Plasmonic antennas based on rectangular graphene nanoribbons with controlled polarization of terahertz and infrared radiation

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Introduction

New wireless communication standards such as 6G require higher bandwidth than current technologies can provide. The terahertz (THz) frequency band is highly promising for high-speed wireless communication networks [1] because it can significantly boost transmission rates compared to ultrahigh-frequency (UHF) bands, especially in 6G Wi-Fi networks [2].

Two-dimensional (2D) materials such as graphene play a crucial role in efficient polarization control in THz devices and antennas. With its unique optoelectronic properties and high doping potential, graphene has become key material in plasmonic antennas [3–7] and THz polarizers [8–11]. It exhibits high carrier mobility and consistently absorbs light across wavelengths. Graphene properties can be tuned by adjusting its chemical potential (Fermi level) via electrical gating and chemical doping. Graphene is chemically and structurally stable owing to the strong covalent bonds among carbon atoms, which enable it to support long-lived and tunable plasmon resonances when excited by THz and infrared (IR) radiation [3].

Graphene analogs of standard metal antennas exhibit better emitting properties [4–7]. This is largely attributed to graphene’s excellent surface conductivity, which is highly responsive to changes in chemical potential when a bias voltage is applied [12]. Such dynamic conductivity control is expected to support operations at terabit-per-second speeds [3].

New THz, ultra-high-performance, and ultra-precise communication systems require efficient control of the polarization direction of the emitted waves. However, most available methods are complex and expensive [2].

The ST MWS and HFSS simulation software packages can solve important problems related to the design of microwave devices, antennas, and phased antenna arrays [13; 14].

This study uses the CST Microwave Studio electromagnetic simulation system to investigate how the polarization of terahertz and IR radiation emitted by plasmonic antennas made of rectangular graphene nanoribbons can be controlled. This control can be achieved by changing the chemical potential by applying an external electric field.

1. Simulation of plasmonic antenna behavior in a rectangular graphene nanoribbon excited by s- and p-polarized TEM wave under chemical potential change

Fig. 1 illustrates the CST MWS 2023 [15] models of excitation of a plasmonic antenna with a rectangular graphene nanoribbon can be excited by a normally incident TEM wave. In these models, the wave is either s-polarized (Fig. 1, a) or p-polarized (Fig. 1, b) using a waveguide port. The antenna model features a rectangular graphene nanoribbon with dimensions of length (l) and width (w) (Fig. 1) installed on a dielectric substrate made of silicon dioxide (SiO₂), characterized by a dielectric constant $(\epsilon =\mathrm{2,2}),$ and with dimensions a, b, and h.

Fig. 1. Excitation of a plasmonic antenna with a rectangular graphene nanoribbon using a waveguide port: a – s-polarized and b – p-polarized TEM. Simulated in CST MWS 2023

Рис. 1. Модели возбуждения плазмонной антенны на основе прямоугольной графеновой наноленты TEM-волной s-поляризации (а) и p-поляризации (б) с помощью волноводного порта в программном комплексе CST MWS 2023

Fig. 1 also depicts the orientation of the incident TEM wave’s electric field strength vector E relative to the graphene nanoribbon. For an s-polarized wave, vector E is oriented along the wide side of the graphene nanoribbon (Fig. 1, a), whereas for a p-polarized wave, vector E aligns with the narrow side (Fig. 1, b).

We used the models presented in Fig. 1 to address the electrodynamic problem using CST Microwave Studio 2023 employing finite integration in the time domain [3].

We calculated an element of the scattering matrix  for a graphene plasmonic antenna excited by s- and p-polarized TEM waves across the THz, far-IR, and mid-IR ranges, considering different values of the graphene’s chemical potential

Fig. 1 also depicts the orientation of the incident TEM wave’s electric field strength vector E relative to the graphene nanoribbon. For an s-polarized wave, vector E is oriented along the wide side of the graphene nanoribbon (Fig. 1, a), whereas for a p-polarized wave, vector E aligns with the narrow side (Fig. 1, b).

We used the models presented in Fig. 1 to address the electrodynamic problem using CST Microwave Studio 2023 employing finite integration in the time domain [3].

