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Terahertz radiation
Terahertz radiation
Terahertz radiation
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Terahertz band
Frequency range
0.1 THz to 30 THz
Wavelength range
3 mm to 30 μm
Radio bands
ITU
1 (ELF) 2 (SLF) 3 (ULF) 4 (VLF)
5 (LF) 6 (MF) 7 (HF) 8 (VHF)
9 (UHF) 10 (SHF) 11 (EHF) 12 (THF)
EU / NATO / US ECM
IEEE
Other TV and radio
Terahertz waves lie mostly at the far end of the infrared band, the longest ones in the microwave band.

Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency[1] (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the International Telecommunication Union-designated band of frequencies from 0.1 to 10 terahertz (THz),[2] (from 0.3 to 3 terahertz (THz) in older texts,[3] which is now called "decimillimetric waves"[4]), although the upper boundary is somewhat arbitrary and has been considered by some sources to be 30 THz.[5]

One terahertz is 1012 Hz or 1,000 GHz. Wavelengths of radiation in the decimillimeter band correspondingly range 1 mm to 0.1 mm = 100 μm and those in the terahertz band 3 mm = 3000 μm to 30 μm. Because terahertz radiation begins at a wavelength of around 1 millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band, and its radiation as submillimeter waves, especially in astronomy. This band of electromagnetic radiation lies within the transition region between microwave and far infrared, and can be regarded as either.

Compared to lower radio frequencies, terahertz radiation is strongly absorbed by the gases of the atmosphere, and in air most of the energy is attenuated within a few meters,[6][7][8] so it is not practical for long distance terrestrial radio communication. It can penetrate thin layers of materials but is blocked by thicker objects. THz beams transmitted through materials can be used for material characterization, layer inspection, relief measurement,[9] and as a lower-energy alternative to X-rays for producing high resolution images of the interior of solid objects.[10]

Terahertz radiation occupies a middle ground where the ranges of microwaves and infrared light waves overlap, known as the "terahertz gap"; it is called a "gap" because the technology for its generation and manipulation is still in its infancy. The generation and modulation of electromagnetic waves in this frequency range ceases to be possible by the conventional electronic devices used to generate radio waves and microwaves, requiring the development of new devices and techniques.

Description

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In THz-TDS systems, since the time-domain version of the THz signal is available, the distortion effects of the diffraction can be suppressed.[11]

Terahertz radiation falls in between infrared radiation and microwave radiation in the electromagnetic spectrum, and it shares some properties with each of these. Terahertz radiation travels in a line of sight and is non-ionizing. Like microwaves, terahertz radiation can penetrate a wide variety of non-conducting materials; clothing, paper, cardboard, wood, masonry, plastic and ceramics. The penetration depth is typically less than that of microwave radiation. Like infrared, terahertz radiation has limited penetration through fog and clouds and cannot penetrate liquid water or metal.[12] Terahertz radiation can penetrate some distance through body tissue like x-rays, but unlike them is non-ionizing, so it is of interest as a replacement for medical X-rays. Due to its longer wavelength, images made using terahertz waves have lower resolution than X-rays and need to be enhanced (see figure at right).[11]

The earth's atmosphere is a strong absorber of terahertz radiation, so the range of terahertz radiation in air is limited to tens of meters, making it unsuitable for long-distance communications. However, at distances of ~10 meters the band may still allow many useful applications in imaging and construction of high bandwidth wireless networking systems, especially indoor systems. In addition, producing and detecting coherent terahertz radiation remains technically challenging, though inexpensive commercial sources now exist in the 0.3–1.0 THz range (the lower part of the spectrum), including gyrotrons, backward wave oscillators, and resonant-tunneling diodes.[citation needed] Due to the small energy of THz photons, current THz devices require low temperature during operation to suppress environmental noise. Tremendous efforts thus have been put into THz research to improve the operation temperature, using different strategies such as optomechanical meta-devices.[13][14]

