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Abstract
The plasmonic metamaterials and metasurfaces play a critical role in manipulating lights in the mid-infrared spectral region. Here, we first propose a novel plasmonic chiral structure with the giant optical activity in the mid-infrared spectral region. The chiral structure consists of the moiré patterns, which are formed by stacking double-layer graphene nanoribbons with a relative in-plane rotation angle. It is demonstrated that the graphene-based plasmonic structure with moiré patterns exhibits the strong circular dichroism. The giant chiroptical response can be precisely controlled by changing the rotation angle and Fermi level of graphene. Furthermore, a dielectric interlayer is inserted between two layers of graphene to obtain the stronger circular dichroism. Impressively, the strongest circular dichroism can reach 5.94 deg at 13.6 µm when the thickness of dielectric interlayer is 20 nm. The proposed structure with graphene-based moiré patterns can be superior to conventional graphene chiral metamaterials due to some advantage of rotation-dependent chirality, flexible tunability and cost-effective fabrication. It will advance many essential mid-infrared applications, such as chiral sensors, thermal imaging and chiroptical detectors.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Chirality is the property of asymmetry, which represents an object that cannot superimpose with its mirror image. The optical response of a chiral object is sensitive to the polarization state of the incident light. It results in differential absorption or transmission of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) lights. The asymmetric interaction between the chiral object and circularly polarized light gives rise to optical activity such as circular dichroism (CD) and optical rotation [1–3]. The optical CD is ubiquitous in many organic molecule systems (e.g., DNA, viruses), whereas their responses are quite weak [4]. The differential absorbance of LCP and RCP is much weaker than the ordinary absorbance for almost all the molecules at all optical wavelength, especially at longer wavelengths [5]. The advent of the metamaterials offers a new way to enhance the artificial chiral response for manipulating the circularly polarization state of light [6,7]. It is possible that obtaining plasmonic chiral metamaterials with strongly enhanced chiral responses and compact size [1,8–10].
There have been several plasmonic chiral metamaterials with great chiroptical responses. For example, some arrays of chiral gold nanostructures with giant specific rotation are proposed and discussed in the visible region [11]. The fabricated helicoidal 3D nanostructure is composed by some arrayed platinum chiral nanohelices. This structure shows the strong chiral activity at visible and near-infrared frequencies [12]. Moreover, it is introduced that the arranged metal nanoparticles based on DNA self-assembly method have giant CD and optical rotary dispersion (ORD) in the visible region. With the composition of the metal nanoparticles changing, the chiroptical response can be adjusted in the visible spectral region [13]. In addition, there are some special metamaterials based on the helical structure, including chiral metal nanoneedles [14], spiral thin films [15] and plasmonic gyroid networks [16]. It shows that the strong chiroptical responses of the mentioned structures are realized by breaking the spatial symmetry of light-matter interactions or with the structural chirality. In order to tune chiroptical responses and simplify the fabrication, a moiré chiral metamaterial has been proposed. It is a novel chiral metamaterial with the moiré patterns. Two layers of achiral Au nanohole arrays are stacked with a relative in-plane rotation angle to form the moiré patterns. The chiroptical response of this moiré chiral metamaterial is strong and tuned by changing in-plane rotation angle in the visible spectral region [17]. The optimized chiral metamaterial is also regarded as an ultrasensitive sensor to detect trace amount of solvent impurities [18]. Many kinds of metamaterials with strong optical activities have been designed and analyzed at visible and near-infrared wavelengths. The development of chiral metamaterial significantly will improve the ability to control and detect electromagnetic waves with circular polarization.
The chiral nanostructures in the mid-infrared (MIR) spectral region also play a key role in distinguishing the chirality of molecules and chemical composites, sensors and polarization imaging. Compared with chiral metamaterials at visible and near-infrared wavelengths, the nanostructures with chiral response are less studied at MIR wavelengths. It has been studied that a broadband circular polarizer at MIR wavelengths, which comprised 3D gold helices arranged on a square lattice. It blocks the incident light that is with the same handedness as the helices. However, the incident light with the inverse handedness circular polarization can transmit this polarizer [19]. A graphene-based nanostructure with a large plasmonic CD has been studied in the MIR spectral interval. The graphene nanodisks are placed along a helix scaffold to form a chiral unit. The differential absorbance of the graphene nanodisk assembly arrays under LCP and RCP illuminations is on the order of 10−2 near the graphene plasmon resonance wavelength [4]. Another graphene achiral metamaterial has been reported to exhibit a strong and tunable chiroptical response at oblique incidence in the MIR region [20]. Moreover, the magnetic resonance of the plasmonic systems plays an important role in generation the CD and circular conversion dichroism (CCD). The circular conversion dichroism is defined as the difference between the left-to-right and right-to-left circularly polarized reflectance (or transmittance) conversion efficiencies [21]. The meta-dielectric-metal (MDM) penetrated by symmetric circular holes array has been designed to create the magnetic dipolar moments, which enhance circular polarization conversion difference in the THz region [22]. Combination meta-dielectric-metal trilayer with the graphene sheet, the CCD spectra can be tuned by electrically controlling the Fermi level of graphene in the MIR region [21]. The graphene-based metamaterials with tunability chiroptical response are gaining concerns to modulate the polarization of light.