We calculated an element of the scattering matrix $\left|S_{12}\right|$ for a graphene plasmonic antenna excited by s- and p-polarized TEM waves across the THz, far-IR, and mid-IR ranges, considering different values of the graphene’s chemical potential ${\mu }_{с}.$

Fig. 2 displays the frequency responses of the scattering matrix element $\left|S_{12}\right|,$ which represents thewave transmission coefficient, for a plasmonic antenna with a rectangular graphene nanoribbon. The nanoribbon dimensions are the following: $w=1$ μm, $l=\mathrm{2,5}$ μm, $a=b=3$ μm, $h=1$ μm). Furthermore. the chemical potential values are the following: ${\mu }_{с1}=\mathrm{0,3}$ eV, ${\mu }_{с2}=\mathrm{0,7}$ eV, ${\mu }_{с3}=1$ eV. The antenna was excited by a TEM wave, which exhibited s-polarization (Fig. 2, a) and p-polarization (Fig. 2, b). The graphene properties considered in this simulation are $Т=300$ K and $\tau =1$ ps.

Fig. 2. Frequency responses of the scattering matrix element $\left|S_{12}\right|$ within a plasmonic antenna containing a rectangular graphene nanoribbon at different chemical potentials ${\mathit{\mu }}_{\mathbf{с}}$ in the THz, far and mid-IR ranges: a – s-polarization; b – p-polarization of the incident TEM wave; curve 1 – ${\mathit{\mu }}_{\mathbf{с}\mathbf{1}}\mathbf{=}\mathbf{0}\mathbf{,}\mathbf{3}$ eV, 2 – ${\mathit{\mu }}_{\mathbf{с}\mathbf{2}}\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{7}$ eV, 3 – ${\mathit{\mu }}_{\mathbf{с}\mathbf{3}}\mathbf{=}\mathbf{1}$ eV; $\mathit{w}\mathbf{=}\mathbf{1}$ μm, $\mathit{l}\mathbf{=}\mathbf{2}\mathbf{.}\mathbf{5}$ μm, $\mathit{a}\mathbf{=}\mathit{b}\mathbf{=}\mathbf{3}$ μm, $\mathit{h}\mathbf{=}\mathbf{1}$ μm

Рис. 2. Частотные зависимости элемента матрицы рассеяния $\mathbf{\left|}{\mathbf{S}}_{\mathbf{12}}\mathbf{\right|}$ плазмонной антенны на основе прямоугольной графеновой наноленты для различных значений химического потенциала ${\mathit{\mu }}_{\mathbf{с}}$ в ТГц, дальнем и среднем ИК-диапазонах: а – s-поляризация; б – p-поляризация падающей TEM-волны; кривая 1 – ${\mathit{\mu }}_{\mathbf{с}\mathbf{1}}\mathbf{=}\mathbf{0}\mathbf{,}\mathbf{3}$ эВ, 2 – ${\mathit{\mu }}_{\mathbf{с}\mathbf{2}}\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{7}$ эВ, 3 – ${\mathit{\mu }}_{\mathbf{с}\mathbf{3}}\mathbf{=}\mathbf{1}$ эВ; $\mathit{w}\mathbf{=}\mathbf{1}$ мкм, $\mathit{l}\mathbf{=}\mathbf{2}\mathbf{.}\mathbf{5}$ мкм, $\mathit{a}\mathbf{=}\mathit{b}\mathbf{=}\mathbf{3}$ мкм, $\mathit{h}\mathbf{=}\mathbf{1}$ мкм

The simulation results (Fig. 2) indicate that frequency shifts and the minima of the transmission coefficient ${\mu }_{с}$ vary with changes in the graphene chemical potential $\left|S_{12}\right|.$ The transmission coefficient $\left|S_{12}\right|$ minima (Fig. 2, a, b) result from the maxima of the absorption coefficient P in graphene. These minima align with the plasmon resonances [16] that occur at frequencies $f_{res}$ driven by the excited main and higher modes of surface plasmon polaritons (SPPs) [11]. These resonant frequencies $f_{res}^{s,p}$ are influenced by the incident wave polarization [11].

