Chenyu Li

Resonance coupling and polarization conversion in terahertz metasurfaces with twisted split-ring resonator pairs

We investigate edge-coupling of twisted split-ring resonator (SRR) pairs in the terahertz (THz) frequency range. Using a simple coupled-resonator model we show that such a system exhibits resonance splitting and cross-polarization conversion. Numerical simulations and experimental measurements agree well with theoretical calculations, verifying the resonance splitting as a function of the coupling strength given by the SRR separation. We further show that a metal ground plane can be integrated to significantly enhance the resonance coupling, which enables the effective control of resonance splitting and the efficiency and bandwidth of the cross-polarization conversion. Our findings improve the fundamental understanding of metamaterials with a view of accomplishing metamaterial functionalities with enhanced performance, which is of great interest in realizing THz functional devices required in a variety of applications.

Manipulating the strength and broadband of the resonators in the terahertz metamaterials

We investigate two dipoles which are attached or separated with the orthogonal arrangement in the terahertz frequency. These results show that the metasurface could achieve the resonance coupling and polarization conversion effect. There are two resonance dips in the transmission spectra, when these two dipoles are attached to form the L-shaped structure. With the spacing between vertical and horizontal dipoles separated, the broadband of the resonator becomes narrower and resonance dips merge into one deeper dip due to the superposition of the interaction of two dipoles. The loss of the energy is not only coupled to the free space but also converted to the cross-polarization. The broadband and the strength of the crosspolarization are modulated by changing the distance between the vertical and horizontal dipoles. Tuning the spacing, we control the co- and cross polarization of the broadband and the strength at the same time. This modulation provides the functionally potential applications in the terahertz modulators and filters.

Studies on electromagnetic response in arc-shaped structures in terahertz region

In this paper, we use the finite-difference time-domain (FDTD) method to simulate and study the electromagnetic response characteristics of ring and arc-shaped resonators. Firstly, we study the terahertz transmission properties of two single-ring resonators with different radii. Either the single-ring resonator with a large radius or with a small radius only has one resonance in the transmission spectra. Then, we combine those resonators into a double-ring resonator structure. The results conclude that the two resonant frequencies of the double-ring resonator are caused by the simple superposition of the resonances of the large and small radius single-ring resonators, respectively. Additionally, on the basis of a single-ring resonator, we also study the influence of the symmetrical and asymmetric arc-shaped resonators on electromagnetic response characteristics. The ring resonator is split from the middle into two symmetrical arc-shaped resonators. As the width of the middle gap gradually increases, the resonant frequency shows blueshift and the intensity of the surface current distribution gradually weakens. Finally, the direction of the current is reversed. In order to further compare the relationship between the single-ring resonators and the double-ring resonators, we study the double arc-shaped resonators. The main purpose of this paper is to study the frequency response characteristics of the ring resonator in the terahertz band and to control the terahertz spectrum by changing the symmetry of the ring resonator. In the future, we can further study the coupling response between the ring structures and the multi-frequency response modulation of the multi-ring structures.

Studies on the resonant properties in the asymmetric dipole-array terahertz metamaterials

Artificial metamaterials with appropriate design can exhibit unique electromagnetic phenomena which do not exist in natural materials. Some studies have shown that the method of breaking the geometric symmetry is capable to modify the electromagnetic response, such as the metamaterial induced transparency in the Fano resonators. In this work, by using the finite-difference time-domain method, we firstly simulate the process that terahertz wave interacts with double-bar structures, in which one bar length is fixed at 36 μm and the other bar length is set to be 12, 24, 36, 48, and 56 μm, respectively. The incident terahertz polarization is along the bar direction. Simulated results show when the variable bar length is less than 36 μm, there is only one obvious resonant dip in transmission spectrum. Meanwhile, with the decreased bar length, this dip frequency presents a slight blueshift. Additionally, by tuning the spacing vertical to bar direction between these two bars, it still exhibits one dip. This result indicates the short bar less than 36 μm does not play important role and the coupling between vertical bars is weak. However, when the variable bar length is larger than 36 μm there are two obvious Fano-shaped resonant dips. With the increased bar length, the low-frequency dip shows a remarkable redshift, while the high-frequency one is almost unchanged. By further tuning the bar spacing vertical to the bar direction, two dips always exist. This phenomenon implies that the coupling between horizontal bars is dominated in this process. Moreover, the metamaterial induced transparency window is found between two resonant dips. The appearance of the resonances is attributed to the excitation of trapped mode. Our obtained results indicate that such metamaterials with very simple configuration could also provide the potential application in the field of terahertz slow-light devices, amplitude and phase modulators.