In this paper, we focus on the optical activity in the MIR spectral region and propose a high-performance tunable chiral structure based on the graphene plasmonic resonance. Two layers of the identical achiral graphene nanoribbons stack with a tailored rotation to generate the moiré patterns. Thus, this structure with moiré configurations leads to the structural chirality. When the incident light is circularly polarized light, polarization-dependent light-matter interactions appear in the graphene-based moiré patterns. The absorption difference between two circularly polarized lights (i.e., CD) can be precisely changed by adjusting the rotation angle between two layers of graphene and Fermi levels of graphene. Furthermore, a dielectric spacer is introduced between two layers of graphene nanoribbons. The plasmonic resonances are excited on the two layers graphene nanoribbons and the spacer-dependent near-field coupling is induced in the dielectric spacer. The simulated results and analytical fittings show that the near-field coupling enables to enhance the chiroptical response of the proposed structure and change the CD spectra in the MIR region. The moiré chiral structure with the tunable optical activity and relatively simple fabrication offer a way for some vital and promising applications, such as realizing adjustable polarization modulator, processing the optical signs in the thermal imaging [4] and sensors at MIR wavelengths.
2. Design of the plasmonic chiral structure and simulation
The proposed structure is schematically depicted in Fig. 1. The double-layer periodic graphene nanoribbons are placed on a homogeneous dielectric, which is set to BaF2. The upper-layer graphene nanoribbons and lower-layer nanoribbons are identical and periodic in the x direction. As shown in Fig. 1(a), the upper-layer nanoribbons are directly placed on the lower-layer nanoribbons. Meanwhile, the upper-layer nanoribbons are rotated by an in-plane angle θ, which is relative to the lower-layer nanoribbons. The thickness L of BaF2 is set to 300 nm. The substrate is the slightly conducting a-Si and the thickness of that is set to 3 µm. To clarify the details, we show the magnified image of the single-layer graphene, as depicted in Fig. 1(b). The periodic of nanoribbons is p = 400 nm and the width of graphene nanoribbon is set to w. The refractive index of BaF2 is not constant and the imaginary part of refractive index is less than the real part in the MIR range [23,24]. The loss of BaF2 can be ignored in the proposed structure [25]. The permittivity of a-Si is set to be 12.05 in this simulation [26].
Fig. 1. (a) Schematic of the plasmonic chiral structure with the moiré patterns. The light perpendicularly illuminates the chiral structure along the z direction. CPL: circularly polarized light. (b) Enlargement of one layer graphene nanoribbons to show the details clearly.
In numerical simulation, the graphene film is modeled as a 2D surface conductivity material model to save the storage space and computing time. Within the random-phase approximation (RPA), the complex optical conductivity of graphene can be approximately written as σ (ω) = ie2EF/[πħ2(ω+iτ−1)] in the MIR region [27]. Here, EF = ħvF (ngπ)1/2 is the Fermi level of graphene, which depends on the carrier concentration. The measured dc mobility is µ = 10,000 cm2V−1s−1 and ħ is the reduced Planck constant. The Fermi velocity is νF = 106 m/s [28]. Importantly, the carrier relaxation time τ satisfies the relationship τ = µEF/(eνF2), which is not a constant and depends on the Fermi level [29]. As we all know, the Fermi level of graphene can be tuned by a bias voltage Vg, according to EF = ħνF(πCVg/e)1/2 [30]. The capacitor model consists of the top contact, BaF2 dielectric spacer and the slightly conducting a-Si [31]. As shown in Fig. 1(a), the material of top contact is Au with the thickness of 100 nm in the z direction and the width of 200 nm in the y direction. The length along the x direction is dependent on the number of graphene nanoribbons in real fabrication. The capacitor model is used for applying the bias voltage to control the Fermi levels of double-layer graphene.
