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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.
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.
“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.
The researchers have created many breakthroughs in terahertz technologies recently that has led to testing and deployment of terahertz wireless links. The Researchers from the Tokyo Institute of Technology have already demonstrated 3Gb/s transmission at 542 GHz. At the heart of the team’s 1mm-square device is what is known as a resonant tunnelling diode, or RTD. During the 2008 Olympic Games in Beijing, scientists from Osaka University and NTT Corp. already demonstrated a 120 GHz data link across a distance of 1 km.
Nevertheless, there still exist many challenges in THz communications requiring innovative solutions, where the well-established technologies may be prohibited. Sensitive to atmospheric attenuation and molecular absorption, the THz signals experience an extremely severe path loss, which leads to a great limitation on communication distance. Meanwhile, the complex structures of THz devices pose extra constraints on the system, where conventional approaches, e.g., completely digital signal processing at baseband for each antenna, are no longer suitable.
THz Communications technologies
In the meantime, marvelous advances in hardware are making THz communications a reality. Currently, high-performance Silicon-Germanium (Si-Ge) based front-ends can generate signals up to 0.84 THz. Besides, the study on transceivers with Gallium-Nitride (GaN) based power amplifiers has demonstrated capabilities over 1 THz.
Many new compact, room temperature, terahertz sources are being developed. Teraphysics has developed miniature helical TWT for operation at 650 GHz under contracts from NASA Jet Propulsion Laboratory (JPL), the Defense Advanced Research Projects Agency (DARPA), the Army Research Office (ARO), and the Air Force Office of Scientific Research (AFOSR).
Photonic devices, such as Quantum Cascade Laser (QCL) sources, are also investigated for the THz band. Specifically, THz digital-to-analog (D/A) converters and analog-to-digital (A/D) converters have been designed using radio-frequency (RF) photonic technology.
Moreover, the magnet or voltage controlled THz phase shifters based on graphene/liquid crystal have been developed at room temperature for THz beamforming. In addition, novel nano-devices, e.g., compact graphene antennas, will be feasible for THz transmission in the near future. Undoubtedly, this ultra-broadband communication is just around the corner.
The significant decrease in the wavelength in terahertz enables packing a large number of antennas in a small area, which could provide more gains to establish reliable links. Thus, gains from multiple antennas should be explored to combat and compensate for such losses.
“We now have many enabling technologies thanks to the recent progress of semiconductor devices and integrated circuits operating at THz frequencies,” Nagatsuma of Osaka University told Nature Photonics. “In addition to the data rate, other expected advantages of THz communications over microwave communications are low power consumption and smaller transceiver size, particularly coming from a reduction in the antenna size,” he added.
Terahertz source breakthrough
Electrical and optical engineers in Australia have designed a novel platform that could tailor telecommunication and optical transmissions. Collaborating scientists from the University of New South Wales in Sydney and Canberra, the University of Adelaide, the University of South Australia and the Australian National University experimentally demonstrated their system using a new transmission wavelength with a higher bandwidth capacity than those currently used in wireless communication.
“I think moving into terahertz frequencies will be the future of wireless communications,” said Shaghik Atakaramians, an author on the paper. However, scientists have been unable to develop a terahertz magnetic source, a necessary step to harness the magnetic nature of light for terahertz devices.
In previous work, Atakaramians and collaborators proposed that a magnetic terahertz source could theoretically be produced when a point source is directed through a subwavelength fiber, a fiber with a smaller diameter than the radiation wavelength. In this study, they experimentally demonstrated their concept using a simple setup — directing terahertz radiation through a narrow hole adjacent to a fiber of a subwavelength diameter. The fiber was made of a glass material that supports a circulating electric field, which is crucial for magnetic induction and enhancement in terahertz radiation.
Creating terahertz magnetic sources opens up new directions for us,” Atakaramians said. Terahertz magnetic sources could help the development of micro- and nanodevices. For example, terahertz security screenings at airports could reveal hidden items and explosive materials as effectively as X-rays, but without the dangers of X-ray ionization.
Another advantage of the source-fiber platform, in this case using a magnetic terahertz source, is the proven ability to alter the enhancement of the terahertz transmissions by tweaking the system. “We could define the type of response we were getting from the system by changing the relative orientation of the source and fiber,” Atakaramians said.
