Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to ‘real world’ applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established.
1. Terahertz quantum cascade lasers
After a decade of research, the operation temperature of THz QCLs has progressively risen to 199.5K in pulsed mode (and in the absence of an applied magnetic field), although this maximum temperature has stayed static now for a number of years.
Although there remain considerable challenges ahead, THz frequency QCLs are currently the only compact source operating with high output powers above 1 THz. Through the potential advances highlighted above, further applications and functionalities of these devices will be found ranging from fundamental science to applied research, including their use as new sources for nonlinear optics, imaging, spectroscopy and trace gas analysis, inter alia. There remain open questions on the operation of these devices, including their high temperature performance, their dynamics related to pulse generation, and the possibility to realize extremely broadband frequency combs. Further, QCLs will accompany new advances in detector technology, such as coherent detection and nano-detection techniques, to provide powerful, inexpensive and compact THz systems.
2. Intense laser-based THz sources
The spectral range between 0.3 and 30 THz has long been described as the ‘THz gap’ because of the lack of strong and compact sources. This has changed drastically in the last two decades. CW-sources based on microwave technology and solid state sources based on quantum cascade laser technology are now available and offer average power in the mW range. Dramatic improvements have also been made in the generation of intense picosecond THz pulses via nonlinear optical methods such as optical rectification. It is now possible to routinely generate pulse energies of tens of micro-joules and field strengths exceeding 1 MV cm−1 with a compact and reliable femtosecond laser source in a small university laboratory without issues of restricted access, low repetition rate, timing stability and beam transport issues in accelerator-based THz facilities.
The most promising applications of these strong-field THz sources are in basic science.
Laser based intense THz sources have progressed tremendously in the last decade. They are now being used to study materials in THz pump experiments that have been previously impossible. Scaling up of current single sources will enhance our understanding of material behavior in the limit of extremely strong THz pulses, where the instantaneous electromagnetic fields exceed the DC breakdown values by a significant amount. Developing tailored pump solutions in the 5–15 THz range will enable novel avenues of control in complex material systems by targeting low energy collective phenomena directly. The future of THz science looks bright indeed.
3. THz vacuum electronics
The progress toward compact, low weight, reliable, affordable, medium-to-high power THz vacuum electron devices (VEDs) is a fascinating adventure in a state-of-the-art, multidisciplinary field. The combined effort of a growing number of research groups active worldwide is yielding new designs, materials and fabrication methods to overcome the formidable obstacle posed by high-power-density VEDs with submillimetre dimensions. The future availability of low cost, high-power-density, THz VED sources will enable transformative advances in the world of THz applications, where the power, size, weight, availability and/or cost of present device options limit the outstanding and unique potential of THz radiation.
4. Accelerator-based sources of terahertz radiation
The development of accelerator-based sources of THz radiation has evolved over the past years towards user facilities that provide either unprecedented high average powers, up to the few 10 W and even kW regime, or more recently high peak fields and/or peak fields at high repetition rates up to 10 MV cm−1 and beyond. These sources complement laser-based and other table-top THz sources, which are limited to lower average powers, lower peak fields and lower repetition rates. The applications of such sources have widened from the brilliance-limited THz spectroscopy applications of the early days towards flux-limited experiments and experiments investigating nonlinear dynamics driven by extreme transient THz fields.
5. Photoconductive devices for THz time-domain spectroscopy
Terahertz photoconductive devices have evolved dramatically since their introduction in the 1980s. The bandwidth, power and reliability of these devices improved with the use of novel semiconductor materials and contact structures. The viability of fabricating these devices at industrial scales is an issue still being addressed, but promising solutions can be foreseen in the years to come.
6. Components for terahertz imaging
Terahertz imaging has been demonstrated using a wide range of methods. These include pulsed time-domain (PTD) and continuous wave (CW) technologies, single detector scanned systems, arrays and focal plane arrays (FPAs). As described in the Friis equation, the overall performance of an imager is determined by its optical properties including source power, system losses, and detector sensitivity. Whatever the means of implementation, all systems rely on the brightness of the illumination source deployed and the responsivity of the detector
Future expectations for terahertz imaging systems include video rate imaging (at least 25 fps) at VGA resolution. Further into the future, HD format will be the normal expectation for any imaging system. Image resolution (as opposed to display resolution), noise and dynamic range are all expected to improve. These improvements will rely on advancement in source, detector and optical/system design technology. Specifically, compact, room temperature terahertz sources in the region of 10 mW average power are essential in order to enable stand-off imaging at distances greater than 1 m. Such sources when coupled with a Si CMOS FPA would render a low-cost THz camera (we estimate less than US$5K per unit) that could find wide spread use in applications such as stand-off detection of hidden objects and non-invasive medical (e.g. oncology) and dental diagnostics.
