Sunday, January 25, 2015

Abstract-Markets, Availability, Notice, and Technical Performance of Terahertz Systems: Historic Development, Present, and Trends

Although a lot of work has already been done under the older terms “far infrared” or “sub-millimeter waves”, the term “terahertz” stands for a novel technique offering many potential applications. The latter term also represents a new generation of systems with the opportunity for coherent, time-resolved detection. In addition to the well-known technical opportunities, an historical examination of Internet usage, as well as the number of publications and patent applications, confirms ongoing interest in this technique. These activities' annual growth rate is between 9 % and 21 %. The geographical distribution shows the center of terahertz activities. A shift from the scientific to more application-oriented research can be observed. We present a survey among worldwide terahertz suppliers with special focus on the European region and the use of terahertz systems in the field of measurement and analytical applications. This reveals the current state of terahertz systems' commercial and geographical availability as well as their costs, target markets, and technical performance. Component cost distribution using the example of an optical pulsed time-domain terahertz system gives an impression of the prevailing cost structure. The predication regarding prospective market development, decreasing system costs and higher availability shows a convenient situation for potential users and interested customers. The causes are primarily increased competition and larger quantities in the future

Saturday, January 24, 2015

First-of-its-kind tube laser created for on-chip optical communications

rolled up tube laser 1
(a) Optical microscope image of the free-standing rolled-up tube laser positioned on top of two electrodes. (b, c, d) Scanning electron microscope images of different views of the laser. Surface corrugations for axial mode confinement can be seen in (d). Credit: M. H. T. Dastjerdi, et al. ©2015 AIP Publishing
By: Lisa Zyga

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(—Nanophotonics, which takes advantage of the much faster speed of light compared with electrons, could potentially lead to future optical computers that transmit large amounts of data at very high speeds. Working toward this goal, researchers in a new study have developed a tiny laser 100 micrometers long and 5 micrometers in diameter—right at the limit of what the unaided human eye can see. As the first rolled-up semiconductor tube laser that is electrically powered, it can fit on an optical chip and serve as the light source for future optical communications technology.

A team of engineers, M. H. T. Dastjerdi, et al., at McGill University in Montreal have reported their development of the tiny laser in a recent issue of Applied Physics Letters.
Future optical chips will require many vital components, such as modulators (which convert electrical signals into optical ones), photodetectors (which do the reverse), and waveguides (which control the path of ). Another essential requirement is, of course, the light itself, which may come from a micro- or nano-scale laser that can be integrated with the other components onto a silicon (Si) platform. 
Although many different types of micro-sized lasers have been studied over the past several years, one promising candidate is a laser made from rolled-up semiconductor tubes. These lasers are fabricated by straining 2D nanomembranes on a substrate, and then selectively releasing parts of the nanomembranes so that they roll up into tiny tubes that act as optical cavities. The rolled-up tube lasers have an advantage over most other types of small lasers in that their optical emission characteristics can be precisely tailored using standard photolithography processes. They can also be easily transferred onto a Si platform, allowing for seamless integration with other chip components.
So far, the only rolled-up tube lasers that have been demonstrated have been powered optically, not electrically, which have certain disadvantages.
rolled up tube laser 2
Schematic of the electrically injected free-standing rolled-up tube laser, showing light emission from the center of the laser. Credit: M. H. T. Dastjerdi, et al. ©2015 AIP Publishing
"In contrast to electrically injected devices, optically pumped devices require additional light sources (lasers, LEDs) to operate that take additional space on the chip and add a significant level of complexity," Zetian Mi, Associate Professor at McGill University, told "Therefore, optically pumped light sources are not practical for integrated chip-level optical communication systems."
As the researchers explain, fabricating electrically powered rolled-up tube lasers is difficult because the very thin nanomembranes make the process of injecting  into the laser very inefficient. To overcome this problem, the researchers designed the laser to lie horizontally on top of two supporting pieces connected to the electrodes in a U-shaped mesa design. In this formation, charge carriers are injected into the laser cavity from the sides. By circumventing the thin membrane walls, this lateral carrier injection scheme emits light from the center of the tube laser, significantly increasing injection efficiency.
"The U-shaped mesa design allows the efficient injection of charge carriers into the center region of the tube, and thereby light emission from this region," Mi said. "However, to achieve lasing, optical confinement in both the axial direction (along the tube) and azimuthal direction are required. The azimuthal confinement is achieved by the free-standing tube itself. The axial confinement is offered by the surface corrugations. The surface corrugations are introduced by modulating the inner edge of the U-shaped mesa during the device fabrication process. It leads to an effective change of the refractive index along the tube direction, which can strongly confine light."
The new rolled-up tube laser is made of the commonly used semiconductor material InGaAsP (indium gallium arsenide phosphide), interspersed with two InGaAs quantum well layers. At just 7 nm thick, the quantum wells confine light in very small areas, contributing to efficient photoluminescence. The laser's light emission peaks in the telecom wavelength range at about 1.5 micrometers. The laser also operates with very low threshold current, which improves efficiency and is essential for chip-scale applications.
"Practical light sources for chip-level communication systems require electrically-injected light sources," Mi said. "To date, the most efficient light sources are made of III-V semiconductors (GaAs, InP, etc.). The monolithic integration of such III-V light sources onto a Si platform has been fundamentally limited by the large lattice mismatch, large thermal coefficient difference, and polar/nonpolar incompatibility between III-V materials and Si, leading to poor performance and extremely short lifetime. The greatest significance of our work is that the electrically injected free-standing semiconductor tube lasers can be transferred directly onto a Si platform, and the device performance is no longer limited by these [three] fundamental issues."
In the future, the researchers plan to integrate the lasers onto chips, taking an important step toward applications.
"Future plans include the achievement of electrically injected semiconductor tube lasers at room-temperature and direct integration with waveguides and other components on a Si-chip," Mi said. "It is worth mentioning that we have previously demonstrated the integration of such devices with Si-waveguides on a Si platform under optical pumping. Therefore we do not foresee any fundamental roadblocks for semiconductor tube lasers to emerge as a viable  for Si photonics."
More information: M. H. T. Dastjerdi, et al. "An electrically injected rolled-up semiconductor tube laser." Applied Physics Letters. DOI: 10.1063/1.4906238

