Tuning terahertz transmission

A mounted device including the new tunable metasurface developed by Ding, Teng and co-workers. (right) When terahertz radiation hits the surface of interlinked p-type and n-type semiconducting silicon fingers, the amount of radiation reflected and transmitted can be controlled precisely using an applied voltage. Credit: A*STAR Institute of Materials Research and Engineering

https://phys.org/news/2019-04-tuning-terahertz-transmission.html

The ability to manipulate light on a subwavelength-scale could lead to a revolution in photonic devices such as antennas, solar panels, and even cloaking devices. Nanotechnology advances have made this possible through the development of metasurfaces, materials covered in features smaller than the wavelength of the light.

Now, a team led by A*STAR researchers has produced a highly promising metasurface that can be precisely controlled using a conventional electrical circuit so that it reflects and transmits different amounts of radiation. It can even reach the condition of 'perfect antireflection' where it reflects no radiation at all. Specifically, the surface works with broadband terahertz radiation, which is found at the far end of the infrared spectrum and has many potential uses, particularly in security or medical fields.

"Terahertz radiation can penetrate a wide variety of non-conducting materials, but is blocked by liquid water or metals," explains Lu Ding, who led the work with Jinghua Teng at the A*STAR Institute of Materials Research and Engineering (IMRE). "This means that terahertz beams can be used for material characterization, layer inspection, and producing high-resolution images of the interior of solid objects. It is non-ionizing radiation, and safer than X-rays."

Previous metasurfaces have been designed to manipulate the reflection of terahertz radiation. However, their application has been limited, as Ding explains: "Conventional terahertz antireflection surfaces are passive and often employ an ultrathin metal coating that, once fabricated, becomes fixed and you can't actively tune its performance."

"An electrically tunable metasurface would produce more versatile devices and render more flexibility in system design," adds Teng. "It is the breakthrough the community is looking for."

Ding and Teng, along with coworkers at the A*STAR Institute of Microelectronics (IME), Nanyang Technological University, National University of Singapore and Jilin University in China, fabricated their new metasurface on a silicon wafer, using a process entirely compatible with the complementary metal–oxide semiconductor (CMOS) technologies that underpin most electronics.

The exposed metasurface contains stripes of semiconducting silicon, doped with other elements. These stripes are alternately n-type, in which the moving charge carriers are electrons, and p-type, in which the carriers are positively-charged 'holes' in the electron structure. When the voltage supplied to the p-n junctions is changed, the reflection and transmission of the radiation also change.

The team realized that the reflection coefficient increased in response to a temperature rise caused by the applied voltage. Meanwhile, the transmission showed a more complex response depending on the voltage polarity, which affected the type of charge carrier that became dominant. Using terahertz time-domain spectroscopy, the team showed that certain voltage conditions caused the echo pulse from the metasurface to vanish, representing complete antireflection.

As well as providing this unprecedented control over reflection and transmission, the metasurface has the benefit of being almost entirely flat at an atomic level. This makes it ideal for building up smooth layers in more complex devices.

"Another big advantage is for our research looking into how 2-D materials interact with 2-D metamaterials or metasurfaces, a topic in our project in A*STAR's 2-D Semiconductors Pharos Program," says Teng. "The atomically smooth surface makes the transfer and formation of 2-D-Si heterostructures much easier than the patterned surfaces of nano-sized pillars or disks seen on conventional metasurfaces."

"We could further exploit this type of metasurface by independently biasing the p-n junctions or designing modular functions, meaning that we would have pre-programmable metamaterials," says Ding. Teng adds that the same platform could be used for studying promising 2-D materials like molybdenum disulfide, which exhibits impressive electronic and optical properties for use in new flexible circuits.

Light-scattering Nanoparticles Could Lead to Invisbility Cloaks and Smaller Optical Antennas

Dexter Johnson 
http://spectrum.ieee.org/nanoclast/semiconductors/materials/light-scattering-nanoparticles-could-lead-to-smaller-optical-antennas

Photo: A*STAR Data Storage Institute

A research team from A*STAR Data Storage Institute in Singapore and St. Petersburg University in Russia has discovered certain light-scattering properties of nanoparticles that could lead to smaller and more effective optical nanoantennas and even invisibility cloaks.

The research, which was published in the journal ACS Photonics, took the form of numerical calculations of the light-scattering properties of dielectric nanoparticles, which are nanoparticles that are electrical insulators and can be polarized by an applied electric field.

One of the key attributes of the transparent nanoparticles modeled by the researchers is that they had a refractive index above two. All materials in nature have a refractive index—a measurement of the speed of light through that material. A refractive index above two it means that light can travel two times faster through a vacuum than through it.