Sources

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Natural

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Terahertz radiation is emitted as part of the black-body radiation from anything with a temperature greater than about 2 kelvin. While this thermal emission is very weak, observations at these frequencies are important for characterizing cold 10–20 K cosmic dust in interstellar clouds in the Milky Way galaxy, and in distant starburst galaxies.[citation needed]

Telescopes operating in this band include the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory and the Submillimeter Array at the Mauna Kea Observatory in Hawaii, the BLAST balloon borne telescope, the Herschel Space Observatory, the Heinrich Hertz Submillimeter Telescope at the Mount Graham International Observatory in Arizona, and at the Atacama Large Millimeter Array. Due to Earth's atmospheric absorption spectrum, the opacity of the atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.[15][16]

Artificial

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Dendrimer Dipole Excitation (DDE) Mechanism - The Rahman-Tomalia Effect

As of 2012, viable sources of terahertz radiation are the gyrotron, the backward wave oscillator ("BWO"), the molecule gas far-infrared laser, Schottky-diode multipliers,[17] varactor (varicap) multipliers, quantum-cascade laser,[18][19][20][21] the free-electron laser, synchrotron light sources, photomixing sources, single-cycle or pulsed sources used in terahertz time-domain spectroscopy such as photoconductive, surface field, photo-Dember and optical rectification emitters,[22] and electronic oscillators based on resonant tunneling diodes have been shown to operate up to 1.98 THz.[23] To the right, image of Dendrimer Dipole Excitation (DDE) Mechanism for broadband 30THz emitter used for sub-nanometer 3D Imaging and Spectroscopy.[24]

There have also been solid-state sources of millimeter and submillimeter waves for many years. AB Millimeter in Paris, for instance, produces a system that covers the entire range from 8 GHz to 1,000 GHz with solid state sources and detectors. Nowadays, most time-domain work is done via ultrafast lasers.

In mid-2007, scientists at the U.S. Department of Energy's Argonne National Laboratory, along with collaborators in Turkey and Japan, announced the creation of a compact device that could lead to portable, battery-operated terahertz radiation sources.[25] The device uses high-temperature superconducting crystals, grown at the University of Tsukuba in Japan. These crystals comprise stacks of Josephson junctions, which exhibit a property known as the Josephson effect: when external voltage is applied, alternating current flows across the junctions at a frequency proportional to the voltage. This alternating current induces an electromagnetic field. A small voltage (around two millivolts per junction) can induce frequencies in the terahertz range.

In 2008, engineers at Harvard University achieved room temperature emission of several hundred nanowatts of coherent terahertz radiation using a semiconductor source. THz radiation was generated by nonlinear mixing of two modes in a mid-infrared quantum cascade laser. Previous sources had required cryogenic cooling, which greatly limited their use in everyday applications.[26]

In 2009, it was discovered that the act of unpeeling adhesive tape generates non-polarized terahertz radiation, with a narrow peak at 2 THz and a broader peak at 18 THz. The mechanism of its creation is tribocharging of the adhesive tape and subsequent discharge; this was hypothesized to involve bremsstrahlung with absorption or energy density focusing during dielectric breakdown of a gas.[27]

In 2013, researchers at Georgia Institute of Technology's Broadband Wireless Networking Laboratory and the Polytechnic University of Catalonia developed a method to create a graphene antenna: an antenna that would be shaped into graphene strips from 10 to 100 nanometers wide and one micrometer long. Such an antenna could be used to emit radio waves in the terahertz frequency range.[28][29]

Terahertz gap

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Until the 2008 manufacture of an EO (electro-optic) Dipole Dendrimer Excitation (DDE[30]) emitter, no practical technologies existed for generating and detecting radiation in a frequency band in the THz region, known as the "terahertz gap". This gap has previously been defined as 0.1 to 10 THz (wavelengths of 3 mm to 30 μm) although the upper boundary is considered by some sources as 30 THz (a wavelength of 10 μm).[31] Until the 2008 DDE[30] implementation by Applied Research & Photonics (ARP) Inc., frequencies within the range from 0.1 to 30THz, useful power generation and receiver technologies were inefficient and unfeasible. Since 2008, ARP has commercially manufactured sub-nanometer resolution 3D Imaging & Spectroscopy tools, known as TeraSpectra.