Furthermore, we introduce a thin dielectric spacer between the double-layer graphene nanoribbons as shown in Fig. 2(a). The material of the dielectric spacer is set to BaF2 with the variational thickness of d. The cross-sectional view is obviously shown in Fig. 2(b). As depicted in Fig. 2(a), the upper-layer and lower-layer graphene nanoribbons are isolated by the dielectric spacer. They are connected to different Au contacts. The thickness of Au contact for lower-layer graphene is dependent on the thickness of the spacer in real fabrication. Other parameters of this structure are the same as Fig. 1. The Fermi levels of double-layer graphene can be tuned by applying a voltage Vg in the experiment [32].
Fig. 2. (a) Schematic of the chiral structure with the tunable dielectric spacer between two layers of graphene nanoribbons. The BaF2 spacer is perspective to enable showing the lower-layer graphene clearly. (b) Cross-sectional view of the designed structure. In order to clearly show the dielectric spacer and two layers of graphene, the dimensions of structure are not drawn to scale.
The optical properties of the proposed structures are simulated by the finite-difference time-domain (FDTD) method. In our simulation, perfectly matched layers (PMLs) are imposed along the x, y and z directions. In general, PML boundaries perform best when structures extend completely through the PML region. Thus, the a-Si substrate is set to extending the PML region in the z-direction. The total-field scattered-field (TFSF) source with LCP or RCP is applied as the incident light along the z direction. As mentioned above, the proposed structure consists of periodic graphene nanoribbons with several periods. Due to the fact that the graphene area should be added very refined mesh to get accurate results, the storage space and computing time increases exponentially as the simulation effective area increasing. Thus, there is a tradeoff between the simulation accuracy and the storage space in the process of numerical simulation. To improve simulation accuracy and save the storage space, the length of the nanoribbon l is set to 4.5 µm in the simulation. The upper-layer graphene nanoribbons are rotated with a varied rotation angle. Thus, the effective simulation area is set to 14 µm by 10 µm to demonstrate the chiral response of the proposed structure. The mesh size inside the graphene nanoribbons region is 1 nm along all directions and the mesh size outside the graphene region gradually increases. The simulation time is set to 8000 fs and the auto shut-off level is set to 1 ×10−5. The 2D frequency-domain power monitors are used for getting the transmission spectra and electric fields of the chiral plasmonic structure.
3. Simulation results and discussion
3.1. Double-layer graphene nanoribbons without the dielectric spacer
Two identical or similar periodic lattices are superimposed with translational or rotational displacement to form the moiré patterns. The moiré structure has the structural chirality, which is used for manipulation circular polarization light. The relative in-plane rotation angle between the two layers of the identical periodic lattices determines the structural handedness of the moiré metamaterial [33]. When the incident light has the left or right circular polarization, the moiré metamaterial has handedness-dependent optical responses. The antisymmetric coupling between structural and light handedness gives rise to the lower light transmission [34]. Thus, the chiroptical response of the moiré metamaterial is dependent on the relative in-plane rotation angle between two layers of graphene. In the MIR spectral region, the periodic graphene nanoribbons placed on a dielectric substrate can support and propagate the strong excited plasmon polaritons modes [35]. In this paper, the plasmonic moiré structure is formed by stacking two layers of identical graphene nanoribbons with a relative in-plane rotation angle. Similarly, the structural handedness of the plasmonic moiré structure can be transferred to the chiroptical response near the surface plasmon resonance wavelengths. As we all know, the intensity and resonance wavelength of plasmonic modes are adjusted by the Fermi level of graphene. Combining the characteristics of moiré patterns and plasmon polaritons modes in graphene, the chiroptical responses of plasmonic moiré structure based on graphene are precisely tuned by the relative rotation angle and the Fermi levels of double-layer graphene.
The structural handedness of plasmonic moiré structure is obviously dependent on the in-plane rotation angle θ between two layers of graphene nanoribbons, as shown in Fig. 1(a). For the moiré structure, the positive value of θ corresponds to a right-handed chiral structure and the negative value of θ corresponds to a left-handed chiral structure. We demonstrate the tunable chiroptical response of plasmonic moiré structure by changing the in-plane rotation angle θ. The plasmonic moiré structure is without the dielectric spacer between double-layer graphene nanoribbons. The CD spectrum can be defined as 32.98°× (TLCP-TRCP), where TRCP and TLCP are the optical transmission of the proposed structure when the incident light is RCP and LCP, respectively [18]. Figure 3 shows the CD spectra of the designed structure with four sets of the rotation angle, i.e., 5°vs −5°, 10°vs −10°, 15°vs −15°, 20° vs −20°, respectively. The CD spectra show similar line shapes with opposite values for each set of the enantiomers. As shown in Fig. 3, the CD peak has a blue-shift with increasing the rotation angle in the MIR spectral region. The highest CD peak is 4.82 deg at 13.8 µm, corresponding to the rotation angle of 15°, as shown by the blue solid line. Figure 3 also reveals that the structural handedness of the proposed structure transfers to the optical chirality response. Thus, the optical chirality of the proposed structure can be precisely tuned by controlling the in-plane rotation angle between two layers of graphene nanoribbons. Due to the fact that the chiral response is from the structural chirality, the translational alignment is not needed for the proposed structure to achieve the desired chiroptical effects in the actual fabrication.
