Monday, July 27, 2015

Abstract-Producing terahertz coherent synchrotron radiation at the Hefei Light Source


Xu De-Rong (许德荣), Xu Hong-Liang (徐宏亮) and Shao Yan (邵琰)
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China
 
Xu De-Rong et al 2015 Chinese Phys. C 39 077003. doi:10.1088/1674-1137/39/7/077003
Received 28 September 2014.
©2015 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd
This paper theoretically proves that an electron storage ring can generate coherent radiation in the THz region using a quick kicker magnet and an AC sextupole magnet. When the vertical chromaticity is modulated by the AC sextupole magnet, the vertical beam collective motion excited by the kicker produces a wavy spatial structure after a number of longitudinal oscillation periods. The radiation spectral distribution was calculated from the wavy bunch parameters at the Hefei Light Source (HLS). When the electron energy is reduced to 400 MeV, extremely strong coherent synchrotron radiation (CSR) at 0.115 THz should be produced.

Sunday, July 26, 2015

OT- LUNA INNOVATIONS BLOG-Testing for Residual Strains in 3D Printed Components using High Definition Distributed Fiber Optic Sensing (HD-FOS)



http://lunainc.com/testing-residual-strains-3d-printed-components-high-definition-distributed-fiber-optic-sensing-hd-fos/


In the May 2015 issue of the Harvard Business Review, Richard D’Aveni states that additive manufacturing is on the brink of completely transforming the design and production ecosystems for manufacturers of tangible goods.   https://hbr.org/2015/05/the-3-d-printing-revolution  Additive manufacturing frees both designers and manufacturers from many of the constraints of the normal product design, prototyping and industrialization process.  The cost and lead time for tooling can be eliminated and designers can create greater numbers of increasingly more complex parts, seemingly limited only by their imaginations. According to the author, the result will be a complete re-thinking of how business operations are conducted.
While the rapid advancements of additive manufacturing technology have begun to change the paradigm for product design and manufacturing, the requirements for test and design validation remain the same. If anything, the growing penetration of additive manufactured parts into structural components along with an ever expanding range of available materials are combining to make traditional test methods completely inadequate for validating designs of additively manufactured components.  In particular is the problem of internal residual stresses that can accumulate during the buildup of material during the printing process.  Residual stress can cause layer delamination, part distortion and cracking and are a significant barrier to the wider adoption of additively manufactured parts in structural applications.
Fortunately, commensurate with the coming 3-D printing revolution is an equally revolutionary technology from Luna Innovations that can be used to measure strain on complex surfaces and, and by embedding the sensor during printing, actually measure strain inside the component.  The fiber optic sensor, with a diameter of only 155 micro meters, can be embedded un-obtrusively during printing and not impact the components inherent characteristics.
To demonstrate this capability Luna engineers constructed a block using a 3D printer.  The block was 3 inches in height with a base of 4 x 1.5 inches and constructed using ABS material.  During printing, the operation was paused and the head lifted to allow laying a section of the fiber lengthwise across the block.  A segment of the fiber was embedded every 0.3 inches resulting in a total of 9 layers vertically spaced 0.3 inches apart.  Figure 1 shows the dimensions and pattern of embedded fiber.
Figure 1. To demonstrate this capability Luna engineers constructed a block using a 3D printer. The block was 3 inches in height with a base of 4 x 1.5 inches and constructed using ABS material. During printing, the operation was paused and the head lifted to allow laying a section of the fiber lengthwise across the block. A segment of the fiber was embedded every 0.3 inches resulting in a total of 9 layers vertically spaced 0.3 inches apart. Figure 1 shows the dimensions and pattern of embedded fiber.
Figure 2 shows the ABS block, nearly completed with a single fiber sensor embedded during its construction.





























Strain measurements were recorded using Luna’s ODiSI high definition fiber optic sensing system both during the printing process and also after its completion once the fabricated part cooled to ambient temperature.  The scan of strain taken at ambient temperature showed a significant amount of residual strain concentrated near the centerline of the ABS block and diminishing in proportion to the distance from the blocks center line.  Figure 3 shows the strain measurements plotted vs the height of the sensor segment from the base of the block.  The strain data shows clearly a very high level of residual stress existing within the bloc
Figure 3.
Figure 3.
Residual stresses and strains accumulate in the 3D printing fabrication process during the build-up of material and can have a significant and detrimental effect on the mechanical strength of a part. These residual stresses can couple additively to stresses from external loading, resulting in unexpected or premature failure.  Residual strains and stresses can be mitigated through a combination of material selection and through a careful optimizing of the various fabrication parameters.  The ODiSI high definition fiber optic sensing system when used in conjunction with this iterative process optimization can ensure parts built with additive manufacturing not only meet the design requirements, but can be built in volume production with a known and consistent process capability for critical parameters.
The data and test methods from Luna’s experiment have been shared with researchers at Oak Ridge National Laboratory (ORNL) and Luna engineers are now working in partnership with ORNL to experiment with embedding sensors in 3D printed components.
If 3D printing is truly going to revolutionize manufacturing and change the way businesses operate then paving the way will be equally revolutionary methods to test and validate these rapid advancements in 3D printing technology.  The accumulation of residual strains and stresses need to be controlled in order to maintain design integrity, reliability and quality.  These stresses and strains can only be controlled if they can be measured and Luna’s ODiSI system, with its ability to embed sensors and provide high definition strain measurements is the perfect solution.

