). The fork grating is capable of generating a vortex beam of one topological charge upon plane wave illumination. Our work shows that phase modulation of the rGO controlled by the laser power holds high potential for the design of novel devices based on rGO, such as the vortex grating fabricated in the present paper.
Funding
National Natural Science Foundation of China (11762009, 61565010, 61865007); Natural Science Foundation of Yunnan Province (2018FB101).
Acknowledgments
We thank Prof. Jinming Cai (Faculty of Materials Science and Engineering, Kunming University of Science and Technology) for his sincere help and support in the aspect of graphene oxides. We also thank Prof. Hongtao Wang and his team of the High-Performance Materials Mechanics Laboratory at Zhejiang University for their valuable suggestions.
Disclosures
The authors declare no conflicts of interest.
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