Fig. 3. CD spectra of the proposed structure with varying rotation angles from −20°to 20°at an interval of 5°.
We have investigated the CD spectra of the designed structure with the different rotation angle θ. The tunable chiroptical response can be realized by controlling the rotation angles between double-layer graphene nanoribbons. In addition, the resonance wavelength and intensity of excited plasmonic polaritons modes are dependent on the dimensions of graphene nanoribbons and Fermi levels of graphene [35]. Therefore, it is significant for optimizing the designed structure to obtain the stronger chiroptical response. We discuss the CD spectra of the proposed structure with varying widths of nanoribbons, as shown in Fig. 4(a). The largest value of CD peak is 4.82 deg with the width of 120 nm. Increasing Fermi level of graphene can be used for enhancing the intensity of plasmonic resonance and modulating the wavelength [36]. In Fig. 4(b), the CD spectra are as a function of variational Fermi level of graphene, from 0.5 eV to 0.9 eV. The value of CD peak increases as the Fermi level increasing and can reach about 7.85 deg with the Fermi level of 0.9 eV. Meanwhile, the corresponding wavelength of CD peak has a blue-shift with increasing the Fermi level of graphene. It is demonstrated that the plasmonic moiré structure based on graphene nanoribbons has tunable strong chiroptical response by controlling the relative rotation angle and the Fermi levels of double-layer graphene.
Fig. 4. (a) CD spectra of the designed structure with the varying width of nanoribbons, from 100 nm to 180 nm with an interval of 20 nm. The Fermi levels of double-layer graphene are both 0.7 eV and the rotation angle is 15° in this simulation. (b) CD spectra of the designed structure under the different Fermi levels of graphene, from 0.5 eV to 0.9 eV with an interval of 0.1 eV. The rotation angle is 15° and the width of nanoribbons is 120 nm in this simulation.
In order to show the chiroptical response of the plasmonic moiré structure, we analyze the electron current densities on the upper-layer and lower-layer graphene nanoribbons. The corresponding electron current distributions of the plasmonic resonances are shown in Fig. 5. It obviously shows that the intensities of electron current under RCP illumination are much larger than that of LCP illumination. Since the handedness of the moiré structure is consistent with RCP light, the incident light with RCP can excite the stronger intensities of plasmonic polaritons modes than the LCP light. The origin of the tunable chiroptical responses of the plasmonic moiré structure is from the in-plane rotation of two layers of graphene nanoribbons [17,33]. The simulated results indicate that the proposed structure based on relatively rotated double-layer graphene nanoribbons has the stronger chiroptical response and larger CD value than the conventional chiral metamaterials based on chiral assembly arrays in the MIR spectral region [4].
Fig. 5. (a, b) Schematics of the electron current distributions on the upper-layer and lower-layer of graphene nanoribbons under RCP illumination at 13.8 µm, respectively. (c, d) Schematics of the electron current distributions on the upper-layer and lower-layer of graphene nanoribbons under LCP illumination at 13.8 µm, respectively. In this simulation, the rotation angle is 15°, the width of nanoribbons is 120 nm and the Fermi levels of double-layer graphene are 0.7 eV.
3.2 Two layers graphene nanoribbons with the dielectric spacer
We have investigated the tunable chiroptical response of the plasmonic moiré structure, which is formed by stacking two layers of identical graphene nanoribbons with a relative in-plane rotation. The simulation results show that the plasmonic moiré structure has the giant optical activity in the MIR spectral region. In further, a thin dielectric spacer with the thickness of d is inserted between the double-layer graphene nanoribbons, as shown in Fig. 2. Inserting the dielectric spacer can induce a thickness-dependent near-field coupling. The near-field coupling plays an important role on improving the transfer of the structural chirality to the chiroptical response in the plasmonic moiré metamaterial [18]. In order to realize the stronger chiroptical response, we discuss the CD spectra by tuning thickness (d) of the dielectric spacer, as shown in Fig. 6. The thickness ranges from 10 nm to 50 nm with an interval of 10 nm. Other parameters are the same as that of the structure without the interlayer. The structure without the interlayer is the contrast. As shown in Fig. 6, the values of CD are varying as the thickness increasing. It is demonstrated that the chiroptical response is thickness-dependent in the moiré structure with the interlayer. Compared with the proposed structure without the interlayer, the chiroptical response is strongest when the thickness d is set to 20 nm. As shown by blue line in Fig. 6, the peak value of CD spectrum can reach 5.94 deg at 13.6 µm. In addition, the wavelengths corresponding to the peaks value of CD spectra exhibit a blue shift by increasing the thickness of d. With the thickness of the spacer increasing from 10 nm to 50 nm, a large shift can reach about 484 nm. It is demonstrated that the enhanced chiral effect of the graphene-based moiré structure can be realized by optimizing the thickness of the dielectric spacer.