Infrared imaging technique operates at high temperatures

By: Amanda Morris

From aerial surveillance to cancer detection, mid-wavelength infrared (MWIR) radiation has a wide range of applications. And as the uses for high-sensitivity, high-resolution imaging continue to expand, MWIR sources are becoming more attractive.
Currently, commercial technologies for MWIR detection, such as indium antimonide (InSb) and mercury-cadmium-telluride (MCT), can only operate at  in order to reduce thermal and electrical noise. In a search for alternatives, a team of researchers at Northwestern University's Center for Quantum Devices (CQD) has incorporated new materials to develop detectors that can work at room temperature.
"A higher operating temperature eliminates the need for liquid nitrogen," said Manijeh Razeghi, Walter P. Murphy Professor of Electrical Engineering and Computer Science and director of the CQD at Northwestern's McCormick School of Engineering and Applied Science. "That makes detectors more compact, less expensive, and more portable."
Depending on its use,  is divided into several wavelength segments. MWIR have a radiation range between 3-5 microns; cameras able to see in this wavelength are capable of passive infrared imaging.
Razeghi and her group developed an indium arsenide/gallium antimonide (InAs/GaSb) type II superlattice that demonstrated high-resolution MWIR images while operating at high temperatures. The new technique was particularly successful at obtaining  of the human body, which has potential for vascular imaging and disease detection.
Supported by DARPA, the Army Research Laboratory, Air Force Research Laboratory, and NASA, the team's findings were reported in paper in the January 1 issue of Optics Letters, the journal of the Optical Society of America.
More information: Optics… t.cfm?uri=ol-40-1-45

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Abstract-Generation of terahertz vector beams with a concentric ring metal grating and photo-generated carriers

Generation of terahertz vector beams with a concentric ring metal grating and photo-generated carriers

Zhenwei Xie, Jingwen He, Xinke Wang, Shengfei Feng, and Yan Zhang  »View Author Affiliations

Optics Letters, Vol. 40, Issue 3, pp. 359-362 (2015)
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A scheme for vector terahertz (THz) beam generation is proposed. A subwavelength metal grating is utilized to adjust the polarization of the THz radiation. The amplitude and phase distributions of the THz beam are dynamically regulated by a THz computer-generated hologram (CGH) pattern of the photo-generated carriers. A radially polarized THz beam and a vector THz vortex beam with a topological charge of 1 are generated to demonstrate the validity and the effectiveness of the proposed scheme. Experimental results correspond to the theoretical simulations well. Moreover, the proposed method is applicative for a broadband THz radiation. These results could be applied in the THz sensing, THz imaging, and THz communication in the future.
© 2015 Optical Society of America

Design of graphene-based terahertz nanoantenna arrays

Microwave and Optical Technology Letters

Author Information

  1. ELEDIA Research Center, Department of Information Engineering and Computer Science, University of Trento, Italy
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This article presents a method for the design of graphene nanoantenna arrays at terahertz. A customized hybrid genetic algorithm (HGA) is used to maximize the plasmonic field enhancements and to minimize the sidelobe levels by acting on the array thinning, the spacing between elements, the size, and the graphene chemical potential. In particular, the graphene chemical potential is controlled by an applied electrostatic bias field. The problem is recasted as an optimization one by defining a suitable cost function, which is then, minimized using the HGA. Numerical results concerning graphene nanoarrays are reported to assess the design methodology. The obtained results are promising, and they demonstrate the potentialities and the advantages of graphene nanoarrays. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:653–657, 2015