For an s-polarized excitation TEM wave, in which the surface electric current resonates along the wide side of the rectangular graphene nanoribbon, longitudinal plasmon resonance occurs [11]. In this case, the first resonant frequency $f_1^{s1}$ is determined by the main mode of SPPs (Fig. 2, a). At this frequency, the electric current density $j_s$ on the graphene surface reaches its peak, leading to maximum absorption. For a p-polarized excitation TEM wave, a transverse resonance of the surface electric current $j_s$ occurs along the narrow side of the rectangular graphene nanoribbon [11], enabling resonance frequencies $f_{res}^p$ to be higher than $f_{res}^s$ for the s-polarization (Fig. 2, b). We observed several resonant frequencies corresponding to the nearest higher SPP modes for both s-polarization (Fig. 2, a) and p-polarization (Fig. 2, b). In a rectangular graphene nanoribbon, the longitudinal SPP resonance occurs at s-polarization (Fig. 2, a), whereas the transverse SPP resonance appears at p-polarization (Fig. 2, b), each at different frequencies.

The table lists the estimated scattering matrix element $\left|S_{12}\right|$ at the resonance frequencies $f_{res}^{s,p}$ of the main and second-order SPP modes excited by an s- and p-polarized TEM wave for different chemical potentials ${\mu }_{с}$ (0,3, 0,7, 1 eV).

Table. Calculated values of the scattering matrix element $\left|S_{12}\right|$ at the resonant frequencies ${\mathit{f}}_{\mathbf{r}\mathbf{e}\mathbf{s}}^{\mathbf{s}\mathbf{,}\mathbf{p}}$ of the main and second SPP modes excited by an s- and p-polarized TEM wave for different chemical potentials ${\mathit{\mu }}_{\mathbf{с}}$

Таблица. Расчетные значения элемента матрицы рассеяния $\mathbf{\left|}{\mathbf{S}}_{\mathbf{12}}\mathbf{\right|}$ на резонансных частотах ${\mathit{f}}_{\mathbf{res}}^{\mathbf{s}\mathbf{,}\mathbf{p}}$ основной и второй мод ППП, возбуждаемых TEM-волной s- и p-поляризации, для различных значений химического потенциала ${\mathit{\mu }}_{\mathbf{с}}$

2. Simulation of THz and IR radiation polarization control for plasmonic antennas with rectangular graphene nanoribbons

Fig. 3 illustrates the simulation results from CST MWS 2023. We modeled the directivity pattern (DP) of a plasmonic graphene antenna on a dielectric substrate and examined the distribution of the surface electric current density vector $j_s$ on the graphene nanoribbon at the resonant frequencies $f_1^{s1},$ $f_2^{s1},$ associated with the main SPP mode. This occurs when the antenna is excited by an s-polarized TEM wave, considering different chemical potentials of 0,3, 0,7, 1 eV.

Fig. 3. DP of a plasmonic antenna with a rectangular graphene nanoribbon on a dielectric substrate at resonance frequencies ${\mathit{f}}_{\mathbf{r}\mathbf{e}\mathbf{s}}^{\mathbf{s}\mathbf{1}}$ corresponding to the main SPP mode when excited by an s-polarized TEM wave. A frequency sweep in the THz range is performed as the chemical potential ${\mathit{\mu }}_{\mathbf{с}}\mathbf{\text{:}}$ changes. 2D RP in the E-plane (depending on $\mathit{\theta }$ $\mathit{\phi }\mathbf{=}\mathbf{90}\mathbf{°}$  (a), in the E-plane (depending on $\mathit{\theta }$ $\mathit{\phi }\mathbf{=}\mathbf{0}\mathbf{°}$ (b) in the polar (a, b) and 3D RP in the spherical (c) coordinate systems and the distribution of the surface electric current density vector ${\mathit{j}}_{\mathbf{s}}$ on the graphene nanoribbon (d); 3.1 – ${\mathit{f}}_{\mathbf{1}}^{\mathbf{s}\mathbf{1}}\mathbf{=}\mathbf{3}\mathbf{,}\mathbf{652}$THz,  3.2 – ${\mathit{\mu }}_{\mathbf{с}\mathbf{1}}\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{3}\mathbf{;}$ ${\mathit{f}}_{\mathbf{2}}^{\mathbf{s}\mathbf{1}}\mathbf{=}\mathbf{5}\mathbf{.}\mathbf{563}$THz, ${\mathit{\mu }}_{\mathbf{с}\mathbf{2}}\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{7}$ eV; 3.3 – ${\mathit{f}}_{\mathbf{3}}^{\mathbf{s}\mathbf{1}}\mathbf{=}\mathbf{6}\mathbf{.}\mathbf{616}$THz, ${\mathit{\mu }}_{\mathbf{с}\mathbf{3}}\mathbf{=}$1 eV. The radiation level and the surface current density are visualized using a color map in the digital edition

