In the past, the frequency spectrum ranging from 0.3 to 3THz (or 300 to 3000GHz) was spoken as infamous “Terahertz Gap” as it lies between traditional microwave and infrared domains but remained “untouchable” via either electronic or photonic means. The conventional “transit-time-limited” electronic devices can hardly operate even at its lowest frequency; the “band-gap-limited” photonic devices on the other hand can only operate beyond its highest frequency. However continuous progress is being made for Terahertz components and devices to overcome electronic/photonic barriers for realizing highly integrated Terahertz systems.
Terahertz can provide hundredfold, increase in the frequency compared to the mmWave addressing spectrum scarcity and capacity limitation in current wireless systems. Terahertz wi-fi could in theory support data rates up to 100Gb/s within ranges of about 10m. THz ad hoc network can be formed in the battlefield to connect soldiers, armoured personnel carriers, tanks, etc. The limited transmission range and highly directional antennas makes eavesdropping extremely difficult.
“Imaging, radar, spectroscopy, and communications systems that operate in the millimeter-wave (MMW) and sub-MMW bands of the electromagnetic spectrum have been difficult to develop because of technical challenges associated with generating, detecting, processing and radiating the high-frequency signals associated with these wavelengths. To control and manipulate radiation in this especially challenging portion of the RF spectrum, new electronic devices must be developed that can operate at frequencies above one Terahertz (THz), or one trillion cycles per second,” says DARPA.
Nanotechnology is key to the 21st Century, involving all aspects of nanoscale science and technology and generating a paradigm shift in diverse areas of physics, chemistry, electronics, materials, engineering, and even medicine and biology, as a result of its interdisciplinary nature. Terahertz electronics are among the fastest growing areas as a result of the discovery, fabrication, and investigation of nanomaterials, in particular carbon nanotubes, graphenes, and compound semiconductors. These advances in nanotechnology have led to the development of nano RF or terahertz devices, which are capable of transcending conventional devices in their compactness, efficiency, performance, and operating frequency.
Terahertz has many applications that require very sensitive technologies such as metal detection, quality assurance, medical spectroscopy, integrity checks, breath-gas analysis, temperature sensing, and biosensing Much progress has been made with terahertz and infrared sensors, each of which employ physical effects — thermoelectrics in semiconductors and plasmonics in noble metals such as gold and silver. Performance is limited by unfavorable physical properties of the sensor materials, and cost and scalability remain challenging. Graphene, however, excels in both of these physical effects. Current technologies would benefit from combination with graphene, making the outlook of creating a highly scalable and cost-effective device very promising
Carbon nanotubes (CNTs) are beginning to take the electronics world by storm, and now their use in terahertz (THz) technologies has taken a big step forward. Researchers have developed flexible terahertz imagers based on chemically ‘tunable’ carbon nanotube materials. The findings expand the scope of terahertz applications to include wrap-around, wearable technologies as well as large-area photonic devices.
Researchers consider CNTs antennas technology will be great success in wireless communication technology This assumption was presented, based on the idea that CNTs can radiate as a small nano-dipole antenna when it is electromagnetically excited. With nanometer length of CNTs dipole antenna, the electromagnetic (EM) radiation from this antenna is expected to cover a range within terahertz and optical frequency
Scientists fine-tune carbon nanotubes for flexible, fingertip-wearable terahertz imagers
Researchers at Tokyo Institute of Technology have developed flexible terahertz imagers based on chemically “tunable” carbon nanotube materials. The findings expand the scope of terahertz applications to include wrap-around, wearable technologies as well as large-area photonic devices.
Due to their excellent conductivity and unique physical properties, CNTs are an attractive option for next-generation electronic devices. One of the most promising developments is their application in THz devices. Increasingly, THz imagers are emerging as a safe and viable alternative to conventional imaging systems across a wide range of applications, from airport security, food inspection and art authentication to medical and environmental sensing technologies.
