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Graphene-based multilayered metamaterials with phototunable architecture for on-chip photonic devices Yunyi Yang, Han Lin, Bao Yue Zhang, Yinan Zhang, Xiaorui Zheng, Aimin Yu, Minghui Hong, and Baohua Jia ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00060 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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Graphene-based multilayered metamaterials with phototunable architecture for on-chip photonic devices Yunyi Yang†#, Han Lin†#, Bao Yue Zhang‡, Yinan Zhang§, Xiaorui Zheng†, Aimin Yu†, Minghui∥ Hong and Baohua Jia†* †Centre

for Micro-Photonics, Faculty of Science, Engineering and Technology, Swinburne

University of Technology, P. O. Box 218, Hawthorn VIC 3122, Australia ‡School

of Engineering, RMIT University, Melbourne, VIC 3000, Australia

§Provincial

Key Laboratory of Optical Fiber Sensing and Communications, Institute of Photonics

Technology, Jinan University, Guangzhou 510632, China ∥Department

of Electrical and Computer Engineering, National University of Singapore,

Singapore 117576, Singapore KEYWORDS：graphene, metamaterial, multilayer, large scale, on-chip device

ACS Paragon Plus Environment

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ABSTRACT

Graphene-based metamaterials have been theoretically demonstrated as an enabler for applications as perfect absorbers, photodetectors, light emitters, modulators, and tunable spintronic devices. However, challenges associated with conventional film deposition techniques have made the multilayered metamaterial difficult to fabricate, which have severely limited experimental validations. Herein, the experimental demonstration of the phototunable graphene-based multilayered metamaterials on diverse substrates by a transfer-free, solution-phase deposition method is presented. The optical properties of the metamaterials are tuned dynamically by controllable laser-mediated conversion from graphene oxide layers into graphene counterparts, which exhibit different degrees of conversion, would offer huge potential for devices design and fabrication. The converted graphene layers present comparable (within 10%) optical conductivity to their chemical vapour deposited analogues. Moreover, laser patterning leads to functional photonic devices such as ultrathin flat lenses embedded in the lab-on-chip device, which maintains consistency and exhibits subwavelength focusing resolution in aqueous environments without any noticeable degradation compared with the original lens. This graphene-based metamaterial provides a new experimental platform for broad applications in on-chip integrated photonic, biomedical, and microfluidic devices.


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Graphene is expected to bring about multiple functionalities in various fields. In particular, its unique electron transfer behaviour and outstanding optical properties1-5 are highly appealing for optoelectronics and energy conversion applications.6-8 However, the ultrathin nature (3.4 Å) and low optical absorption (2.3%) of monolayer graphene3 over a broadband wavelength restrict its ability to provide sufficient optical modulation,9,10 which limits performance in optical applications. Metamaterials comprising alternating graphene and dielectric layers are artificially structured materials designed to attain an extremely high optical response. Graphene-based metamaterials with layered artificial structure can enhance optical modulation; thus, theoretical studies have suggested that these materials can be useful in energy harvesting11,12, light emitting devices14, all optical communication devices15 and spintronic devices.16 However, the fabrication of graphene-based metamaterials remains significantly challenging due to the inaccurate control and sophisticated transfer process of conventional mechanical exfoliation and deposition methods, restricting experimental demonstrations to only a few examples.17,18 Graphene multilayered metamaterials have recently been synthesized by chemical vapour deposition (CVD), atomic layer deposition (ALD) separately and assisted by the graphene layer transfer process.17,18 The quality of the metamaterial is sensitive to the deposition conditions and becomes difficult to control when the number of layers increases due to the transfer process.17,18 The inaccurate transfer and deposition processes, as well as the difficulties in aligning different layers, hinder the fabrication of graphene-based metamaterials. Therefore, the number of periods of graphene multilayers has stayed below six that limits the optical modulation and its further functionalization.17 In addition, the sophisticated deposition and layer transfer process are not viable for real-life device applications.19,20