The other key attribute of the nanoparticle models was their shape. The researchers calculated that if a sphere-shaped nanoparticle had a refractive index above 3.5 and its major axis is just over twice the length of its minor axis, it was possible to maximize its forward scattering of light.

This implies that if you manipulate the size and aspect ratio of the nanoparticle, you can determine how it will scatter light. This would be useful in applications such as photovoltaics as well as in metamaterials, where an artificially structured material is fabricated by assembling different objects to replace the atoms and molecules that one would see in a conventional material. This artificial structure results in a material with very different electromagnetic properties than those found in naturally occurring or chemically synthesized materials, which has lead to several research efforts into things like invisibility cloaks.

"Dielectric particles with optimized shapes which behave as very efficient directional antennas can be used in sensing devices, transmission lines, metasurfaces with numerous uses and in many other devices such as negative refractive index lenses, optical cloaking devices or nanolasers," said Boris Luk'yanchuk of A*Star, the lead researcher in the study, in a press release.

Luk'yanchuk and his colleagues will continue looking at applying their observation of these transparent to creating nanoscale devices and metamaterials.

A*STAR AND RIKEN CELEBRATE 10 YEARS OF RESEARCH COLLABORATION

http://www.news.gov.sg/public/sgpc/en/media_releases/agencies/astar/press_release/P-20150803-1

Singapore—The Agency for Science, Technology and Research (A*STAR) and RIKEN, Japan's largest comprehensive research institute in the natural sciences, have marked a 10-year milestone of research partnership.

A*STAR and RIKEN inked their first MOU September 2005 to encourage more opportunities for scientific exchange between Singapore and Japan. The MOU has since been renewed three times. For a timeline of milestones, refer to Annex A.

RIKEN established its first overseas international liaison office in 2006, attesting to its long-term commitment to the partnership with Singapore. The partnership has catalysed joint projects in fields ranging from the biomedical sciences to the physical sciences and engineering domains (enclosed within and in Annex B). This partnership has also offered opportunities to broaden scientific exchange through RIKEN’s prestigious summer programmes in brain science and immunology.

The most recent renewal of the MOU will further build on the existing partnership to include mutual areas of interest in material science. This will continue to encourage sharing of ideas, co-advancing scientific capabilities, broadening research networks, and developing research talent.

Collaborative projects between A*STAR and RIKEN

·         Biomaterials and hydrogels: Dr Loh Xian Jun from A*STAR’s Institute of Materials Research and Engineering (IMRE) and Prof Yoshihiro Ito from RIKEN have successfully pioneered a new method of cell detachment using a novel temperature-sensitive biomaterial. This method has many applications in basic research, allowing scientists to better study adult human stem cells more efficiently. Currently, the process requires complicated chemical synthesis techniques whereas the new approach uses a “Drop-and-Dry” coating method. This method allows non-chemists to prepare their own temperature responsive cell culture surface for non-enzymatic detachment. This method will simplify experiments and facilitate the understanding of the cell differentiation process..

·         Identification of novel immune cell subset: Dr Florent Ginhoux from A*STAR’s Singapore Immunology Network (SIgN) is collaborating with Dr Ichiro Taniuchi from the RIKEN Center for Integrative Medical Sciences, to characterize novel lymphoid populations in the epidermis. Tapping on IMS’s strength in generating gene-manipulated mice and on SIgN’s flow cytometry and bioinformatics analyses of small cell populations, the collaboration identified a novel lymphoid cell subset in the epidermis in mice and humans. This finding, which will soon be published in Nature Scientific Reports, has provided new insights into the skin’s immune system and will deepen our understanding of the physiological and pathological roles of these new cell types during immune responses at the interface between our body and the environment.

Mr Lim Chuan Poh, Chairman A*STAR, said: “I am pleased that A*STAR’s and RIKEN’s longstanding partnership has catalysed many opportunities for collaboration in R&D between Singapore and Japan. We will continue to build on the complementary research capabilities of both countries through joint projects and scientific exchange. I look forward to many more fruitful years of collaboration.”

Dr Hiroshi Matsumoto, President of RIKEN, said: “RIKEN’s collaborations with Singapore through A*STAR have been extremely meaningful to us. A major mission of science today is to ensure the continued survival of humanity, and this mission cannot be accomplished without international cooperation. I strongly hope that our partnership will continue to develop, leading to important research breakthroughs that will benefit all of humanity.”

__________________________________________________________

.....

ii)            Safer X-Rays – Improving T-Rays for Potential Applications in Diagnostics and Security

 

A*STAR	RIKEN
Dr Gong Yandong Scientist, Institute for Infocomm Research (I2R)	Dr Hiroaki Minamide Team Leader, Tera-Photonics Research Team, RIKEN Center for Advanced Photonics
Expertise in Terahertz polarization, focus of collaboration is on the polarization sub-system and algorithms	Strength lies in basic terahertz frequency domain spectroscopy (THz-FDS) system

Terahertz (THz) is an underexplored band of light located between microwave and infrared frequency in the electromagnetic (EM) spectrum. THz radiation, also known as T-rays, produces images faster than X-rays and has a lower photon energy, making it safer and more efficient for use in various applications, such as in the detection of cancerous tumours for non-invasive, high-sensitivity medical diagnostics, or the detection of concealed objects during security screening.

