Multi-functional Device with Switchable Functions of Absorption and Polarization Conversion at Terahertz Range
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Terahertz electromagnetic (EM) wave components usually have a single function, such as they can only convert the polarization state of an incident wave or absorb the incident energy, which would be a limitation for their applications. To make a breakthrough, a multi-functional device (MFD) is proposed in this paper, and it is capable of switching between absorption mode and polarization conversion mode. The device has a low-profile and simple structure, and it is constructed by graphene-based absorbing metasurface (AM) and gold-based polarization conversion metasurface (PCM). By controlling the chemical potential (μc) of the graphene, the leading role is transferred between the AM and the PCM, which leads to steerable absorption and polarization conversion (PC) modes. For the PC mode, the simulated polarization conversion ratio (PCR) is larger than 0.9 in the 2.11–3.63-THz band (53.0% at 2.87 THz). For the absorption mode, the simulated absorptivity is larger than 80% in the 1.59–4.54-THz band (96.4% at 3.06 THz). The physical mechanisms and operating characteristics of the MFD are discussed. This research has potential applications in terahertz imaging, sensors, photodetectors, and modulators.
KeywordsAbsorption mode Polarization conversion mode Terahertz Graphene Metasurface
Polarization conversion metasurface
Polarization conversion ratio
Surface plasmon polaritons
Absorbers and polarization converters, capable of regulating electromagnetic (EM) wave, are two crucial devices for terahertz technology. They have significant applications in sensors, photodetectors, and modulators, and they are indispensable in medical imaging/diagnostics, environmental monitoring and surveillance, chemical spectroscopy, high-resolution radar, and high-speed communication [1, 2, 3, 4]. The absorbers are utilized to absorb and dissipate the impinging EM wave, while the polarization converters have the capacity of polarization state regulating of the illuminating wave. These devices are widely studied in recent years [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24].
Metasurfaces are found to have perfect absorption in the terahertz wave range [5, 6, 7, 8]. This metasurface can be constructed by gold patterns or graphene patterns. The gold patterns include coupled ring resonator and cross-shaped structure , cross-shaped gold resonator , and three-layer cross-shaped gold resonators . However, the bandwidths of these gold metasurface absorbers are quite narrow. Graphene, which supports surface plasmons in the terahertz range [10, 11], is a good material to design metasurface-based absorber with a wide bandwidth. The fishnet graphene pattern achieves a bandwidth of 59.4% at 3.2 THz , the dual-ring structure with hybridized plasmonic resonances obtains a bandwidth of 1.18–1.64 THz (32.6%) , the nine layers of different size graphene ribbons realizes good absorption from 3 to 7.8 THz (88.9%) , and the three-layer asymmetrically pattern graphene strips etched with holes in  has a bandwidth of 84.6% (4.7–11.6 THz). Though the monolayer of transition metal dichalcogenides and periodic metal nano-groove array has a narrow bandwidth, it absorbs light in a wide angle . In , monolayer MoS2 is applied to titanium nitride nano-disk array, which achieves an average absorption of 98.1% in the band from 400 to 850 nm (72%).
On the other hand, metasurfaces have high performance in polarization conversion. Noble metals, such as gold, have high efficiency for metasurface-based polarization converter designing. Double L-shaped pattern with two metallic gratings in  rotates a linear polarization (LP) by 90°. The bandwidth of the converter in  is 0.2–0.4 THz (66.7%). Double L-shaped pattern and grating with Fabry-Perot-like resonance achieve a bandwidth from 0.55 to 1.37 THz (85.4%) . Three-layer metasurfaces form a quarter-wave converter to convert a LP incident wave to a circular polarization (CP) wave, in a bandwidth of 2.1–8 THz (116.8%) . The strip-loaded half elliptical ring structure in  is capable of cross-polarization converting both LP and CP with a bandwidth of 2.1–2.9 THz (32%). The graphene metasurfaces applied for polarization converter usually realize the function of frequency or polarization state tuning. The designs in [22, 23] obtain polarization rotation by etching slots/hollows periodically on graphene sheets, and the operating frequencies can be dynamically tuned by adjusting the chemical potential (μc). The periodic graphene patterns  and dual crossed graphene gratings  tune the polarization states. The design in  applies graphene strips on the ground to disturb the field distributions; then, the polarization conversion ratio can be regulated.
Though the above-reported absorbers and polarization converters are very efficient, these devices are a single function. They are not accommodated with terahertz systems that require portable, compact, and multi-functional devices. Therefore, multi-functional devices (MFDs) are significant. In this research, an MFD, capable of switching between absorption mode and polarization conversion mode, is proposed. The proposed MFD has a low-profile and simple structure by assembling a gold-based polarization conversion metasurface (PCM) and a graphene-based absorbing metasurface (AM). Then, by setting the chemical potential of graphene μc = 0 eV, the AM is neutralized and the PCM plays a dominant role, and the device rotates the polarization of an incident EM wave. By setting μc = 0.7 eV, the AM takes the main role and the device absorbs the incident EM wave.