Mass production of devices in this range and operation at room temperature (at which energy kT is equal to the energy of a photon with a frequency of 6.2 THz) are mostly impractical. This leaves a gap between mature microwave technologies in the highest frequencies of the radio spectrum and the well-developed optical engineering of infrared detectors in their lowest frequencies. This radiation is mostly used in small-scale, specialized applications such as submillimetre astronomy. Research that attempts to resolve this issue has been conducted since the late 20th century.[32][33][34][35][36]

In 2024, an experiment was published by German researchers[37] where a TDLAS experiment at 4.75 THz was performed in "infrared quality" with an uncooled pyroelectric receiver. The THz source was a cw DFB-QC-Laser operating at 43.3 K, with laser currents between 480 mA and 600 mA.

Closure of the terahertz gap

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See DDE[30] as exception to, "Most vacuum electronic devices that are used for microwave generation can be modified to operate at terahertz frequencies, including the magnetron,[38] gyrotron,[39] synchrotron,[40] and free-electron laser.[41] " Similarly, microwave detectors such as the tunnel diode have been re-engineered to detect at terahertz[42] and infrared[43] frequencies as well. However, many of these devices are in prototype form, are not compact, or exist at university or government research labs, without the benefit of cost savings due to mass production.

Research

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Molecular biology

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Terahertz radiation has comparable frequencies to the motion of biomolecular systems in the course of their function (a frequency 1THz is equivalent to a timescale of 1 picosecond, therefore in particular the range of hundreds of GHz up to low numbers of THz is comparable to biomolecular relaxation timescales of a few ps to a few ns). Modulation of biological and also neurological function is therefore possible using radiation in the range hundreds of GHz up to a few THz at relatively low energies (without significant heating or ionisation) achieving either beneficial or harmful effects.[44][45]

Medical imaging

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Unlike X-rays, terahertz radiation is not ionizing radiation and its low photon energies in general do not damage living tissues and DNA. Some frequencies of terahertz radiation can penetrate several millimeters of tissue with low water content (e.g., fatty tissue) and reflect back. Terahertz radiation can also detect differences in water content and density of a tissue. Such methods could allow effective detection of epithelial cancer with an imaging system that is safe, non-invasive, and painless.[46] In response to the demand for COVID-19 screening terahertz spectroscopy and imaging has been proposed as a rapid screening tool.[47][48]

The first images generated using terahertz radiation date from the 1960s; however, in 1995 images generated using terahertz time-domain spectroscopy generated a great deal of interest.[citation needed]

Some frequencies of terahertz radiation can be used for 3D imaging of teeth and may be more accurate than conventional X-ray imaging in dentistry.[citation needed]

Security

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Terahertz radiation can penetrate fabrics and plastics, so it can be used in surveillance, such as security screening, to uncover concealed weapons on a person, remotely. This is of particular interest because many materials of interest have unique spectral "fingerprints" in the terahertz range. This offers the possibility to combine spectral identification with imaging. In 2002, the European Space Agency (ESA) Star Tiger team,[49] based at the Rutherford Appleton Laboratory (Oxfordshire, UK), produced the first passive terahertz image of a hand.[50] By 2004, ThruVision Ltd, a spin-out from the Council for the Central Laboratory of the Research Councils (CCLRC) Rutherford Appleton Laboratory, had demonstrated the world's first compact THz camera for security screening applications. The prototype system successfully imaged guns and explosives concealed under clothing.[51] Passive detection of terahertz signatures avoid the bodily privacy concerns of other detection by being targeted to a very specific range of materials and objects.[52][53]

