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Abstract
In this manuscript, we propose a tunable multifunctional reflection polarization converter based on a graphene sheet, which is decorated by a π-shaped carved-hollow array. Our design can achieve the linear-to-linear (LTL) polarization conversion with 3.58 THz bandwidth and two linear-to-circular (LTC) polarization conversion bands with opposite handedness, which originates from the excitation of the three graphene surface plasmons (GSPs) modes. The LTL polarization converter can be switched to the LTC polarization converter by only changing the medium height. The performances of this multifunctional reflection polarization converter under different geometric parameters, incident angles, and electron scattering time of graphene are investigated. Besides, it is found that the broadband of LTL and double bands of LTC have the blue shift with increasing the Fermi energy (Ef) of graphene. Especially, the conversion between line-to-left-circular and line-to-right-circular polarization converter can be achieved by changing Ef. All the simulation results are conducted by finite element method, and our design has potential applications in integrated terahertz systems.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Terahertz wave manipulation is a technology with great potential, which is of great significance in the research and application of wireless communication, space exploration and other fields [1–4]. In recent years, manipulating the polarization state of terahertz waves has become a hot research in high resolution imaging, terahertz communication, remote sensing and other mid-infrared applications [5–7]. The traditional polarization converter is usually based on the birefringence effect generated by grating [8] and dichroic crystal [9] based on natural birefringent materials. The realization of polarization manipulation requires phase delay between two orthogonal components [10,11]. However, it is difficult for traditional devices to achieve nanometer size because of their large structure and long distance to achieve phase accumulation [12,13]. In this case, the metasurface provides a possible way for a highly integrated polarizer. As a kind of artificial subwavelength structure, metasurface makes up for the shortage of the traditional photoelectric element and becomes a hot research [14–18]. Surface plasmon can manipulate the amplitude, phase and polarization [19–21] of the electromagnetic wave in nanoscale and enhance the interaction between light and matter. This may provide scientific basis for the highly integrated miniaturization of nanoscale optoelectronic devices and the application of metamaterial design [22,23]. Due to its ultra-thin shape and efficient operation, several metasurface structures have been proposed to control the polarization of the terahertz electromagnetic waves such as linear-to-circular (LTC) [24–28], linear-to-linear (LTL) [29–31] and circular-to-circular [32,33] polarization conversion. Metasurface devices have developed a unique method to manipulate the polarization state of terahertz waves. However, most of these polarized operating devices are not dynamically adjustable.
Graphene is a single two-dimensional plane of carbon atoms packed tightly together. Novoselov et al. prepared graphene by mechanical stripping in 2004 [34] and found that graphene has excellent photoelectric properties. Graphene supports low loss surface plasmons in the mid-infrared region [35–37], which provides a possible way to manipulate the amplitude and phase of electromagnetic wave. The thickness of the graphene is ultra-thin and the electrical properties of graphene can be adjusted by doping concentration and bias voltage [38–42]. So far, the graphene metasurface has been extensively studied for tunable polarization control [38,44,45]. But, the multifunctional polarization converter, only based on one simple design of graphene metasurface, has been rarely reported.
Multifunctional polarization converters can achieve the switch between the LTL and LTC polarization conversion by changing the parameters of the metasurface [43,46,47]. Yao et al. proposed a multifunctional polarization converter based on the cross double-ellipse graphene patches, but the bandwidths of LTL and LTC polarization converters are both smaller than 1THz [43]. Guo et al. proposed the L-shaped and rectangular graphene patches to achieve LTL and LTC polarization conversion, respectively [46]. The LTC and LTL polarization converters were realized by the single-layer and double-layer graphene metasurfaces, respectively [47]. Our design can achieve broadband LTL polarization conversion and two LTC polarization conversion bands with opposite handedness by only changing the medium height.
In this paper, a tunable multifunctional reflective polarization converter is proposed, which is composed of a graphene sheet decorated by a π-shaped carved-hollow array. The LTL polarization conversion with 3.58 THz bandwidth can be achieved, and the polarization conversion ratio is larger than 80%. By carefully choosing the medium height, the broad band of LTL is changed to the two bands of LTC polarization conversion, including linear to left-handed and right-handed circular polarization conversion simultaneously. The multifunctional reflection polarization converter originates from the excitation of three graphene surface plasmons (GSPs) modes. The influences of incident angle, electron scattering time of graphene and geometric parameter over LTL and LTC polarization converter are investigated. In addition, the broad band of LTL and double bands of LTC all have blue shifts with increasing the Fermi energy (Ef) of graphene, and the band of line-to-left-circular-polarization (LTLCP) can be dynamically tuned to the one of line-to-right-circular-polarization (LTRCP), by changing Ef. All the simulation results based on finite element method verify the design of this paper. The broadband effect for LTL polarization converter has been reported a lot [29–31], but our proposed graphene metasurface can realize the switch from broadband LTL to LTC polarization converter just by changing the medium height and without changing the shape of metasurface, which will improve the practicability and adjustability of this design. So the proposed polarization converter has promising potential on the miniaturization and integration of the terahertz system.
