Terahertz (THz) optronic technologies aren’t new, but it’s only recently that they’ve been explored for more commercial applications, specifically those tied to enabling imaging and communications.
Researchers look beyond the barriers to terahertz-technology adoption
Imaging capabilities based on terahertz technologies span a wide range of use cases, including sensors for medical, industrial, and security applications. The terahertz frequency band could also be leveraged to build faster wireless communications systems, further speeding up the transmission of information between electronic devices, as well as building next-generation wireless local area networks.
But along with all the opportunities for terahertz technologies come challenges that must be overcome before these use cases can be fully realized.
Terahertz potential defined by pros and cons
The terahertz frequency band is part of the larger electromagnetic spectrum (EMS), and to date, terahertz technologies have been primarily used for astronomical observation.
The properties of terahertz waves are a key part of the potential of terahertz technologies. Their location between infrareds and microwaves in the EMS mean that terahertz waves can penetrate many more materials than infrareds and provide greater spatial resolution than microwaves. The penetration of terahertz waves is also relatively deep for dielectric materials such as paper, plastics, cardboard, plaster, ceramics, and textiles. This makes terahertz imaging useful for analyzing the internal structure of materials to characterize their physical properties as well as detect and locate faults and contaminations.
Another property of terahertz waves that makes them appealing for many applications including communications is the fact that this technology can easily propagate through the air, even when it’s damp, which makes them even more useful than ultrasounds for contactless measurement. The frequency range of terahertz waves also achieved good spatial resolution required for rendering quality imaging when compared with microwave radiation. They also exhibit properties that make them more attractive and useful than X-rays and near-infrared radiation in that they are non-ionizing. A very appealing property of terahertz waves is that they won’t harm humans, animals, and plants, because unlike X-rays, they’re non-invasive and have no ionizing radiation impact.
Terahertz waves do have limitations, however. A major challenge being addressed by terahertz research is when an “intense” terahertz source is relatively weak, and the need for exotic materials, such as nonlinear crystals, makes them unwieldy and expensive. Their inherent bulkiness and faintness mean there remain many logistical challenges for the integration of terahertz sources with other technologies, including sensors and communications. These hurdles are already being addressed by scientists around the world, including those at the EPic Lab at University of Sussex, which has developed terahertz sources from extremely thin materials to elicit the emission of short bursts of terahertz radiation.
Another challenge is that even though terahertz sources enable the ability to “see” through many materials and are used for transmitting information between devices that are close together, a terahertz signal will degrade quickly in the atmosphere. While advances in power sources will likely prevent this degradation, the immediate applications for terahertz technologies are limited to more controlled settings.
Controlled environments offer earliest opportunities
Even with the present limitations, there are plenty of potential applications for terahertz technologies today. Fertile growth areas include terahertz sensors and terahertz imaging devices for scientific, industrial, and medical applications.
For the latter, there are many examples of non-invasive imaging use cases for terahertz technologies. For example, terahertz spectroscopy can identify substances or characterize the chemical properties of medicines and narcotics, including their composition, molecular, and spatial arrangement. Terahertz-based imaging is increasingly in demand in research laboratories, as high-end terahertz instruments are useful for biomedical research and medical science applications such as bacterial identification, biological tissue discrimination, blood cell detection, and cancer cell characterization.
In addition to being able to detect cancer, including skin and breast cancers at early stages, terahertz imaging enables the analysis of the upper layers of a human body, including skin, vessels, joints, and muscles, to visualize the current conditions of wounds under layers of bandages.
Other scientific applications, aside from medical uses, include the analysis of a variety of materials, including those used in the semiconductor industry, as terahertz frequency range is convenient for the creation and study of metamaterials and plasmonic effects. For example, electro-optical terahertz pulse reflectometry can be applied in the form of an ultra-fast laser-source–driven electro-optical device that can be used to isolate faults and detect defects in microelectronics.
Because of the current limitations of terahertz, other applications such as those for security and defense are in their infancy, but terahertz imaging could potentially scan passageways or be mounted on vehicles for IED detection in a combat environment, as well as detect plastic or minimal metal land mines on current or former battlefields. Other potential security applications include surveillance in urban environments such as shopping malls and subway stations and tunnels.
Terahertz could enable 6G
Aside from the imaging applications, terahertz frequencies are being touted as an enabler of future wireless communications systems. These frequencies cover a range of 0.1 THz to 30 THz, laying between microwave and millimeter waves but before the infrareds on the EMS.
It’s still early days, however, as terahertz technologies do still have challenges specific to communications applications; however, researchers are actively looking at the sub-terahertz domain by establishing scalable testbeds for 6G research. By combining high-performance, multichannel equipment and hardware with flexible signal generation and analysis software, these testbeds are designed to evaluate candidate waveforms for 6G. A key part of the research is determining the level error-vector–magnitude system performance possible in these new frequency bands and extreme modulation bandwidths.
Many characteristics of the terahertz frequency band are a double-edged sword in that they create limitations and opportunities for future terahertz-based wireless systems. For example, the quasi-opticality of the terahertz band means the molecular absorption effect can limit the propagation of terahertz waves, but this same effect does present sensing opportunities that are otherwise not available with other frequency bands. Overall, there’s a lot of opportunity for terahertz frequencies to co-exist with mmWave and sub-6–GHz bands, services, and infrastructure that could create new opportunities for sensors and communications.
Growing interest drives terahertz innovation
The inherent properties of terahertz frequencies make them a compelling area of research and development for many commercial applications. The many imaging applications could improve human health and safety as well as open doors for communications and sensing capabilities. Interest and investment in the health-care and security markets alone are already driving the adoption of terahertz technologies, which, in turn, will spur development and lead to even more commercial applications.