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In this paper, to address these challenges, we report a low-cost solution-phase method that generates a multilayered metamaterial consisting of alternating monolayer graphene oxide (GO)/graphene and dielectric layers without a transfer step.21-26 The single-step method produces metamaterial on diverse substrates with arbitrary surfaces, shapes, and sizes. The quality of the metamaterial is independent of the number of layers and surface area up to 100 layers. The surface roughness in the overall metamaterial thickness can be maintained at 2 nm, which is comparable to that obtained by state-of-the-art vacuum deposition techniques17. In addition, laser-mediated photoreduction can convert GO to graphene and effectively decrease the bandgap of the graphenebased metamaterial (Figure 1) by removing the oxygen-functional groups. As a result, the optical response can be manipulated, which is attractive for the production of functional photonic devices.27-29 Optical responses including effective complex refractive indices and optical conductivities of the multilayered graphene-based metamaterial are measured by spectroscopic ellipsometry. The calculated optical conductivity of photo-reduced graphene is nearly identical to that of CVD graphene.11 Moreover, this solution-phase method enables applications of the graphene-based metamaterial in aqueous environments. To harness the laser processability and water resistance of the metamaterial, we have created a water-immersion ultrathin flat lens of 18 nm for a microfluidic biophotonic device. The lens achieves a stable subwavelength focal resolution in water, which is valuable for lab-on-a-chip biological devices. The graphene-based metamaterial provides a new experimental platform for potential applications ranging from on-chip integrated photonic, biomedical, and microfluidic devices to perfect absorbers and directional light emitters.30, 31


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RESULTS AND DISCUSSION

Figure 1. Schematic of the dynamic process for in-situ phototunable graphene-based metamaterial. (a) The initial state of the GO multilayer metamaterial consists of alternating monolayer GO and the dielectric layer. (b) Schematic of phototuning process. The bandgap is narrowed by laser photoreduction and the permittivity ε is ascending with the decreasing of the bandgap. (c) The final state of the graphene metamaterial consists of fully reduced graphene layers and dielectric layers. Rational design of the graphene-based metamaterial. The conceptual design of the graphenebased metamaterial architecture, which includes two boundary states and the dynamic phototuning process, is illustrated in Figure 1. In the initial state (Figure 1a), the metamaterial consists of alternating a monolayer GO (thickness 𝑡𝑔) and a dielectric layer (thickness 𝑡𝑑, Figure 1a). The laser photoreduction is introduced to dynamically manipulate the bandgap of the graphene-based metamaterials and the schematic process is shown in Figure 1b. In the final state (Figure 1c), GO is completely converted to graphene layers. The intermediate states between the two boundary states are all accessible depending on laser irradiation, which controls the extent of conversion of the material. Because each layer is much thinner than the wavelength of light, the structure is a uniform uniaxial metamaterial film with effective permittivity ε.32 The graphene-dielectric multilayer structure can be homogenized and viewed as a metamaterial using the effective medium


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approximation. The effective in-plane (𝜀 ∥ ) and out-of-plane permittivities (𝜀 ⊥ ) are defined by the long-wavelength limit of the Bloch’s theorem.32 (1)

𝜀 ∥ = 𝑓𝜀𝑔 + (1 ― 𝑓)𝜀𝑑 𝜀 ⊥ = (𝑓𝜀𝑔―1 + (1 ― 𝑓)𝜀𝑑―1)

―1

(2)

where 𝑓 = 𝑡𝑔/(𝑡𝑔 + 𝑡𝑑) is the graphene filling fraction and 𝜀𝑑 is the permittivity of the dielectric. The frequency-dependent, in-plane permittivity of a graphene layer (𝜀𝑔) is given by17 𝑖𝜎

(3)

𝜀𝑔 = 1 + 𝜀0𝜔𝑡𝑔

where σ is the surface conductivity and 𝜀0 is the permittivity of vacuum. The theoretical conductivity of graphene is written as17

𝜎(𝜔) =

(

𝜎0 2

tanh

ħ𝜔 + 2𝐸𝐹 4𝑘𝐵𝑇

+ tanh

ħ𝜔 ― 2𝐸𝐹 4𝑘𝐵𝑇

𝜎0

[

― 𝑖2𝜋𝑙𝑜𝑔

(ħ𝜔 + 2𝐸𝐹)2

]+𝑖

(ħ𝜔 ― 2𝐸𝐹)2 + (2𝑘𝐵𝑇)2

4𝜎0

𝐸𝐹

𝜋 ħ𝜔 + 𝑖ħ𝛾

)