2. Design and theories
The structural diagram of proposed polarization converter is shown in Fig. 1(a). It consists of a graphene sheet decorated with the periodical π-shaped hollows. The graphene sheet is placed over an intermediate silica layer, which is supported by Au substrate to improve the reflection. One unit of the proposed structure is shown in Fig. 1(b). The lengths L1 and L2 of the lateral and vertical arms are 505 and 665 nm, respectively. The width of arms and the distance between the two vertical arms are d(=60 nm) and g(=110 nm), respectively. In our simulation, the periods (Px and Py) along the x and y directions are both set as 990 nm. Our proposed graphene metasurface could be fabricated by the existing preparation methods. For example, Helium Ion Beam direct writing technique could be employed to pattern graphene in nanometer-scale graphene metasurface under 5nm extreme precision [48,49], which meets our demand. The incident angle is defined as θ as shown in Fig. 1(c).
Fig. 1. (a) Schematic diagram of multifunctional polarization converter, in which yellow and gray regions represent gold and silicon dioxide layer, respectively. (b) One unit of (a). Geometric parameters include: L1 = 505 nm, L2=665 nm, d=60 nm, g=110 nm and Px= Py = 990 nm. Besides, the incident angle is defined as θ as shown in Fig. 1(c).
In this paper, all the simulation results are obtained by finite element method (FEM) via frequency domain solver in COMSOL Multiphysics. The Floquet boundary condition is employed along the x and y directions. The graphene thickness (Δ) is chosen as 1 nm, and the graphene permittivity ε is as follows:
where η0 (≈ 377 Ω) is the impedance of air and k0 is the wave vector of the incident light in air. The optical conductivity of graphene σg can be calculated as:Here, Ef is Fermi energy of graphene, T(=300K) is temperature, ω is frequency of the incident light, e is electronic charge, τ is electron scattering time, ℏ is reduced Planck’s constant, and kB is Boltzmann constant. For dynamically controlling the electromagnetic property of graphene, a thin transparent ion-gel layer can be covered at top of the structure so that each cell could be electrically biased through transparent electrodes. If large Fermi energy is required, the chemical doping is employed before electrical stimuli, and the targeted Fermi energy is obtained. Assume that the dielectric constant of silica is 2.1. The Drude model is used to simulate the metal layer. The typical carrier concentration was 4.11×1027 m−3 and the scattering time was 20 fs.
The incident and reflected electric fields can be calculated by the following formula:
where EAi and EBi represent the electric field of incident light in the +45° and −45° to positive x-direction respectively. EAr and EBr represent the electric field of the reflected light in the +45° and −45° directions, respectively. The general reflection matrix of the reflective graphene metasurface can be expressed as:
where ${r_{jk}} = E_j^r/E_k^i$ (j, k = A, B)) represents the reflection coefficient. In this manuscript, the polarization angle of incident beam is 45° respect to the x axis, so rAA and rBA are the co-polarization and cross-polarization reflection coefficients, and ${\varphi _{AA}}$ and ${\varphi _{BA}}$ are the corresponding phases, respectively. Hence, the polarization conversion ratio (PCR) can be described as [19]:Here, we define co-polarization reflection RAA and cross-polarization reflection RBA as:
Here, the proposed design is believed to achieve cross polarization conversion when PCR is over 80%.
