Ex) Article Title, Author, Keywords
Current Optics
and Photonics
Ex) Article Title, Author, Keywords
Curr. Opt. Photon. 2022; 6(6): 590-597
Published online December 25, 2022 https://doi.org/10.3807/COPP.2022.6.6.590
Copyright © Optical Society of Korea.
Corresponding author: *leejujc@chonnam.ac.kr, ORCID 0000-0002-4624-8795
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
We carry out numerical simulations of the responses of planar metamaterials composed of metamolecules under obliquely incident terahertz waves. A Fano-like-resonant planar metamaterial, with two types of resonance modes originating from the two meta-atoms constituting the meta-molecules, exhibits high performance in terms of resonance strength, as well as the outstanding ability to manipulate the resonance frequency by varying the incident angle of the terahertz waves. In the structure, the fundamental electric dipole resonance associated with Y-shaped meta-atoms is highly tunable, whereas the inductive-capacitive resonance of C-shaped meta-atoms is relatively omnidirectional. This is attributed to the electric near-field coupling between the two types of meta-atoms. Our work provides novel opportunities for realizing terahertz devices with versatile functions, and for improving the versatility of terahertz sensing and imaging systems.
Keywords: Metamaterials, Optical devices, Terahertz
OCIS codes: (060.2630) Frequency modulation; (160.3918) Metamaterials; (230.5750) Resonators; (300.6495) Spectroscopy, terahertz
Metamaterials are artificial composites with periodic structures consisting of subwavelength geometric elements called meta-atoms. The remarkable advantage of these materials is that they exhibit unique electromagnetic characteristics, such as negative refraction, invisible cloaking, super-resolution imaging, and outstanding optical behaviors across the electromagnetic spectrum, especially from microwave to optical frequencies [1–6].
Various devices with desirable optical characteristics and versatile functions can be realized using metamaterials created by selecting a specific design of meta-atoms and engineering their spatial arrangement. Optical metamaterials exhibit interesting optical phenomena, such as reflective-index engineering, chirality, frequency selectivity, and high spectral tunability [7–10]. Furthermore, the fabricated metamolecules, composed of two or more different types of meta-atoms, provide high-level functionalities such as plasmon-induced transparency, generation and manipulation of Fano-type resonance, tunable multiple resonators, and resonance-mode engineering, owing to their strong electric near-field coupling and structural asymmetry [11–17].
Many such metamaterials exhibit favorable properties in response to terahertz (THz) frequencies, which are much lower than those in the optical and infrared regions; therefore, it is easier to realize multifunctional devices based on metallic subwavelength structures [18–20]. In particular, Jo
In this paper, we report planar THz metamaterials based on metamolecules composed of two types of meta-atoms, and analyze their coupling with obliquely incident THz waves. We design metamolecules with two basic components, a C- and a Y-shaped metallic rod, exhibiting inductive-capacitive (
The proposed THz metamaterial is illuminated by a polarized THz electromagnetic wave along the x axis, as shown in Fig. 1(a). The angle of incidence
To calculate the angle-dependent transmission spectra at THz frequencies, we carry out a frequency-domain study using the COMSOL Multiphysics software, which is a finite-element analysis, solver, and simulation package with a variety of applications in physics and engineering. Periodic boundary conditions are used in the
The simulated THz transmission amplitude spectra of each resonator at a normal incidence angle of
The transmission spectrum of the metamolecule structure obtained by combining the two meta-atoms is shown as a black solid line in Fig. 2(a). The metamolecule is designed in such a way that the dipole resonance of the Y-shaped rod and the second-order
The underlying physical mechanisms of the resonance characteristics can be understood by analyzing the distributions of the surface current and electric near field, shown in Figs. 2(b) and 2(c). The fundamental
To understand the effects of the interaction of the two adjacent resonators, the angle of incidence
The THz transmission amplitude spectra for each meta-atom, simulated at different incidence angles of
On the other hand, Figs. 3(c) and 3(d) reveal that the simulated spectra in TM mode exhibit more pronounced changes with varying angle of incidence than those in TE mode. The position of the fundamental
The resonance exhibits a larger blueshift in TM than in TE mode, due to the decrease in the electric field component parallel to the surface when the incident wave is tilted, although the difference was not significant [28]. Despite the difference in the overall shape of the spectra plotted in the two figures, the degree of change of the resonance position does not seem large enough to be meaningful.
