Chiral graphene plasmonic Archimedes’ spiral nanostructure with tunable circular dichroism and enhanced sensing performance

Hengjie Zhou; Shaojian Su; Shaojian Su; Huanxi Ma; Zeyang Zhao; Zhili Lin; Weibin Qiu; Weibin Qiu; Weibin Qiu; Pingping Qiu; Pingping Qiu; Beiju Huang; Beiju Huang; Qiang Kan; Qiang Kan

doi:10.1364/OE.403041

1. Introduction

Circularly polarized light (CPL), of which the electric field vector travels along a clockwise or anticlockwise helical trajectory [1], has significant applications in the fields of magnetic recording [2], quantum computation [3–5], and circular dichroism (CD) spectroscopy [6–9]. By measuring the differential absorption of left-handed circularly polarized (LCP) light and right-handed circularly polarized (RCP) light, CD spectroscopy is used to characterize chiral molecules, which have been widely researched in DNA [8,10], single chiral nanocrystals [11], amplification of chiroptical activity of chiral biomolecules [12], charge transfer in inorganic complexes and conformational changes in biomolecules [13]. However, conventional semiconductors lack intrinsic chirality, which is difficult to distinguish the two polarizations of CPL and hard to cause excellent optical properties such as CD and optical rotatory dispersion (ORD) [1]. Due to the strong localized surface plasmon resonances (LSPRs) under the excitation of LCP light or RCP light and obvious CD characteristics [1,7,14], plasmonic chiral nanostructures are natural candidates for applications in optical materials and chiral sensing [15–18]. However, due to the obstacle in varying permittivity of noble metal and the large damping rate [19–21], the quality of LSPR produced by chiral plasmonic metamolecules (CPMs) composed of noble metal nanoparticles (NPs) is degraded [22] and the resonant wavelength (RW) is hard to tune when the nanostructure of CPMs is constructed [1,11,23], which restrict the flexible applications in the fields of biochemical sensing, photodetector, nanoantenna, and chirality sensor [12,15,24–26].

Graphene, a two-dimensional material made up of sp² hybridized carbon atoms, demonstrates remarkable electromagnetic properties and peculiar behaviors in electronics due to the unique electronic properties. These exciting behaviors induce research groups world widely to investigate more surpassing properties with distinctive methods, such as higher confinement and tunability of the electromagnetic (EM) fields [27–29]. Due to the unique Dirac conical band structure, graphene plasmons (GPs) display lower damping, significant wave localization for certain frequencies and intrinsically more prominent ultra-high confinement of EM fields than noble metals [30–33]. Furthermore, monolayer graphene behaves like a metallic film under the illumination of the CPL, which enables it to generate the working frequency ranges from near-infrared (NIR) to terahertz (THz) and results in that GPs in the infrared region have practical applications of surface-enhanced infrared absorption (SEIRA) [34], high-speed electronic [35] (i.e., field-effect transistors), photonic devices [36] (i.e., low noise sensors and THz oscillators) and light trapping [37,38]. In addition, compared with the noble metal plasmonic molecules (PMs), graphene PMs have the decisive advantage that surface conductivity can be manipulated by adjusting chemical potential (Fermi energy) of graphene, leading to that graphene PMs have flexibility and high quality [39,40]. However, the research of CD properties and EM behaviors of CPMs based on graphene material under the coupling with the two polarizations of CPL is still insufficient. Therefore, the studies of chiral graphene plasmonic molecules are of great significance for the applications of graphene-based optical detections and sensors.

In this paper, the low damping graphene plasmon is employed to research CD performance, and the optimized parameters are used to design an enhanced refractive index sensor. We propose a chiral graphene plasmonic Archimedes’ spiral (AS) nanostructure, which presents a right-hand (RH) mode and a left-hand (LH) mode under the excitation of RCP light and LCP light, respectively. Moreover, by adjusting the number of turns and chemical potential of the AS, the CD performance can be improved and the RW can be effectively tuned. Using the optimized plasmonic nanostructure, the sensitivity can reach 7000nm/RIU, and the corresponding FOM is 68.75. The simulation results demonstrate that the proposed plasmonic nanostructure has potential applications in the fields of photodetectors and optical sensors.