To verify the polarization state of the reflected wave, the Stokes parameters [50] are introduced as follows:
3. Broadband of LTL polarization converter
In this section, the graphene sheet is placed over an intermediate silica layer (the thickness of silica layer m = 3600 nm). A tunable broadband LTL polarization converter in mid-infrared region can be achieved when Fermi energy (Ef) of graphene is considered to be 1 eV. The incident beam with polarization angle +45° (the angle between polarization angle and positive x-direction) is converted to reflected beam with polarization angle −45° under normal incidence. The results of PCR, co-polarization (RAA) and cross-polarization (RBA) reflections are shown in Fig. 2. There are three dips in the co-polarization (RAA) reflection spectrum, which are located at 14.02, 15.78 and 17.36 THz respectively. And there are three cross-polarization (RBA) reflection peaks around the same frequencies, respectively. The PCRs reach the three peaks at 14.02, 15.78 and 17.36 THz, whose values are 91.16%, 99.78% and 97.69%, respectively. In the PCR spectrum from 13.94 to 17.52 THz, PCRs are over 80%, proving that the structure can realize LTL polarization conversion with the bandwidth of 3.58 THz, yielding a fractional bandwidth (the ratio of bandwidth to central frequency) about 23% with reference to the central frequency of 15.73 THz. Thus, a broadband LTL polarization converter is realized in reflective mode.
Fig. 2. The simulation results of (a) co-polarization and cross-polarization reflection (RAA and RBA), and (b) PCR are plotted in black, blue and black solid line, respectively.
We attempt to further explain the physical mechanism of broadband LTL polarization converter, and find that there are three eigenmodes related to the physical origin. The incident polarization light (polarization angle +45°) can be divided into two orthogonal polarization light components Ex and Ey, whose polarization direction is along the x-axis and y-axis respectively. Rxx is the co-polarization reflection under the incident Ex and Ryy is the co-polarization reflection under the incident Ey. In Fig. 3(a), the two reflectance spectra (drawn with blue and green solid lines, respectively) show three dips at 13.84, 16.00 and 17.58 THz which are very close to three PCR peaks (14.02, 15.78 and 17.36 THz). The three dips originate from three eigenmodes, which are excited by these two components (Ex and Ey). Ex excited one mode at 16.00 THz and Ey excited two modes at 13.84 and 17.58 THz. As shown in Figs. 3(b), (c) and (d), the three modes show symmetric (16.00THz) and antisymmetric (13.84 and 17.58 THz) magnetic field distributions about the y-axis, which originate from the GSPs resonances, respectively. In addition, the phase delay (PD) between Rxx and Ryy in the reflected wave occurs in the three resonant modes, which is approximately 180° at 14.02, 15.78 and 17.36 THz respectively, shown in Fig. 3(a). Thus, the superposition of two reflected orthogonal components produces a polarization rotation of 90° at 14.02, 15.78 and 17.36 THz. Thus, the three PCR peaks happen at these three frequencies as shown in Fig. 2(b), and the broadband LTL converter with 3.58 THz bandwidth is achieved.
Fig. 3. (a) The co-polarized reflection spectrum under x-polarized Ex and y-polarized Ey incidence, respectively. The simulation results of Rxx and Ryy are drawn with blue and green solid lines. The dotted red line represents the PD between Rxx and Ryy. Magnetic field distributions (Hz) of 13.84 THz(b) and 17.58 THz (d) under Ey incidence, and 16.00 THz (c) under Ex incidence, respectively.
Next, we study the broadband performance of LTL polarization converter with different geometric parameters L1, L2 and d. When L1 increases from 465 to 545 nm, other geometric parameters remain unchanged, and PCRs are shown in Fig. 4(a). The third PCR peak corresponding to antisymmetric GSPs mode has a significant red shift, the first and second PCR peaks are almost unchanged, and the bandwidth of LTL polarization converter narrows with increasing L1. Besides, the LTL polarization converter shows the bad broadband performance (PCR<80%) when L1 is smaller than 485 nm. In Fig. 4(b), when L2 increases from 625 to 705 nm, the frequency of first PCR peak has a red shift from 14.74 to 13.36 THz, and other two PCR peaks are almost unchanged. Thus, the bandwidth of LTL polarization converter broadens with increasing L2. In Fig. 4(c), the positions of first and third PCR peak are almost unchanged, but the second PCR peak corresponding to symmetric GSPs mode has a blue shift from 15.10 to 16.86 THz with the increase of d. The bandwidth of LTL polarization converter broadens with increasing d, and LTL polarization converter shows bad broadband performance when d is larger than 60 nm.
Fig. 4. Simulated PCR of the LTL polarization converter under normal incidence with different (a) length L1, (b) length L2 and (c) width d of the π-shaped hollow array. Other parameters are the same with those of Fig. 2.