Рис. 3. ДН плазмонной антенны на основе прямоугольной графеновой наноленты на диэлектрической подложке на резонансных частотах ${\mathit{f}}_{\mathbf{res}}^{\mathbf{s}\mathbf{1}}$ основной моды ППП при s-поляризации возбуждающей TEM-волны и сканирование по частоте в ТГц–диапазоне при изменении значения химического потенциала ${\mathit{\mu }}_{\mathbf{с}}\mathbf{\text{:}}$ 2D ДН в E-плоскости (в зависимости от $\mathit{\theta }$ $\mathit{\phi }\mathbf{=}\mathbf{90}\mathbf{°}$ (а), в E-плоскости (в зависимости от $\mathit{\theta }$ $\mathit{\phi }\mathbf{=}\mathbf{0}\mathbf{°}$ (б) в полярной (а, б) и 3D ДН в сферической (в) системах координат и распределение вектора плотности поверхностного электрического тока ${\mathit{j}}_{\mathbf{s}}$ на графеновой наноленте (г); 3.1 – ${\mathit{f}}_{\mathbf{1}}^{\mathbf{s}\mathbf{1}}\mathbf{=}\mathbf{3}\mathbf{,}\mathbf{652}$ ТГц,  3.2 – ${\mathit{\mu }}_{\mathbf{с}\mathbf{1}}\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{3}\mathbf{;}$ТГц, ${\mathit{f}}_{\mathbf{2}}^{\mathbf{s}\mathbf{1}}\mathbf{=}\mathbf{5}\mathbf{.}\mathbf{563}$${\mathit{\mu }}_{\mathbf{с}\mathbf{2}}\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{7}$ эВ; 3.3 – ${\mathit{f}}_{\mathbf{3}}^{\mathbf{s}\mathbf{1}}\mathbf{=}\mathbf{6}\mathbf{.}\mathbf{616}$ТГц, ${\mathit{\mu }}_{\mathbf{с}\mathbf{3}}\mathbf{=}$1 эВ; интенсивность излучения и плотности поверхностного электрического тока  обозначены цветом в электронной версии статьи

Fig. 3 – 3.1–3.3, a–c illustrate how the frequency of a plasmonic graphene antenna can be tuned, or frequency-swept, at resonant frequencies $f_{res}^{s1}$ corresponding to the main SPP mode when excited by an s-polarized TEM wave in the terahertz range, as the chemical potential ${\mu }_{с}$ varies from 0,3 to 1 eV by applying an external electric field.

At the resonant frequencies of the main SPP mode, excited by an s-polarized TEM wave, a resonance of the electric current is generated by the standing SPP half-wave along the wide side of the rectangular nanoribbon [11]. This results in a half-wave distribution of the surface electric current $j_s$ along the length of the rectangular graphene nanoribbon, peaking at the center (Fig. 3 – 3.1–3.3, d). The resulting 3D DP of the radiation produced by this half-wavelength (half of the SPP wavelength) electric emitter is toroidal (Fig. 3 – 3.1–3.3, c). The axis of the 2D DP in the equatorial plane aligns with the longitudinal emitting current (Fig. 3 – 3.1–3.2, b). The DP in the E-plane, which depends on $\theta$ at $\phi =0°),$ matches that of a half-wave symmetric vibrator [17]. In the E-plane (depending on $\theta$ at $\phi =90°),$ the DP forms a circle (Fig. 3 – 3.1–3.3, a).

As the chemical potential ${\mu }_{с}$ increases, the surface electric current density rate $j_s$ on the graphene nanoribbon increases (Fig. 3 – 3.1–3.3, d). This occurs because the q-value of the resonance decreases, leading to a higher absorption coefficient in graphene [11], which in turn boosts the emitting efficiency of the plasmonic graphene antenna (Fig. 3 – 3.1–3.3, c).

