Terahertz (THz, 1012 Hz) technology is based on light sources with frequencies much lower than of visible light, and slightly higher than of microwaves. Such light sources have a number of unique capabilities, for example that it can propagate through most insulating materials such as paper, plastic, cardboard, clothes etc., while being highly sensitive to the chemical composition of most crystalline materials. For this reason, the obvious applications of THz radiation are within detection of explosives, airport scanners and finding hidden layers in artwork.
The terahertz (THz) frequency range is located between radiofrequency and infrared—a largely unreachable bandwidth for many decades and has often been referred to as the “THz gap.” Conventional electronics were incapable of generating such high frequencies, while photonic devices could not emit low enough frequencies. As such, no known wave generators could reach into this regime. Approaching the THz regime from either of these regions comes with unique challenges. Increasing the operating frequency of microwave devices is limited by the carrier mobility of the oscillating semiconductor.2 On the other hand, reducing the energy of emitted photons generated by electron transitions in a semiconductor is inhibited by the fact that the energy of THz photons is less than the thermal energy at room temperature. The THz region , however, with the rapid progression in the development of sources and detectors over the past 30 years finally broke through the THz frequency range.
One of the main applications of THz technology is characterization of materials: THz light can probe processes that occur on picosecond timescales. These include hydrogen bonds such as those in water and proteins as well as carrier dynamics in semiconductor materials and even certain types of glass. Continuous progress is being made for Terahertz components and devices to overcome electronic/photonic barriers for realizing highly integrated Terahertz systems. THz spectroscopy and imaging are today considered as well-established techniques.
The field of THz science and technology has already gained a very large international interest due to its numerous applications ranging from ultrahigh speed communication systems to medical imaging and diagnostics, industrial quality control, and security screening. 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.
Terahertz radiation can pass through objects, clothing and packages, identify what materials and substances are inside and, unlike for example X-rays, is completely harmless. Its potential in industries such as security (airports, maritime transport, etc.), food, pharmaceuticals or even aeronautics (for example, to perform checks on the condition of the wings of an aircraft, without needing to dismantle them) is immense.
Promising medical and industrial applications of THz technology soon followed. New systems such as THz time-domain spectroscopy (TDS) were developed as well. THz TDS is powerful for material spectroscopy, layer inspection, and transmission imaging of packaged objects. THz-TDS systems are utilized in authentication, nondestructive inspection, three-dimensional (3-D) imaging, quality control, airport security, art investigations, detecting damages on wood caused by insects, tomography, characterization of astrophysical ice, biomedical
diagnosis and imaging, assessment of burn injuries, material characterization, thickness measurements, aerospace application, detection of the dielectric function in biological fluids, and holography.
Although the resolution of THz spectroscopy and imaging is significantly higher than that of ultrasound and radiofrequency, its depth of penetration is limited as the absorption of the beam inside materials increases exponentially with respect to the frequency of the photons. Hence, increasing both the photon density and frequency of emission has been the primary focus of researchers in the device sector. In biophotonics applications, challenges for THz imaging and spectroscopy are even more significant as molecules of water have high absorption in the THz regime. The strong absorption of THz waves by water molecules means that significantly more photons must be emitted to maintain respectable signal-to-noise ratio (SNR) and depth penetration. However, increasing the power of the THz source can lead to structural and functional changes in tissues.
To tackle this challenge, THz measurements on wet tissues are primarily done in reflection mode. Alternatively, thin slices of tissues or dried
tissues can be imaged in transmission mode. As compared with optical and x-ray imaging, the resolution of THz is limited due to the low frequency of the THz photons. Considering the frequency of visible light in the color red to be 428 THz and the upper limit of the commonly used THz beam to be around 4 THz, the resolution of THz images is at least 100 times lower than that of images captured in visible light. To fit this ever-increasing range of applications for industrial and medical fields., researchers from different fields have consistently worked toward improving the resolution and photon density of the beam in THz spectroscopy and imaging.
In this respect, research groups in the photonics and electron device sectors have proposed several innovative device architectures and semiconductor materials. Researchers in the field of optics have developed innovative architectures of optical setups and lenses toward achieving high numerical aperture and low absorption. Groups in image processing and computer data processing have developed computational techniques for improving the resolution of the THz spectroscopy and imaging. Data scientists have developed automated software programs for analyzing THz measurements and improving the results.
“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.
Security & Military Applications
Unfortunately, two limitations stifle THz growth. First, atmospheric attenuation is a fundamental problem. THz signal strength diminishes at an extreme rate. A 1-watt transmission at a frequency of 1 THz diminishes to almost nothing (10-30 percent of original strength) after one kilometer. This degradation can be even more severe if water is present in the atmosphere. Second, the power requirement to overcome this attenuation is not presently realistic outside of a laboratory environment — the immense power requirement is a critical subcategory of common design considerations: Size, Weight, Power and Cooling (SWaP-C). Collectively, SWaP-C presents a barrier known as the ‘terahertz wall.’ Currently, SWaP technology for mobile transport of THz devices does not exist to enable stand-off capabilities.
