If you've ever heard the term "THz gap" but didn't know what it meant, this article is for you.
The Terahertz Spectrum
Terahertz (THz) radiation is generally defined as the region of the electromagnetic spectrum in the range of 100 GHz (3 mm) to 10 THz (30 μm), which is between the millimeter and infrared frequencies. The THz band has been called by several names, such as sub-millimeter, far infrared, and near-millimeters wave.
At 1 THz, the radiated signal has the following characteristics:
- Wavelength: 300 μm in free space
- Period: 1 ps,
- Photon energy: 4.14 meV
Additionally, hf/kB = 48 K temperature where h is Planck’s constant (6.62607004 × 10-34 J.s), f is the frequency, and kB is Boltzmann’s constant (1.380649×10−23 J/K).
The THz band in the electromagnetic spectrum is shown in Figure 1.
Figure 1. Schematic diagram showing the location of THz band in the electromagnetic spectrum
This portion of the electromagnetic spectrum is the least investigated area when compared to neighbor regions, i.e., the microwave and optical bands.
This is why the term “THz gap” is used to explain the infancy of this band as compared to well-developed neighboring spectral regions. This has led researchers from various disciples (such as physics, material science, electronics, optics, and chemistry) to investigate various unexplored or less-explored aspects of THz waves.
Properties of Terahertz Waves
Although interest in the THz region dates back to the 1920s, extensive studies have been devoted to this region only within the past three decades. A key motivation for this is the exceptional wave properties and vast possible applications in the THz frequency range.
THz waves have mid-characteristics of the two bands they have sandwiched in between.
These properties can be summarized as follows:
- Penetration: The wavelength of THz radiation is longer than the infrared wavelength; hence, THz waves have less scattering and better penetration depths (in the range of cm) compared to infrared ones (in the range of μm). Therefore, dry and non-metallic materials are transparent in this range but are opaque in the visible spectrum.
- Resolution: THz waves have shorter wavelengths in comparison to the microwave ones; this gives a better spatial imaging resolution.
- Safety: The photon energies in the THz band are much lower than X-rays. Therefore, THz radiation is non-ionizing.
- Spectral fingerprint: Inter- and intra-vibrational modes of many molecules lie in THz range.
Challenges in Developing the THz Band
Although the THz band has several fascinating characteristics, there are some challenges specific to THz technologies. The primary reason that the THz field has been underdeveloped as compared to the neighboring bands is the lack of efficient, coherent, and compact THz sources and detectors.
These characteristics for the sources can be found in the common microwave-frequency sources such as transistors or RF/MW antennas, and in devices working in the visible and infrared range like semiconductor laser diodes. It is not possible, however, to adopt these technologies for operation in the THz region without a significant reduction in power and efficiency.
At the lower end of the THz frequency range, solid-state electronic devices are employed in general; however, such devices have roll-offs of 1/f2 due to reactive-resistive effects and long transit times. On the other hand, optical devices such as diode lasers do not perform well at THz range limit because of the lack of materials with adequately small bandgap energies.
Another challenge in the THz band is high losses. THz waves have high absorption in the atmospheric situation and the moist environment. The atmospheric attenuation across the electromagnetic spectrum is depicted in Figure 2.
Figure 2. Attenuation at sea level for different atmospheric situations: Rain = 4 mm/h; Fog = 100 m visibility; STD = 7.5 g/m3 water vapour; 2×STD = 15 g/m3 water vapour. Image from M. C. Kemp via IEEE Xplore
It is obvious that signal degradation in the THz range is considerably more than microwave and infrared bands. That is partly because the water molecules resonate in this range.
The adverse atmospheric characteristics of THz waves make them a suitable working frequency region for the following two cases:
- Aerospace: In space, the ambient is near-vacuum so signal absorption and attenuation due to water drops are not problems. Also, the spectral signature of interstellar dust is located in the THz region. Therefore, THz technology has been widely used in radio astronomy such as the launch of Herschel Space Observatory by the European Space Agency.