2. Simulated methods and models

In the Cartesian coordinate system, the equation of AS can be expressed by

(1)$$\left\{ \begin{array}{l} x = (a + b\theta )cos\theta \\ y = (a + b\theta )sin\theta \end{array} \right.,$$

where a=0 is the initial spiral radius, b=(g + w)/2π is the spiral growth rate, where g is the gap between two adjacent circles and w is the width of the graphene spiral, and θ is the angle of rotation of the spiral. Simultaneously, θ₀=0 indicates the initial angle and θ_f=2πN represents the final angle, where N=3.01 stands for the number of turns. Furthermore, θ varies from θ₀ to θ_f. As demonstrated in Fig. 1(a), the graphene AS is placed on a calcium fluoride (CaF₂) substrate. In the mid-infrared (MIR) region, the broadband transparency of calcium fluoride can eliminate the substrate phonon effect, which prevents graphene plasmonic field enhancement from decreasing drastically. In addition, the two polarizations of CPL (i.e., LCP and RCP) are respectively incident from the top for chiral graphene PM, and the top view of the graphene PM nanostructure is schematically presented in Fig. 1(b).

Fig. 1. (a) Schematic of the graphene nanostructure. Here, the thickness of graphene H is set as 0.334 nm, w=40 nm is the width of spiral, and g=30 nm, which stands for the gap between two adjacent circles. The graphene spiral is laid flat on a CaF₂ substrate. Simultaneously, the spiral is surrounded by the air. (b) The top view of the graphene spiral nanostructure. The chemical potential of graphene AS is labelled as μ_c.

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In our model, graphene, composed of a single atomic layer of carbon atoms arranged in a hexagonal crystal lattice, is modeled by an equivalent relative permittivity. Here, the equivalent relative permittivity can be described by [19]

(2)$$\varepsilon = 1 + \frac{{i{\sigma _g}{\eta _0}}}{{{k_0}H}},$$

where H=0.334nm described in Fig. 1(a) stands for the equivalent thickness when monolayer graphene is treated as a thin film, η₀=377Ω corresponds to the impedance of free space, k₀=2π/λ, which represents the wavenumber of light in free space, and σ_g is the complex surface conductivity of graphene, which decides the equivalent relative permittivity (ε). Furthermore, the surface conductivity (σ_g) is modeled by Kubo’s formulae and contains two contributions (i.e., interband electron-electron transition and intraband electron-photon scattering), which can be described as the equations below [20,41]:

(3)$${\sigma _g} = {\sigma _{intra}} + {\sigma _{inter}},$$

(4)$${\sigma _{intra}} = \frac{{2{e^2}{k_B}T}}{{\pi {\hbar ^2}}} \cdot \frac{i}{{\omega + i{\tau ^{ - 1}}}}\left[ {ln(2cosh(\frac{{{\mu_c}}}{{{k_B}T}}))} \right],$$

(5)$${\sigma _{inter}} = \frac{{{e^2}}}{{4\hbar }}\left[ {\frac{{sinh(\frac{{\hbar \omega }}{{2{k_B}T}})}}{{cosh(\frac{{{\mu_c}}}{{{k_B}T}}) + cosh(\frac{{\hbar \omega }}{{2{k_B}T}})}} - \frac{i}{{2\pi }}ln\frac{{{{(\hbar \omega + 2{\mu_c})}^2}}}{{{{(\hbar \omega - 2{\mu_c})}^2} + {{(2{k_B}T)}^2}}}} \right],$$