Figure 5 shows the broadband performance of LTL polarization converter under different incident angles from 0° to 70°, and other parameters are the same with those of Fig. 2. As the incident angle increases from 0° to 40°, the operating frequency range of the LTL polarization converter (PCR is over 80%) remains from 13.94 to 17.52 THz and the fractional bandwidth is unchanged. The resonant frequencies of three GSPs modes are insensitive to the incident angle. The simulation results clearly show that there is no significant degradation of PCR performance in the range from 0° to 40°. When the incident angle is larger than 40°, the broadband performance turns down, because the interaction between graphene and electromagnetism weakens the intensity of the reflected lights due to the large incident angle.
Fig. 5. Simulation of the PCR under different incident angles from 0° to 70°. Other parameters are the same with those of Fig. 2.
We discuss the tunable performance of the LTL polarization converter by changing Fermi energy (Ef). The Fermi energy (Ef) of graphene can be dynamically regulated by changing chemically doped or electronically gated [39], showing the great potential of tunable devices. As the Fermi energy (Ef) decreases from 1 to 0.84 eV, the ideal wideband LTL effect with 3.58 THz bandwidth is almost maintained. The resonant frequency positions have red shifts from 14.02, 15.78 and 17.36 to 12.84, 14.18 and 15.90 THz, and the corresponding PCR values decrease from 91.16%, 99.78% and 97.69% to 80.04%, 99.40% and 94.96%. The operating frequency range of the LTL polarization converter (PCR is over 80%) has a red shift from the frequency range (13.94 to 17.52 THz) to one (12.78 to 16.02THz). When the Fermi energy (Ef) decreases from 0.84 to 0.7 eV, the performance of wideband LTL turns down.
The red shift behavior of the GSPs’ resonant frequency with decreasing Fermi energy can be interpreted by the following way. The wave vector of surface plasmon is satisfied with the equation
α0 is the fine structure constant. The resonant frequency f can be written as
Lg is the resonance characteristic length of graphene carved-hollows. Equation (8) shows that decreasing Fermi energy (Ef) can cause red shift behavior. The dynamically tunable characteristic makes our proposed structure more practical than metal integrated metasurface polarization converter, which can be achieved by adjusting Fermi energy rather than geometric parameters.
Secondly, we change the electron scattering time τ to investigate the influence on the performances of the LTL polarization converter. Previous studies have shown that electron scattering time τ can vary over a wide range of ps or sub-ps [51]. Figure 6(b) shows PCR at different electron scattering time τ. It is obvious that as the electron scattering time τ decreases from 1 to 0.2 ps, the frequency position of the PCR peak is almost unchanged, but the PCR values of the three peaks will gradually decrease. When the electron scattering time τ is less than 0.6 ps, the PCR values will drop rapidly, and the broadband performance of LTL converter turns down. This is because a decrease in the scattering time will increase the loss, resulting in a decrease in the amplitude of the reflected wave.
Fig. 6. Simulation of the PCR under different (a) Fermi energy (Ef) varying from 1 to 0.7 eV and (b) electron scattering time τ varying from 1 to 0.2ps. Other parameters are the same with those of Fig. 2.
As shown in Fig. 7, we find that the resonant frequencies of the symmetric GSPs mode blue shift and those of two antisymmetric GSPs modes are almost unchanged, when increasing the medium height m. Besides, as the medium height increases from 3600 to 4500 nm, the PCR at the resonant frequencies of the two antisymmetric GSPs modes gradually decreases, which leads to the possible emergence of the double LTC polarization conversion bands. In the next part, we will continue a further discussion about the double LTC bands.
Fig. 7. Simulation of the PCR when the medium height m varying from 3600 to 4500 nm. Other parameters are the same with those of Fig. 2.