To analyze the coupling effects on the above characteristics, the transmission amplitude spectra of the metamolecule resonator are simulated for both TE and TM modes, as shown in Fig. 4. No particular effect is observed in TE mode, as seen in Fig. 4(a). The resonance position corresponding to the C shape is still independent of the angle of incidence, and the degree of blueshift corresponding to the Y shape is similar to that observed for the individual meta-atom resonator. Apart from the shift of the resonance frequency position of each metamolecular resonator due to the Fano effect (shown in Fig. 2), no other angle-dependent coupling effects are not observed in TE mode.
In TM mode, the position of the second-order
The most noteworthy effect observed in TM mode is associated with the resonance of the Y-shaped rod appearing near 0.735 THz. The blueshift of the resonant frequency shows a completely different trend, compared to the results obtained before combining the meta-atoms, and also those observed in TE mode. The strong blueshift is due to the electric near-field coupling effect, and particularly to its asymmetric distribution on one side of the C-shaped SRR. The resonance depth, which is greatly reduced in the meta-atomic structures, remains similar to that observed in the normal-incidence case, due to the electric field coupling effect of the two meta-atoms.
To elucidate the properties of the resonances, we simulate the electric near-field distributions at the incidence angles of
To quantify the coupling effects, we obtain the rate of change of the resonant frequencies for various types of metastructures. Except for the electric dipole resonance of the metamolecule, M-Y(TE), all results were obtained in TM mode. The corresponding frequency shifts are plotted in Fig. 6. The values of the fundamental
In conclusion, we have shown that metamolecule resonators in the THz region exhibit resonance frequencies that are tunable according to the angle of incidence. Each resonance mode of the two meta-atoms constituting the metamolecule can be independently controlled by varying the angle of incidence. The frequencies of the
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.
The authors thank the funding agencies for supporting our research work. In addition, the authors would like to thank the Editor in Chief, the Associate Editor, and the anonymous referees for their insightful suggestions.
National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Grant Number: NRF- 2022R1F1A1071757).
Curr. Opt. Photon. 2022; 6(6): 590-597
Published online December 25, 2022 https://doi.org/10.3807/COPP.2022.6.6.590
Copyright © Optical Society of Korea.
Department of Physics and Optoelectronics Convergence Research Center, Chonnam National University, Gwangju 61186, Korea
Correspondence to:*leejujc@chonnam.ac.kr, ORCID 0000-0002-4624-8795
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
We carry out numerical simulations of the responses of planar metamaterials composed of metamolecules under obliquely incident terahertz waves. A Fano-like-resonant planar metamaterial, with two types of resonance modes originating from the two meta-atoms constituting the meta-molecules, exhibits high performance in terms of resonance strength, as well as the outstanding ability to manipulate the resonance frequency by varying the incident angle of the terahertz waves. In the structure, the fundamental electric dipole resonance associated with Y-shaped meta-atoms is highly tunable, whereas the inductive-capacitive resonance of C-shaped meta-atoms is relatively omnidirectional. This is attributed to the electric near-field coupling between the two types of meta-atoms. Our work provides novel opportunities for realizing terahertz devices with versatile functions, and for improving the versatility of terahertz sensing and imaging systems.
Keywords: Metamaterials, Optical devices, Terahertz
Metamaterials are artificial composites with periodic structures consisting of subwavelength geometric elements called meta-atoms. The remarkable advantage of these materials is that they exhibit unique electromagnetic characteristics, such as negative refraction, invisible cloaking, super-resolution imaging, and outstanding optical behaviors across the electromagnetic spectrum, especially from microwave to optical frequencies [1–6].
Various devices with desirable optical characteristics and versatile functions can be realized using metamaterials created by selecting a specific design of meta-atoms and engineering their spatial arrangement. Optical metamaterials exhibit interesting optical phenomena, such as reflective-index engineering, chirality, frequency selectivity, and high spectral tunability [7–10]. Furthermore, the fabricated metamolecules, composed of two or more different types of meta-atoms, provide high-level functionalities such as plasmon-induced transparency, generation and manipulation of Fano-type resonance, tunable multiple resonators, and resonance-mode engineering, owing to their strong electric near-field coupling and structural asymmetry [11–17].
