The terahertz (THz) region (nominally 0.1-10THz) separates electronics from photonics, and has historically been difficult to access. Semiconductor electronics run out of steam after ~100GHz due to transport time limitations. Photonic devices falter below ~10THz because photon energy drops to thermal energy. At the high-frequency end, infra-red (IR) optoelectronics cannot operate significantly below 10THz. Since these devices employ photon-electron particle interactions, as photon energy hv decreases below thermal energy kT, the device ceases to operate efficiently unless it is cooled down. This adds significant cost and weight. At the low-frequency end, semiconductor electronic devices cannot operate at frequencies significantly above 100GHz. Transport time across the semiconductor junction is limited by drift and diffusion speeds. The largely-untapped frequency region between 100GHz and 10THz (the THz region) holds promise for a wide range of commercial and military applications. Terahertz electronics (TE) is a new technology that extends the range of electronics into the THz-frequency region.

Fig. 1: Terahertz gap
Fig. 1: Terahertz gap

Terahertz electronics
Terahertz electronics technology opens up practical applications in high-speed data interconnects, THz imaging, and highly-integrated radar and communication systems. The gap between electronics and photonics has closed. Further use and development of such technological devices will make TE a reality in the near future. It does not use semiconductors; instead, it is based on metal-insulator tunnelling structures to form diodes for detectors and ultra-high-speed tunnelling transistors for oscillator based transmitters. With these devices, detectors and transistors for operation in the THz region have been designed. Besides being extremely fast, TE devices are made entirely of thin-film materials—metals and insulators—and so may be fabricated on top of complementary metal oxide semiconductor (CMOS) circuitry—a technology for constructing integrated-circuits circuitry or on a wide variety of substrate materials.

Based on metal-insulator tunnel junctions, TE technology extends the range of electronics beyond the 100GHz barrier to 10THz. In these devices, charge transport through the junction occurs via electron tunnelling, which has a time constant of ~10–15 seconds. Charge transport to and from the junction occurs via plasma oscillation in metals, which is easily supported with low loss in the THz range of interest. Furthermore, epitaxial growth and high process temperatures are not required. Thus, integration of TE onto low-loss insulating substrates (for example, glass, sapphire, ceramic or plastic) or onto silicon-integrated circuits is possible. Industries are employing this concept in practical components, like detectors, transistors, mixers and other devices, to provide a full suite of ultra-fast integrated components for building high-frequency electromagnetic wave circuits and systems. The result is complete integrability, and consequently, low cost. In addition, devices may be fabricated onto large-area flat panels or flexible sheets, enabling completely integrated microwave/millimetre-wave/sub-millimetre-wave sensor and emitter arrays. These devices operate at low voltages, allowing compatibility with CMOS circuitry, coupled with innovations in antenna design and travelling wave devices. This leap in performance and flexibility enables a host of new applications for TE.

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Sources of terahertz radiations

Fig. 2: Mercury arc lamps generate light in terahertz
Fig. 2: Mercury arc lamps generate light in terahertz

One of the main reasons that THz applications have not fully materialised yet is the lack of a small, low-cost, moderate-power THz source. THz radiations are generally emitted as a part of black-body radiation from anything with temperatures greater than 10 kelvin. While this thermal emission is very weak, observations at these frequencies are important for characterising the cold 10-20K dust in the interstellar medium. However, the opacity of the Earth’s atmosphere to sub-millimetre radiation restricts these observatories to very high altitude sites, or to space. About a decade back the only viable sources of THz radiation were:
1. The gyrotron
2. The backward-wave oscillator (BWO)
3. The far IR laser (FIR laser)
4. Quantum cascade laser
5. The free electron laser (FEL)
6. Synchrotron light sources
7. Photo-mixing sources
8. Single-cycle sources used in THz time domain spectroscopy, such as hoto-conductive, surface field, photo-dember and optical-rectification emitters

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Fig. 3: Close-up of the electron source
Fig. 3: Close-up of the electron source

There have also been solid-state sources of millimetre and sub-millimetre waves for many years. Nowadays, most time-domain work is done via ultra-fast lasers. In mid 2007, scientists announced the creation of a compact device that can lead to portable, battery-operated sources of T-rays, or THz radiation. This new T-ray source uses high-temperature superconducting crystals that comprise stacks of Josephson junctions that exhibit a unique electrical property—when an external voltage is applied, an alternating current will flow back and forth across junctions at a frequency proportional to the strength of the voltage (phenomenon known as Josephson effect). These alternating currents then produce electro-magnetic fields whose frequency is tuned by the applied voltage. Even a small voltage, around two millivolts per junction, can induce frequencies in the THz range.