If harnessed correctly, the THz band of the EMS has the potential to accelerate our military capabilities ahead of an adversary. The conceivable military applications of THz sensors are broad for intelligence, surveillance and reconnaissance (ISR), to include detection of isolated personnel behind enemy lines, fixing targets and terminal guidance of precision weapons. So far, most THz research is in the proof-of-concept phase for use in a combat or security environment. This significant power limitation must be overcome before THz can exist in an operational setting. Once the power limitation is solved, several military applications can be realized: Ground-based ISR, radar system integration, airborne and space-based ISR, and precision targeting and communications are capabilities that can offer a massive advantage.
Regarding ground-based THz systems, security corridors or mobile patrols could soon ‘see-through’ structures, clothing, vessels or transport containers, and most other non-liquid material to probe for concealed materials. A future security environment where threats can be detected and countered could thwart criminals and belligerents manufacturing or smuggling explosives. It could also help locate bomb-making materials, weapons/mines, illegal drugs, and rare earth minerals.
As the frequency of the radiation in the THz range increases, the absorption of photons in the material increases exponentially. For increasing the probability of detecting a higher number of high-frequency THz photons by a THz detector, the number of emitted high-frequency THz photons from the THz emitter, the sensitivity, and the frequency band of the detector need to increase. Hence, the main focus of research has been developing THz emitting devices capable of emitting THz radiation with higher photon densities and at the same time capable of emitting THz photons in higher frequency bands, together with developing THz sensors with higher detection frequency and higher sensitivities.
Advances in Terms of Terahertz Devices
Terahertz (THz) light is now conveniently generated using a number of methods, such as photoconductive antennas, optical rectification in nonlinear crystals, or via laser-induced air plasma, thanks to major advancements in photonics and the development of turn-key lasers. Common methods of 1 to 5 THz bandwidth radiation involved rectifying the oscillations of the femtosecond lasers using a nonlinear crystal. In such a way, a pulse wave with a central frequency in the optical band and spectral width in the THz range can be downconverted into a pulse of THz radiation.
Terahertz Photoconductive Antennas
As mentioned, the THz beam has been traditionally generated through extracting the envelope of the oscillations in the femtosecond laser beam using a nonlinear crystal. As nanotechnology advances, the tendency has been shifting toward photoconductive THz emitters and
detectors. In 1995, Hu and Nuss reported the first transmission of a THz image developed through optoelectronic THz TDS using photoconductive antennas.
A major source of THz radiation is the quantum cascade laser (QCL), semiconductor device that could emit high power narrowband THz radiation between 1 and 5THz, is revolutionizing the THz field. It achieves laser emission by exploiting phenomena that emerge from a repeated stack of semiconductor multiple quantum well heterostructures. QCLs have shown remarkable performances over the range 1–5 THz range, with demonstration of high powers (>1 W), photonic and far-field engineering, a quantum limited linewidth, frequency combs and pulse generation. These advances have permitted THz QCLs to be made commercially available and, although cryogenic cooling is still required, this can be achieved conveniently and inexpensively with Stirling coolers.
At room temperature, continuous terahertz emission with 3 microwatts is realized in a monolithic nonlinear QCL device with a tiny packaging dimension (as small as 2x5x8 mm3). This research was partially supported by the National Science Foundation, Department of Homeland Security, Naval Air Systems Command, and NASA.
Breakthrough in Terahertz Photonics
Scientists from Regensburg, Pisa and Leeds developed a key photonic component. By strongly coupling electronic resonances with the light field of a microresonator, they were able to operate a saturable absorber even at extremely low intensities, which in future could enable ultra-short pulses from terahertz lasers. The international research team presented their results in Nature Communications. Terahertz radiation is electromagnetic radiation in the inaccessible frequency window between microwave electronics and long-wave infrared. It opens up a diverse spectrum of applications, ranging from security scanners at airports and trace gas detection to ultra-fast communication technology and medical technology. Many other technologies could be added if ultrashort pulses could be generated directly from electrically pumped, compact terahertz lasers, so-called quantum cascade lasers. So far, however, these have only worked in continuous wave operation, i.e. without any variation in performance over time.
Using so-called saturable absorbers, inexpensive quantum cascade lasers can easily be used to elicit short terahertz pulses. The way a saturable absorber works can be compared to that of a fogged mirror, which becomes clear again as soon as sufficient intense light falls on it. If such an element is incorporated into a quantum cascade laser, the light intensity in the case of continuous light emission is not sufficient to make the mirror clear – the high losses mean that the laser emits only weak light or no light at all. If, on the other hand, the entire power of the laser is concentrated in a single, short light pulse, this is intense enough to saturate the absorber. Here the light experiences significantly lower losses, so that he develops a preference for this operating mode. Up to now, however, saturable absorbers for the terahertz spectral range have been difficult to implement and, moreover, required light intensities far beyond the capabilities of a quantum cascade laser.