However, most conventional THz systems can only provide basic parameters for the user. In this project with RIKEN, the researchers demonstrated the world-first polarimetric THz-FDS system. The ability to provide polarization information makes the THz system more powerful in detection and sensing. Out of this collaboration, the team also developed a new device on the market – the achromatic Terahertz waveplate. This device can be adapted for use in conventional THz systems, allowing THz polarization to be manipulated for greater control and more precise results. The THz polarization technology has been licensed to two local SMEs in the imaging and photonics industry.

Quantum effects in nanometer-scale metallic structures

An electron nanoprobe (yellow) placed near the functionalized silver nanoparticles measured plasmon-assisted quantum tunneling at terahertz frequencies. Credit: Shu Fen Tan, National University of Singapore

 http://phys.org/news/2014-10-quantum-effects-nanometer-scale-metallic.html#jCp

Plasmonic devices combine the 'super speed' of optics with the 'super small' of microelectronics. These devices exhibit quantum effects and show promise as possible ultrafast circuit elements, but current material processing limits this potential. Now, a team of Singapore-based researchers has used a new physical process, known as quantum plasmonic tunneling, to demonstrate the possibility of practical quantum plasmonic devices.

Tunneling is an intriguing aspect of quantum mechanics whereby a particle is able to pass through a classically insurmountable barrier. Theoretically, quantum plasmonic tunneling is only noticeable when plasmonic components are very closely spaced—within half a nanometer or less. However, researchers from the A*STAR Institute of Materials Research and Engineering, the A*STAR Institute of High Performance Computing and the National University of Singapore were able to observe quantum effects between materials spaced more than one nanometer apart.

They investigated the tunneling of electrons across a gap between two nanoscale cubes of silver coated with a monolayer of molecules. High-resolution transmission electron microscopy showed that these nanocubes self-assembled into pairs. The separation, and hence the tunneling distance, between the nanoparticles could be controlled by the choice of surface molecule—between 0.5 and 1.3 nanometers in the cases tested.

The monolayer of molecules had an another function—to provide molecular electronic control over the frequency of the oscillating tunnel current, which could be tuned between 140 and 245 terahertz (1 terahertz = 1012 hertz), as was shown by monochromated electron energy-loss spectroscopy.

Theoretical predictions, supported by experimental results, confirmed the nature of the plasmon-assisted tunnel currents between the silver cubes. "We show that it is possible to shine light onto a small system of two closely spaced silver cubes (see image) and generate a tunnel current that oscillates very rapidly between these silver electrodes," explains A*STAR researcher Michel Bosman. "The oscillation is several orders of magnitude faster than typical clock speeds in microprocessors, which currently operate in the gigahertz (= 109 hertz) regime." At the same time, the results also demonstrate the possibility of terahertz molecular electronics.

Two factors contributed to the success of the experiments. First, the nanocubes had atomically flat surfaces, maximizing the tunneling surface area between the two nanoparticles. Second, the molecule-filled gap increased the rate of tunneling, making it possible to measure plasmon-assisted quantum tunneling.

"We will now use different molecules in the tunnel gap to find out how far the tunnel currents can be carried, and in what range we can tune the oscillation frequency," says Bosman.

 Explore further: Scientists develop novel, ultra-fast electrical circuits using light-generated tunneling currents

More information: Tan, S. F., Wu, L., Yang, J. K. W., Bai, P., Bosman, M. & Nijhuis, C. A." Quantum plasmon resonances controlled by molecular tunnel junctions." Science 343, 1496–1499 (2014). dx.doi.org/10.1126/science.1248797

Improving terahertz optics with efficient broadband antireflection coatings

Thin strips of chromium form an efficient antireflection coating for terahertz light and can be applied to a broad range of surfaces. Credit: A*STAR Institute of Materials Research and Engineering

http://phys.org/news/2014-05-terahertz-optics-efficient-broadband-antireflection.html#jCp

Antireflection coatings are familiar from their use in everyday optical devices, such as glasses and lenses. They can increase the amount of light that passes through optical instruments by reducing the fraction of light reflected (and hence lost) at surfaces. Antireflection coatings have applications beyond visible light: for instance, in the infrared and terahertz regimes they are useful for chemical sensing and imaging applications, such as those employed at airport security checks.

