Metasurfaces for manipulating terahertz waves

 

The associated THz responses include focusing, holograms, polarization modulation, special beams and active controlling. Credit: Xiaofei Zang, Bingshuang Yao, Lin Chen, Jingya Xie, Xuguang Guo, Alexei V. Balakin, Alexander P. Shkurinov, and Songlin Zhuang

https://phys.org/news/2021-03-metasurfaces-terahertz.html

by Chinese Academy of Sciences

THz waves have a plethora of applications ranging from biomedical and medical examinations, imaging, environmental monitoring, to wireless communications, because of abundant spectral information, low photon energy, strong penetrability, and shorter wavelength. THz waves with technological advances not only determined by high-efficiency sources and detectors but also decided by a variety of high-quality THz components/functional devices. However, traditional THz devices should be thick enough to realize the desired wave-manipulating functions, hindering the development of THz integrated systems and applications. Although metamaterials have shown groundbreaking discoveries due to tunable electric permittivity and magnetic permeability of a meta-atom, they are limited by technical challenges of fabrication and high loss of the metal-based unit cell.

In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Professor Songlin Zhuang from Terahertz Technology Innovation Research Institute, University of shanghai for Science and Technology, and co-workers have summarized recent advancements of metasurfaces for the manipulation of THz waves. These ultra-compact devices with unusual functionalities render metasurface devices very attractive for applications such as imaging, encryption, information modulation and THz communications.

Actually, metasurfaces typically consist of planar antennas that enable predesigned EM responses. The antennas are made of metals or traditional high-refractive index dielectrics that can be easily fabricated based on standard fabrication processes. In addition, metasurfaces with functionality in manipulating EM waves are dependent on abrupt phase changes at planar antenna interfaces, and thus the thickness of metasurfaces is much thinner than the incident wavelength. Metasurfaces can locally control the wavefront of EM waves at subwavelength resolution, leading to various practical applications such as metalenses, waveplates, vortex beam generators, beam steering and holograms. The ultrathin nature of metasurfaces, ease of fabrication, and subwavelength resolution in manipulation of EM waves make metasurfaces ideal candidates for THz device miniaturization (ultra-compact THz devices) and system integration.

The metasurface-based approach for manipulatig THz waves enables remarkable contributions in designing ultra-thin/ultra-compact and tunable THz components. The main advantages/contributions of THz metasurfaces can be concluded as follows: (1) THz components with reduced size: The functionalities of focusing, OAM, and polarization conversion realized by metasurfaces can be traditionally obtained by using a THz lens, helical phase plate, and half-wave (or quarter-wave) plate, respectively; (2) THz components with multiple functions: The traditional THz devices, e.g. THz lenses, waveplates, etc, always show a single function. Metasurfaces not only provide a flexible platform to realize ultra-thin/ultra-compact THz devices with single function, but also enable the unprecedented capability in designing multifunctional THz devices. (3) THz components with tunable function: Metasurfaces combined with VO2, graphene, etc, open a new avenue for designing THz components with active functions.

In conclusion, metasurfaces with planar structures can locally modify the wavefront of THz waves at subwavelength resolution. Metasurfaces not only provide an ultra-compact platform for manipulating the wavefront of THz waves, but also generate a plethora of applications that are difficult to achieve with conventional functional devices. As an overview, the recent developments of metasurfaces for manipulating THz waves were presented in this paper, and this progress report may open a new avenue to design ultra-thin or ultra-compact THz functional devices and systems.

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Researchers build a silicon-graphene-germanium transistor for future THz operation

Device design and fabrication. a A Si–Gr–Ge transistor is built by directly stacking a Si membrane, single-layer graphene and a Ge substrate. b Optical image of a Si–Gr–Ge transistor (scale bar: 20 μm). c SEM image of a Si membrane on graphene (scale bar: 4 μm). d Illustration of the cross-section of the transistor. e Illustration of the basic operating principle of the transistor. Credit: Nature Communications

https://phys.org/news/2019-11-silicon-graphene-germanium-transistor-future-thz.html


In 1947, the first transistor, a bipolar junction transistor (BJT), was invented in the Bell Laboratory and has since led to the age of information technology. In recent decades, there has been a persistent demand for higher frequency operation for a BJT, leading to the inventions of new devices such as heterojunction bipolar transistors (HBT) and hot electron transistors (HET). The HBTs have enabled terahertz operations, but their cut-off frequency is ultimately limited by the base transit time; for the HETs, the demand of a thin base without pinholes and with a low base resistance usually causes difficulties in material selection and fabrication.

Recently, researchers have proposed graphene as a base material for transistors. Because of the atomic thickness, the graphene base is almost transparent to electron transport, leading to a negligible base transit time. At the same time, the remarkably high carrier mobility of graphene will benefit the base resistance compared with a thin bulk material. Graphene-based transistors (GBTs) generally use a tunnel emitter that emits an electron through an insulator. However, the emitter potential barrier height seriously limits the cut-off frequency. Theoretical study has indicated that a Schottky emitter may solve this potential barrier limitation.

A team of researchers at the Institute of Metal Research, Chinese Academy of Sciences, has built the first graphene-based transistor with a Schottky emitter, which is a silicon-graphene-germanium transistor. Using a semiconductor membrane and graphene transfer, the team stacked three materials including an n-type top single-crystal Si membrane, a middle single-layer graphene (Gr) and an n-type bottom Ge substrate. 

Compared with the previous tunnel emitters, the on-current of the Si-Gr Schottky emitter shows the maximum on-current and the smallest capacitance, leading to a delay time more than 1,000 times shorter. Thus, the alpha cut-off frequency of the transistor is expected to increase from about 1 MHz by using the previous tunnel emitters to above 1 GHz by using the current Schottky emitter. THz operation is expected using a compact model of an ideal device. The electrical behavior and physical activity of the working transistor are discussed in detail in the published paper in Nature Communications.

With further engineering, the vertical semiconductor-graphene-semiconductor transistor is promising for high-speed applications in future 3-D monolithic integration because of the advantages of atomic thickness, high carrier mobility, and the high feasibility of a Schottky emitter.