In January 2013, the NYPD announced plans to experiment with the new technology to detect concealed weapons,[54] prompting Miami blogger and privacy activist Jonathan Corbett to file a lawsuit against the department in Manhattan federal court that same month, challenging such use: "For thousands of years, humans have used clothing to protect their modesty and have quite reasonably held the expectation of privacy for anything inside of their clothing, since no human is able to see through them." He sought a court order to prohibit using the technology without reasonable suspicion or probable cause.[55] By early 2017, the department said it had no intention of ever using the sensors given to them by the federal government.[56]

Scientific use and imaging

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In addition to its current use in submillimetre astronomy, terahertz radiation spectroscopy could provide new sources of information for chemistry and biochemistry.[57]

Recently developed methods of THz time-domain spectroscopy (THz TDS) and THz tomography have been shown to be able to image samples that are opaque in the visible and near-infrared regions of the spectrum. The utility of THz-TDS is limited when the sample is very thin, or has a low absorbance, since it is very difficult to distinguish changes in the THz pulse caused by the sample from those caused by long-term fluctuations in the driving laser source or experiment. However, THz-TDS produces radiation that is both coherent and spectrally broad, so such images can contain far more information than a conventional image formed with a single-frequency source.[citation needed]

Submillimeter waves are used in physics to study materials in high magnetic fields, since at high fields (over about 11 tesla), the electron spin Larmor frequencies are in the submillimeter band. Many high-magnetic field laboratories perform these high-frequency EPR experiments, such as the National High Magnetic Field Laboratory (NHMFL) in Florida.[citation needed]

Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old buildings, without harming the artwork.[58]

In additional, THz imaging has been done with lens antennas to capture radio image of the object.[59][60]

Particle accelerators

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New types of particle accelerators that could achieve multi Giga-electron volts per metre (GeV/m) accelerating gradients are of utmost importance to reduce the size and cost of future generations of high energy colliders as well as provide a widespread availability of compact accelerator technology to smaller laboratories around the world. Gradients in the order of 100 MeV/m have been achieved by conventional techniques and are limited by RF-induced plasma breakdown.[61] Beam driven dielectric wakefield accelerators (DWAs)[62][63] typically operate in the Terahertz frequency range, which pushes the plasma breakdown threshold for surface electric fields into the multi-GV/m range.[64] DWA technique allows to accommodate a significant amount of charge per bunch, and gives an access to conventional fabrication techniques for the accelerating structures. To date 0.3 GeV/m accelerating and 1.3 GeV/m decelerating gradients[65] have been achieved using a dielectric lined waveguide with sub-millimetre transverse aperture.

An accelerating gradient larger than 1 GeV/m, can potentially be produced by the Cherenkov Smith-Purcell radiative mechanism[66][67] in a dielectric capillary with a variable inner radius. When an electron bunch propagates through the capillary, its self-field interacts with the dielectric material and produces wakefields that propagate inside the material at the Cherenkov angle. The wakefields are slowed down below the speed of light, as the relative dielectric permittivity of the material is larger than 1. The radiation is then reflected from the capillary's metallic boundary and diffracted back into the vacuum region, producing high accelerating fields on the capillary axis with a distinct frequency signature. In presence of a periodic boundary the Smith-Purcell radiation imposes frequency dispersion.[citation needed]

A preliminary study with corrugated capillaries has shown some modification to the spectral content and amplitude of the generated wakefields,[68] but the possibility of using Smith-Purcell effect in DWA is still under consideration.[citation needed]

Communication

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The high atmospheric absorption of terahertz waves limits the range of communication using existing transmitters and antennas to tens of meters. However, the huge unallocated bandwidth available in the band (ten times the bandwidth of the millimeter wave band, 100 times that of the SHF microwave band) makes it very attractive for future data transmission and networking use. There are tremendous difficulties to extending the range of THz communication through the atmosphere, but the world telecommunications industry is funding much research into overcoming those limitations.[69] One promising application area is the 6G cellphone and wireless standard, which will supersede the current 5G standard around 2030.[69] In particular, 6G is expected to leverage advanced technologies such as terahertz and full duplex (FD) communications, combined with dynamic spectrum sharing to meet the growing demand for higher data rates and more efficient spectrum efficiency.[70]