Now, Jing Hua Teng from the A*STAR Institute of Materials Research and Engineering and colleagues from the A*STAR Institute of Microelectronics and Osaka University, Japan, have developed ultrathin antireflection coatings for terahertz waves that can be applied to almost any surface. "Their fabrication is very straightforward, as it takes only one step of photolithography, metal deposition and lift-off," explains Teng.

Antireflection coatings are usually based on interference effects, which requires them to be at least as thick as the wavelength of light. This is practical for visible light, with wavelengths in the range of hundreds of nanometers. However, it is a serious limitation for infrared or terahertz radiation, which has much longer wavelengths of the order of hundreds of microns. Moreover, as these coatings are often functional only over narrow frequency ranges, they do not operate across the broad ranges needed for terahertz sensing applications.

The research team developed antireflection coatings based on metamaterials, which are metallic structures that are much smaller than the wavelength used. These structures completely alter the optical properties of a material in a predetermined way, enabling the generation of a much broader range of optical effects than those that occur naturally. One application of the unusual optical effects they produce is invisibility cloaks.

In the new design for metamaterial surfaces developed by the researchers, thin strips of chromium are fabricated on a silicon surface to form a grating (see image). Silicon, being flexible, is a typical material for terahertz optics. When terahertz light passes through the stripes and into the silicon, its phase is changed in the same way as for the much thicker coatings based on interference effects; this suppresses surface reflection.

These metasurfaces have the advantage that they can function across an unprecedentedly wide frequency range, namely 0.06 to 3 terahertz. The flexibility of the coatings for other wavelengths and applications also enhances their commercial appeal, comments Teng. "The beauty of this method is that it is very flexible and can be easily adapted to other metals and substrates."

 Explore further: Nonlinear optical materials convert terahertz radiation into infrared light

More information: Ding, L., Wu, Q. Y. S., Song, J. F., Serita, K., Tonouchi, M. & Teng, J. H. "Perfect broadband terahertz antireflection by deep-subwavelength, thin, lamellar metallic gratings." Advanced Optical Materials 1, 910–914 (2013). dx.doi.org/10.1002/adom.201300321

Researchers create circuits that operate at ‘hundreds of terahertz’

Author Graham Pitcher

http://www.newelectronics.co.uk/electronics-news/researchers-create-circuits-that-operate-at-hundreds-of-terahertz/60698/

Circuits that are said to operate at hundreds of terahertz have been designed and fabricated by researchers at the National University of Singapore. The team says its work has the potential to revolutionise high speed electronics, nanoscale optoelectronics and nonlinear optics. 

The development is based on a new physical process called quantum plasmonic tunnelling. Current photonic elements are large, but operate at frequencies of 100THz, while current nanoelectronic devices are much smaller, making it difficult to combine the properties of both.

According to researchers, it has long been known that light can interact with certain metals and can be captured in the form of plasmons – ultra fast oscillations of electrons that can be manipulated at the nanoscale. Quantum plasmon modes have been predicted to occur at atomic scales and have been difficult to investigate.

In its study, the research team demonstrated that quantum plasmonics is possible at scales that are useful for real applications, then fabricated an element of a molecular electronic circuit using two plasmonic resonators. These structures, which can capture light in the form of plasmons, are bridged by a single molecule thick layer that switches on the quantum plasmonic tunnelling effects, enabling the circuits to operate at terahertz frequencies.

The work was led by Assistant Professor Christian Nijhuis from the National University of Singapore's Faculty of Science and Dr Bai Ping and Dr Michel Bosman from A*STAR.

Dr Bosman used advanced electron microscopy techniques to visualise and measure the optoelectronic properties of these structures at nanometre resolution. The measurements revealed the existence of the quantum plasmon mode and that its speed could be controlled by varying the molecular properties of the devices.

By performing quantum corrected simulations, Dr Bai confirmed that quantum plasmonic properties could be controlled in the molecular electronic devices at high frequencies.

Asst Prof Nijhuis said: "We are excited by the new findings. Our team is the first to observe the quantum plasmonic tunneling effects directly. This is also the first time that a research team has demonstrated theoretically and experimentally that very fast switching at optical frequencies are possible in molecular electronic devices."

The researchers will now address the integration of these devices into real electronic circuits.

Quantum Plasmon Resonances Controlled by Molecular Tunnel Junctions

https://www.google.com/search?q=national+university+of+singapore+logo&tbm=isch&tbo=u&source=univ&sa=X&ei=ZtQ5U5HcJfSl2AWU2IHoBg&sqi=2&ved=0CCgQsAQ&biw=1680&bih=892

Asst Prof Christian Nijhuis from the Department of Chemistry, has collaborated with Dr Michel Bosman from the Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR) and Dr Bai Ping Institute of High Performance Computing (IHPC), A*STAR, to design and fabricate ultra-fast electrical circuits that operate using the new physical process, quantum plasmonic tunneling, at terahertz frequencies to potentially bypass the inherent speed limit of copper-based interconnects.