4. Double LTC polarization bands with opposite handedness
Based on the results of Fig. 7, when we set the medium height as m( = 4485 nm), and other parameters are as shown in Fig. 2, the perfect line-to-right-circular-polarization (LTRCP) and line-to-left-circular-polarization (LTLCP) conversion are simultaneously achieved. We use ellipticity χ and axial ratio to measure the performance of LTC polarization converter. Figure 8 shows the performance of LTC polarization converter under normal incident angle. Excited by the incident THz wave polarized at 45° with respect to x-direction, our proposed structure converts linear to right-handed and left-handed circular polarized waves. Figure 8(a) shows co-polarization (RAA) and cross-polarization (RBA) reflections in black and red lines, respectively. As shown in Fig. 8(b), ${\varphi _{BA}}$, ${\varphi _{AA}}$ and $\mathrm{\Delta }\varphi $($= {\varphi _{\textrm{BA}}} - {\varphi _{\textrm{AA}}}$) are plotted in black dashed line, blue dashed line and red solid line, respectively. RAA, and RBA are equal at 14.88 and 17.32 THz, while $\mathrm{\Delta }\varphi $ is equal to −90° and 90°, respectively. So, the perfect right-hand and left-hand circular polarization conversion are simultaneously formed at 14.88 and 17.32 THz in the reflection mode, respectively. Figure 8(c) shows that the band of LTRCP conversion is obtained at the lower frequency range, and the band of LTLCP conversion could be realized at the higher frequency range. Results indicate that the two bands of LTRCP and LTLCP operate in the frequency range of 13.82-16.08 THz and 16.84-17.74 THz with an axial ratio ($\textrm{ = 10log}|{{\textrm{r}_{\textrm{BA}}}\textrm{/}{\textrm{r}_{\textrm{AA}}}} |$) of less than 3 dB, respectively. The fractional bandwidths reach 15.1% and 5.2%, respectively. Besides, the ellipticity χ is also calculated in Fig. 8(c) to measure the bandwidth of the two bands of LTC polarization conversion. It can be found that χ is smaller than −0.95 from 14.24 to 15.74 THz, which indicates that LTRCP conversion is achieved and the fractional bandwidth is 10.0%. χ is −1 at 14.88 THz, which indicates that perfect LTRCP conversion is achieved. From 17.02 to 17.48 THz, χ is larger than 0.95, which indicates that LTLCP is achieved and the fractional bandwidth is 2.7%. χ is 1 at 17.32 THz, which indicates that perfect LTLCP is achieved. Obviously, the bandwidth defined by χ is a little smaller than the one defined with axial ratio.
Fig. 8. (a) Simulation results of the co-polarization reflection (RAA), cross-polarization reflection (RBA). (b) Phase of φAA and φBA together with their phase difference Δφ. (c) Ellipticity and axial ratio. The medium height m is 4485 nm, and other parameters are the same with those of Fig. 2.
We further explore the influence of different geometric parameters L1, L2 and d on the performances of two bands of LTC polarization converter with opposite handedness, including ellipticity χ, phase difference Δφ, and the axial ratio. As shown in Figs. 9(a)-(c), the band of LTLCP conversion disappears when L1 is larger than 505 nm, because χ is smaller than 0.95. The band of LTLCP (χ>0.95) narrows from the frequency range (17.06 to 17.80 THz) to one (17.02 to 17.48 THz), and the band with 3 dB bandwidth of LTLCP narrows from frequency range (16.90 to 18.00 THz) to one (16.84 to 17.74 THz), with increasing L1 from 485 to 505 nm. Besides, the bandwidth of LTRCP is almost unchanged. It is because that there is a red shift of the third GSPs mode, and the first and second GSPs modes are almost unchanged with the increase of L1 from 465 to 545 nm (shown as Fig. 4(a)). It can be found that the band of LTRCP broadens, and the one of LTLCP is almost unchanged with increasing L2, as shown in Figs. 9(d)-(f). The band with |χ|> 0.95 of LTRCP broadens from the frequency range (14.94 to 15.92THz) to one (13.62 to 15.62THz), and the band with 3 dB bandwidth of LTRCP broadens from frequency range (14.52 to 16.22THz) to one (13.20 to 15.98THz), with increasing L2 from 625 to 705 nm. It is because that there is a red shift of the first GSPs mode, and the second and third GSPs mode are almost unchanged with the increase of L2 from 625 to 705 nm (shown as Fig. 4(b)). As shown in Figs. 9(g)-(i), it is found that the band of LTRCP broadens, and the one of LTLCP narrows with increasing d. The band with |χ|> 0.95 of LTRCP broadens from the frequency range (14.50 to 14.88 THz) to one (14.00 to 16.72 THz), and the one of LTLCP narrows from frequency range (16.64 to 17.20 THz) to one (17.54 to 17.74 THz), with increasing d from 50 to 80 nm. In the similar way, the band with 3 dB bandwidth of LTRCP broadens from frequency range (13.74 to 15.44 THz) to one (13.78 to 16.86 THz), and the one of LTLCP narrows from frequency range (16.48 to 17.52 THz) to one (17.40 to 17.94 THz). It is because that there is a blue shift of the second GSPs mode, and the first and third GSPs modes are almost unchanged with the increase of d from 50 to 80 nm (shown as Fig. 4(c)).
Fig. 9. Simulated (a), (d), (g) ellipticity, (b), (e), (h) phase difference Δφ, and (c), (f), (i) the axial ratio of the LTC polarization converter under different L1, L2 and d, respectively. Other parameters are the same with those of Fig. 8.