The development of imaging systems to meet a range of growing applications for terahertz is presently limited by the available technologies, irrespective of
the image system configuration and modality. Whether the intended application entails the development of a microscope, a far-field or near-field imager, or the use of scalar or timedomain techniques, there is a demand for improved source and detector technologies. Work is required to improve every attribute including, but not limited to: source power; terahertz bandwidth; operating temperature; responsivity and NEP; and array size where deployed. Improvements in these basic component measures will lead to improved imaging systems
7. Passive THz components
With the relentless advances in the performance of active devices and circuits that make up sources, amplifiers, active modulators and detectors, the associated passive components (implemented with either guided-wave structures or free-space quasi-optics) will ultimately limit the overall performance of front-end THz subsystems. Existing and emerging passive component technologies, used to perform vital functions (e.g. within impedance/amplitude/phase matching networks, power couplers, filters, antennas, polarizers and even switches), must find their niche location(s) within the ever-expanding cost-performance application space. Be-spoke passive solutions will be needed to keep pace with developments in active technologies, ideally without the need for cryogenic cooling.
8. Developments in THz time domain spectroscopy
Despite the fact that THz time-domain spectroscopy has become a mature field and the technique is used in hundreds of research labs worldwide, there is still
significant room for improvement of THz systems. The key could be the further development of mode-locked edge emitting semiconductor lasers and improved photoconductive semiconductor quantum structures. Furthermore, mass production and modern fabrication techniques like 3D printing will have an impact.
9. Terahertz spectroscopy of semiconductors and semiconductor nanostructures
Terahertz spectroscopy, both static and time-resolved, will continue to be a leading method for characterization of semiconductors and semiconductor nanostructures. In the future, it will be used to probe increasingly smaller regions and will also be used to drive systems out of equilibrium. One can also envision using photoexcited semiconductors as transient optical components such tunable filters when coupled with metamaterials.
10. THz microscopy
THz microscopy has recently made the transition from proof-of-concept experiments to an established class of diagnostic techniques with unique capabilities. Subjects range from evanescent fields confined to the surfaces of subwavelength objects to the local material properties of inhomogeneous media and nanoparticles. Future advances in THz generation technology and near-field tip preparation will improve near-field signal-to-noise ratios. This will, in turn, lead to a greater ability to distinguish between small changes in the local dielectric function. Advances in THz generation may ultimately even enable nonlinear THz near-field experiments at the apex of a tip. Meanwhile, cleaner sample conditions could play a key role in improving the spatial resolution of THz microscopy. THz-STM in particular will benefit greatly from ultraclean cryogenic operation under ultrahigh vacuum. It is poised to enter a completely new experimental regime of ultrafast and ultrasmall
11. Biological applications of THz technology
Immediately after the development of THz time domain spectroscopy there was a great rush to make startling claims about THz biomedical capabilities, with the unfortunate result that some deeply flawed results were published. These problematic studies have caused skepticism within the biomedical community. This skepticism is compounded with a natural mistrust of technology that is not familiar. This has meant that the necessary communication and collaboration between optical engineers and physicists with biologist and medical doctors has been limited. Nevertheless the collaborations that have formed contribute to an increasing number of THz studies published in primarily biological journals. An international consortia that includes biomedical researchers is needed to define specific targets of inquiry to take the field to the next level.
12. Medical applications
There remains are a number of potential advantages in the application of terahertz technology to medical imaging. The low photon energy means that the radiation is non-ionising; there is negligible scattering in tissues, the high sensitivity to water content provides contrast between diseased states; time-domain systems can provide quasi 3D information and the broad frequency range the opportunity to investigate a range of diagnostic parameters. Although there are a number of alternative well established clinical imaging techniques and those translating from the research laboratory to the clinic there remains a number of interesting clinical problems where terahertz could be applied and aid clinical decision making, for example, there is a need to improve the surgical removal of cancer by accurately locating tumor margins, especially in the case where conservation of normal tissue is essential, as in breast or brain surgery. Several challenges remain from understanding contrast to the engineering of suitable devices but terahertz technology is still relatively young and although there have been no major commercial breakthroughs in the field of terahertz medical application to date; niche applications will likely evolve.