Asst Prof Nijhuis and his team are the first to observe the quantum plasmonic tunneling effects directly. Before their research, state-of-the-art nanoelectronic devices operate at length scales that are much smaller than that of visible light, making it difficult to combine the ultra-fast properties of photonic elements with nano-scale electronics to be able to observe the quantum plasmonic tunneling effects directly.

It is shown for the first time, experimentally and theoretically, that very fast-switching at optical frequencies are indeed possible in molecular electronic devices. In addition, by simply changing the molecules in the device, the frequency of the circuits can be altered in the hundreds of terahertz regime. This novel discovery by the multidisciplinary research team opens up possibilities for real applications such as high speed electronics at terahertz frequencies.

To further their research, Asst Prof Nijhuis and his team are looking into resolving challenges such as the integration of these devices into real electronic circuits.

This study was funded by the Singapore National Research Foundation (NRF) of Singapore and has been published in the findings were published in Science on 28 March 2014.

 The figure above shows the two plasmonic resonators (silver nanocubes) bridged by a layer of molecules with a length of 0.5 nm. A focused electron beam (in yellow) was used to characterise the structures and to probe the optical properties.

Novel laser produces random mid-infrared light for improved imaging applications

Random lasers remove speckling while maintaining brightness and could be used for applications where imaging quality is important, such as checking mail or airport security. Credit: Tomasz Wyszołmirski/iStock/Thinkstock

http://phys.org/news/2014-02-laser-random-mid-infrared-imaging-applications.html#jCp

Most lasers produce coherent light, meaning that the light waves are perfectly synchronized with each other. Spatially coherent waves, however, can interfere with one another and produce speckles in an image. With this in mind, scientists are turning to so-called random lasers, which not only show promise for applications such as biological and environmental imaging, but also mimic natural, disordered scattering from objects such as clouds.

Hou Kun Liang and co-workers at the A*STAR Singapore Institute of Manufacturing Technology and Nanyang Technological University, Singapore, have now developed a random laser that emits light in the mid-infrared range1. Moreover, the random laser is driven by electricity, making it more suitable for practical applications.

"Most random lasers are driven by optical pumping—this requires another laser to excite the random media," says Liang. "With electrical pumping we can make the laser smaller, less complex and cheaper."

The researchers modified a design known as a quantum cascade laser that contains several thin layers of compound semiconductors. When an external voltage is applied, electrons are driven across the layers and emit photons at every step. The frequency of the emitted light can be controlled by adjusting the thickness of the layers.

"A quantum cascade laser is like an electron reservoir," says Liang. "After an electron relaxes to a lower energy level, instead of becoming inactive, it flows to the subsequent active region where it is 're-used'. This is important for our laser, because loss in the mid-infrared region is high, and so we need a high gain to compensate for it."

Crucially, Liang and co-workers used plasma etching to create a random pattern of small holes—each only three micrometers in diameter—on the top surface of their laser. This design causes the laser light to be randomly scattered before it is emitted through the holes.

Currently, the random laser must be cooled to very low temperatures using liquid nitrogen to maximize the gain, but Liang and co-workers anticipate that their design can be improved to reduce the loss of mid-infrared radiation at room temperature. Liang also points out that their design gives them great freedom to explore other laser frequencies.

"For example, terahertz lasers can penetrate thick plastics and papers and, unlike X-rays, are harmless to humans. These lasers could be used for applications, such as checking mail or airport security, where imaging quality is important—a random laser would remove speckling while maintaining brightness."

 Explore further: Harnessing randomness to improve lasers

More information: Liang, H. K., Meng, B., Liang, G., Tao, J., Chong, Y. et al. "Electrically pumped mid-infrared random lasers." Advanced Materials 25, 6859–6863 (2013). DOI: 10.1002/adma.201303122

Journal reference: Advanced Materials 

Terahertz technology: Seeing more with less

Terahertz radiation can penetrate materials such as a paper envelope and reveal the contents (left) in an accurate image (right). Credit: 2013 A*STAR Institute of Microelectronics

http://phys.org/news/2013-05-terahertz-technology.html#jCp
Terahertz technology is an emerging field that promises to improve a host of useful applications, ranging from passenger scanning at airports to huge digital data transfers. Terahertz radiation sits between the frequency bands of microwaves and infrared radiation, and it can easily penetrate many materials, including biological tissue. The energy carried by terahertz radiation is low enough to pose no risk to the subject or object under investigation

Before terahertz technology can take off on a large scale, however, developers need new kinds of devices that can send and receive radiation in this frequency range. Worldwide, electronic engineers are developing such devices. Now, Sanming Hu and co-workers from the A*STAR Institute of Microelectronics (IME), Singapore, have designed novel circuits and antennas for terahertz radiation and efficiently integrated these components into a transmitter–receiver unit on a single chip. Measuring just a few millimeters across, this area is substantially smaller than the size of current commercial devices. As such, it represents an important step towards the development of practical terahertz technologies.