Mona Jarrahi- New Frontiers in Terahertz Technology


Speaker: A/Prof Mona Jarrahi, University of California Los Angeles, CA, USA
Seminar Date: Tue, 28/07/2015 - 14:00
Venue: Advanced Engineering Building (Building 49), 49-502
Host: Konstanty Bialkowski

https://www.blogger.com/blogger.g?blogID=124073320791841682&pli=1#editor/target=post;postID=5778916195061277540

Although unique potentials of terahertz waves for chemical identification, material characterization, biological sensing, and medical imaging have been recognized for quite a while, the relatively poor performance, higher costs, and bulky nature of current terahertz systems continue to impede their deployment in field settings. In this talk, I will describe some of our recent results on developing fundamentally new terahertz electronic/optoelectronic components and imaging/spectrometry architectures to mitigate performance limitations of existing terahertz systems. In specific, I will introduce new designs of high-performance photoconductive terahertz sources that utilize plasmonic antennas to offer terahertz radiation at record-high power levels of several milliwatts – demonstrating more than three orders of magnitude increase compared to the state of the art. I will describe that the unique capabilities of these plasmonic antennas can be further extended to develop terahertz detectors and heterodyne spectrometers with single-photon detection sensitivities over a broad terahertz bandwidth at room temperatures, which has not been possible through existing technologies. To achieve this significant performance improvement, plasmonic antennas and device architectures are optimized for operation at telecommunication wavelengths, where very high power, narrow linewidth, wavelength tunable, compact and cost-effective optical sources are commercially available. Therefore, our results pave the way to compact and low-cost terahertz sources, detectors, and spectrometers that could offer numerous opportunities for e.g., medical imaging and diagnostics, atmospheric sensing, pharmaceutical quality control, and security screening systems. And finally, I will briefly highlight our research activities on development of new types of high-performance terahertz passive components (e.g., modulators, tunable filters, and beam deflectors) based on novel reconfigurable meta-films.

Saturday, July 25, 2015

Abstract-A simple method to enhance terahertz radiation from femtosecond laser filament array with a step phase plate



Jiayu Zhao, Lanjun Guo, Wei Chu, Bin Zeng, Hui Gao, Weiwei Liu, and Ya Chen
https://www.osapublishing.org/ol/upcoming_pdf.cfm?id=236756


  • Abstract: In this work, we experimentally demonstrate a 200% enhancement of terahertz (THz) wave amplitude generated by femtosecond laser filamentation in air. The experimental setup simply uses a semicircular phase plate to generate two parallel filaments. Temporally overlapped THz pulses from two filaments coherently add up, giving rise to significant enhancement of the THz pulse amplitude. It has been foreseen that further enhancement would be achieved if the design of phase plates could be optimized to generate filament array. This simple method makes full use of the laser energy and might potentially open a new approach to remotely enhance the THz emission in air.

Friday, July 24, 2015

Illuminating the electronic properties of graphene


http://phys.org/news/2015-07-illuminating-electronic-properties-graphene.html

Danish researchers have for the first time mapped the carrier mobility and density of large sheets of graphene with electromagnetic radiation.
For the last decade, the usual way of measuring the electronic properties of graphene – in particular the carrier mobility and carrier density, which together give the sheet conductance – has been to fabricate a transistor-like device and electronically measure how the conductance changes as a function of applied electrostatic gate voltage. This all-electronic approach is best when dealing with small pieces of graphene, such as the microscopic flakes produced by micromechanical cleavage (also known as the 'scotch-tape method') – however, advances in graphene production techniques now allow us to continuously produce large areas of graphene meters across. Producing and measuring thousands or millions of microscopic devices from such sheets would be impractical and would reduce the useful area of graphene for the intended application. We need to be able to check the electronic properties of such large regions without destroying them in the process.

Researchers at the Technical University of Denmark (DTU) have shown that both the carrier mobility and the carrier density of graphene can be measured in a spatially resolved and non-destructive way – providing 'maps' of the electronic properties critical for the successful use of graphene in photovoltaics, electronics, spintronics and optics – using terahertz (THz) radiation and doing away with the need to fabricate devices. Using a procedure known as THz time-domain spectroscopy, Jonas Buron and colleagues from DTU research teams led by Peter Uhd Jepsen and Peter Bøggild measured the carrier mobility and carrier density at tens of thousands of points in a centimetre sized single layer of graphene.
A key enabling step in these first contact-free measurements of the electronic properties of graphene was the realisation that the graphene conductance could be tuned during the measurements using a back-gate, which is transparent to THz radiation. "While we still need to transfer the graphene to a special substrate with the THz-invisible gate, it is far easier and less destructive than conventional techniques... and much, much faster", says Jonas Buron. For many electronic applications of graphene, the fabrication of a back-gate is a necessary step anyway. "With some optimisation we could potentially map the  and density of a graphene-coated 4-inch wafer in minutes."
The maps of the  of graphene are already providing insight and surprises about the origin of their spatial variation – in one sample, the researchers observed twice as much variation in mobility as in carrier density. Variations in conductance are usually ascribed to carrier density changes due to doping variations, but the researchers proved that here this was not the case. "We have often noted such slow variations of the conductivity across many centimeters in THz measurements." Peter Bøggild explained. "But since graphene is so easily doped due to its extreme surface-to-volume ratio, we always expected these to be related to local doping level variations. In this case, we have the exact opposite situation, and this is puzzling. Without this mobility mapping technique we would never have known."
The THz-TDS technique has a strong potential, adds Peter Uhd Jepsen. "It is already surprising how deep information we can extract from transmitting radiation through a just 0.3 nm thin sheet of carbon atoms, which is supported by a 1.5 million times thicker piece of silicon. We are still learning how to characterise the electric properties of  without electric contacts, and there seem to be excellent options for improving and speeding up the technique."


Read more at: http://phys.org/news/2015-07-illuminating-electronic-properties-graphene.html#jCp