Thursday, October 8, 2015

Abstract-Broadband, Spectrally Flat, Graphene-based Terahertz Modulators

    Fenghua Shi1
  1. Yihang Chen1,*
  2. Peng Han1 and
  3. Philippe Tassin2,*
Article first published online: 8 OCT 2015
DOI: 10.1002/smll.201502036

Advances in the efficient manipulation of terahertz waves are crucial for the further development of terahertz technology, promising applications in many diverse areas, such as biotechnology and spectroscopy, to name just a few. Due to its exceptional electronic and optical properties, graphene is a good candidate for terahertz electro-absorption modulators. However, graphene-based modulators demonstrated to date are limited in bandwidth due to Fabry–Perot oscillations in the modulators’ substrate. Here, a novel method is demonstrated to design electrically controlled graphene-based modulators that can achieve broadband and spectrally flat modulation of terahertz beams. In our design, a graphene layer is sandwiched between a dielectric and a slightly doped substrate on a metal reflector. It is shown that the spectral dependence of the electric field intensity at the graphene layer can be dramatically modified by optimizing the structural parameters of the device. In this way, the electric field intensity can be spectrally flat and even compensate for the dispersion of the graphene conductivity, resulting in almost invariant absorption in a wide frequency range. Modulation depths up to 76% can be achieved within a fractional operational bandwidth of over 55%. It is expected that our modulator designs will enable the use of terahertz technology in applications requiring broadband operation.

Abstract-Active metasurface terahertz deflector with phase discontinuities

Xiaoqiang Su, Chunmei Ouyang, Ningning Xu, Wei Cao, Xin Wei, Guofeng Song, Jianqiang Gu, Zhen Tian, John F. O’Hara, Jiaguang Han, and Weili Zhang

Metasurfaces provide great flexibility in tailoring light beams and reveal unprecedented prospects on novel functional components. However, techniques to dynamically control and manipulate the properties of metasurfaces are lagging behind. Here, for the first time to our knowledge, we present an active wave deflector made from a metasurface with phase discontinuities. The active metasurface is capable of delivering efficient real-time control and amplitude manipulation of broadband anomalous diffraction in the terahertz regime. The device consists of complementary C-shape split-ring resonator elements fabricated on a doped semiconductor substrate. Due to the Schottky diode effect formed by the hybrid metal-semiconductor, the real-time conductivity of the doped semiconductor substrate is modified by applying an external voltage bias, thereby effectively manipulating the intensity of the anomalous deflected terahertz wave. A modulation depth of up to 46% was achieved, while the characteristics of broadband frequency responses and constant deflected angles were well maintained during the modulation process. The modulation speed of diffraction amplitude reaches several kilohertz, limited by the capacitance and resistance of the depletion region. The scheme proposed here opens up a novel approach to develop tunable metasurfaces.
© 2015 Optical Society of America
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Abstract-Evaluating PM2.5 at a Construction Site Using Terahertz Radiation

Zhan, H. Li, Q. ; Zhao, K. ; Zhang, L. ; Zhang, Z. ; Zhang, C. ; Xiao, L.
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China

Economic and industrial development has led to increasing problems with particulate pollution in many countries. Particulate matter with the diameter of less than 2.5 $mu$m (PM2.5) is of concern in many cities. In this study, a total of 70 samples of PM2.5 were collected from dusty environments and analyzed using terahertz (THz) radiation. The transmission spectrum of PM2.5 had two distinct absorption bands between 2.5 and 7.5 THz. Their center frequencies were 3.36 and 6.91 THz, respectively. Based on the THz absorbance spectra, the elemental compositions were studied by monitoring PM2.5 masses in conjunction with two-dimensional correlation spectroscopy. Correlations between absorption bands and cross peaks in the synchronous and asynchronous plots indicated that the metallic oxides showed absorption features in the range from 2.5 to 7.5 THz. The vibration modes of anions and cations were located at relatively low and high frequencies, respectively. Statistical methods, including partial least squares and back propagation artificial neural network, were used to quantitatively characterize the PM2.5 content with the input of absorbance over the whole frequency range. This research indicates that THz spectral analysis of PM2.5 is a promising tool for investigating the composition and mass of pollutants.