Fig. 6. CD spectra of the proposed structure with a thin dielectric spacer. The thickness ranges from 10 nm to 50 nm. The CD spectrum of the structure without the interlayer is used for the contrast. In simulation, other parameters are the same as that of the designed structure without the interlayer.
As mentioned above, surface plasmon polaritons (SPPs) can be excited in the graphene-dielectric interfaces by the MIR light. In analogy to the moiré chiral metamaterial based on the Au nanohole arrays at visible wavelengths, the dielectric spacer between two layers of graphene nanoribbons enables to induce the internal SPPs [18,37]. The near-field coupling between SPPs and internal SPPs is modulated by the thickness of spacer layer. It plays a vital role in enhancing chiroptical response. It is demonstrated that the near-field coupling and thus CD of the designed structure are tunable by changing the thickness of d, as shown in Fig. 6. To reveal the effect of near-field coupling further, we make the analytical fitting for the simulated transmission spectra of the designed structure under the RCP and LCP illumination, respectively. The transmission spectra are obtained with the thickness of 20 nm in the simulation. The simulated transmission spectra under different circular polarization illumination are drawn by black dotted lines in Figs. 7(a) and 7(b). When the incident light is RCP, the transmission spectrum has an obviously asymmetric dip at 22.06 THz, corresponding to the strongest chiroptical response. In contrast, the transmission dip has relatively small amplitude and the asymmetry of dip largely deceases under LCP illumination, as shown in Fig. 7(b). It indicates that the near-field coupling between SPPs and internal SPPs under RCP illumination is stronger than under LCP illumination. As a result of the polarization-dependent near-field coupling, the differential transmission and thus the enhanced chiroptical response are realized at the coupling wavelength.
Fig. 7. (a) Simulated transmission spectra of the graphene-based moiré structure with the dielectric spacer and analytical fitting when the incident light is RCP. (b) Simulated transmission spectra and analytical fitting when the incident light is LCP. The thickness of the dielectric spacer is 20 nm.
In order to discuss the origin of near-field coupling clearly, the x-z cross-sectional views of the electric fields under RCP and LCP illuminations are shown in Figs. 8(a) and 8(b), respectively. The wavelength of incident light is 13.6 µm, corresponding to the transmission dip in Fig. 7. The locations of double-layer graphene nanoribbons are marked in white. It can be seen that the intensities of graphene plasmonic modes for RCP light are stronger than that of for LCP. The internal SPPs obviously appear in the dielectric spacer under RCP incident light, as shown in Fig. 8(a). Compared with RCP illumination, the intensities of internal SPPs in the spacer significantly decrease under LCP illumination. Moreover, the x-z cross-sectional views of the z component of electric fields in Figs. 8(c) and 8(d) correspond to RCP and LCP, respectively. The plus and minus sighs are added to illustrate the positive and negative surface charges. As shown in Figs. 8(c) and 8(d), the positive and negative surface charges are distributed on the double-layer graphene nanoribbons, respectively. It is indicated that the lowest-order dipole modes (bright modes) supported by the SPPs are excited on the lower-layer and upper layer graphene nanoribbons. Similarly, the high-order dipole modes, which are corresponding to the dark modes, are shown by plus/minus signs in the dielectric spacer [38]. In order to show the dark modes clearly, the black dotted rectangles are added to highlight in Figs. 8(c) and 8(d). The bright modes supported by the SPPs both are excited by two circular polarization lights on double-layer graphene. However, the strong dark modes in the dielectric spacer, which are supported by the internal SPPs, are excited under RCP illumination at 13.6 µm. When the incident light is LCP light, the intensities of the dark modes significantly decrease, as shown in Fig. 8(d). The polarization-dependent dark modes couple with the bright modes to lead to the asymmetric transmission under two circularly polarized lights illumination. Thus, introducing the polarization-sensitivity near-field coupling can enhance the transfer of the structural chirality to the optical chirality in the graphene-based moiré structure with the interlayer.