where σ_intra stands for the intraband electron-photon scattering, σ_inter corresponds to the interband electron-electron transition, e represents the charge of an electron, ћ is the reduced Planck constant, T=300K represents the absolute temperature at room temperature, k_B is the Boltzmann’s constant, ω=2πυ is the angular frequency, ${\mu _c} = \hbar {\upsilon _f}\sqrt {\pi n}$ corresponds to the chemical potential of the doped graphene, where υ_f =c/300 is the Fermi velocity and n stands for the carrier density, τ=μμ_c/eυ2 f corresponds to the momentum relaxation time (inverse of the electron-phonon scattering rate), where μ=10000cm² V^-1S^-1 stands for the carrier mobility of graphene. For simplicity, the momentum relaxation time (τ) is set as a constant value (i.e., τ=0.5ps). Herein, it is consistent with the ballistic transport features of graphene. Moreover, an almost purely imaginary conductivity of graphene may be provided by the significant features of ultrahigh carrier mobility and ballistic transport, and it effectively produces a low-loss inductive atomic surface. This moment, graphene acts like a lossless reactive FSS at RF, even the surface does not need to pattern [42]. Furthermore, the free path of electrons has been measured up to 500nm at room temperature, moreover, larger than 4μm can be measured at low temperature [42,43]. From these equations, the surface conductivity of graphene is sensitively dependent on the chemical potential, which means that σ_g can be operated by doping profile [36,44] (i.e., type or density of carriers), or providing an isotropic scalar surface conductivity via external static electric field [36], or using carboxylation and thiolation to realize the chemical surface modification [45], or providing anisotropic and tensor surface conductivity by an external static magnetic field [46].

Herein, the EM fields and spectra are obtained by utilizing COMSOL Multi-Physics, which is a commercial finite element method (FEM) based software. Simultaneously, the extinction spectra are collected by calculating the extinction cross-section σ_ext of chiral graphene PM nanostructure. Nevertheless, the extinction cross-section consists of two terms. The first term is the absorption cross-section σ_abs, and the second term is the scattering cross-section σ_sc, which are given by [19,21]

(6)$${\sigma _{ext}} = {\sigma _{abs}} + {\sigma _{sc}},$$

(7)$${\sigma _{abs}} = \frac{1}{{{I_0}}}\int\!\!\!\int\!\!\!\int {QdV} ,$$

(8)$${\sigma _{sc}} = \frac{1}{{{I_0}}}\int\!\!\!\int {(\overrightarrow n \cdot {{\vec{S}}_{sc}})dS} ,$$

where I₀ represents the incident intensity, Q corresponds to the power loss density in the chiral graphene molecule, $\vec{n}$ represents the normal vector that points outwards from the graphene plasmonic nanocluster, ${\vec{S}_{sc}}$ stands for the scattered intensity electromagnetic energy intensity, the integral demonstrated in Eq. (7) is taken over its volume and the integral presented in Eq. (8) is taken over the closed surface of the scatter. In addition, the Perfectly Matched Layer (PML) is arranged around the nanostructure to further prevent the reflected light fields and avoid the interference to the calculation results. Furthermore, the thickness of graphene monolayer is meshed at least 5 layers and the maximum mesh size in each layer is set as 2nm to ensure the accuracy of calculation results. Under the excitation of an external light field, surface plasmon polaritons (SPPs) produced on the surface of graphene PM have excellent characteristics of local field enhancement effect and strong binding capacity, which can constrain the EM field energy around the PM, resulting in the gradual decrease of the EM field energy in the distance from the PM to PML. Therefore, the mesh size from the chiral graphene molecule to PML increases gradually for the convenience of calculation.