For comparison, Fig. 4 presents the simulated DP of a plasmonic antenna with a rectangular graphene nanoribbon on a metalized dielectric substrate, maintaining identical dimensions $(w=1$ μm, $l=\mathrm{2,5}$ μm, $a=b=3$ μm, $h=1$ μm). This is presented at the resonant frequency $f_3^{s1}$ that matches the main SPP mode, equal to $f_3^{s1}=\mathrm{5,252}$ THz when excited by an s-polarized TEM wave at a given chemical potential of ${\mu }_{с}=1$ eV.

Fig. 4. DP of a plasmonic antenna with a rectangular graphene nanoribbon on a metalized dielectric substrate excited by an s-polarized TEM at the resonant frequency corresponding to the main SPP mode ${\mathit{f}}_{\mathbf{3}}^{\mathbf{s}\mathbf{1}}\mathbf{=}\mathbf{5}\mathbf{.}\mathbf{252}$ THz, ${\mathit{\mu }}_{\mathbf{с}\mathbf{3}}\mathbf{=}$1 eV: 2D RP in the H-plane (depending on $\phi ,$ $\mathit{\theta }\mathbf{=}\mathbf{90}\mathbf{°}$ (a) and in the E-plane (depending on $\mathit{\theta }\mathbf{,}$ $\mathit{\phi }\mathbf{=}\mathbf{90}\mathbf{°}$ (b) in the polar (a, b) and 3D RP in the spherical (c) coordinate systems

Рис. 4. ДН плазмонной антенны на основе прямоугольной графеновой наноленты на металлизированной диэлектрической подложке при s-поляризации возбуждающей TEM-волны на резонансной частоте основной моды ППП ${\mathit{f}}_{\mathbf{3}}^{\mathbf{s}\mathbf{1}}\mathbf{=}\mathbf{5}\mathbf{.}\mathbf{252}$ ТГц, ${\mathit{\mu }}_{\mathbf{с}\mathbf{3}}\mathbf{=}$1 эВ: 2D ДН в H-плоскости (в зависимости от $\phi ,$ $\mathit{\theta }\mathbf{=}\mathbf{90}\mathbf{°}$ (а) и в E-плоскости (в зависимости от $\mathit{\theta }\mathbf{,}$ $\mathit{\phi }\mathbf{=}\mathbf{90}\mathbf{°}$ (б) в полярной (а, б) и 3D ДН в сферической (в) системах координат

The comparison of the DPs for plasmonic antennas containing rectangular graphene nanoribbons on dielectric substrates (Fig. 3 – 3.3) and metalized substrates (Fig. 4) substrates at resonant frequencies $f_3^{s1}$ corresponding to the main SPP mode indicates that the directivity axis, and therefore, the polarization plane of the half-wave THz radiation (half of the SPP wavelength) emitted by the electric emitter, is rotated by 90° in the latter case.

Plasmonic antennas containing rectangular graphene nanoribbons on dielectric and metalized substrates emit THz waves with two orthogonal polarizations. This means that by applying metallization to the dielectric substrate, we can control the polarization of the THz radiation.

Fig. 5 presents the simulation results from CST MWS 2023, showing the DP of a plasmonic graphene antenna on a dielectric substrate. It also depicts the distribution of the surface electric current density vector $j_s$ on the graphene nanoribbon at resonant frequencies $f_1^{p1},$ $f_1^{p1},$ $f_1^{p1}$ corresponding to the main SPP mode, when the antenna is excited by a p-polarized TEM wave at different chemical potentials (0,3, 0,7, 1 eV).

Fig. 5. TDP of a plasmonic antenna containing a rectangular graphene nanoribbon on a dielectric substrate at the resonance frequencies ${\mathit{f}}_{\mathbf{r}\mathbf{e}\mathbf{s}}^{\mathbf{p}\mathbf{1}}$ corresponding to the main SPP mode when excited by a p-polarized TEM wave and frequency sweep in the THz range as the chemical potential ${\mathit{\mu }}_{\mathbf{с}}\mathbf{\text{:}}$ changes. 2D RP in the E-plane (depending on $\mathit{\theta }\mathbf{,}$$\mathit{\phi }\mathbf{=}\mathbf{90}\mathbf{°}$ (a), in the E-plane (depending on $\mathit{\theta }\mathbf{,}$ $\mathit{\phi }\mathbf{=}\mathbf{0}\mathbf{°}$(b) in the polar (a, b) and 3D RP in the spherical (c) coordinate systems and the distribution of the surface electric current density vector ${\mathit{j}}_{\mathbf{s}}$ on a rectangular graphene nanoribbon (d); 5.1 – ${\mathit{f}}_{\mathbf{1}}^{\mathbf{p}\mathbf{1}}\mathbf{=}$7,747 THz, ${\mathit{\mu }}_{\mathbf{с}\mathbf{1}}\mathbf{=}$0,3; 5.2 – ${\mathit{f}}_{\mathbf{2}}^{\mathbf{p}\mathbf{1}}\mathbf{=}$11,803 THz, ${\mathit{\mu }}_{\mathbf{с}\mathbf{2}}\mathbf{=}$0,7 eV; 5.3 – ${\mathit{f}}_{\mathbf{3}}^{\mathbf{p}\mathbf{1}}\mathbf{=}$14,104 THz, ${\mathit{\mu }}_{\mathbf{с}\mathbf{3}}\mathbf{=}$1 eV. The radiation level and the surface current density  are visualized using a color map in the digital edition