(4)

where 𝜎0 = 𝑒2/4ħ, EF is the Fermi energy relative to the Dirac point, γ is the intraband scattering rate, T is the temperature, and kB is the Boltzmann constant. In this expression, the first two terms correspond to interband transitions, while the third term is the Drude-like intraband conductivity. 𝑒2

( ), E

For GO, 𝜎0 = 4ħ𝑒𝑥𝑝

― 𝐸𝑔

2𝑘𝐵𝑇

g

is the bandgap of GO, which indicates that the conductivity can be

manipulated by the bandgap of the graphene-based metamaterial (Figure 1). When GO is fully reduced to graphene, Eg =0, the 𝜎0 is the same as that of graphene. Most previous theoretical work has concentrated on using high-mobility graphene obtained by mechanical exfoliation or epitaxial growth. We use GO in solution because it is compatible with solution-phase film synthesis and can be directly deposited onto any surface without


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requiring a transfer step that is necessary for conventional graphene deposition methods. More importantly, the as-synthesized metamaterial undergoes insulator–semiconductor–semi-metal transitions during reduction33 from GO to graphene due to the bandgap decreases. Thus, the GO to graphene conversion presents a much wider tuning range compared with that achieved via doping pristine graphene.17 Furthermore, the bandgap can be accurately controlled by varying the degree of reduction, which facilitates the tuning of the effective metamaterial parameters. Additionally, initial and final states are controlled by the filling ratio of graphene (f) and the permittivity of the dielectric (εd). The effective refractive indices of pristine graphene should be reduced to minimize reflection at the metamaterial–water interface. In addition, optical modulation should be maximized to achieve high performance. The tuning range is discussed in detail in the Supplementary Information (Figure S1). The graphene-based metamaterial is carefully designed to balance all these requirements. The thickness of the graphene layers was controlled down to a couple of nanometers while keeping the dielectric layer thickness at 2 nm. The dielectric layer has a refractive index of approximately 1.5 and near-zero absorption in the visible to near-infrared region.


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Figure 2. Fabrication of graphene-based metamaterial. (a) Schematic showing the layer-by-layer fabrication of a five-period graphene-based metamaterial. The inset shows the molecular structure of the film. (b) SEM image of the five-layer GO structure clearly showing the layer-by-layer structure. AFM thickness profiles and images of (c) a PDDA monolayer, (d) GO flake and (e) a five-layer GO multilayer film, respectively. Graphene-based metamaterial formation and characterization. The designed graphenebased metamaterial is fabricated by a solution-phase layer-by-layer film deposition technique (Figure 2a). GO is a graphene-based material decorated with oxygen-functional groups, which promote the uniform dissolution of GO flakes in water.34 Its negatively charged surface can firmly attach to any positively charged surface by electrostatic forces. The opposing electrostatic force between the same negatively charged GO layers would help the GO flakes to maintain a monolayer structure and prevent aggregation. Here, we use the positively charged polyelectrolyte polydiallyldimethylammonium chloride (PDDA) as a dielectric material, which is deposited to


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the negatively charged substrate before the adsorption of GO flakes. Proper control of the flake number, flake size and GO concentration can lead to a monolayer GO film (see Experimental Section). Repeating these successive PDDA and GO depositions gives multilayered GO films with a precisely controlled number of layers and layer thicknesses. The morphological features of the resulting metamaterial are characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM images clearly show five ultrathin layers in the metamaterial (Figure 2b). According to the AFM measurements, the thicknesses of the PDDA (Figure 2c) and GO monolayers (Figure 2d) are approximate 2 and 1 nm, respectively. In addition, a five-layer structure displays an overall thickness of ca. 18 nm, which corresponds to an average thickness of 3.6 nm for each GO–PDDA bilayer (Figure 2e). The quality and monolayer control of the GO-based metamaterial rest on the key parameters, such as the flake size, the deposition time in each iteration, and the GO concentration. The optimization of these parameters leads to a concentration of 5 mg/mL and an immersion time of 1 min to generate a highly uniform film (Figure S2). The absorbance of the layer numbers from 1-5 is shown in Figure S3a. The thickness and roughness of different layer numbers from 1-10 are presented in Figure S3b.