3. Simulation results and discussions

3.1 Circular dichroism of graphene AS under LCP light and RCP light

The trajectory of the end point of the light vector is a circle when circularly polarized light (CPL) propagate. Therefore, during the investigation of the interaction between the LCP light or the RCP light and the chiral molecules, three-dimensional (3D) chiral molecular nanostructures, such as two stacked L-shaped [14], active chiral plasmonic dimer stack [47] and single chiral nanocrystals [11], are used as a preferred object, due to the unique advantage that the rotational sense of the induced electric dipole moments can be demonstrated. However, non-stacked chiral PMs also present outstanding CD properties [1]. In this paper, we investigate the mechanism of the interaction between the graphene AS nanostructure and two polarizations of CPL. In this context, the number of turns of AS is set as 3.01 and the gap between two adjacent circles and the width of AS are fixed to 30nm and 40nm, respectively. Simultaneously, the chemical potential of AS is 0.5eV. Figure 2 depicts the calculated spectra of extinction cross-section for LCP light and RCP light incident on an AS nanostructure, and the normalized electronic field distributions (|E|) in corresponding plasmonic resonance peaks. It is obvious that one plasmonic resonance peak is observed each for LCP and RCP excitation from 8.6μm to 9.2μm. For the LCP excitation, the plasmonic resonance peak is marked as A, as presented in Fig. 2(a). Simultaneously, Fig. 2(b) illustrates the plasmonic resonance peak labeled as B. Because the proposed chiral graphene nanostructure is laid flat on a level surface and the thickness of the PM is analogous to the height of a carbon atom, the differences in working wavelength and plasmonic resonance peak are minute under the excitation of two polarizations of CPL. However, it is obvious that the AS has a right-hand (RH) character at higher energy (shorter wavelength) and a left-hand (LH) character at lower energy (longer wavelength) by observing the CD spectrum, which is defined as the difference of the extinction cross-section for the LCP and the RCP excitation, as sketched in Fig. 2(c). Simultaneously, in order to understand the mechanism of interaction between the graphene AS and the two different polarizations of CPL, the electronic field distributions (|E|) of peak A and peak B are demonstrated in Fig. 2(d). For convenience, we define the mode of plasmonic resonance peak A under the LCP excitation as LH mode and the mode of plasmonic resonance peak B under the RCP excitation as RH mode. Since the graphene AS is a special structure that is equidistantly expanded in each rotation period, a coupling and hybridization effect of plasmons exists between the adjacent turns of the spiral, making the EM field of spiral enhance sharply. By comparing the near-field distributions of the LH mode and the RH mode, the difference is mainly concentrated in the EM field distribution at the center of the spiral. For the LH mode, the electromagnetic hot spots distributed in the center of the spiral are significantly enhanced. Strikingly, the electromagnetic hot spots at the same position in the RH mode are weakened. It can be seen that the LCP light and the RCP light have significantly different manipulation capabilities for the initial position of the AS. Intriguingly, we can use the proposed graphene AS as a photodetector to distinguish two different polarizations of CPL by this characteristic. This indicates that the proposed chiral graphene plasmonic AS nanostructure is of great significance in the field of optical detection.

Fig. 2. Simulated spectra of extinction cross-section and normalized electronic field distributions (|E|) for AS nanostructure. (a) The extinction cross-section spectrum for LCP excitation. An obvious plasmonic resonance peak exists in the working wavelength. (b) The extinction cross-section spectrum for RCP excitation. (c) Calculated CD spectrum. Herein, the CD spectrum is defined as the difference of the extinction cross-section for LCP and RCP excitation. The CD spectrum can clearly see that a RH character plays a dominant role at higher energy and a LH character exists at lower energy. (d) The normalized electronic field distributions (|E|) for peak A and B. Different electromagnetic hot spot distributions display in the center of AS under the excitation of LCP light and RCP light.

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3.2 Effect of the number of turns of AS on CD