Figure 10 shows performances of the double LTC polarization bands with opposite handedness under different incident angles from 0° to 70°, and other parameters are the same with those of Fig. 8. Figure 10(a) shows the band with |χ|> 0.95 of LTRCP narrows from the frequency range (14.24 to 15.74 THz) to one (14.44 to 15.68 THz), and the one of LTLCP narrows from frequency range (17.02 to 17.48 THz) to one (17.04 to 17.42 THz), with the incident angle increasing from 0° to 30°. When the incident angle is larger than 30°, the double bands of LTC polarization are destroyed due to |χ|< 0.95. Figure 10(b) shows that the band with 3 dB bandwidth of LTRCP narrows from frequency range (13.82 to 16.08 THz) to one (14.28 to 15.58 THz), and the one of LTLCP narrows from frequency range (16.84 to 17.74 THz) to one (16.84 to 17.44 THz), with the increase of incident angle from 0° to 40°. When the incident angle is larger than 40°, the double LTC polarization bands with 3 dB bandwidth are destroyed.
Fig. 10. (a) The ellipticity and (b) axial ratio of the proposed polarization converter under the incident angle ranging from 0°to 70°. Other parameters are the same with those of Fig. 8.
With the decrease of Fermi energy (Ef), LTRCP and LTLCP bands both have an obvious red shift, and it is because the three GSPs’ modes have a red shift with decreasing Fermi energy based on Eq. (9). As shown in Fig. 11(a), the frequency range of LTRCP band with |χ|> 0.95 covers from 11.84 to 15.74 THz, the one of LTLCP covers from 14.02 to 17.48 THz with |χ|>0.95 with the Fermi energy (Ef) decreasing from 1 to 0.7 eV. In a similar way, as shown in Fig. 11(b), the 3dB band of LTRCP covers from 11.54 to 16.08 THz, the one of LTLCP covers from 14.02 to 17.74 THz. Thus, by changing the Fermi energy, the two broad bands of LTC are also achieved. Especially, the simulation results demonstrate the conversion between LTLCP and LTRCP can be achieved by changing Ef. For example, when Ef=0.7eV, the working frequency 14.1 THz is located at the LTLCP band (χ> 0.95 and axial ratio<3dB); when Ef=1eV, the working frequency 14.1 THz is located at the LTRCP band (χ<−0.95 and axial ratio<3dB).
Fig. 11. Simulation of the (a) ellipticity, and (b) the axial ratio of the LTC polarization converter under different Fermi energy (Ef) varying from 1 to 0.7 eV. Other parameters are the same with those of Fig. 8.
With the decrease of electron scattering time τ, the performances (ellipticity and the axial ratio) of the LTC polarization converter are investigated as shown in Fig. 12. It is obvious that as the electron scattering time τ decreases from 1 to 0.2 ps, the two LTC bands have no frequency shift when other parameters are the same with those of Fig. 8. The bandwidths for the two LTC bands are almost unchanged, and the perfect right-hand and left-hand circular polarization always happens at 14.88 and 17.32 THz, respectively. Three peaks of axial ratio decrease with decreasing electron scattering time, which originates from increased free-carrier loss in graphene. When the electron scattering time τ is smaller than 0.2 ps, the performance of double bands of LTC polarization conversion turns down.
Fig. 12. Simulation of the (a) ellipticity, and (b) the axial ratio of the LTC polarization converter under different electron scattering time τ varying from 1.0 to 0.2ps. Other parameters are the same with those of Fig. 8.
5. Conclusion
In summary, we have proposed a novel design of a tunable multifunctional polarization converter, which consists of a single graphene layer decorated with a π-shaped carved-hollow array. The broadband LTL and double bands of LTC polarization converter with opposite handedness can be realized respectively, by carefully choosing the medium height. The polarization conversions, including broadband LTL and double bands of LTC, are all related to the excitation of three GSPs modes. By increasing the Fermi energy, the switch between LTRCP and LTLCP can be realized, and the bands of polarization conversions all blue shift. In addition, the performance of the multifunctional polarization converter is also investigated under different geometrical parameters, incident angles and electron scattering time. This design provides a feasible method for the tunable multifunctional polarization control of terahertz field.
Funding
National Natural Science Foundation of China (1148081606193050); National College Students Innovation and Entrepreneurship Training Program (202010295062); The Key Research and Development Program of Jiangsu Province (BE2020756).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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