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Figure 3 Large-area coating graphene-based metamaterial on arbitrary substrates (Silicon, acrylic and polyester). (a) Optical images of the five-layer GO-based metamaterial on a 4-inch silicon wafer. (b) Raman spectra of uncoated and metamaterial-coated Si wafers. (c) Thickness mapping of the five-layer metamaterial on a Si wafer. (d) Optical images showing the side-view (left panel) of a 72-mm-diameter curved acrylic lens, and the top-view (right panel). (e) Curved lenses coated with 10- (left panel) and 20-layer metamaterials (right panel). (f) Bending of a five-layer metamaterial deposited on a flexible transparent substrate. To demonstrate that the proposed method is suitable for large-scale fabrication, a five-layer graphene-based metamaterial is deposited on a 4-inch silicon wafer (Figure 3a). The integration of the metamaterial on the silicon substrate is assessed by Raman spectroscopy (Figure 3b). These spectra show the representative D and G peaks of GO at 1345 and 1590 cm-1, respectively. The D to G ratio as low as 0.7 is achieved, indicating the low defect density of the GO layers

35.

The


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surface profile characterization by optical microscopy (Figure S4a) and SEM (Figure S4b) indicates that the metamaterial presents an ultra-smooth surface. The thickness mapping of the five-layer GO-based metamaterial (Figure 3c) demonstrates that the thickness remains within 18.0±1.5 nm over the entire wafer area, which approaches the accuracy of the state-of-the-art vacuum film deposition methods. 17 More importantly, the GO–PDDA multilayer material is coated onto a 72-mm optical acrylic lens with a curved surface to prove that the solution-phase method enables direct coating on substrates of arbitrary shapes without a transfer process (Figure 3d). Such curved surfaces are typically challenging to accurately control the thickness over the entire surface by vacuum coating methods. 10- (Figure 3e, left) and 20-layer (Figure 3e, right) metamaterials form uniform films on the curved lens with high transmission, which can find many applications in vision correction and virtual reality. To illustrate the mechanical robustness and flexibility of the metamaterial, a five-layer thin film is deposited on a 30 cm × 20 cm bendable and twistable polyester substrate (Figure S5). After numerous bending and twisting cycles (Figure 3f), the metamaterial retains its mechanical integrity without any visible wrinkles or cracks observed by a microscope, consistent with its excellent mechanical strength and flexibility. We deposit the metamaterial onto various substrates, such as glass, Si, acrylic, and polyester, demonstrating its versatility as a coating.


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Figure 4. Laser tuning of graphene-based metamaterials. (a) Optical micrographs of eight reduction levels and ablation. Changes in refractive index (n) (b) and extinction coefficient (k) (c) as a function of laser power for graphene-based metamaterials at broadband wavelengths ranging from 200 to 1600 nm. (d) Bandgap tuning range as a function of laser power. (e) Real and imaginary parts of the optical conductivity of GO-metamaterials, laser-reduced graphenemetamaterials and CVD-grown graphene films. Laser-mediated tuning and optical characterization of the graphene-based metamaterial. The laser photoreduction has emerged as appealing alternatives to thermal and chemical methods