Next, our attention is switched to the effect of the number of turns of AS on the two modes and CD under the excitation of two polarizations of CPL, which is significant for obtaining outstanding CD. LSPR is sensitive to the size and shape of NP. For instance, at the tip of a NP or where the radius of curvature is smaller, the density of free charge is much greater than other areas, resulting in a strong local surface electromagnetic field distribution, which is analogous to the lightning rod effect. Here, according to the relationship between the final angle (θ_f) and the number of turns (i.e., θ_f=2πN), the adjustment in the number of turns of AS is equivalent to operating the length of the outermost circle of AS. In this context, the AS nanostructure remains constant at the part where N is less than 3.01, as shown in Fig. 3(a). Moreover, the gap between two adjacent circles and the width of AS are fixed to 30nm and 40nm, respectively. Simultaneously, the chemical potential of AS sets as 0.5eV. Figures 3(b) and 3(c) depict the calculated spectra of extinction cross-section with the variation of N under the excitation of LCP light and RCP light. It can be seen that a phenomenon of red shift in spectra occurs as N increases. This is because the increase in the length of the spiral leads to an increase in the area of plasmon interaction and hybridization, which eventually leads to the enhancement of the localized electromagnetic fields. This is confirmed in the extinction spectra. Moreover, the increase of the length is more effective at absorbing lower frequencies. Simultaneously, in order to investigate the effect of the manipulation for the length of the spiral on the LH mode and the RH mode, we make a deeper analysis of the extreme cases (i.e., N=3.01 and N=3.04) in the extinction spectra. For the excitation by the LCP light, the two plasmonic resonance peaks from higher energy to lower energy are sequentially marked as a and b. Similarly, two peaks under the excitation of RCP light are labeled as c and d, respectively. Simultaneously, the simulated CD spectra reveal the CD under the manipulation of N, as presented in Fig. 3(d). With the increase of N, the absolute value of the CD appears an upward tendency, which means that a more excellent CD can be obtained by operating N. Moreover, comparing the CD spectra at N=3.01 and N=3.04, the absolute value of CD at N=3.04 is greatly enhanced, which is attributed to the enhancement of difference in absorption between the excitation of LCP light and RCP light with the increase in N. Figure 3(e) demonstrates the normalized electronic field distributions at the peak a, b, c and d. It is obvious that the two modes can maintain their unique characteristics when N is operated, which is of great significance for tuning the plasmonic resonance frequency by adjusting N. In addition, due to the increase of length in the spiral, the distributions of electromagnetic hot spots have a slight deviation in the direction of the spiral extending outwards. In this context, our proposed chiral graphene nanostructure might find significant application in the field of circularly polarized light photodetector.

Fig. 3. Simulated spectra of extinction cross-section and normalized electronic field distributions (|E|) for the varying number of turns. (a) The schematic geometry of the proposed nanostructure when N changes. (b) The extinction cross-section spectra with the variation of N under the excitation of LCP light. With the increase of N, the plasmonic resonance peak increases gradually, and the resonance frequency appears a phenomenon of red shift. (c) The extinction cross-section spectra with the variation of N under the excitation of RCP light. (d) Simulated CD spectra with the increase of N. It is obvious that the absolute value of the CD increases when the variation of N in the region from 3.01 to 3.04. (e) The normalized electronic field distributions (|E|) at the labeled plasmonic resonance peaks in the extinction spectra. When N varies from 3.01 to 3.04, the LH mode and the RH mode are unchanged, but the distributions of electromagnetic hot spots appear slight deviation.

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3.3 Tunability of the plasmon frequency by the chemical potential of graphene

The chemical potential of graphene can be flexibly controlled by electrostatic doping or chemical doping to manipulate the conductivity of graphene, and then tunes the resonance frequency of graphene PMs, which is extremely difficult in noble metals. Herein, the research on the operation of the chemical potential of the chiral graphene molecule has extremely significance for this structure in the fields of optical devices and biosensing. Due to the superior CD, N=3.04 is preferentially used as the parameter of the number of turns of the AS. In addition, the gap between two adjacent circles and the width of AS are still fixed at 30 nm and 40 nm, respectively. The adjustment of the chemical potential of graphene is usually achieved by two methods of electrostatic and chemical doping. For electrostatic doping, the chiral graphene nanostructure can be p/n-doped under negative/positive electrostatic bias, which can be manipulated by a top gate configuration and providing the appropriate top gate voltage. The injection of charge carriers deviates the graphene chemical potential from the Dirac point, allowing the surface conductivity to be adjusted. For chemical doping, it can use carboxylation and thiolation to realize the chemical surface modification. In experiment, the graphene nanostructure is exposed to nitrogen dioxide or nitric acid vapor, thereby manipulating the change of carrier concentration. The carrier concentration is expressed by the equation below [42].

