Shen Qiao

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.

Photoinduced active terahertz metamaterials with nanostructured vanadium dioxide film deposited by sol-gel method

Applying the photoexcitation characteristics of vanadium dioxide (VO(2)), a dynamic resonant terahertz (THz) functional device with the combination of VO(2) film and dual-resonance metamaterial was suggested to realize the ultrafast external spatial THz wave active manipulation. The designed metamaterial realizes a pass band at 0.28-0.36 THz between the dual-resonant frequencies, and the VO(2) film is applied to control the transmittance of the spatial THz wave. More than an 80% modulation depth has been observed in the statics experiment, and the dynamic experimental results illustrate that this active metamaterial realizes up to a 1 MHz amplitude modulation signal loaded on a 0.34 THz carrier wave without any low noise amplified devices. The electromagnetic properties and photoinduced dynamic characteristics of this structure may have many potential applications in THz functional components, including modulators, intelligent switches, and sensors.

Terahertz transmission characteristics across the phase transition in VO2 films deposited on Si, sapphire, and SiO2 substrates

Vanadium dioxide (VO2) films were deposited on high-purity Si, sapphire, and SiO2 substrates by an organic sol-gel method. The effect of the substrate on the structure, morphology, and phase transition properties of the VO2 films was demonstrated. We proposed that the film-substrate interaction induced the differences in the fraction of the +4 valence state vanadium oxide phase, surface morphology, and grain size for the VO2 films. The VO2 film on the Si substrate exhibited a switching property of about 2 orders of change in electrical resistivity. By contrast, the VO2 films on the sapphire and SiO2 substrates exhibited a switching property of about 3 orders of change in resistivity. The THz transmission across the phase transition in the VO2 films was quite different in the transmission modulation ratio, the width, and the slope of the hysteresis loop. In particular, the VO2 films on the sapphire and SiO2 substrates have the same reduction in THz transmission by about 46% comparing with about 35% in the ...

Asymmetric single-particle triple-resonant metamaterial in terahertz band

This paper presents the design, simulation, and measurement of an asymmetric triple-band metamaterial composed of single geometry electric field coupled resonators in the terahertz region. Theoretical and experimental results show that the structure has three distinct and strong absorption frequency peaks near 0.38, 0.58, and 0.74 THz, all of which are related to the inductance-capacitance resonance of the metamaterial. Due to the well-separating of different resonances in the particle, this metamaterial shows potentially application promises in the design of multiband terahertz devices.

Synthesis and terahertz transmission properties of nano-porous vanadium dioxide films

Vanadium dioxide (VO2) films were prepared by the sol–gel method on high-purity silicon substrates, in which cetyltrimethyl ammonium bromide (CTAB) was used as a functional additive to form nano-porous structure in the VO2 films. The morphology, crystalline structure and stoichiometry of the films were investigated by field emission scanning electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy. Furthermore, the effects of nano-scaled grain size and pores on the THz transmission properties across the phase transition in the VO2 films were studied. The results indicated that the film modified with CTAB can form a nano-porous VO2 structure with a uniform grain size of about 30 nm. The nano-porous VO2 film exhibited significantly broader hysteresis loops and a slight decrease in THz transmission reduction across the phase transition, compared with the common film without a nano-porous structure. A tentative interpretation is given for these phenomena.

Nanostructured VO2 film with high transparency and enhanced switching ratio in THz range

We investigated the terahertz (THz) transmission characteristics of semiconductor VO2 film and its THz suppression behavior after the phase transition. The VO2 films were deposited by the sol-gel method, and an in situ growth with surface nanocrystallization occurring in the films with increasing thickness was presented. Morphology-induced percolation leads to high THz transparency in the semiconductor VO2 film, and the more compact nanostructure could account for the enhanced THz switching ratio in the metallic film. These results may offer insights into the artificial design of VO2 films for THz device applications.

Multiband Frequency-Selective Surface With Five Resonance Peaks in Terahertz Band

We discuss a multiband frequency-selective surface (FSS) device with an irregular electromagnetic structure that has five resonance peaks in the terahertz (THz) regime. The compact asymmetric structure utilizes all components efficiently. Different modes, including LC and quasi-quadrupole resonances, are induced in each unit cell. Simulations indicate that each of the five resonance peaks has independent characteristics with different electric field and surface current distributions. The multiband FSS provides an alternative way to design THz multiband filters, modulators, absorbers, and sensors at small planar dimensions.

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.

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.