Рис. 5. ДН плазмонной антенны на основе прямоугольной графеновой наноленты на диэлектрической подложке на резонансных частотах ${\mathit{f}}_{\mathbf{res}}^{\mathbf{p}\mathbf{1}}$ основной моды ППП при p-поляризации возбуждающей TEM-волны и сканирование по частоте в ТГц, дальнем и среднем ИК-диапазонах при изменении значения химического потенциала ${\mathit{\mu }}_{\mathbf{с}}\mathbf{\text{:}}$ 2D ДН в E-плоскости (в зависимости от $\mathit{\theta }\mathbf{,}$ $\mathit{\phi }\mathbf{=}\mathbf{90}\mathbf{°}$ (а), в E-плоскости (в зависимости от $\mathit{\theta }\mathbf{,}$ $\mathit{\phi }\mathbf{=}\mathbf{0}\mathbf{°}$(б) в полярной (а, б) и 3D ДН в сферической (в) системах координат и распределение вектора плотности поверхностного электрического тока ${\mathit{j}}_{\mathbf{s}}$ на прямоугольной графеновой наноленте (г); 5.1 – ${\mathit{f}}_{\mathbf{1}}^{\mathbf{p}\mathbf{1}}\mathbf{=}$7,747 ТГц, ${\mathit{\mu }}_{\mathbf{с}\mathbf{1}}\mathbf{=}$0,3; 5.2 – ${\mathit{f}}_{\mathbf{2}}^{\mathbf{p}\mathbf{1}}\mathbf{=}$11,803 ТГц, ${\mathit{\mu }}_{\mathbf{с}\mathbf{2}}\mathbf{=}$0,7 эВ; 5.3 – ${\mathit{f}}_{\mathbf{3}}^{\mathbf{p}\mathbf{1}}\mathbf{=}$14,104 ТГц, ${\mathit{\mu }}_{\mathbf{с}\mathbf{3}}\mathbf{=}$1 эВ; интенсивность излучения и плотности поверхностного электрического тока  обозначены цветом в электронной версии журнала

Fig. 5 – 5.1–5.3, a-c demonstrate the capability of tuning the operating frequency of a plasmonic graphene antenna (frequency scanning), allowing it to scan from the THz range to the far and mid-IR ranges. This frequency tuning is achieved at resonant frequencies $f_{res}^{p1}$ of the main SPP mode when the antenna is excited by a p-polarized wave. The tuning is facilitated by varying chemical potential ${\mu }_{с}$ between 0,3 and 1 eV using an external electric field).

At the resonant frequencies of the main SPP mode, excited by an s-polarized wave, a resonance of the electric current is generated by the standing SPP half-wave along the wide side of the rectangular nanoribbon [11]. This results in a half-wave distribution of the surface electric current $j_s$ along the length of the rectangular graphene nanoribbon, peaking at the center (Fig. 5 – 5.1–5.3, d). The resulting 3D DP for the radiation emitted by this half-wavelength (half of the SPP wavelength) electric emitter is toroidal (Fig. 5 – 5.1–5.3, c). The axis of the 2D DP in the equatorial plane aligns with the longitudinal emitting current (Fig. 5 – 5.1–5.3, d). In the E-plane (depending on $\theta$ at $\phi =90°),$ the DP matches that of a half-wave symmetric vibrator [17] (Fig. 5 – 5.1–5.2, а), and in the E-plane, (depending on $\theta$ at $\phi =0°)$ it forms a circle (Fig. 5 – 5.1–5.3, b).