(9)$${n_s} = \frac{2}{{\pi {\hbar ^2}\upsilon _f^2}}\int_0^\infty {\varepsilon [{{f_d}(\varepsilon - {\mu_c}) - {f_d}(\varepsilon + {\mu_c})} ]} d\varepsilon ,$$

where ћ stands for the reduced Planck constant, υ_f is the Fermi velocity, ε is the energy and f_d=1/{1+exp[(ε-μ_c)/(k_BT)]} represents Fermi-Dirac distribution. The manipulation of carrier concentration makes the chemical potential tunable, leading to the widespread use of graphene in the fields of THz optoelectronics applications [48], adaptive camouflage systems [49], acoustic plasmon resonator [50] and biosensing [51]. We can adjust the chemical potential from 0.502eV to 0.518eV by the above approaches, and calculate the extinction spectra, as demonstrated in Fig. 4. Figures 4(a) and 4(b) plot the calculated spectra of two polarizations of CPL on chiral nanostructure with tunable chemical potential. With the increase of chemical potential, an obvious phenomenon of blue shift occurs in spectra, which is attributed to the fact that the plasmon frequency is proportional to the chemical potential, as shown the equation below [31,37].

(10)$${\omega _{pl}} = \sqrt {\frac{{{e^2}{\mu _c}q}}{{2\pi {\hbar ^2}{\varepsilon _0}{\varepsilon _r}}}} ,$$

where q is the wave vector, ε_r=1/2·(ε_CaF2+ε_air) stands for the average dielectric constant of the environment around chiral graphene molecule, where ε_CaF2 and ε_air represent the dielectric constant of CaF₂ and Air, respectively. As the chemical potential increases, the carrier concentration increases, resulting in a decrease in the kinetic inductance of graphene. In this context, we propose chiral nanostructure that can be applied to selectively adjust the plasmonic resonance frequency by manipulating the chemical potential, which makes the nanostructure more flexible in application. With the increase of chemical potential, the absorption of graphene nanostructure increases dramatically due to the less loss of graphene SPR and the consequence of promotion of carrier density, which results that plasmons avoid being quenched through interacting with electron-hole pairs (Landau damping) [20,52]. Simultaneously, the graphene reveals the behavior of metal due to the forbidden interband transitions by the Pauli blocking. Moreover, the enhancement of plasmonic oscillation is realized with the increase of the number of allowed virtual electron-hole pair transitions [28]. Furthermore, coherently coupled nanostructures present significant drastic coupling, which enhances the extinction. Meanwhile, our attention is switch to the effect of adjustment of chemical potential on CD performance. Figure 4(c) displays the CD spectra with the operation of chemical potential. As the chemical potential increases, the CD spectra have an obvious phenomenon of blue shift, while the absolute value of CD improves slightly. Strikingly, the chemical potential mainly plays a role in manipulating the working wavelength on the chiral nanostructure, which has an important contribution to the application of adjusting the resonance frequency without changing other properties. Compared with noble metal PMs, the graphene PMs effectively solve the obstacle of non-tunable resonant wavelength, making the chiral graphene plasmonic AS nanostructure promising candidates for future biosensing applications.

Fig. 4. Simulated spectra of extinction cross-section and CD for AS nanostructure with the variation of chemical potential. (a and b) The calculated spectra with the variation of chemical potential under the excitation of two polarizations of CPL. The left panel represents the effects of the adjustment of the chemical potential under the LCP light, while the right panel depicts the manipulation under the RCP light. (c) Simulated CD spectra under the operation of chemical potential.