In 2008, engineers announced they had built a room-temperature semiconductor source of coherent THz radiation. Until then, sources had required cryogenic cooling, greatly limiting their use in everyday applications.

Many other THz-source technologies have been investigated in the past four decades. Numerous groups worldwide are producing tuneable CW THz radiation using photo-mixing of near-IR lasers. Direct multiplied (DM) sources take millimetre-wave sources and directly multiply their output up to THz frequencies. DM sources with frequencies up to a little more than 1THz and approximately 1µW of output have been used as local oscillators for heterodyne receivers in select applications, most of which are in radio astronomy.

However, these can produce substantially more output power at lower frequencies, and are often well-suited to applications requiring frequencies of less than 500GHz. In addition, physicists have recently demonstrated quantum-cascade semiconductor lasers operating at wavelengths in the 4.4THz regime. These lasers are made from 1500 alternating layers (or stages) of gallium-arsenide and aluminium-gallium-arsenide and have produced 2mW of peak power (20nW average power), and advances in output power and operating wavelength continue at a rapid pace.

Recently, researchers at JILA (formerly known as Joint Institute for Laboratory Astrophysics), jointly operated by University of Colorado and National Institute of Standards and Technology, USA, have developed a laser based source of THz radiation that is unusually efficient and less prone to damage than similar systems. JILA instruments for generating THz radiation make use of ultra-fast pulses of near-IR laser light that enter through the lens on left, striking a semiconductor wafer studded with electrodes (transparent square that is barely visible under the white box connected to orange wires) bathed in an oscillating electric field. The light dislodges electrons, which accelerate in the electric field and emit waves of THz radiation. Fig. 3 shows the close-up of the electron source.

General applications
THz radiations are non-ionising, and therefore safe to humans. These penetrate a wide variety of non-conducting materials, including clothing, paper, plastics and ceramics, and can also penetrate fog and clouds, but are strongly absorbed by metal and water. Until recently, researchers did not extensively explore the material interactions occurring in the THz-spectral region because they lacked reliable sources of THz radiation. However, pressure to develop new THz sources arose from two dramatically different groups—ultra-fast time-domain spectroscopists who wanted to work with longer wavelengths, and long-wavelength radio astronomers who wanted to work with shorter wavelengths. Today, with continuous wave (CW) and pulsed sources readily available, investigators are pursuing potential THz-wavelength applications in many fields.

Fig. 4: Terahertz transistor
Fig. 4: Terahertz transistor

Companies are planning to exploit the commercial applications of TE. THz applications span the physical (security imaging), biological (cell formation) and medical (cancerous-tumour detection) sciences with a growing interest in the application of THz frequencies from security imaging through clothing in airport scanners to non-destructive pharmaceutical and manufacturing inspection through multilayered or opaque surfaces. The unique properties of THz radiation also include high-frequency radars to produce high-resolution images of objects through cloud, fog and dust storms to support aircraft landing in harsh environments.

Much of the recent interest in THz radiation stems from its ability to penetrate deep into many organic materials without the damage associated with ionising radiation such as X-rays. Also, because THz radiation is readily absorbed by water, it can be used to distinguish between materials with varying water content, for example, fat versus lean meat. These properties lend themselves to applications in process and quality control as well as biomedical imaging. Tests are currently underway to determine whether THz tomographic imaging can augment or replace mammography, and some people have proposed THz imaging as a method of screening passengers for explosives at airports as well as for detecting the presence of cancerous cells in humans. However, all these applications are still in the research phase.

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THz radiation can also help scientists understand the complex dynamics involved in condensed-matter physics and processes such as molecular recognition and protein folding. CW THz technology has long interested astronomers because approximately one-half of the total luminosity and 98 per cent of the photons emitted since the Big Bang fall into the sub-millimetre and far-IR, and CW THz sources can be used to help study these photons. One type of CW THz source is the optically pumped THz laser (OPTL). These lasers are in use around the world, primarily for astronomy, environmental monitoring and plasma diagnostics. The emerging field of time-domain spectroscopy (TDS) typically relies on a broadband short-pulse THz source. A split antenna is fabricated on a semiconductor substrate to create a switch. A DC bias is placed across the antenna, and an ultra-short pump-laser pulse (<100fs) is focused in the gap in the antenna. The bias-laser pulse combination allows electrons to rapidly jump the gap, and the resulting current in the antenna produces a THz electromagnetic wave. This radiation is collected and collimated with an appropriate optical system to produce a beam.