13. Non-destructive testing and molecular spectroscopy
The field of NDT and molecular spectroscopy applications at terahertz frequencies has grown rapidly over the past decade and is showing signs of establishing itself as a well-recognised measurement technique. Innovative applications of using terahertz radiation in this context are being developed and are starting to be implemented in real-world situations.
14. THz technology—the rocket road to space
Deployment of THz technology in space has gained considerable heritage through scientific applications that include astronomy and Earth observation. Excellent potential also exists for increased spaceborne commercial exploitation in support of, for example, weather monitoring and future ultra-high frequency telecommunications. Next generation instrumentation must, however, be compliant with small satellite payload platforms and exhibit low mass, minimal volume, and efficient power consumption. Evolution of sensors to higher frequencies with greater sensitivity and improved imaging capability together with enhanced digital signal processing is also necessary. These technical enhancements present considerable challenges and require developments in, for example, detector materials, circuit miniaturisation, advanced machining, lightweight composites, and improved cooling technologies. Addressing these challenges will allow a wider exploitation of the THz domain from space and important advancements are correspondingly being made within relevant organisations world-wide. Spaceborne THz technology will continue to flourish during coming decades providing both scientific and commercial return.
15. Terahertz components and systems for defence and security imaging
The challenges confronting THz body scanners are likely representative of THz security systems more broadly, and here we highlight two primary ones: component cost and signal acquisition. The cost of RF components increases sharply with frequency. For example, a $50 mixer at 5 or 10 GHz might be two orders of magnitude more expensive at 0.5 or 1 THz. With other THz-specific subsystems that lack a mass market, such as precision reflector optics and closed-cycle cryogenics (used in some passive imagers), system costs can become prohibitive
Whereas solid state transistors will likely replace Schottky diodes as sources and receivers in active systems, for passive systems above 150 GHz the large number of receivers (1000s) required to achieve reasonable contrast mean that bolometer arrays continue to be critical, until packaging makes MMICs competitive. For both active and passive systems we therefore predict, with moderate confidence, that integrated circuit and packaging technologies are the highest priority research challenges to achieve lower costs and more widely applicable capabilities. But whatever the route THz technology development takes, we are much more certain that THz systems generally can open sensing modalities simply not possible in any other electromagnetic band.
16. Semiconductor device based THz detection
Schottky devices, made from GaAs and its alloys, will continue to be the most widely applied terahertz detector technology, due to their critical role in test equipment and scientific instrumentation. This includes mixers, with associated frequency multipliers for LO provision, and power detectors. For specific video detection applications, CMOS transistor detectors may be employed, but the high initial cost of a wafer run is a barrier to widespread application. High CMOS device impedance restricts the IF bandwidth, making the technology unattractive for mixer applications. However, both direct and mixing functions have been demonstrated in GaAs alloy based heterostructure field effect transistors. Space limitations prevents discussion of other detector devices, e.g. ternary tunnel diodes.
Investment in the parallel application of numerical simulations by high power computers, composite semiconductor growth and ambitious integration projects will deliver a step change in the way GaAs devices perform, will physically shrink interconnects and packaging, and achieve the needed integration with other semiconductor technologies.
17. Status of THz communications
The need for bandwidth to support high capacity wireless data transmission is a strong driver for the development of THz communication systems. There are a number of transmission windows in the spectrum below 400 GHz that are well suited to short range and indoor wireless systems.
The most important challenges for commercial realisation of this technology are the development of compact and efficient THz sources providing continuous wave output power levels up to 100 mW and the development of compact electronically steerable antenna arrays to minimise wireless link loss.
18. Metrology for THz technologies
The broad area of THz measurements is set to continue its rapid expansion, encompassing all types of instrumentation platforms, free-space and waveguidebased. These will require robust metrological underpinning in order to support both scientific research and industrial applications. Several issues demand to be addressed in particular. For TDS these are: establishment of standardised measurement, calibration, and data analysis procedures. For VNA, engineering solutions are needed for high-precision waveguides and interconnects. In addition to these, inter-operability and inter-comparability must be achieved between TDS and VNA measurements.
The ultimate goal is to establish a robust framework of metrological traceability to the international system of units (SI) for the portfolio of measurands (i.e. for physical quantities that need to be measured) that are, and will be, needed to underpin scientific and technological developments in the years to come. This framework will ensure the reliability and equivalence of all measurements made at terahertz frequencies. This is a prerequisite for effective science, trade and industry that exploits the terahertz region of the electromagnetic spectrum.
SOURCE- Journal of Physics D- Applied Physics