Influence of asymmetric structures on electromagnetic response characteristics of terahertz metamaterials

Asymmetric split-ring resonators (SRRs) exhibit different resonant modes and phenomena that do not have in symmetric structure, such as Fano resonance, electromagnetic induced transparency, and plasma resonance hybridization. The asymmetric SRR was first confirmed to produce narrow linewidth resonance and has a high quality factor. Then it extends to the terahertz and near infrared bands. It has been found that the most common way to modulate the electromagnetic response characteristics is to change the asymmetry of the SRR and the coupling strength between the resonators. Here we use the finite-difference time-domain (FDTD) method to simulate the electromagnetic response characteristics of asymmetric structures. When the polarization direction along the bottom bar of the U-shaped structure, there are two similar resonance dips like those in typical SRR structure. When the incident wave is perpendicular to the bottom bar, there is only one dipole resonance. However, with the broken of the symmetry, the resonant behaviors will change. In horizontal direction, both the resonant frequency and transmittance has changed. In the vertical polarization, there are three resonant dips in transmission spectrum. Meanwhile a sharp window appears in transmission spectrum. In addition, when we turn the bottom bar of the U-shaped structure into the arc shape, we just find very slight change in frequency-shift and modulation depth in both cases, showing the impact of the short arc is nearly equivalent to the linear dipole resonance. Our obtained results indicate that we could tune the electromagnetic resonances in metamaterials and the interaction mechanism with terahertz wave.

Studies on the trapped-mode resonant properties in asymmetric terahertz metamaterial

Artificial metamaterials with appropriate design can exhibit unique electromagnetic phenomena which do not exist in natural materials. Some studies have shown that the method of breaking the geometric symmetry is capable to modify the electromagnetic responses. Here, we simulated and measured the transmission spectra of period arrays of subwavelength double-bar structure. The obtained results show the trapped-mode resonance with Fano-shaped spectrum can be induced in terahertz metamaterial with asymmetric double-bar structure, accompanied with a metamaterial induced transparency window between two resonant dips. And the bar spacing and lattice constant have great impact on the coupling strength concerned with the transparency position and spectral lineshape. We attribute there are two mechanisms together determine the coupling pattern between the bar array and the terahetz wave, the coupling between the bars of the same unit cell and the coupling between the bars of the neighbouring cells. Our obtained results indicate that such metamaterial with very simple configuration could also provide the potential application in the field of terahertz slow-light devices, amplitude and phase modulators.

The birefringence of two liquid crystals in terahertz band

Compared with the wide application of liquid crystals (LCs) in the visible frequency band, their properties in the terahertz band have not been investigated extensively yet. In this paper, we have investigated the optical anisotropy of LCs TEB30A and 9023 at room temperature using terahertz time-domain spectroscopy (THz-TDS). The extraordinary and ordinary refraction indices of LC TEB30A are ne≈1.84 and no≈1.65, or a birefringence of 0.19 from 0.5 to 2.2 THz. The extraordinary and ordinary refraction indices of LC 9023 are ne≈1.83 and no≈1.62, or a birefringence of 0.21 from 0.5 to 2.2 THz. LC 9023 exhibits a little larger terahertz birefringence than that of LC TEB30A.

Influence of substrate refractive index and metallic structure size on resonant properties in terahertz split ring resonators

Metamaterials with subwavelength structural features show unique electromagnetic responses that are unattainable with natural materials. Modulation property is regarded as one of the most important features of metamaterials. At present, the development of such terahertz switches and modulators are relatively slow. So the research of the terahertz metamaterial is very meaningful. The light-control modulation, for example, could control the resonance characteristics of split ring resonators (SRRs) by changing dielectric property of the light layer. Due to the complicated effect in photo-excited layer, we could simplify the research to firstly study the influence of substrate’s refractive index on the resonant behaviors, providing the way to further the investigation of complex problems. In addition, the shape and size of metal microstructure can produce important effect on electromagnetic response. Therefore, based on the finite-difference time-domain method, we have also simulated several SRR structures with different geometry. We find the calculated terahertz transmission spectra exhibit remarkable change, showing that the resonant dips have a red-shift phenomenon and the bandwidth gets narrow with the increased refractive index as well as the structure size. Compared with the geometry effect, the red-shift is more sensitive to the change in refractive index. This work could help us to choose the suitable substrate materials for sample fabrication to realize the specific features.

Transmitted spectral modulation of double-ring resonator using liquid crystals in terahertz range

Metamaterials with subwavelength structural features show unique electromagnetic responses that are unattainable with natural materials. Recent research on these artificial materials has been pushed forward to the terahertz region because of potential applications in biological fingerprinting, security imaging, remote sensing, and high frequency magnetic and electric resonant devices. Active control of their properties could further facilitate and open up new applications in terms of modulation and switching. Liquid crystals, which have been the subject of research for more than a century, have the unique properties for the development of many other optical components such as light valves, tunable filters and tunable lenses. In this paper, we investigated the transmitted spectral modulation in terahertz range by using liquid crystals (5CB and TEB300) covering on the fabricated double-ring resonators to realize the shift of the resonance frequency. Our obtained results indicate the low frequency resonance shows the obvious blue-shift, while the location of high frequency resonance is nearly unchanged. We believe this phenomenon is related to not only the refractive index of the covering liquid crystals but also the resonant mechanism of both resonances.