Terahertz electronic devices

Fig. 5: A terahertz mixer component containing a Schottky diode
Fig. 5: A terahertz mixer component containing a Schottky diode

Metal oxide semiconductor (MOS) transistor is the building block of integrated circuits (ICs) in electronics and is the engine that powers these. Today’s most complex ICs, such as microprocessors, graphics and DSP chips, pack more than 100 million MOS transistors on a single chip. Integration of one billion transistors into a single chip will soon become a reality. The semiconductor industry faces an environment that includes increasing chip complexity, continued cost pressures, increasing environmental regulations and growing concern about energy consumption. The observation that the number of transistors per integrated circuit doubles every 18 to 24 months is well-known to industry analysts and many of the general public. New materials and technologies are needed to support the continuation of Moore’s law.

TE holds promise of greatly-expanding and numerous applications in detection of biological and chemical hazardous agents, building and airport security, and explosive detection, as well as in radio astronomy, biology and medicine. Many companies are thinking towards exploiting the commercial applications of TE beyond traditional aerospace and medical markets. The technology being exploited depends on the fabrication of electronic devices that operate above 100GHz, where traditional electronic circuits no longer function. It is a generic device technology that can be used as both a detector and source of THz radiation, opening the potential for very-high-frequency communication systems and radars. The devices, Schottky diodes, operate at room temperature, rather than under cryogenic conditions like most competitor technologies, significantly simplifying system infrastructure and reducing cost.

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Companies are focusing on the development of microwave monolithic integrated circuit (MMIC) technologies at frequencies approaching 1THz, including corresponding circuits, components and modules. Voltage-controlled oscillators and amplifiers, power amplifiers, frequency multipliers and mixers will soon be developed with operating frequencies beyond 500GHz. These devices shall be used to realise compact integrated front-end modules for radar and communication systems.

THz transistor technology is now emerging and short-channel Si CMOS, InGaAs based hetero-structure bipolar transistors and high-electron mobility transistors have reached cut-off frequencies and maximum frequencies of oscillation in the THz range. Si Schottky diodes have demonstrated millimetre-wave detection. GaN based FETs have additional advantages at THz frequencies with a different design. The device feature-sizes have shrunk to the point where ballistic mode of electron transport becomes important or even dominant. THz radiation excites oscillations of the electron density (plasma waves) in transistor channels. Plasma waves propagate with velocities much larger than electron drift velocities and have characteristic frequencies in the THz range even for devices with feature-sizes exceeding a few hundred nanometres. The rectification of plasma waves by the device non-linearity can be used for detecting THz radiation and for imaging and in-situ testing of transistor structures. Using synchronised THz transistor arrays, it is expected to yield dramatic performance improvements of plasmonic THz electronic detectors and sources.

Today’s research of TE is also focused on a variety of different compact optoelectronic devices, like photoconductive antennae (PCA), which operate at room temperature but need an external laser excitation, or THz quantum cascade lasers that are powerful but currently operate at cryogenic temperatures. One of the main sources of THz radiation, the inter-digitated photoconductive antenna, can now be tuned to THz frequencies that were previously difficult to reach. Researchers found that changing the spacing between electrodes in the antenna’s structure enables the emission spectrum to be centred at a chosen frequency—a property that will be useful for spectroscopy and imaging, where access to particular parts of the THz spectrum is needed.

We will need to think about new geometries and new materials to improve these issues, possibly combined with the use of cheaper and compact fibre based laser systems. Improving these types of sources will permit the THz technological range to become mature and comparable to those used in microwave electronics and IR optical systems.

Challenges ahead
The objective of the TE program is to develop critical devices and integration technologies necessary to realise compact, high-performance electronic circuits that operate at centre frequencies exceeding 1THz. The main focus will be on the development of two critical THz technical areas. One is THz transistor electronics to develop multi-THz InP HBT and InP HEMT transistor technologies to enable TMICs along with THz low-loss inter-element interconnect and integration technologies to build compact THz transmitter and receiver modules. Another is THz high-power amplifier modules for compact, micro-machined vacuum electronics devices to produce a significant increase of output power at frequencies beyond 1THz and to radiate this energy at an antenna.

The success of TE will lead to 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-tuneable synthesisers for explosive-detection spectroscopy. Despite intense research efforts, there have been many challenges that have not yet been overcome in achieving a miniature, efficient THz source. Further research in TE should investigate innovative approaches that enable revolutionary advances in electronic devices and ICs achieving THz frequencies. Efforts are being made to use band-gap engineering and the unique properties of graphene to develop basic building blocks of graphene TE and to accelerate its applications.