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due to its unique advantages including reliability, amenability, low cost, and flexible patterning.36 It can trigger the insulator–semiconductor–semi-metal transition in graphene-based metamaterial by effectively decreasing the bandgap with removing the oxygen-functional groups.27,28,33,36 Owing to its unique mechanism of the reduction process, the laser photoreduction has been chosen as the method of choice to tune the bandgap of the metamaterials. The tuning process is controlled by varying the laser power (Figure 4a), yielding multiple (eight in case of Figure 4) distinct reduction levels when the power increased from 3 to 10 µW, which can be visually confirmed by optical microscopy. The tuning range is identified experimentally, with clear contrast, after reduction to the laser ablation threshold (11 µW). The laser scanning speed is 20 µm/s. The intermediate levels are all continuously accessible by finely tuning the laser power. The Raman mapping (G band of GO-based film) image in Figure S6 clearly shows the edge of the substrate and GO film. The GO and laser-reduced metamaterial are characterized by Raman spectroscopy, which shows significant changes in the D (1345 cm-1) and G (1590 cm-1) bands and the emergence of 2D (2671 cm-1) band (Figure S7). This also proves the successful reduction of the graphenebased metamaterials. The effective refractive index (n) (Figure 4b) and extinction coefficient (k) of the metamaterial (Figure 4c) are measured before and after laser tuning by spectroscopic ellipsometry. The details of the ellipsometry measurements are provided in Supporting Information Section 1. Before laser tuning, the metamaterial exhibits a moderate refractive index of approximately 2, within the measured wavelength range. After increasing the laser power to 10 µW, its refractive index increases significantly beyond 2.5 at 500 nm and approaches 2.7 at 1600 nm. This dramatic refractive index change (∆n ≈ 0.5) is more than one order of magnitude greater than previously observed for conventional refractive materials.37 This is indicative of a large dynamic tuning range,


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which is useful for potential phase modulation in photonic devices. On the other hand, before tuning, the metamaterial exhibits a low extinction coefficient over the measured wavelength range. At a laser power of 10 µW, the extinction coefficient changes by 0.25 over a wide wavelength range that results in high contrast in light absorption of the laser-reduced metamaterial, providing a mechanism for achieving efficient amplitude modulation. The measured n and k values during the laser photoreduction provide the bandgap of the GO/graphene layers. The bandgap gradually decreases from 2.1 eV for GO to 0.1 eV after complete reduction (Figure 4d). The non-zero bandgap results from the presence of defects consisting of graphene nanoflakes in the reduced material. Furthermore, the optical conductivities (σ) of the GO/graphene metamaterials are also evaluated using Equations (1–4). The real (orange solid line, Figure 4e) and imaginary (orange dashed line, Figure 4e) parts of the calculated surface conductivity of the graphene metamaterial are extremely close to those of CVD grown graphene (black solid and dashed lines in Figure 4e),17 which suggests that the properties of graphene are maintained in the metamaterial. Moreover, the layer thickness of GO (8.1 Å) gradually decreases to that of graphene (3.4 Å), which reduces the filling ratio of the metamaterial. This results in an overall increase in the effective permittivity and absorption. To quantify the laser-fabricated features, the thickness and width of laser-inscribed lines (Figure S8) are measured by AFM and correlated to the laser power for a 22-nm-thick GO film (Figure S9). Here, an NA=0.8 dry lens and the femtosecond laser at 800 nm are used. The calculated laser spot size is around 500 nm. During the fabrication, the linewidth can be tuned from 300 nm to 1200 nm (Fig. S9) by controlling the laser power. The line thickness and width increase with ascending laser power from 3 to 10 µW. This is caused by the enhanced reduction degree of the GO film. When the laser power is over 11 µW, both the line thickness and width


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show a monotonic nonlinear dependence with a saturation trend due to the laser ablation. This phenomenon presents the ablation of the GO material by high power laser irradiation, which is consistent with the observation in Figure 4a. The smallest realizable feature is measured as 300 nm, which approaches the diffraction limit of the fabrication laser. The fabrication resolution can be further improved by using a high NA objective lens, for example, 100× 1.4NA oil immersion objective. Optical microscopy and profiling images show that pre-designed arbitrary patterns can be produced (Figure S10). A functional quick response (QR) code pattern (Figure S11) covering an entire cover glass (22 mm × 22 mm) is also fabricated within one minute, which demonstrates that laser fabrication is suitable for large-area production. This demonstration leads to the direct inscription of information in the functional metamaterial device.

