Gaochao Zhou

Electrically tunable superconducting terahertz metamaterial with low insertion loss and high switchable ratios

With the emergence and development of artificially structured electromagnetic materials, active terahertz (THz) metamaterial devices have attracted significant attention in recent years. Tunability of transmission is desirable for many applications. For example, short-range wireless THz communications and ultrafast THz interconnects require switches and modulators. However, the tunable range of transmission amplitude of existing THz metamaterial devices is not satisfactory. In this article, we experimentally demonstrate an electrically tunable superconducting niobium nitride metamaterial device and employ a hybrid coupling model to analyze its optical transmission characteristics. The maximum transmission coefficient at 0.507 THz is 0.98 and decreases to 0.19 when the applied voltage increases to 0.9 V. A relative transmittance change of 80.6% is observed, making this device an efficient narrowband THz switch. Additionally, the frequency of the peak is red shifted from 0.507 to 0.425 THz, which means that the device can be used to select the frequency. This study offers an alternative tuning method to existing optical, thermal, magnetic-field, and electric-field tuning, delivering a promising approach for designing active and miniaturized THz devices.

Excitation of terahertz plasmon-polariton in a grating coupled two-dimensional electron gas with a Fabry-Pérot cavity

Terahertz plasmon-polariton modes are excited and probed in a grating coupled GaN/AlGaN two-dimensional electron gas embedded in a Fabry-Perot cavity using a terahertz time-domain spectroscopy at 8K. A strong coupling between the plasmon modes and the cavity modes was observed. Electromagnetic simulations confirmed that both the cavity and the grating coupler play important roles in coupling the terahertz field and the plasmons. The finding suggests that the manipulation of terahertz plasmon-polariton could be achieved by engineering the Q-factor and the mode number of the cavity to further enhance the coupling strength

Designing perfect linear polarization converters using perfect electric and magnetic conducting surfaces

We propose a kind of general framework for the design of a perfect linear polarization converter that works in the transmission mode. Using an intuitive picture that is based on the method of bi-directional polarization mode decomposition, it is shown that when the device under consideration simultaneously possesses two complementary symmetry planes, with one being equivalent to a perfect electric conducting surface and the other being equivalent to a perfect magnetic conducting surface, linear polarization conversion can occur with an efficiency of 100% in the absence of absorptive losses. The proposed framework is validated by two design examples that operate near 10 GHz, where the numerical, experimental and analytic results are in good agreements.

Broadband and high modulation-depth THz modulator using low bias controlled VO_2-integrated metasurface

An active vanadium dioxide integrated metasurface offering broadband transmitted terahertz wave modulation with large modulation-depth under electrical control is demonstrated. The device consists of metal bias-lines arranged with grid-structure patterned vanadium dioxide (VO2) film on sapphire substrate. Amplitude transmission is continuously tuned from more than 78% to 28% or lower in the frequency range from 0.3 THz to 1.0 THz, by means of electrical bias at temperature of 68 °C. The physical mechanism underlying the devices electrical tunability is investigated and found to be attributed to the ohmic heating. The developed device possessing over 87% modulation depth with 0.7 THz frequency band is expected to have many potential applications in THz regime such as tunable THz attenuator.

Electrical dynamic modulation of THz radiation based on superconducting metamaterials

We demonstrate an electrically tunable superconducting metamaterial capable of modulating terahertz waves dynamically. The device is based on electromagnetically induced transparency-like metamaterials, and the maximum modulation depth reaches 79.8% in the transmission window. Controlled by an electrical sinusoidal signal, such a device could achieve a modulation speed of approximately 1 MHz. The superior property and simplicity of design make this device promising for the development of high performance THz systems.

Terahertz narrow bandstop, broad bandpass filter using double-layer S-shaped metamaterials

In this study, double-layer S-shaped metamaterials (MMs) are analyzed by terahertz time-domain spectroscopy. These materials exhibit narrow bandstop and broad bandpass transmission properties at both horizontal and vertical electric-field polarizations. A 117% increase in the unloaded quality factor is experimentally observed for these materials. The center frequency is approximately 0.45 THz, with a 3-dB bandwidth of 0.52 THz from 0.20 to 0.72 THz at normal incidence. The measured average insertion loss is 0.5 dB with a ripple of 1 dB. These results show that double-layer S-shaped MMs are effective in designing tunable terahertz devices.