Fig. 8. (a) X-z cross-sectional view of the simulated electric field E in the designed structure under RCP light illumination; (b) under LCP light illumination. (c) X-z cross-sectional view of the z component of electric fields Ez under RCP light illumination; (d) under LCP light illumination. The wavelength incident light is 13.6 µm in this simulation. The thickness of the dielectric interlayer is 20 nm.
As depicted in Figs. 7(a) and 7(b), the transmission spectra of the designed structure with the interlayer appears an evident asymmetric dip. The near-field coupling between the bright modes and dark modes is dependent on the polarization state of the incident light. The Fano resonances have been proved as a result of the spectral and spatial overlap between a bright mode and a dark mode in the plasmonic systems [39]. The spectrum of Fano resonance is an asymmetric line shape and follows the product of a symmetric resonance (σs) and an asymmetric resonance (σa) [39]:
The line shape of symmetric resonance (σs) can be fitted as follows:
Table 1. Parameters for the Fano fittings in Fig. 7
4. Conclusion
In summary, we propose a graphene-based moiré structure with the strong chiroptical response in the MIR spectral region. This structure with the moiré patterns is formed by stacking two layers of graphene nanoribbons with a relative rotation angle. Due to the dependence on the relative in-plane rotation and Fermi level of the graphene, the chiroptical response can be precisely controlled by the rotation angle and bias voltage. The value of CD is high as 4.82 deg at 13.8 µm when the rotation angle is 15° and the Fermi level is 0.7 eV. We have explained the rotation-dependent chiroptical properties of the moiré structure by discussing the electron current distributions in two layers of graphene nanoribbons. Further, we introduce the dielectric spacer between two layers of graphene in the plasmonic moiré structure. The strong SPPs are excited in the graphene-dielectric interfaces and the internal SPPs can be induced in the dielectric spacer. The near-field coupling between two plasmonic modes plays a vital role on enhancing and tuning the chiroptical response. It is demonstrated that the near-field coupling is highly sensitive to the thickness of spacer and thus enable modulation of the chiroptical response. With the spacer thickness of 20 nm, the high value of CD is 5.94 deg at 13.6 µm. According to the good match of the simulation results and analytical fittings, it indicates that the chiral moiré structure with the spacer can support the polarization-sensitive Fano resonance, resulting in an enhanced chiroptical response. Further, the spacer between two layers of graphene may be set to different dielectrics to be promising for the sensor. The double-layer graphene with the dielectric spacer can be fabricated by two independent transfers of the single-layer graphene. Two layers of graphene nanoribbons are patterned respectively [40]. The graphene transfer techniques can control the positions and twist angles [4]. The graphene-based plasmonic structure with moiré patterns provides a new way for realizing giant optical activity in the MIR spectral region. The magnetic resonances of graphene-based plasmonic structure may be used for enhancing the chiroptical response in future research. With the strong chiroptical response and high tunability, the plasmonic moiré structure will be potential for tunable polarizers, on-chip polarization imaging and molecular sensing in the MIR spectral region.
Funding
National Natural Science Foundation of China (11604377, 61775234); Qingdao National Laboratory for Marine Science and Technology (QNLM2016ORP0111); West Light Foundation of the Chinese Academy of Sciences (XAB2017B18); Natural Science Foundation of Shaanxi Province (2018JQ6067).
Disclosures
The authors declare no conflicts of interest.
