Yaxin Zhang

Gbps Terahertz External Modulator Based on a Composite Metamaterial with a Double-Channel Heterostructure

The past few decades have witnessed a substantial increase in terahertz (THz) research. Utilizing THz waves to transmit communication and imaging data has created a high demand for phase and amplitude modulation. However, current active THz devices, including modulators and switches, still cannot meet THz system demands. Double-channel heterostructures, an alternative semiconductor system, can support nanoscale two-dimensional electron gases (2DEGs) with high carrier concentration and mobility and provide a new way to develop active THz devices. In this Letter, we present a composite metamaterial structure that combines an equivalent collective dipolar array with a double-channel heterostructure to obtain an effective, ultrafast, and all-electronic grid-controlled THz modulator. Electrical control allows for resonant mode conversion between two different dipolar resonances in the active device, which significantly improves the modulation speed and depth. This THz modulator is the first to achieve a 1 GHz modulation speed and 85% modulation depth during real-time dynamic tests. Moreover, a 1.19 rad phase shift was realized. A wireless free-space-modulation THz communication system based on this external THz modulator was tested using 0.2 Gbps eye patterns. Therefore, this active composite metamaterial modulator provides a basis for the development of effective and ultrafast dynamic devices for THz wireless communication and imaging systems.

Multi-band terahertz active device with complementary metamaterial

We describe a multi-band terahertz-active device using a composite structure made of complementary metamaterial and doped silicon that can be dynamically controlled. This special complementary metamaterial exhibits three resonances that produce three pass-bands. The pass-bands can be uniformly manipulated by exploiting the photoinduced characteristics of the doped silicon. Simulations were performed to analyze the magnetic field and surface current distributions. The simulation results agree well with experimental results obtained from terahertz time-domain spectroscopy. Using an 808-nm-wavelength laser beam, a modulation depth of up to 80% was obtained. In numerical simulations, we used a conductivity mode to characterize photoinduction. The development of multi-band terahertz-active devices has many potential applications, for example, in filters, modulators, switches, and sensors.

Controlling the transparency window in terahertz band using mode coupling metamaterials

Mode-coupling metamaterials are typically composite structures with different resonance modes. Controlling couplings among these modes results in a sharp transparency window within the absorption spectrum of the metamaterials. Here, we present a composite structure of ring and split-ring resonators to constitute a new structure with entirely new mode. Experimental results show that the asymmetric combination of these resonators can result in a variation in transparency strength. The dimensions and relative positions of the split-ring resonator are discussed particularly with a series of experimental results. Simulation results show that the coupling intensity is the main reason for this behavior. Exploiting this aspect, a way to control the transparency window between composite structure metamaterials is proposed.

A terahertz-band branch waveguide directional coupler based on micro-machining

A compact E plane of 10 dB rectangular waveguide directional coupler with different lengths of branch waveguide operating at the center frequency of 340 GHz is presented. The coupler is fabricated using the micro-fabrication technology of deep reactive ion etching (DRIE). The S-parameter is measured by the two-port vector network analyzer and the result is consistent with that of simulation, which indicates that the isolation of the coupler is better than 30 dB, the return loss is greater than 15 dB, and the best insertion loss is 2.9 dB within 330~350 GHz. With simple, compact structure and superior performance, the coupler provides a new way for designing terahertz passive circuit.

Terahertz metamaterial and its sensing application

Summary form only given. Spatial overlap between the electromagnetic fields and the analytes is a key factor for strong light-matter interaction leading to high sensitivity for label-free optical biosensors. Usually, the exponential fields of cavity modes or surface plasmon resonances are applied to monitor the refractive index variation from bio-reactions. The sensitivity is therefore limited by the influence of local index variation to the weak exponential field. In this paper, by constructing a metallic microstructure array-dielectric-metal (MDM) structure, a novel metamaterial integrated microfluidic (MIM) sensor is demonstrated in terahertz (THz) range, where the dielectric layer of the MDM metamaterial is hollow and acts as the microfluidic channel. Tuning the electromagnetic parameters of metamaterial, greatly confined electromagnetic fields can be obtained in the channel resulting in significantly enhanced interaction between the analytes and the THz wave. A record high sensitivity of 3.5 THz/RIU is predicted by numerical simulation. Normalized the sensitivity to the working frequency, the calculated and measured normalized sensitivity is 0.55/RIU and 0.31/RIU, respectively. The proposed idea to integrate metamaterial and microfluid with a large light-matter interaction can be extended to other frequency regions and has promising applications in biosensing and matter detection.