Effect of loss and coupling on the resonance of metamaterial: An equivalent circuit approach

In this work we establish an equivalent circuit model to analyze the resonace of the metamaterial considering the loss of the unit cell and coupling effect between them. From this model, we find that metamaterial can be divided into three categories: weak, critical and strong couplings, depending on the values of the loss and coupling strength, where the different resonant properties are presented. The physical reason of the division is whether the loss in each unit cell can be offset by energy coupling from the adjunct unit cells. Full-wave electromagnetic simulations have also been carried out to verify the equivalent circuit analysis. Our circuit analysis provides a simple and effective way to understand the coupling of the metamaterial and gives guidance for the analysis and design of the metamaterial.概要创新点人工电磁材料的结构之间存在电磁耦合, 特别是结构距离很近的时候. 这种电磁耦合以及结构的损耗对其谐振特性起很重要的影响. 本文建立了人工电磁材料的等效电路模型, 在模型中分别考虑了电场耦合和磁场耦合以及结构损耗, 并根据耦合强度和损耗值的关系, 把人工电磁材料中的谐振特性分成三类: 强耦合, 临界耦合和弱耦合. 通过对等效电路模型的分析, 得出了人工电磁材料的谐振频率的劈裂和这三类耦合之间的关系, 并从能量的角度解释了其物理机理.

Tailoring electromagnetically induced transparency effect of terahertz metamaterials on ultrathin substrate

Electromagnetically induced transparency (EIT) is a fascinating phenomenon in optical physics and has been employed in slow light technology. In this work, we use terahertz (THz) metamaterials to mimic EIT phenomenon and study their spectral dependence on the coupling strength between bright and dark resonators. In these metamaterials, two kinds of resonators are located on two different layers separated by a 10-µm-thick polyimide (PI) film. The whole sample is supported by a 5-µm-thick flexible PI film, so the Fabry-Perot resonance at THz can be avoided. The coupling strength is tuned by the translational offset of symmetry axes between two different kinds of resonators, resulting in the change of EIT-like spectra.

A broadband reflective-type half-wave plate employing optical feedbacks

We propose and demonstrate a type of a broadband half-wave plate that operates in the reflective mode. It consists of a metal grating embedded in a dielectric slab and placed on top of a grounded metal surface. We theoretically show that owing to the optical feedback effect which originates from the wave reflections at the air-dielectric interface, the proposed half-wave plate exhibits a broadened and flattened response when comparing to the case where the feedback effect is absent. Such a prediction is validated using both numerical and experimental works carried out on a half-wave plate designed at 10 GHz. Moreover, our theoretical analysis also reveals that the half-wave plate has an interesting feature of broad angular response. Taking advantage of these features, we experimentally demonstrate that the proposed device can function as a freely tunable linear polarization converter with polarization conversion residues less than −20 dB in a wide frequency band, under the condition that the incident angle is as large as 45 degrees.

Normal and inverse bulk spin valve effects in single-crystal ruthenates

The current-perpendicular-to-plane magnetoresistivity (CPP-MR) /ρc(B) is investigated in single crystal ruthenates Ca3(Ru1−xTix)2O7 (x = 0.02). This material is naturally composed of ferromagnetic metallic bilayers (Ru,Ti)O2 separated by nonmagnetic insulating layers of Ca2O2, resulting in tunneling magnetoresistivity. Non-monotonic ρc(B) curves as well as the inverse spin valve effect are observed around the magnetic phase transition associating with the metal-to-insulator transition. A spin dependent tunneling model with alternate distribution of hard and soft magnetic layers [(Ru,Ti)O2] is proposed to explain the exotic CPP-MR behavior. This eccentric CPP-MR behavior highlights the strong spin-charge coupling in double-layered ruthenates and provides a potential material for spintronic devices.