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.

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.

T-Rays technology could help develop Star Trek-style hand-held medical scanners

http://www.nanowerk.com/news/newsid=24012.php (Nanowerk News) Scientists who have developed a new way to create a type of radiation known as Terahertz (THz) or T-rays - the technology behind full-body security scanners - say their new, stronger and more efficient continuous wave T-rays could be used to make better medical scanning gadgets and may one day lead to innovations similar to the "tricorder" scanner used in Star Trek.

In a study published recently in Nature Photonics ("Greatly enhanced continuous-wave terahertz emission by nano-electrodes in a photoconductive photomixer"), researchers from the Institute of Materials Research and Engineering (IMRE), a research institute of the Agency for Science, Technology and Research (A*STAR) in Singapore and Imperial College London in the UK have made T-rays into a much stronger directional beam than was previously thought possible and have efficiently produced T-rays at room-temperature conditions. This breakthrough allows future T-ray systems to be smaller, more portable, easier to operate, and much cheaper.

Optical microscope picture of an antenna structure with the nano-antennas built into its centre (highlighted, top left) and the electric field distribution (top right). Bottom: An optical microscope image showing the unique nano-antennas and their effect on the THz waves generated.

The scientists say that the T-ray scanner and detector could provide part of the functionality of a Star Trek-like medical "tricorder" - a portable sensing, computing and data communications device - since the waves are capable of detecting biological phenomena such as increased blood flow around tumorous growths. Future scanners could also perform fast wireless data communication to transfer a high volume of information on the measurements it makes.

T-rays are waves in the far infrared part of the electromagnetic spectrum that have a wavelength hundreds of times longer than visible light. Such waves are already in use in airport security scanners, prototype medical scanning devices and in spectroscopy systems for materials analysis. T-rays can sense molecules such as those present in cancerous tumours and living DNA as every molecule has its unique signature in the THz range. T-rays can also be used to detect explosives or drugs, in gas pollution monitoring or non-destructive testing of semiconductor integrated circuit chips. However, the current continuous wave T-rays need to be created under very low temperatures with high energy consumption. Existing medical T-ray imaging devices have only low output power and are very expensive.

In the new technique, the researchers demonstrated that it is possible to produce a strong beam of T-rays by shining light of differing wavelengths on a pair of electrodes - two pointed strips of metal separated by a 100 nanometre gap on top of a semiconductor wafer. The unique tip-to-tip nano-sized gap electrode structure greatly enhances the THz field and acts like a nano-antenna that amplifies the THz wave generated. The waves are produced by an interaction between the electromagnetic waves of the light pulses and a powerful current passing between the semiconductor electrodes from the carriers generated in the underlying semiconductor. The scientists are able to tune the wavelength of the T-rays to create a beam that is useable in the scanning technology.

Lead author Dr Jing Hua Teng, from A*STAR's IMRE, said: "The secret behind the innovation lies in the new nano-antenna that we had developed and integrated into the semiconductor chip." Arrays of these nano-antennas create much stronger THz fields that generate a power output that is 100 times higher than the power output of commonly used THz sources that have conventional interdigitated antenna structures. A stronger T-ray source renders the T-ray imaging devices more power and higher resolution.

Research co-author Stefan Maier, a Visiting Scientist at A*STAR's IMRE and Professor in the Department of Physics at Imperial College London, said: "T-rays promise to revolutionise medical scanning to make it faster and more convenient, potentially relieving patients from the inconvenience of complicated diagnostic procedures and the stress of waiting for accurate results. Thanks to modern nanotechnology and nanofabrication, we have made a real breakthrough in the generation of T-rays that takes us a step closer to these new scanning devices. With the introduction of a gap of only 0.1 micrometers into the electrodes, we have been able to make amplified waves at the key wavelength of 1000 micrometers that can be used in such real world applications."

The research was led by scientists from A*STAR's IMRE and Imperial College London, and involved partners from A*STAR Institute for Infocomm Research (I2R) and the National University of Singapore. The research is funded under A*STAR's Metamaterials Programme and the THz Programme, as well as the Leverhume Trust and the Engineering and Physical Sciences Research Council (EPSRC) in the UK.

Source: A*STAR