Atakaramians emphasized that this ability to selectively enhance radiation isn’t limited to terahertz wavelengths. “The conceptual significance here is applicable to the entire electromagnetic spectrum and atomic radiation sources,” said Shahraam Afshar, the research director. This opens up new doors of development in a wide range of nanotechnologies and quantum technologies such as quantum signal processing
Terahertz Multiplexers / Demultiplexers
Multiplexers / Demultiplexers are devices that are used to enable separate streams of data to travel through a single medium, by combining the signals through multiplexers at the transmitter end and separating them through demultiplexers at the receiver end, common examples are cable carrying multiple TV channels or fiber optic line carrying thousands of phone calls at the same time.
Researchers from Brown University have developed deices for multiplexing / demultiplexing terahertz waves. The multiplexer that Mittleman and his colleagues have been working on makes use of what’s known as a leaky wave antenna. In this case, the antenna is made from two metal plates placed in parallel to form a waveguide. One of the plates has a small slit in it. As terahertz waves travel down the waveguide, some of the radiation leaks out of the slit. It turns out that terahertz waves leak out different angles depending on their frequency.
“That means if you put in 10 different frequencies between the plates — each of them potentially carrying a unique data stream — they’ll come out at 10 different angles,” Mittleman said. “Now you’ve separated them and that’s demultiplexing.” On the other end, a receiver could be tuned to accept radiation at a particular angle, thus receiving data from only one stream.
“We think it’s definitely a reasonable solution to meet the needs of a terahertz communication network,” said Nicholas Karl, a graduate student at Brown and the paper’s lead author. Karl led the experiments on the device with fellow graduate student Robert McKinney. Other authors on the study are Rajind Mendis, a research professor at Brown, and Yasuaki Monnai from Keio University in Tokyo.
The group plans to continue its work to refine the device. A research group from Osaka University is collaborating with Mittleman’s group to implement the device in a prototype terahertz network they’re building.
Tufts Researchers Build A Chip-Sized, High-Speed Terahertz Modulator
Tufts University engineers claim to have a breakthrough with their successful fabrication of an on-chip device that can perform gigahertz-rate amplitude modulation, and switching of broadband terahertz electromagnetic waves confined within a novel slot waveguide with tunable, two-dimensional electron gas.
“A prototype device is fabricated which shows THz intensity modulation of 96% at 0.25 THz carrier frequency with low insertion loss and device length as small as 100 microns. The demonstrated modulation cutoff frequency exceeds 14 GHz indicating potential for the high-speed modulation of terahertz waves. The entire device operates at room temperature with low drive voltage (<2 V) and zero DC power consumption,” the researchers wrote in a paper published in Scientific Reports.
Previously-built THz modulators were capable of reaching speeds of only up to a few kilohertz (kHz). The Tufts University team claims to have experimental results showing gigahertz speed modulation of THz waves for the first time.
“This is a very promising device that can operate at terahertz frequencies, is miniaturized using mainstream semiconductor foundry, and is in the same form factor as current communication devices. It’s only one building block, but it could help to start filling the THz gap,” said Sameer Sonkusale, Ph.D., of Nano Lab, Department of Electrical and Computer Engineering, Tufts University, and the paper’s corresponding author, in a news release.
A well-known application for building fast and compact terahertz modulators is to achieve high data rate wireless communication, where an inherently high carrier frequency of THz wave will support much wider signal bandwidth compared to the radio frequency (RF) bands used today, according to the researchers. But wider applications abound, such as in material identification, imaging, wireless communications, chemical and biological sensing.
THz Antenna
Due to the enormous path loss for long distance in space and tiny power capacity, the high gain antenna is preferred. The traditional reflector antenna used for THz band will bring some problems because of the extremely higher requirement of surface precision, although this kind of antenna has larger gain.
A team headed by physicists Alexander Holleitner and Reinhard Kienberger from the Technical University of Munich (TUM) has generated ultrashort electric pulses on a chip using metal antennas only a few nanometers in size. Pulses of femtosecond length from the pump laser generate on-chip electric pulses in the terahertz frequency range. With another laser, the information is read out again.
