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.
Abstract. An improved analysis of polarization (as the ratio of vertical magnetic field component to the horizontal one) has been developed, and applied to the approximately four years data (from 1 March 2003 to 31 December 2006) observed at Kashi station in China. It is concluded that the polarization ratio has exhibited an apparent increase only just before the earthquake on 1 September 2003 (magnitude = 6.1 and epicentral distance of 116 km).
Abstract. The long-term data (during the period of 1 March 2003 through 31 December 2006) of ULF geomagnetic variations observed at Kashi station (geographic coordinates: 39.5 • N, 76.0 • E) in China have been used to investigate the long-term variation of fractal dimension of ULF emissions. We have studied the changes in fractal dimension in association with several earthquakes around the observation station. It is then found that a significant change (or decrease) in the fractal dimension of the Z component took place before the 1 September 2003 earthquake, which lends a further support to our previous finding based on our improved polarization analysis for the same earthquake. The results obtained are discussed in the contexts of a few aspects (detectability of seismogenic emissions, comparison with previous results by other analysis methods, the importance of fractal analysis in the nonlinear process of the lithosphere and earthquake prediction).
The zinc blend nonlinear crystal
of zinc telluride (ZnTe) is currently
one of the most commonly used electro-optical material for terahertz
(THz) probe and imaging. We report herein how to engineer the surface
behavior of a ZnTe single crystal to design subwavelength structures
(SWSs) for enhancing ultrabroadband transmission. Polystyrene (PS)
nanoparticle monolayers with a maximum coverage of 85.2% were produced
on the ZnTe crystal by an eccentric spin-coating technique combined
with surface wettability engineering. Subsequently, the well-defined
conical SWS arrays were fabricated on the ZnTe crystal by reactive
ion etching over the PS monolayer template, with the size of the SWS
arrays customized by optimizing the etching process. Finally, we demonstrated
ultrabroadband antireflection on the surface structured ZnTe crystals
in the visible-near-infrared, infrared, and terahertz regions with
transmittance increase of 11.6%, 10.0%, and 24.8%, which are attributed
to the decrease of surface Fresnel reflection by SWS. Notably, in
0.2–1.0 THz, the transmittance reached over 70%. Our work provides
a new strategy to enhance the THz generation efficiency and detection
sensitivity based on ZnTe crystals by surface engineering.
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