Gbps THz external modulator based on the high electron mobility transistor-metamaterial

Utilizing THz waves to transmit data for communication and imaging places high demands in phase and amplitude modulation. Therefore, active devices including modulators and switches have been intensively studied in the THz regime. However, till now these devices still cannot meet the demands of THz systems. In this article we demonstrate an effective, ultra-fast and all electronic grid-controlled THz modulator, which combines an equivalent collective dipolar metamaterial array with an AlGaN/GaN hetero structure. By controlling the carrier concentration of two-dimensional electron gas (2DEG) of the modulator, we realize a resonant mode conversion with blue-shift that significantly improves the modulation speed and depth. This THz modulator achieved 1 GHz modulation speed and 67% modulation depth in real-time dynamic test. Moreover, a 1.19 rad phase shift has also been realized. This active metamaterial modulator can be applied as an effectively and ultra-fast dynamic device in THz wireless communication systems.

Mode coupling in terahertz metamaterials using sub-radiative and super-radiative resonators

We theoretically and experimentally explored the electromagnetically induced transparency (EIT) mode-coupling in terahertz (THz) metamaterial resonators, in which a dipole resonator with a super-radiative mode is coupled to an inductance-capacitance resonator with a sub-radiative mode. The interference between these two resonators depends on the relative spacing between them, resulting in a tunable transparency window in the absorption spectrum. Mode coupling was experimentally demonstrated for three spacing dependent EIT metamaterials. Transmittance of the transparency windows could be either enhanced or suppressed, producing different spectral linewidths. These spacing dependent mode-coupling metamaterials provide alternative ways to create THz devices, such as filters, absorbers, modulators, sensors, and slow-light devices.

Estimation of directive radiator's beam-width based on iteration-free near to far field transformation

This paper proposes an iteration-free near to far field transformation for estimation of directive radiators beam-width. Near field amplitudes are scanned on two planes, and they are used to scale the radiation matrix that relates the complex-valued fields on the two planes. It is shown that an eigen-vector of the scaled radiation matrix represents the phase of the field on the first plane. With the scanned amplitude and the retrieved phase, the radiation pattern is calculated based on Huygens principle. Due to the limited size of the scanning plane, the side-lobe of the radiation pattern is shifted, but the main lobe is accurately reconstructed. Therefore, the proposed method is useful for estimating the beam-width of directive radiators.

Terahertz superradiation with a cylindrical surface wave structure within a 3-mirror quasi-optical cavity

In this paper, a cylindrical surface wave-hollow electron beam interaction in a special 3-mirror quasi-optical cavity is presented and explored. The study demonstrates THz free electron superradiation from the interaction of cylindrical surface wave and hollow electron beam that forms a resonance within the structure, with the 3-mirror cavity enhancing the intensity of superradiation. The results show that the cylindrical surface wave-hollow electron beam interaction is much stronger than that of planar surface wave and sheet electron beam. Moreover, this system can work in the high-harmonic superradiation region with relatively high efficiency and low current density.

Compact bandstop filter using novel coupled-line hairpin unit

A novel coupled-line hairpin unit is proposed to design a compact bandstop filter. In order to improve the performance of the filter, defected microstrip structure is used in the parallel coupled line section. Design equations are obtained using a lossless transmission line model and parallel coupled line model. To validate the analysis, a bandstop filter with about 67% fractional bandwidth at 5 GHz has been fabricated. The implementation area is 0.241λg × 0.068λg .