References
1. V. K. Valev, J. J. Baumberg, C. Sibilia, and T. Verbiest, “Chirality and chiroptical effects in plasmonic nanostructures: fundamentals, recent progress, and outlook,” Adv. Mater. 25(18), 2517–2534 (2013). [CrossRef]
2. C. J. Kim, A. Sánchez-Castillo, Z. Ziegler, Y. Ogawa, C. Noguez, and J. Park, “Chiral atomically thin films,” Nat. Nanotechnol. 11(6), 520–524 (2016). [CrossRef]
3. Y. Kim, B. J. Yeom, O. Arteaga, S. J. Yoo, S. G. Lee, J. G. Kim, and N. A. Kotov, “Reconfigurable chiroptical nanocomposites with chirality transfer from the macro- to the nanoscale,” Nat. Mater. 15(4), 461–468 (2016). [CrossRef]
4. X. T. Kong, R. B. Zhao, Z. M. Wang, and A. O. Govorov, “Mid-infrared plasmonic circular dichroism generated by graphene nanodisk assemblies,” Nano Lett. 17(8), 5099–5105 (2017). [CrossRef]
5. C. Gautier and T. Bürgi, “Chiral N-Isobutyryl-cysteine Protected Gold Nanoparticles: Preparation, Size Selection, and Optical Activity in the UV-vis and Infrared,” J. Am. Chem. Soc. 128(34), 11079–11087 (2006). [CrossRef]
6. X. L. Ma, W. B. Pan, C. Huang, M. B. Pu, Y. Q. Wang, B. Zhao, J. H. Cui, C. T. Wang, and X. A. Luo, “An active metamaterial for polarization manipulating,” Adv. Opt. Mater. 2(10), 945–949 (2014). [CrossRef]
7. Y. Zhao, M. A. Belkin, and A. Alù, “Twisted optical metamaterials for planarized ultrathin broadband circular polarizers,” Nat. Commun. 3(1), 870 (2012). [CrossRef]
8. Y. M. Liu and X. Zhang, “Metamaterials: a new frontier of science and technology,” Chem. Soc. Rev. 40(5), 2494–2507 (2011). [CrossRef]
9. A. Ben-Moshe, B. M. Maoz, A. O. Govorov, and G. Markovich, “Chirality and chiroptical effects in inorganic nanocrystal systems with plasmon and exciton resonances,” Chem. Soc. Rev. 42(16), 7028–7041 (2013). [CrossRef]
10. X. Lan and Q. B. Wang, “Self-assembly of chiral plasmonic nanostructures,” Adv. Mater. 28(47), 10499–10507 (2016). [CrossRef]
11. M. K. Gonokami, N. Saito, Y. Ino, M. Kauranen, K. Jefimovs, T. Vallius, J. Turunen, and Y. Svirko, “Giant Optical activity in quasi-two-dimensional planar nanostructures,” Phys. Rev. Lett. 95(22), 227401 (2005). [CrossRef]
12. M. Esposito, V. Tasco, M. Cuscunà, F. Todisco, A. Benedetti, I. Tarantini, M. D. Giorgi, D. Sanvitto, and A. Passaseo, “Nanoscale 3D chiral plasmonic helices with circular dichroism at visible frequencies,” ACS Photonics 2(1), 105–114 (2015). [CrossRef]
13. A. Kuzyk, R. Schreiber, Z. Y. Fan, G. Pardatscher, E. M. Roller, A. Högele, F. C. Simmel, A. O. Govorov, and T. Lied, “DNA-based self-Assembly of chiral plasmonic nanostructures with tailored optical response,” Nature 483(7389), 311–314 (2012). [CrossRef]
14. K. Toyoda, K. Miyamoto, N. Aoki, R. Morita, and T. Omatsu, “Using optical vortex to control the chirality of twisted metal nanostructures,” Nano Lett. 12(7), 3645–3649 (2012). [CrossRef]
15. D. Barada, G. Juman, I. Yoshida, K. Miyamoto, S. Kawata, S. Ohno, and T. Omatsu, “Constructive spin-orbital angular momentum coupling can twist materials to create spiral structures in optical vortex illumination,” Appl. Phys. Lett. 108(5), 051108 (2016). [CrossRef]
16. S. Vignolini, N. A. Yufa, P. S. Cunha, S. Guldin, I. Rushkin, M. Stefik, K. Hur, U. Wiesner, J. J. Baumberg, and U. Steiner, “A 3D optical metamaterial made by self-assembly,” Adv. Mater. 24(10), OP23–OP27 (2012). [CrossRef]
17. Z. L. Wu and Y. B. Zheng, “Moiré chiral metamaterials,” Adv. Opt. Mater. 5(16), 1700034 (2017). [CrossRef]
18. Z. L. Wu, X. D. Chen, M. S. Wang, J. W. Dong, and Y. B. Zheng, “High-performance ultrathin active chiral metamaterials,” ACS Nano 12(5), 5030–5041 (2018). [CrossRef]
19. J. K. Gansel, M. Thiel, M. S. Rill, M. Decker, K. Bade, V. Saile, G. V. Freymann, S. Linden, and M. Wegener, “Gold helix photonic metamaterial as broadband circular polarizer,” Science 325(5947), 1513–1515 (2009). [CrossRef]
20. S. E. Zhou, P. T. Lai, G. H. Dong, P. Li, Y. X. Li, Z. Zhu, C. Y. Guan, and J. H. Shi, “Tunable chiroptical response of graphene achiral metamaterials in mid-infrared regime,” Opt. Express 27(11), 15359–15367 (2019). [CrossRef]
21. C. W. Cao, L. B. Wei, S. Mao, and Wang, “Tuning of giant 2D-chiroptical response using achiral metasurface integrated with graphene,” Opt. Express 23(14), 18620–18629 (2015). [CrossRef]
22. T. Cao, C. W. Wei, and Y. Li, “Dual-band strong extrinsic 2D chirality in a highly symmetric metal-dielectric-metal achiral metasurface,” Opt. Mater. Express 6(2), 303–311 (2016). [CrossRef]
23. M. R. Querry, Optical constants of minerals and other materials from the millimeter to the ultraviolet (The Department of Commerce International Trade Center Bookstore, 1987).