Physicists from the Technical University of Munich (TUM) exploited femotsecond photoswitches based on the nanoscale metal structures to drive the pulses. The nonlinear ultrafast response was based on a plasmonically enhanced, multiphoton absorption, resulting in a field emission of ballistic hot electrons propagating across the nanoscale structures.
According to researchers, a femtosecond laser pulse with a frequency of 200 THz could generate an ultrashort terahertz signal with a frequency of up to 10 THz in the circuits on the chip. Researchers used sapphire as the chip material because sapphire cannot be stimulated optically and would therefore cause no interference. Lasers with a 1.5-μ wavelength were used in traditional internet fiber optic cables.
THz Signal Processing
The classical signal processing method cannot fully benefit from the properties of the THz band signal. The new channel models are required for the THz wave propagation. New coding schemes are needed to overcome the channel errors in the THz band. Different bandwidths and transmission distances of THz wave applications require various adaptable modulations.
Low-cost system architectures and communication schemes are needed, which should be adaptive and stable to the whole THz band.
Terahertz Photonics
“Scientists are turning to the development of photonic, rather than electronic, devices for THz communications because it is easier to achieve higher data rates using photonic components,” said Nagatsuma of Osaka University. “In addition, photonics-based systems might be deployed in the future convergence of fibre optic and wireless communications networks,” commented Nagatsuma. He believes that ultrawideband amplifiers and antennas are the most crucial components needed to make full use of the bandwidth. “Even for photonics-based systems, amplifiers are necessary to boost the output power in the transmitter and to increase the sensitivity in the receiver,” he stressed.
“Therefore, over the past 10 years many developments have been made to prepare for the future convergence between fiber optic and mobile end users, in backhaul — point to point (P2P) — or fronthaul schemes,” writes Guillaume Ducournau, Institute of Electronics, Microelectronics and Nanotechnology. “Both require very high frequency transceivers, and electronic/optic approaches are under investigation. In addition, the massive development of multilevel encoding combined with standard WDM (wavelength division multiplexing) and the context of coherent networks and core signal processing is now established. Thus, the quest for direct optical to radio transceivers has become very attractive and would enable direct bridges between optical data rates and mobile data delivery.
THz communication devices will require innovation in integration and packaging to be practical. Guillermo Carpintero of Universidad Carlos III de Madrid in Spain described how he and his co-workers are tackling this challenge and have developed integrated photonics-based sources of millimetre and THz waves during 40th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz).
“Although we tried to use available generic integration-platform building blocks, there is no building block for Bragg mirrors,” said Carpintero. As a result, the team developed the concept of integrated multimode interference reflector mirrors for mode-locked lasers. The optical spectrum of the optical heterodyne source based on the mode-locked photonic integrated circuit around 1,560 nm showed a carrier wave frequency of 90 GHz. The team has used this on-chip optical heterodyne source to perform broadband wireless data transmission.
“For nomadic usages, development of siliconbased photonics and/or CMOS might enable advanced integration and miniaturization, which is especially required for mass market applications, such as those embedded in mobile terminals,” writes Guillaume Ducournau.
InP Devices
To date, the terahertz band is mostly unused for a lack of suitable electronic components, which are commercially available only up to around 100 GHz.” In order to reach these high frequencies, both intrinsic and extrinsic parasitic capacitances need to be reduced, and semiconductor materials with high electron mobility need to be used. For this purpose, we developed a transfer-substrate Indium Phosphide (InP) Hetero-Bipolar Transistor technology. Besides high electron mobility, the InP material system offers a high breakdown field due to its large energy gap, enabling higher output power at THz frequencies than any other semiconductor material,” write Leibniz-Institute IHP Frankfurt. Capacitances are effectively reduced with the transfer-substrate approach.
“InP HBTs with an emitter size of 0.5 × 5 µm2 are defined by electron beam lithography, demonstrating an fmax of more than 450 GHz at a breakdown voltage of BVCEO = 4.5 V. Monolithically integrated circuits such as amplifiers, mixers, and oscillators operating in the frequency range from 100 GHz to over 300 GHz have been fabricated and tested.”