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3.4 Refractive-index sensing performance of the optimized chiral graphene PM

We now proceed to evaluate the sensing performance of this nanostructure as a sensor. Herein, the optimized parameters (i.e., N = 3.04 and μ_c= 0.518eV) are used to construct the nanostructure of sensor. Simultaneously, the g and w are fixed at 30nm and 40nm, respectively. Figure 5(a) demonstrates the environment around the nanostructure. Graphene PM is laid on a calcium fluoride substrate with a refractive index n₂=1.4. Under the excitation of LCP light, the extinction spectra with a refractive index n₁ from 1 to 1.03 are calculated, as depicted in Fig. 5(b). It is obvious that the shape of the spectrum is highly maintained with the variation of the environmental refractive index. Intriguingly, a remarkable phenomenon of red shift occurs when n₁ varies slightly, which indicates that the chiral nanostructure is a candidate for an outstanding refractive index sensor. Therefore, the relationship between the resonant wavelength and the environmental refractive index is considered, as presented in Fig. 5(c). The shift in RW per unit change of refractive index n₁ is calculated, and the calculated sensitivity is 7000 nm/RIU, which is higher than other nanostructures, such as graphene pentamer [19], quadrumer [53], and cut-out structure in a homogeneous gold ﬁlm [54]. In addition, the energy of RW as a function of environmental refractive index n₁, as depicted in Fig. 5(d). The definition of Figure of Merit (FOM) can more intuitively describe the sensing performance of the proposed structure as a sensor, and its value can be obtained by the ratio of the plasmon energy shift of each environmental refractive index unit change and the full width at half maximum (FWHM) of the spectral peak, which is written as the equation below [55].

(11)$$\textrm{FOM = }\frac{{{K_0}(\textrm{eVRI}{\textrm{U}^{ - 1}})}}{{\textrm{FWHM}(\textrm{eV})}},$$

where K₀ stands for the linear regression slope for the refractive index, which can be obtained in Fig. 5(d). Intriguingly, the calculated FOM is 68.75, which means that the proposed nanostructure has a potential application in the field of high-performance optical sensor.

Fig. 5. The refractive sensing effect in graphene AS nanostructure. (a) A schematic diagram of the surrounding environment of graphene nanostructure. (b) Calculated spectra with different n₁. (c) The curve of the wavelength of plasmonic resonance peak with the change of the refractive index of environment. The calculated nm/RIU is 7000. (d) The energy of resonance peaks as a function of the refractive index of surrounding environment. The calculated figure of merit is 68.75.

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4. Conclusions

In summary, the chiral graphene plasmonic Archimedes’ spiral nanostructure we proposed displays the left-hand mode and the right-hand mode under the excitation of two polarizations of CPL, respectively, and has an obvious CD response. As the number of turns of AS increases, the absolute value of CD can be significantly improved, and the two modes are not affected by the number of turns. In addition, the chemical potential of graphene can effectively tune the resonant wavelength and improve CD to a certain extent. Within the range of the geometric parameters researched, the optimized parameters are selected to research the sensing performance. The optimized plasmonic AS nanostructure has excellent refractive index sensing performance, where the sensitivity and the FOM reach 7000nm/RIU and 68.75 respectively. Our proposed chiral graphene plasmonic nanostructure might find important applications in the fields of optical detection and high-performance optical sensors.

Funding

Project for Cultivating Postgraduates’ Innovative Ability in Scientific Research of Huaqiao University (18013082026, 18014082040); Quanzhou City Science & Technology Program of China (2018C003); The open project of Fujian Key Laboratory of Semiconductor Materials and Applications (2019001); National key R&D Program of China (2018YFA0209000); Natural Science Fund of China (11774103).

Disclosures

The authors declare no conflicts of interest.

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Chiral graphene plasmonic Archimedes’ spiral nanostructure with tunable circular dichroism and enhanced sensing performance

Abstract

1. Introduction

2. Simulated methods and models

3. Simulation results and discussions

3.1 Circular dichroism of graphene AS under LCP light and RCP light

3.2 Effect of the number of turns of AS on CD

3.3 Tunability of the plasmon frequency by the chemical potential of graphene

3.4 Refractive-index sensing performance of the optimized chiral graphene PM

4. Conclusions

Funding

Disclosures

References

Cited By

Figures (5)

Equations (11)

Optics Express