The demand for THz detectors that can deliver real-time imaging for a broad range of industrial applications has spurred research into low-cost, flexible THz imaging systems. Yukio Kawano is of the Laboratory for Future Interdisciplinary Research of Science and Technology, Tokyo Institute of Technology (Tokyo Tech). In 2016 he announced the development of wearable terahertz technologies based on multiarrayed carbon nanotubes.
Kawano and his team have since been investigating THz detection performance for various types of CNT materials, in recognition of the fact that there is plenty of room for improvement to meet the needs of industrial-scale applications. Now, they report the development of flexible THz imagers for CNT films that can be fine-tuned to maximize THz detector performance. Publishing their findings in ACS Applied Nano Materials, the new THz imagers are based on chemically adjustable semiconducting CNT films.
By making use of a technology known as ionic liquid gating, the researchers demonstrated that they could obtain a high degree of control over key factors related to THz detector performance for a CNT film with a thickness of 30 micrometers. This level of thickness was important to ensure that the imagers would maintain their free-standing shape and flexibility.
“Additionally,” the team says, “we developed gate-free Fermi-level tuning based on variable-concentration dopant solutions and fabricated a Fermi-level-tuned pn junction CNT THz imager.” In experiments using this new type of imager, the researchers achieved successful visualization of a metal paper clip inside a standard envelope.
The bendability of the new THz imager and the possibility of even further fine-tuning will expand the range of CNT-based devices that could be developed in the near future. Moreover, low-cost fabrication methods such as inkjet coating could make large-area THz imaging devices more readily available.
Terahertz spectroscopy goes nano
Brown University researchers have demonstrated a way to bring a powerful form of spectroscopy — a technique used to study a wide variety of materials — into the nano-world. Laser terahertz emission microscopy (LTEM) is a burgeoning means of characterizing the performance of solar cells, integrated circuits and other systems and materials. Laser pulses illuminating a sample material cause the emission of terahertz radiation, which carries important information about the sample’s electrical properties.
“This is a well-known tool for studying essentially any material that absorbs light, but it’s never been possible to use it at the nanoscale,” said Daniel Mittleman, a professor in Brown’s School of Engineering and corresponding author of a paper describing the work. “Our work has improved the resolution of the technique so it can be used to characterize individual nanostructures.” Typically, LTEM measurements are performed with resolution of a few tens of microns, but this new technique enables measurements down to a resolution of 20 nanometers, roughly 1,000 times the resolution previously possible using traditional LTEM techniques.
The research, published in the journal ACS Photonics (“Nanoscale Laser Terahertz Emission Microscopy”), was led by Pernille Klarskov, a postdoctoral researcher in Mittleman’s lab, with Hyewon Kim and Vicki Colvin from Brown’s Department of Chemistry.
For their research, the team adapted for terahertz radiation a technique already used to improve the resolution of infrared microscopes. The technique uses a metal pin, tapered down to a sharpened tip only a few tens of nanometers across, that hovers just above a sample to be imaged. When the sample is illuminated, a tiny portion of the light is captured directly beneath the tip, which enables imaging resolution roughly equal to the size of the tip. By moving the tip around, it’s possible to create ultra-high resolution images of an entire sample.
Klarskov was able to show that the same technique could be used to increase the resolution of terahertz emission as well. For their study, she and her colleagues were able to image an individual gold nanorod with 20-nanometer resolution using terahertz emission.
The researchers believe their new technique could be broadly useful in characterizing the electrical properties of materials in unprecedented detail.
“Terahertz emission has been used to study lots of different materials — semiconductors, superconductors, wide-band-gap insulators, integrated circuits and others,” Mittleman said. “Being able to do this down to the level of individual nanostructures is a big deal.”
One example of a research area that could benefit from the technique, Mittleman says, is the characterization of perovskite solar cells, an emerging solar technology studied extensively by Mittleman’s colleagues at Brown.
“One of the issues with perovskites is that they’re made of multi-crystalline grains, and the grain boundaries are what limits the transport of charge across a cell,” Mittleman said. “With the resolution we can achieve, we can map out each grain to see if different arrangements or orientations have an influence on charge mobility, which could help in optimizing the cells.” That’s one example of where this could be useful, Mittleman said, but it’s certainly not limited to that.
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