The cutoff frequency of ultra-high frequency transistors is increased with geometrical device scaling. The cooling of these transistors becomes ever more important as the power density increases with shrinking device dimensions. The heat can be efficiently extracted from the transistors with the integration of an electrically isolating diamond heat spreading layer, without having to compromise the high frequency performance. The thermal resistance of diamond-integrated InP HBT could be reduced by more than a factor of three compared to standard InP HBT, reaching a value below 1 K/mW. The RF output power of analog amplifier circuits operating at around 100 GHz could be doubled with the inclusion of the diamond heat sink.
System integration of integrated terahertz circuits requires a suitable mounting technology, foremost to connect the terahertz circuit to an antenna structure. The required mounting and connection technology needs to be sufficiently broadband, should not incur significant RF losses, must be reproducibly manufacturable from the initial electromagnetic design, and needs to be low cost. Classic bond wire connections are difficult to implement beyond 100 GHz due to manufacturing tolerances.
A flip-chip mounting technology based on gold-tin with 10 µm design rule was developed, including multilayer passive submount substrates with shielded transmission lines, which were manufactured in a process sequence similar to the InP DHBT process flow. Passive and active InP HBT circuits were mounted onto these submounts and measured. A bandwidth from DC to 450 GHz could be demonstrated. The insertion loss of the flip-chip transitions was less than 1 dB even at the highest frequencies.
Wafer-scale InP DHBT – SiGe BiCMOS hetero-integration process – SciFab
“Based on the transferred-substrate concept, we developed a wafer-scale 3D integration approach of InP DHBT technology onto Silicon Germanium Bipolar-CMOS (SiGe BiCMOS) wafers using face-to-face adhesive wafer bonding, subsequent InP substrate removal, and formation of vertical RF interconnects between the InP DHBT and SiGe BiCMOS subcircuits. In this complementary approach, we combine the advantages of both technologies: highly complex BiCMOS analog and digital circuits are augmented with the high bandwidth and output power of InP DHBT amplification and mixing stages. Signal sources including a BiCMOS VCO and InP mixing and power amplification stages operating at up to 330 GHz were demonstrated.”
DARPA’s thrust in Terahertz
DARPA has made a series of strategic investments in terahertz electronics through its HiFIVE, SWIFT and TFAST programs. “To fully exploit the sub-MMW band will require monolithic microwave integrated circuits (MMICs) that can operate up to THz frequencies. And to make these THz MMICs (or “TMICs”) will require THz transistors with maximum oscillation frequencies (fmax) well above 1 THz.
The objective of the Terahertz (THz) Electronics program is to develop the critical device and integration technologies necessary to realize compact, high-performance electronic circuits that operate at center frequencies exceeding 1.0 THz.
Successes in the THz Electronics program could catalyze the development of revolutionary applications by enabling coherent THz processing techniques such as THz imaging systems; sub-MMW, ultra-wideband, ultra-high-capacity communication links; and sub-MMW, single-chip widely-tunable synthesizers for explosive detection spectroscopy.
The program is focused on two critical THz technical areas:
Terahertz Transistor Electronics
This part of the program has aggressively developed multi-THz Indium Phosphide-based transistors (heterojunction bipolar transistors, or HBTs, and High Electron Mobility Transistors, or HEMTs) and has demonstrated TMICs operating up to and above 1 THz. In addition, THz low-loss inter-element interconnect and integration technologies have been developed, enabling compact THz transmitter and receiver modules to demonstrate wireless communications at 220 GHz, 670 GHz, and 850 GHz – hundreds of times faster than your cell phone.
Terahertz High Power Amplifier Modules
This part of the program aims to develop compact, micromachined vacuum electronics devices to produce a significant increase of output power at frequencies beyond 1.0 THz and to radiate that energy at an antenna. Already, micromachined traveling wave tube amplifiers (TWTs) operating at 670 GHz and 850 GHz have been built and tested and have produced the highest linear output power available at these frequencies.
Northrop Grumman Corporation, under DARPA’s Terahertz Electronics, has created a fastest solid-state amplifier integrated circuit ever measured, operating at a speed of one terahertz. The ten-stage common-source amplifier Terahertz Monolithic Integrated Circuit (TMIC) exhibits power gains several orders of magnitude beyond the current state of the art, using a super-scaled 25 nanometer gate-length. The Northrop Grumman TMIC showed a measured gain of nine decibels at 1.0 terahertz and 10 decibels at 1.03 terahertz.
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