Hu and his co-workers based their terahertz design on a fabrication technology known as BiCMOS, which enables full integration of devices on a single chip of only a few cubic millimeters in size. "Currently, commercial products for terahertz technologies use discrete modules that are assembled into a device," explains Hu. These module-based devices tend to be considerably more bulky than fully integrated systems.

"In a commercial terahertz transmitter–receiver unit, the central module alone measures typically around 190 by 80 by 65 millimeters, which is roughly 1 million cubic millimeters," says Hu. The novel design of Hu's team unites the essential components of a terahertz device in a smaller two-dimensional area of just a few millimeters along each side. According to Hu and his co-workers, this compact device paves the way towards the mass production of a fully integrated terahertz system.

As the next step, the team will use the IME's cutting-edge technologies to build more complex structures composed of several two-dimensional layers, which will be based on their new designs. Although the team is not pursuing any specific applications, their devices potentially open up a wide range of possibilities. These include wireless short-range transfers of data sets—the content of a Blu-ray disc could be sent in as little as a few seconds, for example—high-resolution biosensing, risk-free screening of patients and passengers, and see-through-envelope imaging (see image at top).

More information: Hu, S. et al. A SiGe BiCMOS transmitter/receiver chipset with on-chip SIW antennas for terahertz applications. IEEE Journal of Solid-State Circuits 47, 2654–2664 (2012). ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6301781

 

THz radiation leads to powerful nanoscale sensors

My Note: More on the recent article relating to the work at A*STAR
http://news.radio-electronics.co/manufacturing/thz-radiation-leads-to-powerful-nanoscale-sensors/
A team of researchers from the A*STAR Institute of Materials Research and Engineering (IMRE) has observed that microstructures made up by pairs of touching semiconductor disks yield enhanced terahertz radiation in a tiny V-shaped gap, just a fraction of a micrometre wide. According to the scientists, the effects seen in the microfabricated semiconductor structure could be used in applications such as biosensing and airport security scanners.
Hua Teng and his co-workers developed tiny semiconductor structures made of the chemical elements indium and antimony. From this material, they produced disks of 20µm in diameter, which they arranged such that pairs just touched. The gap between contiguous disks was merely tens to hundreds of nanometers wide. When the researchers exposed the structures to THz radiation, they found that the radiation intensity in the gap was enhanced by more than a hundred times.

Source: Wiley-VCH Verlag. Terahertz radiation is greatly enhanced in the tiny V-shaped gap, just a fraction of a micrometer wide, between pairs of touching semiconductor disks.

Confining and enhancing THz radiation is significant for two reasons, according to Teng. First, electromagnetic waves in the THz range can be used in a range of applications, for example, to study the structure of large biomolecules. As this sort of radiation can penetrate textiles but is less energetic than X-rays—or microwaves—it is also well suited for use in body scanners at airports. The second reason as to why the new results are important is more fundamental. "We have produced this particular touching-disc structure to test, in the THz regime, intriguing theoretical predictions made for optical radiation," noted Teng. "Building a device such as ours for visible light is much more challenging, as it would involve even smaller structures."
The now-verified theoretical predictions came from collaborators at Imperial College London in the UK. "For the present work, IMRE is in charge of the materials growth and the structure fabrication, while Imperial College contributes structure design and characterisation," stated Teng. The A*STAR researchers are now focused on practical applications: they will further explore the unique properties of their semiconductor materials and try to develop devices for THz technology. The group has already succeeded in tuning the THz response of their structure, meaning that they can conveniently adjust the frequency response of their device for different applications.

Microstructures made of adjoining semiconductor disks could lead to powerful nanoscale sensors

http://thznetwork.net/index.php/archives/1737

Many users of microwave ovens have had the frightening experience of leaving a fork, crumpled piece of aluminum foil or some other pointy metal item inside the cooking chamber. The sharp metal object acts as an antenna for the oven’s microwave radiation, causing strong local heating or sparking. Jing Hua Teng from the A*STAR Institute of Materials Research and Engineering (IMRE) and colleagues in Singapore and the UK have now observed a similar antenna effect, involving a different sort of electromagnetic radiation — known as terahertz (THz) radiation — in a microfabricated semiconductor structure (“Broadband Terahertz Plasmonic Response of Touching InSb Disks“). Their discovery could find application in areas ranging from biosensing to airport security scanners.