The comparison of the DPs at the resonance frequencies of the main SPP mode $f_3^{s1}=$6,616 THz for s-polarization (Fig. 3 – 3.3) and $f_3^{p1}=$14,104 THz for p-polarization (Fig. 5 – 5.3) indicates that the directivity axis is parallel to the longitudinal emitting current in the first case and to the transverse emitting current in the second case. Therefore, the polarization plane of the half-wave THz radiation (half of the SPP wavelength) from the electric emitter [17] rotates 90° in the meridian plane.

A plasmonic antenna with rectangular graphene nanoribbons on a dielectric substrate can emit waves with two orthogonal polarizations. This happens when the resonance frequency shifts from the SPP mode $f_{res}^s$ for s-polarization to the SPP mode $f_{res}^p$ for p-polarization of the exciting TEM wave. Consequently, the polarization of THz/IR radiation from such antennas can be controlled by selecting the signal frequency depending on the type of plasmon resonance, either longitudinal or transverse, in the rectangular graphene nanoribbon.

Conclusion

The simulation results reveal that plasmonic antennas with rectangular graphene nanoribbons can emit THz/IR waves with two orthogonal polarizations. We identified methods to control the polarization of these waves by selecting signal frequencies that align with the plasmon resonances of SPP modes and through metallization of the dielectric substrate.

This ability to control THz/IR radiation enables the creation of both new plasmonic antenna array elements [18] and new communication technologies, including future 6G networks [2]. These advancements will support energy-efficient communication, enable cognitive radio networks that can intelligently change channels, and facilitate in-band full-duplex technology that can double the bandwidth [2].

About the authors

Galina S. Makeeva

Author for correspondence.
Email: galina.makeeva@gmail.com

Doctor of Physical and Mathematical Sciences, Professor of the Department of Radio Engineering and Radioelectronic Systems

References

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2. Fig. 1. Excitation of a plasmonic antenna with a rectangular graphene nanoribbon using a waveguide port: a – s-polarized and b – p-polarized TEM. Simulated in CST MWS 2023

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3. Fig. 2. Frequency responses of the scattering matrix element within a plasmonic antenna containing a rectangular graphene nanoribbon at different chemical potentials in the THz, far and mid-IR ranges: a – s-polarization; b – p-polarization of the incident TEM wave; curve 1 – eV, 2 – eV, 3 – eV; μm, μm, μm, μm

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4. Fig. 3. DP of a plasmonic antenna with a rectangular graphene nanoribbon on a dielectric substrate at resonance frequencies corresponding to the main SPP mode when excited by an s-polarized TEM wave. A frequency sweep in the THz range is performed as the chemical potential changes. 2D RP in the E-plane (depending on (a), in the E-plane (depending on (b) in the polar (a, b) and 3D RP in the spherical (c) coordinate systems and the distribution of the surface electric current density vector on the graphene nanoribbon (d); 3.1 – THz, 3.2 – THz, eV; 3.3 – THz, 1 eV. The radiation level and the surface current density are visualized using a color map in the digital edition

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5. Fig. 4. DP of a plasmonic antenna with a rectangular graphene nanoribbon on a metalized dielectric substrate excited by an s-polarized TEM at the resonant frequency corresponding to the main SPP mode THz, 1 eV: 2D RP in the H-plane (depending on (a) and in the E-plane (depending on (b) in the polar (a, b) and 3D RP in the spherical (c) coordinate systems

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6. Fig. 5. TDP of a plasmonic antenna containing a rectangular graphene nanoribbon on a dielectric substrate at the resonance frequencies corresponding to the main SPP mode when excited by a p-polarized TEM wave and frequency sweep in the THz range as the chemical potential changes. 2D RP in the E-plane (depending on (a), in the E-plane (depending on (b) in the polar (a, b) and 3D RP in the spherical (c) coordinate systems and the distribution of the surface electric current density vector on a rectangular graphene nanoribbon (d); 5.1 – 7,747 THz, 0,3; 5.2 – 11,803 THz, 0,7 eV; 5.3 – 14,104 THz, 1 eV. The radiation level and the surface current density are visualized using a color map in the digital edition

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