24. W. Kaiser, W. G. Spitzer, R. H. Kaiser, and L. E. Howarth, “Infrared Properties of CaF2, SrF2, and BaF2,” Phys. Rev. 127(6), 1950–1954 (1962). [CrossRef]
25. G. G. Zheng, Y. Y. Chen, L. B. Bu, L. H. Xu, and W. Su, “Waveguide-coupled surface phonon resonance sensors with super-resolution in the mid-infrared region,” Opt. Lett. 41(7), 1582–1585 (2016). [CrossRef]
26. S Adachi, Optical constants of crystalline and amorphous semiconductors (Springer, 1997).
27. F. H. L. Koppens, D. E. Chang, and F. J. G. de Abajo, “Graphene plasmonics: a platform for strong light–matter interactions,” Nano Lett. 11(8), 3370–3377 (2011). [CrossRef]
28. W. L. Gao, J. Shu, C. Y. Qiu, and Q. F. Xu, “Excitation of plasmonic waves in graphene by guided-mode resonances,” ACS Nano 6(9), 7806–7813 (2012). [CrossRef]
29. S. Y. Xiao, T. Wang, T. T. Liu, X. C. Yan, Z. Li, and C. Xu, “Active modulation of electromagnetically induced transparency analogue in terahertz hybrid metal-graphene metamaterials,” Carbon 126, 271–278 (2018). [CrossRef]
30. Z. Y. Fang, S. Thongrattanasiri, A. Schlather, Z. Liu, L. L. Ma, Y. M. Wang, P. M. Ajayan, P. Nordlander, N. J. Halas, and F. J. García de Abajo, “Gated tunability and hybridization of localized plasmons in nanostructured graphene,” ACS Nano 7(3), 2388–2395 (2013). [CrossRef]
31. X. Y. He, “Tunable terahertz graphene metamaterials,” Carbon 82, 229–237 (2015). [CrossRef]
32. R. Hao, Z. W. Ye, X. L. Peng, Y. J. Gu, J. Y. Jiao, H. X. Zhu, W. E. I. Sha, and E. P. Li, “Highly efficient graphene-based optical modulator with edge plasmonic effect,” IEEE Photonics J. 10(3), 1–7 (2018). [CrossRef]
33. Z. L. Wu, Y. R. Liu, E. H. Hill, and Y. B. Zheng, “Chiral metamaterials via moiré stacking,” Nanoscale 10(38), 18096–18112 (2018). [CrossRef]
34. M. Esposito, V. Tasco, M. Cuscunà, F. Todisco, A. Benedetti, I. Tarantini, M. D. Giorgi, D. Sanvitto, and A. Passaseo, “Nanoscale 3D chiral plasmonic helices with circular dichroism at visible frequencies,” ACS Photonics 2(1), 105–114 (2015). [CrossRef]
35. J. Christensen, A. Manjavacas, S. Thongrattanasiri, F. Koppens, and J. G. de Abajo, “Graphene plasmon waveguiding and hybridization in individual and paired nanoribbons,” ACS Nano 6(1), 431–440 (2012). [CrossRef]
36. H. S. Chu and C. H. Gan, “Active plasmonic switching at mid-infrared wavelengths with graphene ribbon arrays,” Appl. Phys. Lett. 102(23), 231107 (2013). [CrossRef]
37. R. Ortuño, C. García-Meca, F. J. Rodríguez-Fortuño, and J. Martí, “A. Role of surface plasmon polaritons on optical transmission through double layer metallic hole arrays,” Phys. Rev. B 79(7), 075425 (2009). [CrossRef]
38. S. X. Xia, X. Zhai, L. L. Wang, and S. C. Wen, “Plasmonically induced transparency in double-layered graphene nanoribbons,” Photonics Res. 6(7), 692–702 (2018). [CrossRef]
39. B. Gallinet and O. J. F. Martin, “Influence of electromagnetic interactions on the line shape of plasmonic Fano resonances,” ACS Nano 5(11), 8999–9008 (2011). [CrossRef]
40. D. Rodrigo, A. Tittl, O. Limaj, F. J. G. de Abajo, V. Pruneri, and H. Altug, “Double-layer graphene for enhanced tunable infrared plasmonics,” Light: Sci. Appl. 6(6), e16277 (2017). [CrossRef]