Figure 5. (a) Schematic design of graphene-based metamaterial lens (top) and in a micro-fluidic environment (bottom). The image of a microfluidic device (bottom). (b) Theoretical design of the flat lens (left) and its corresponding simulated focal spot (right). (c) Microscopic image (left) and focal spot of the laser-inscribed flat lens on an 18-nm-thick GO film (right). (d) Microscopic image (left) and focal spot of the flat lens after immersion in a biocompatible fluorescence solution


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(labelling dye, ThermoFisher Alexa Fluor 647) within a microfluidic device for one month (right). (e) Intensity distributions of the theoretical and laser-inscribed graphene-based metamaterial lens before and after immersion in a microfluidic device for one month. Ultrathin microfluidic flat lens. Unlike normal GO materials, which are soluble in the water environment, the graphene-based metamaterial is inherently water-resistant due to the solutionphase synthesis and its adhesion by electrostatic force,34 which makes it a versatile platform for microfluidic and bio-sensing/imaging devices integrated with lab-on-a-chip systems. To demonstrate this unique property and flexibility for real-world application, a water-immersion ultrathin flat lens was designed and fabricated on an 18 nm GO film for use in a microfluidic device (Figure 5a). The details of the flat lens design are presented in Supporting Information Section 2. The theoretical design of the flat lens (left, Figure 5b) and the simulated focal spot (right, Figure 5b) are compared with the fabricated flat lens (left, Figure 5c) and its measured focal spot (right, Figure 5c). The corresponding experimental intensity distribution (black line, Figure 5e) agrees well with its theoretical counterpart (blue dotted line, Figure 5e), which can be attributed to the small roughness of the graphene-based metamaterial and precise control of the laser fabrication process. During the tuning of the graphene-based metamaterial, there are three effects happening simultaneously, namely the increase of the refractive index and absorption, and a decrease of the film thickness. Therefore, the overall results of the three effects are the phase and amplitude can be modulated simultaneously. Here the lens was designed based on both phase and amplitude modulations. In the current flat lens design, the maximal modulation (10 µW) is used to achieve the optimized focusing efficiency. By taking advantages of both phase and amplitude modulations, the full width at half maximum approximates 500 nm (0.7 λ), indicative of a near-diffraction-


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limited performance, which is comparable with the state-of-the-art flat lens and among the thinnest flat lens demonstrated so far.38 To assess the stability of the graphene-based metamaterial in an aqueous environment, we repeated these measurements, including optical microscopy (left, Figure 5d) and intensity distribution (right, Figure 5d), after immersing the lens in a biocompatible fluorescence solution (labeling dye, ThermoFisher Alexa Fluor 647) within a microfluidic device in ambient water for one month. The focal spot of the lens remained identical after immersion (Figure 5d), demonstrating that the graphene-based metamaterial is water-resistant. The intensity distribution curves overlapped (Figure 5e), indicating that the performance of the flat lens does not noticeably degrade when operating in the microfluidic device. In addition, optical profiling confirms that the lens maintained its surface profile in an aqueous environment instead of swelling (Figure S12). All these results demonstrate that the graphene-based metamaterial can persist without compromising its morphology or optical performance. Such a flat lens design demonstrates that the combined phase and amplitude modulation in an ultrathin graphene-based metamaterial is sufficient for fabricating multifunctional on-chip diffractive optical devices in an aqueous environment. CONCLUSIONS In summary, we have demonstrated the first synthesis of the graphene-based metamaterial by a low-cost and scalable solution-phase method without transfer. This unique method provides unprecedented control over the monolayer thickness and smoothness and enables threedimensional surface coating on arbitrarily shaped substrates. The effective metamaterial parameters are controlled by modifying the filling ratio and the permittivity of the spacing material and can be locally tuned by laser photoreduction. Moreover, the graphene-based metamaterial is