Many such metamaterials exhibit favorable properties in response to terahertz (THz) frequencies, which are much lower than those in the optical and infrared regions; therefore, it is easier to realize multifunctional devices based on metallic subwavelength structures [18–20]. In particular, Jo
In this paper, we report planar THz metamaterials based on metamolecules composed of two types of meta-atoms, and analyze their coupling with obliquely incident THz waves. We design metamolecules with two basic components, a C- and a Y-shaped metallic rod, exhibiting inductive-capacitive (
The proposed THz metamaterial is illuminated by a polarized THz electromagnetic wave along the x axis, as shown in Fig. 1(a). The angle of incidence
To calculate the angle-dependent transmission spectra at THz frequencies, we carry out a frequency-domain study using the COMSOL Multiphysics software, which is a finite-element analysis, solver, and simulation package with a variety of applications in physics and engineering. Periodic boundary conditions are used in the
The simulated THz transmission amplitude spectra of each resonator at a normal incidence angle of
The transmission spectrum of the metamolecule structure obtained by combining the two meta-atoms is shown as a black solid line in Fig. 2(a). The metamolecule is designed in such a way that the dipole resonance of the Y-shaped rod and the second-order
The underlying physical mechanisms of the resonance characteristics can be understood by analyzing the distributions of the surface current and electric near field, shown in Figs. 2(b) and 2(c). The fundamental
To understand the effects of the interaction of the two adjacent resonators, the angle of incidence
The THz transmission amplitude spectra for each meta-atom, simulated at different incidence angles of
On the other hand, Figs. 3(c) and 3(d) reveal that the simulated spectra in TM mode exhibit more pronounced changes with varying angle of incidence than those in TE mode. The position of the fundamental
The resonance exhibits a larger blueshift in TM than in TE mode, due to the decrease in the electric field component parallel to the surface when the incident wave is tilted, although the difference was not significant [28]. Despite the difference in the overall shape of the spectra plotted in the two figures, the degree of change of the resonance position does not seem large enough to be meaningful.
To analyze the coupling effects on the above characteristics, the transmission amplitude spectra of the metamolecule resonator are simulated for both TE and TM modes, as shown in Fig. 4. No particular effect is observed in TE mode, as seen in Fig. 4(a). The resonance position corresponding to the C shape is still independent of the angle of incidence, and the degree of blueshift corresponding to the Y shape is similar to that observed for the individual meta-atom resonator. Apart from the shift of the resonance frequency position of each metamolecular resonator due to the Fano effect (shown in Fig. 2), no other angle-dependent coupling effects are not observed in TE mode.
In TM mode, the position of the second-order
The most noteworthy effect observed in TM mode is associated with the resonance of the Y-shaped rod appearing near 0.735 THz. The blueshift of the resonant frequency shows a completely different trend, compared to the results obtained before combining the meta-atoms, and also those observed in TE mode. The strong blueshift is due to the electric near-field coupling effect, and particularly to its asymmetric distribution on one side of the C-shaped SRR. The resonance depth, which is greatly reduced in the meta-atomic structures, remains similar to that observed in the normal-incidence case, due to the electric field coupling effect of the two meta-atoms.
To elucidate the properties of the resonances, we simulate the electric near-field distributions at the incidence angles of
To quantify the coupling effects, we obtain the rate of change of the resonant frequencies for various types of metastructures. Except for the electric dipole resonance of the metamolecule, M-Y(TE), all results were obtained in TM mode. The corresponding frequency shifts are plotted in Fig. 6. The values of the fundamental
In conclusion, we have shown that metamolecule resonators in the THz region exhibit resonance frequencies that are tunable according to the angle of incidence. Each resonance mode of the two meta-atoms constituting the metamolecule can be independently controlled by varying the angle of incidence. The frequencies of the
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at the time of publication, but may be obtained from the authors upon reasonable request.
The authors thank the funding agencies for supporting our research work. In addition, the authors would like to thank the Editor in Chief, the Associate Editor, and the anonymous referees for their insightful suggestions.
National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Grant Number: NRF- 2022R1F1A1071757).