Terahertz radiation is greatly enhanced in the tiny V-shaped gap, just a fraction of a micrometer wide, between pairs of touching semiconductor disks.

Teng and his co-workers developed tiny semiconductor structures made of the chemical elements indium and antimony. From this material, they produced disks of 20 micrometers in diameter, which they arranged such that pairs just touched. The gap between contiguous disks was merely tens to hundreds of nanometers wide (see image). When the researchers exposed the structures to THz radiation, they found that the radiation intensity in the gap was enhanced by more than a hundred times.

Confining and enhancing THz radiation is significant for two reasons, according to Teng. First, electromagnetic waves in the THz range can be used in a broad range of applications, for example, to study the structure of large biomolecules. As this sort of radiation can penetrate textiles but is less energetic than X-rays — or microwaves — it is also well suited for use in body scanners at airports. The second reason as to why the new results are important is more fundamental. “We have produced this particular touching-disk structure to test, in the THz regime, intriguing theoretical predictions made for optical radiation,” explains Teng. “Building a device such as ours for visible light is much more challenging, as it would involve even smaller structures.”

The now-verified theoretical predictions came from collaborators at Imperial College London in the UK. “For the present work, IMRE is in charge of the materials growth and the structure fabrication, while Imperial College contributes structure design and characterization,” says Teng. The A*STAR researchers are now focused on practical applications: they will further explore the unique properties of their semiconductor materials and try to develop devices for THz technology. The group has already succeeded in tuning the THz response of their structure (“Direct Optical Tuning of the Terahertz Plasmonic Response of InSb Subwavelength Gratings“), meaning that they can conveniently adjust the frequency response of their device for different applications.

Photonics: Graphene's Flexible Future

Plots showing that surface plasmons are more confined when propagating along on a monolayer of graphene (G) than they are along a thin film of gold (Au). (Credit: © 2012 A*STAR Institute of High Performance Computin

My Note: This is a follow-up to the article posted on December 5th, which is found here:
http://www.blogger.com/blogger.g?blogID=124073320791841682#editor/target=post;postID=7516187338046434277

Dec. 10, 2012 — Theoretical calculations show graphene's potential for controlling nanoscale light propagation on a chip
http://www.sciencedaily.com/releases/2012/12/121210080425.htm
Semiconductors have revolutionized computing because of their efficient control over the flow of electrical currents on a single chip, which has led to devices such as the transistor. Working towards a similar tunable functionality for light, researchers from the A*STAR Institute of High Performance Computing (IHPC), Singapore, have shown how graphene could be used to control light at the nanometer scale, advancing the concept of photonic circuits on chips1.
Graphene, which is made from a single layer of carbon atoms, has excellent electronic properties; some of these are also useful in photonic applications. Usually, only metals are able to confine light to the order of a few nanometers, which is much smaller than the wavelength of the light. At the surface of metals, collective oscillations of electrons, so-called 'surface plasmons', act as powerful antennae that confine light to very small spaces. Graphene, with its high electrical conductivity, shows similar behavior to metals so can also be used for plasmon-based applications, explains Choon How Gan of IHPC, who led the research.
Gan and co-workers studied theoretically and computationally how surface plasmons travel along sheets of graphene. Even though graphene is a poorer conductor than a metal, so plasmon propagation losses are higher, it has several key advantages, says team member Hong Son Chu. "The key advantage that makes graphene an excellent platform for plasmonic devices is its large tunability that cannot be seen in the usual noble metals," he explains. "This tunability can be achieved in different ways, using electric or magnetic fields, optical triggers and temperature."
The team's calculations indicated that surface plasmons propagating along a sheet of graphene would be much more confined to a small space than they would traveling along a gold surface (see image). However, the team also showed that surface plasmons would travel far better between two sheets of graphene brought into close contact. Furthermore, by adjusting design parameters such as the separation between the sheets, as well as their electrical conductivity, much better control over surface plasmon properties is possible.
In the future, Gan and his co-workers plan to investigate these properties for applications. "We will explore the potential of graphene plasmonic devices also for the terahertz and mid-infrared regime," he explains. "In this spectral range, graphene plasmonic structures could be promising for applications such as molecular sensing, as photodetectors, or for optical devices that can switch and modulate light."
The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance Computing

Theoretical calculations show graphene's potential for controlling nanoscale light propagation on a chip

Plots showing that surface plasmons are more confined when propagating along on a monolayer of graphene (G) than they are along a thin film of gold (Au). Credit: 2012 A*STAR

http://phys.org/news/2012-12-theoretical-graphene-potential-nanoscale-propagation.html



Institute of High Performance Computing Semiconductors have revolutionized computing because of their efficient control over the flow of electrical currents on a single chip, which has led to devices such as the transistor. Working towards a similar tunable functionality for light, researchers from the A*STAR Institute of High Performance Computing (IHPC), Singapore, have shown how graphene could be used to control light at the nanometer scale, advancing the concept of photonic circuits on chips.