In order to develop a new class of saturable absorbers, the working group of Professor Dr. Rupert Huber at the Institute for Experimental and Applied Physics at the University of Regensburg, together with Professor Miriam Vitiello, NEST Pisa, and Professor Edmund Linfield, University of Leeds, are inspired by music: where does the unique sound of a Steinway piano come from, for example? The secret is not in the strings, but rather in the sound box. There sound and dynamics arise after a keystroke. “Basically, we are adopting this idea in the terahertz optics,” says Jürgen Raab, the first author of the publication. Miriam Vitiello’s group developed a micro-structured arrangement consisting of a gold mirror and a gold grid, which together act as a resonance body for terahertz radiation.
In a high-precision slow-motion camera developed in Regensburg, the scientists observed how the new structures react to a strong “keystroke”, i.e. the stimulation with an intense terahertz pulse. On the time scale of femtoseconds – the millionth part of a billionth of a second – an astonishing result was shown: the absorber was already saturated at an intensity ten times less than the pure semiconductor structure alone. In addition, this reaction set in faster than a single light oscillation of the terahertz pulse and the “tone” changed during saturation in such a way that almost no absorption of the intense terahertz pulse took place. Vitiello is enthusiastic: “We now have all the necessary components in our hands,
Since terahertz radiation oscillates a thousand times faster than the clock rates of modern computers, ultra-short terahertz pulses could enable a new generation of telecommunications connections – far faster than 5G. Important advances in the field of chemical analysis and medical diagnostics are also conceivable. An important milestone on this path has now been reached.
Advances in Terms of Materials
Large bandgap semiconductor materials have proven to be promising semiconductor materials for achieving THz devices with higher photon frequencies and emission powers. Among these materials, GaN and GaAs have been the most favorable materials. The bandgap energy, saturation velocity, and thermal conductivity of GaN are all more than twice that of GaAs. As a result, GaN devices offer higher output power
and operation frequency compared with other conventional III to V devices. The mentioned characteristics of GaN together with its capabilities of providing high 2-D electron densities and high longitudinal-optical (LO) phonon of ∼90 meV make it one of the most promising semiconductors for the future of generation, detection, mixing, and frequency multiplication of electromagnetic waves in the THz frequency regime.
Super resolution can be achieved in near-field THz imaging. However, in near-field imaging systems, objects must be placed at a subwavelength distance from the aperture. Thus, transmission imaging of objects that are thicker than roughly a 100 μm is not possible in near-field THz imaging. As a result, in most of the applications, near-field THz imaging cannot replace far-field THz imaging. For this reason, a tremendous amount of research is dedicated to the enhancement of far-field THz imaging. In addition to digital image reconstruction techniques, high-resolution THz imaging based on utilizing aperture synthesis, dielectric cube terajet generation, solid immersion imaging, confocal THz laser microscope, and wide-aperture spherical lens for 3-D and flat diffractive optics were proposed.
In 2018 and 2019, researchers from A.M. Prokhorov General Physics Institute demonstrated a 0.15λ resolution of the proposed imaging modality at λ ¼ 500 μm, which is beyond the Abbe diffraction limit and represents a considerable improvement over the previously reported arrangements of solid immersion imaging setups. The proposed technique does not involve any subwavelength near-field probes and diaphragms; thus, it avoids the THz beam attenuation due to such elements. The mentioned work has been demonstrated to be promising for microscopy of soft biological tissues
Digital Image Processing and Data Science
Image reconstruction techniques improve the resultant images of the optics-based resolution enhancement techniques even further, and thus developments in both areas need to be pursued in parallel. In addition, since THz imaging is fairly a new science, theories and mathematical models for describing the THz imaging systems are not matured yet. Advancing the research and development in THz optics and image reconstruction cannot be done efficiently without welldeveloped cohesive models and theories.
Researchers have proposed a near-field THz imaging of hidden objects using a single-pixel detector. However the drawback of near-field imaging is the fact that objects thicker than a few hundred micrometers cannot be imaged. Researchers have also realized conventional image processing techniques to increase the quality of THz imaging systems and taking material dispersion into account to enhance the quality of THz images. For suppressing the absorption in the physical lenses, diffraction lenses with low absorptions were proposed. Signal
processing and suppression of diffraction through filtering out the delayed beams have also proved significant enhancements in THz images.
Data postprocessing methods in this field open up the possibility of reconstructing holograms recorded with spatially restricted THz detectors and overcoming the diffraction limit even for the lower-frequency spectral components. In addition to resolution enhancement, a combination of temporal and complex-domain filters allows for expansion of the dynamic range of THz frequencies. This means that amplitude/phase information as a result of utilizing holography can also be obtained.
Automation of Terahertz Imaging and Inspection
As new applications for THz imaging and inspection are being proposed, the automation of these applications through software programs and robotics becomes more appealing. Biological and medical applications are certainly among the fields in which automation can contribute to the expansion of the THz footprint. In this respect, a robotic THz imaging system for subsurface analysis of ancient human remains was proposed. The advantage of this system over the conventional computed tomography scan is that THz provides higher depth resolution and is noninvasive.
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