- Short-range: For short-range applications, atmospheric attenuation is negligible, especially the frequencies with high absorption. This makes the removal/recognition of the effect of these narrow lines easier. Hence, THz technology is a very resourceful tool for fundamental investigations in various disciplines such as physics and chemistry. Also, it is an attractive option for short-range wireless communication with high data rates.
Applications of Terahertz Radiation
THz radiation can be used in many potential applications including terahertz imaging, spectroscopy, and wireless communication.
Biomedical imaging is one of the subcategories of THz imaging. THz waves can penetrate up to a few hundred micrometers in human tissues; so THz medical imaging can be applied for body surface diagnoses such as skin, mouth, and breast cancer detection, and dental imaging. Also, THz systems have the potential market for security applications, solid explosive material detection, and mail screening. Last but not least, THz imaging is a convenient method for semiconductor packaging inspections.
THz spectroscopy is a very powerful technique to characterize material properties and understand their signature in this band. THz spectroscopy has enhanced understanding of absorption features in many single-crystal, microcrystalline, and powder samples of organic molecules.
Figure 3 indicates a sample of measurement result to identify the vibrational modes of maltose molecules.
Figure 3. The measured vibrational spectrum of maltose in a THz time domain spectroscopy system, the upper graph shows the measured THz signal without a maltose sample. Arrows on below graph show the vibrational frequencies of maltose molecules. Inset shows the molecular structure of maltose. Image from Y. C. Shen et al via Applied Physics Letters.
THz spectroscopy has applications in biochemical science such as analysis of DNA signatures and protein structures. In-line control of production processes is another potential application of THz spectroscopy which could provide contactless and real-time measurements. THz spectroscopy can be manipulated positively to distinguish the hydrated substances from dried ones because of high water absorption in THz frequencies. For instance, in the paper industry, THz spectroscopy has been used for monitoring the thickness and moisture content of papers by manufacturers.
In some applications, such as non-destructive testing, both THz imaging and spectroscopy are employed. For instance, in an art-history investigation, THz imaging and spectroscopy help imaging antiquities, to reveal the thickness of the different layers of the artwork and to show material types.
Figure 4 shows a visible photograph of the Madonna in Preghiera (left) and THz image of the painting based on the integrated spectrum between 0.5 – 1 THz (right).
Figure 4. (a) visible photograph of the Madonna in Preghiera (b) THz image of the Madonna in Preghiera on the integrated spectrum between 0.5 and 1 THz. Image from J. Dong et al via Scientific Reports
THz imaging provides information on the painting’s underlayers with a groundbreaking degree of details on the order of tens of microns.
Furthermore, THz imaging and spectroscopy are two strong quantitative and qualitative non-invasive methods for examining pharmaceutical solid dosage forms, tablet coatings, and active pharmaceutical ingredients. For example, Figure 5 shows the inter-tablet variation of the coating layer thickness of eight tablets with the same coating time in the coating process in the THz region.
Figure 5. The averaged coating thickness of each individual tablet against the coating time, The inset shows the coating thickness map (μm) of eight tablets with the same coating time of 120-min. A large tablet-to-tablet variation of coating thickness is obvious. Image from Y. C. Shen via the International Journal of Pharmaceutics
The Potential of the Terahertz Band
During the end of the 20th century and the first decade of the 21st century, when the huge amount of THz lab experiments were taking place, researchers mostly focused on various potential THz applications and very promising results have been achieved. In fact, those fascinating experimental results were a great motivation and driving force for many researchers to dig into the THz field and explore it from different aspects.
Due to continuous progress in the THz research field in recent years, THz systems and applications are finding their spots in some commercial applications. However, in order for THz waves to be able to compete and overcome other technologies in real-world scenarios, various issues must be tackled and/or improved. For instance, high-power and compact THz sources are required, THz measurement systems should be miniaturized, methods for a faster THz beam scan are required, and THz systems should have a lower cost.
Another rising research field is THz wireless communication. This is particularly in demand because it allows high-speed wireless communications for beyond 5G. Therefore, various studies are in demand to ripen and achieve full potentials of the THz band.
What would you like to learn about regarding terahertz band technologies? Share your questions in the comments below.