Graphene, which is made from a single layer of carbon atoms, has excellent electronic properties; some of these are also useful in photonic applications. Usually, only metals are able to confine light to the order of a few nanometers, which is much smaller than the wavelength of the light. At the surface of metals, collective oscillations of electrons, so-called 'surface plasmons', act as powerful antennae that confine light to very small spaces. Graphene, with its high electrical conductivity, shows similar behavior to metals so can also be used for plasmon-based applications, explains Choon How Gan of IHPC, who led the research. Gan and co-workers studied theoretically and computationally how surface plasmons travel along sheets of graphene. Even though graphene is a poorer conductor than a metal, so plasmon propagation losses are higher, it has several key advantages, says team member Hong Son Chu. "The key advantage that makes graphene an excellent platform for plasmonic devices is its large tunability that cannot be seen in the usual noble metals," he explains. "This tunability can be achieved in different ways, using electric or magnetic fields, optical triggers and temperature." The team's calculations indicated that surface plasmons propagating along a sheet of graphene would be much more confined to a small space than they would traveling along a gold surface (see image). However, the team also showed that surface plasmons would travel far better between two sheets of graphene brought into close contact. Furthermore, by adjusting design parameters such as the separation between the sheets, as well as their electrical conductivity, much better control over surface plasmon properties is possible. In the future, Gan and his co-workers plan to investigate these properties for applications. "We will explore the potential of graphene plasmonic devices also for the terahertz and mid-infrared regime," he explains. "In this spectral range, graphene plasmonic structures could be promising for applications such as molecular sensing, as photodetectors, or for optical devices that can switch and modulate light."

 More information: Gan, C. H., Chu, H. S. & Li, E. P. Synthesis of highly confined surface plasmon modes with doped graphene sheets in the midinfrared and terahertz frequencies. Physical Review B 85, 125431 (2012). prb.aps.org/abstract/PRB/v85/i12/e125431 Journal reference: Physical Review B 

A new approach to generating terahertz radiation will lead to new imaging and sensing applications

http://www.nanowerk.com/news/newsid=25168.php (Nanowerk News) Terahertz (THz) electromagnetic radiation has promising properties for a wide range of applications. The low energy of the radiation means that it can pass through materials that are otherwise opaque, opening up uses in imaging and sensing — for example, in new security scanners. In practice, however, applications have been difficult to implement. Terahertz radiation is a difficult portion of the electromagnetic spectrum to utilize. The frequencies of the region are higher than the mega and gigahertz frequencies achievable with conventional electronic circuits, but are too low-frequency to be compatible with optical instruments.

"The key challenges for THz technology are the development of a compact high power source and high sensitivity detector operating at room temperature," explains Jinghua Teng of the A*STAR Institute of Materials Research and Engineering. A recent discovery made by Teng's team of a new, efficient protocol for THz wave generation that utilizes the enhancement of light between nanometer-scale electrical contacts may provide a solution (see paper in Nature Photonics: "Greatly enhanced continuous-wave terahertz emission by nano-electrodes in a photoconductive photomixer").

Terahertz (THz) generation. A strong THz emission from the center of the device is observed in the tip-to-tip design (top). The electrodes are the black lines in the center of the device. The colours show the electric field from low (blue) to high (red) values. Much weaker electric fields and THz emission are seen in the interdigitated electrode design (bottom).

One method for creating continuous THz radiation involves directing two optical laser beams of almost similar frequencies at a suitable nonlinear material, such as certain semiconductors causing light emission exactly at the frequency difference of the two laser beams. If this difference is sufficiently small, the radiation produced falls within the THz spectrum.

However, this process is rather inefficient and requires strong light fields. Fortunately, strong amplification of light can occur near small metallic objects that act as mini antennas. This antenna effect occurs with the small metal contacts that are needed to link the non-linear material that creates the THz emission — in the current case a variant of the common semiconductor gallium arsenide.

Normally, these electrical contacts are arranged such that they resemble the fingers of interlocked hands reaching into each other. However, the A*STAR researchers developed a revised design in which the electrodes are arranged tip to tip (see top of the above image). This means that the gap between the electrodes is much narrower and also results in the alignment of the electrical field with the THz light waves, which leads to a considerably stronger antenna enhancement.

Using the new arrangement the A*STAR team were able to generate THz radiation of about 100 times the strength of that produced by conventional systems. The work suggests that these devices can be miniaturized significantly for compact yet powerful THz sources. "This approach will greatly facilitate the applications of THz technology in areas such as gas sensing, non-destructive inspection and testing, high resolution spectroscopy, product quality monitoring and bio-imaging," says Teng.