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water-resistant, yielding a high-performance microfluidic flat lens that can focus optical energy with subwavelength resolution. The novel metamaterial platform with laser-mediated patterning provides new solutions for ultrathin, water-proof, highly integratable and flexible optical systems, opening up new avenues for various multidisciplinary applications including biomedical imaging, virtual reality, optical data storage, encryption, all-optical processing photonic chips, light harvesting, aerospace photonics, optical microelectromechanical systems, microfluidic and labon-chip devices. EXPERIMENTAL SECTION Preparation of the GO dispersion. The high-quality GO solution was synthesized by the chemical oxidation of graphite via a modified Hummers method and sonicated vigorously using a Branson Digital Sonifier.34 Solution-phase preparation of the graphene-based metamaterials. The layer-by-layer process can be divided into four steps. (1) The substrate was immersed into a 2.0% (w/v) aqueous PDDA (Sigma-Aldrich) solution and (2) rinsed with a stream of deionized distilled water and dried with N2. (3) The PDDA-coated substrate was then immersed into an aqueous GO solution with optimized concentration and immersion time and (4) the sample was rinsed with a stream of deionized water and dried with N2. Upon completing steps 1-4, a polymer/GO layer film was directly assembled on the desired substrate.39 These steps were repeated to construct multilayer films, and the process is highly scalable. In principle, the area of the film is only limited by the solution container. Characterization of the metamaterials. The morphology and optical properties of the graphene-based metamaterial films were characterized by a scanning electron microscope


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(RAITH150-TWO), an atomic force microscope (Bruker Dimension Icon), an optical profiler (Bruker ContourGT InMotion), an ellipsometer (J.A. Woollam M-2000), and an UV–visible spectrometer (PerkinElmer UV/Vis Spectrophotometer). Femtosecond laser patterning. The ultrathin flat lens was directly fabricated on the asprepared graphene-based metamaterial using a custom-designed femtosecond laser writing system. To minimize thermal effects and enhance the fabrication resolution, GO layers were converted into graphene layers by direct laser writing using a low-repetition-rate, femtosecond pulsed laser beam (Coherent Libra, femtosecond laser, 800 nm, 10 kHz repetition rate, 100 fs pulse width, focused by a 100×, 0.8 NA objective lens). The sample was mounted on a three-dimensional nanometric piezo stage. A computer-controlled LabVIEW system was employed to create flat lens structures. Imaging system. The laser-inscribed ultrathin microfluidic flat lens was characterized using a Nikon N-STORM microscope with 700-nm plane-wave illumination. The cross-sectional distributions of the generated focal spots of the flat lens were captured using a charge-coupled device (CCD) camera equipped with a 100× objective at a numerical aperture of 1.4. The lateral cross-sectional intensity distribution was captured by normalizing the sensitivity and exposure time of the CCD camera, and the peak focusing intensities obtained in different situations could be compared directly. ASSOCIATED CONTENT Supporting Information


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Graphene metamaterial simulation, optical characterization, surface profile characterization, large-scale demonstration, Raman mapping, AFM characterization, laser patterned photonic device, the theoretical design of the flat lens, topographic characterization of water immersion lens. Supporting sections, figures, tables, and equations (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions B.J. and H.L. proposed the idea and developed the strategy of the project. Y.Y., H.L. developed the theoretical model and designed the experiments. Y.Y., H.L. conducted the experiments and data analysis. Y.Y and B.Y.Z performed the AFM measurements. Y. Z. advised on the thickness mapping experiment. X. Z., M. H. and A. Y. participated in early discussions. All authors contributed to manuscript writing. Y. Yang and H. Lin contributed equally to this work.

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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Baohua Jia thanks the Australian Research Council for its support (DP150102972). We thank Dr. Jie Zhang for assisting in large-scale film preparation, Dr. Ye Chen for assisting in optical


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characterization, Dr. Jiayang Wu for the helpful discussion, Chong Lei for assisting in the microfluidic device fabrication, Dr. Xijun Li and Dr. Xiaohan Yang for technical support.


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For Table of Contents Use Only Graphene-based

multilayered

metamaterials

with

phototunable architecture for on-chip photonic devices Yunyi Yang†#, Han Lin†#, Bao Yue Zhang‡, Yinan Zhang§, Xiaorui Zheng†, Aimin Yu†, Minghui∥ Hong and Baohua Jia†*

This figure presents that the experimental realization of phototunable graphene-based metamaterials on diverse substrates by a scalable, transfer-free solution-phase deposition method has been achieved. The laser-induced reduction can manipulate the optical properties, thickness and bandgap of the graphene metamaterials. Then laser-patterned functional photonic devices such as ultrathin flat lenses and lab-on-chip devices are achieved, which exhibits excellent performance even in extreme conditions.


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Graphene-Based Multilayered Metamaterials with ... - ACS Publications

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