where e, ℏ, kB, T, and μc represent the charge of an electron, the reduced Planck’s constant, Boltzmann’s constant, Kelvin’s temperature, and chemical potential, respectively. The Γ is a phenomenological scattering rate, and it is assumed to be independent of energy ε. Thus, the complex conductivity σs can be adjusted by tuning the chemical potential (μc) with biasing voltage. It is found in Eq. (1) that for μc = 0 eV, the conductivity of the graphene is very small owing to the low carrier density at this case. Therefore, the graphene operates as a dielectric substrate. Moreover, as the graphene layer is very thin, it has little impact on the illuminated EM waves for μc = 0 eV. However, the carrier density of the graphene would be raised with increasing chemical potential (μc), and the complex conductivity (σs) of the graphene is boosted with increasing chemical potential (μc) [26, 27]. Therefore, the graphene supports surface plasmon polaritons (SPPs) for large μc [26, 28, 29, 30], and the SPPs confine the incident waves. To further enhance the SPPs and achieve wave absorption in certain frequencies, periodical structures should be etched in the graphene layer to form a metasurface, which is called AM. Therefore, by setting μc = 0, the AM can be deemed as a thin dielectric substrate, and it is almost transparent to EM wave. Thereby, the incident EM wave can be concentrated on the PCM layer, and the device operates in the PC mode. For an appropriate large μc, the enhanced SPPs of the AM confine most of the incident EM wave, which makes the PCM layer of no avail. Thereby, the incident EM waves are dissipated in the AM layer.
Results, Physical Mechanisms, and Discussion
As shown in Fig. 3b, the MFD operates at PC mode with μc = 0 eV, and it works at absorption mode with μc = 0.7 eV. At the PC mode, the structure operates as a polarization converter, and it rotates a linear polarized incident wave to its orthogonal polarization wave. For the PC mode, the PCR is larger than 0.9 in the 2.11–3.63-THz band (53.0% at 2.87 THz), while the absorptivity is small and it ranges from 0.14 to 0.27 in the band. For the structure without AM, it has almost the same PCR band as the PC mode while its absorptivity ranges from 0.06 to 0.09. In the absorption mode, most of the incident wave is absorbed in the band as demonstrated in the figure. Note that the PCR curve for absorption mode is not presented as it is meaningless. The absorptivity is larger than 80% in the 1.59–4.54-THz band (96.4% at 3.06 THz). Therefore, by adjusting the chemical potential, the proposed structure can switch between PC mode and absorption mode.
For the PC mode (μc = 0 eV), two frequencies of 2.56 THz and 3.22 THz are chosen to present their field distributions at Fig. 4a and b, respectively. The left parts of the figures are the electric energy densities, and the right parts are the currents. As shown in the figures, the field distributions of 2.56 THz and 3.22 THz are very similar to each other, which imply a wide operating band. From the electric energy densities at the left parts of Fig. 4a, b, the energies are mainly concentrated on the L-shaped structures (PCM). It is indicated that the PCM plays a leading role for μc = 0 eV. From the currents at the right parts of Fig. 4a, b, the currents of both 2.56 THz and 3.22 THz are also concentrated on the PCM, and the currents on the AM are weak. The dotted line arrows indicate the vectors of the currents. The y-polarized illuminations generate x-vector currents on the L-shaped structures, which achieve polarization conversion.
For the absorption mode (μc = 0.7 eV), the electric energy densities of 1.7 THz and 3.3 THz are painted in Fig. 5a and b, respectively. As shown in the figure, the electric energy densities of the two frequencies are mainly distributed on the AM. It is also found that the energies are focused in the cross-slot patterns; therefore, SPP effects are enhanced by the cross-slots on the AM. The strong SPP effects lead to field enhancement on the AM, which endow the AM a dominant role. Thereby, the incident waves are confined and dissipated in the AM. It is also found that there are still some energies spread on the PCM, which make no perfect absorption, such as 80–90% absorptivity in the band.
For the absorption mode, the absorptivity plots of s- and p-polarized incident waves are plotted in Fig. 9a and b, respectively, with the incident angle (θ) ranged from 0 to 80°. Generally speaking, the absorptivity of the s-polarized incidence reduced with increasing θ, and the absorptivity is larger than 0.8 for θ smaller than 30°. It is interesting to find that the absorptivity of p-polarized incident EM wave increased with increasing θ.
In summary, a low-profile and simple structure MFD is proposed by combining gold-based PCM and graphene-based AM. The chemical potential (μc) can be utilized to activate or neutralize the graphene-based AM, and then, the structure can be transformed from absorber to polarization converter. For the PC mode, the PCR is larger than 0.9 in the 2.11–3.63-THz band (53.0% at 2.87 THz). For the absorption mode, the absorptivity is larger than 80% in the 1.59–4.54-THz band (96.4% at 3.06 THz). The design may be applied to terahertz imaging, sensing, photodetection, and modulation systems.
National Natural Science Foundation of China under Grant Nos. 61661011 and 61761012.
Availability of Data and Materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
LP developed the concept. LP conducted the simulations. LP, JX, and SML analyzed the results and physical mechanisms. All authors reviewed the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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