Wednesday, October 7, 2015

Scientists create mini linac prototype, could alter radiation therapy

                               Terahertz accelerator modules easily fit into two fingers.
                                   Credit: DESY/Heiner 

by Lisa Chamoff

A team of scientists has built a prototype for a miniature particle accelerator with a single module that is 1.5 centimeters long and 1 millimeter thick, which could enable new diagnostic imaging and radiation therapy techniques. 

The researchers, part of the Hamburg-based Center for Free-Electron Laser Science — a joint enterprise of DESY, the Max Planck Society and the University of Hamburg — presented the prototype in the journal Nature Communications. The prototype, which was set up in DESY scientist Franz Kärtner's lab at the Massachusetts Institute of Technology (MIT), uses terahertz radiation instead of radio frequency structures, which the scientists say has the potential to miniaturize the entire accelerator by at least a factor of 100. 

“The compact accelerator we are building enables the construction of very bright and potentially fully coherent X-ray sources, which enable also new medical diagnostic imaging techniques, like phase contrast imaging and potentially also new radiation therapy techniques like image-guided small tumor radiation therapy and micro-beam radiation therapy or brain tumors,” Kärtner, a professor at the University of Hamburg and at MIT, as well a member of the Hamburg Centre for Ultrafast Imaging, told HCB News. “Currently, high brightness X-ray beams are only available from large synchrotron or free-electron laser facilities, which is not practical for health care. A compact highly coherent X-ray source would address this shortcoming.” 

For the prototype, the physicists used a type of electron gun to fire fast electrons into an accelerator module that was tailored to be used with terahertz radiation, which was fed into the module, further accelerating the electrons. The prototype was able to increase the energy of the particles by 7 kiloelectronvolts, according to the scientists. While not a particularly large acceleration, the experiment demonstrated that the principle works in practice, said co-author Arya Fallahi of CFEL in a press release. 

"The theory indicates that we should be able to achieve an accelerating gradient of up to one gigavolt per meter,” Fallahi said in the release. This is more than 10 times what can be achieved with the top conventional accelerator modules currently available, the scientists said. Plasma accelerators could product higher accelerations, but the scientists said this experimental technology requires much more powerful lasers than the ones needed for terahertz accelerators. 

Recently, scientists from the European Organization for Nuclear Research (CERN) built a miniature linear accelerator made up of four modules that are each roughly 20 inches long, for a total size of a little more than 6.5 feet. They doubled the operating frequency used for the radiofrequency quadrupole (RFQ), a linear accelerator component used in the acceleration of low-velocity ion beams. 

Kärtner said his group’s prototype works at an even higher frequency, roughly 100 times the usual frequency of 1.3 gigahertz, and generates electrons, which Kärtner said is good for making accelerator-driven X-ray sources. 

The scientists are looking to make a 20 mega-electronvolt accelerator within the next three to four years, Kärtner said.

Abstract-Terahertz meets sculptural and architectural art: Evaluation and conservation of stone objects with T-ray technology

Conservation of cultural heritage is an area where novel scientific techniques are having enormous impact. Given the value and uniqueness of art pieces, non-invasive diagnostic methods are highly appreciated by conservators. Terahertz radiation has shown enormous potential as non-contact probe that can be used for the three-dimensional reconstruction of internal structure of stone-made objects. In this article we report the evaluation of the internal damage state of two art pieces, a medallion from the Castle of Celle and a window sill from the St. Peter of Trier Cathedral. We also used terahertz radiation to follow and assess the restoration process of the window sill. We found that terahertz spectroscopy is an excellent non-destructive evaluation method for stone artwork that shows enormous potential as a tool for conservation.