Thursday, October 8, 2015
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.
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
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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
Wednesday, October 7, 2015
Credit: DESY/Heiner Mueller-Elsner
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
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