For a given antenna aperture, the gain of directive antennas scales with the square of frequency, while for low power transmitters the power efficiency is independent of bandwidth. So the consumption factor theory of communication links indicates that, contrary to conventional engineering wisdom, for a fixed aperture it is more efficient in bits per second per watt to use higher frequencies in the millimeter wave and terahertz range.[69] Small directive antennas a few centimeters in diameter can produce very narrow 'pencil' beams of THz radiation, and phased arrays of multiple antennas could concentrate virtually all the power output on the receiving antenna, allowing communication at longer distances.

In May 2012, a team of researchers from the Tokyo Institute of Technology[71] published in Electronics Letters that it had set a new record for wireless data transmission by using T-rays and proposed they be used as bandwidth for data transmission in the future.[72] The team's proof of concept device used a resonant tunneling diode (RTD) negative resistance oscillator to produce waves in the terahertz band. With this RTD, the researchers sent a signal at 542 GHz, resulting in a data transfer rate of 3 Gigabits per second.[72] It doubled the record for data transmission rate set in November 2011.[73] The study suggested that Wi-Fi using the system would be limited to approximately 10 metres (33 ft), but could allow data transmission at up to 100 Gbit/s.[72][clarification needed] In 2011, Japanese electronic parts maker Rohm and a research team at Osaka University produced a chip capable of transmitting 1.5 Gbit/s using terahertz radiation.[74] According to nature journal, researchers reported to transfer two videos error free at the speed of 50 Gbps.[75] Which was way more than the previous record.

Potential uses exist in high-altitude telecommunications, above altitudes where water vapor causes signal absorption: aircraft to satellite, or satellite to satellite.[citation needed]

Amateur radio

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Main article: Submillimeter amateur radio

A number of administrations permit amateur radio experimentation within the 275–3,000 GHz range or at even higher frequencies on a national basis, under license conditions that are usually based on RR5.565 of the ITU Radio Regulations. Amateur radio operators utilizing submillimeter frequencies often attempt to set two-way communication distance records. In the United States, WA1ZMS and W4WWQ set a record of 1.42 kilometres (0.88 mi) on 403 GHz using CW (Morse code) on 21 December 2004. In Australia, at 30 THz a distance of 60 metres (200 ft) was achieved by stations VK3CV and VK3LN on 8 November 2020.[76][77] [78]

Manufacturing

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Many possible uses of terahertz sensing and imaging are proposed in manufacturing, quality control, and process monitoring. These in general exploit the traits of plastics and cardboard being transparent to terahertz radiation, making it possible to inspect packaged goods. The first imaging system based on optoelectronic terahertz time-domain spectroscopy were developed in 1995 by researchers from AT&T Bell Laboratories and was used for producing a transmission image of a packaged electronic chip.[79] This system used pulsed laser beams with duration in range of picoseconds. Since then commonly used commercial/ research terahertz imaging systems have used pulsed lasers to generate terahertz images. The image can be developed based on either the attenuation or phase delay of the transmitted terahertz pulse.[80]

Since the beam is scattered more at the edges and also different materials have different absorption coefficients, the images based on attenuation indicates edges and different materials inside of objects. This approach is similar to X-ray transmission imaging, where images are developed based on attenuation of the transmitted beam.[81]

In the second approach, terahertz images are developed based on the time delay of the received pulse. In this approach, thicker parts of the objects are well recognized as the thicker parts cause more time delay of the pulse. Energy of the laser spots are distributed by a Gaussian function. The geometry and behavior of Gaussian beam in the Fraunhofer region imply that the electromagnetic beams diverge more as the frequencies of the beams decrease and thus the resolution decreases.[82] This implies that terahertz imaging systems have higher resolution than scanning acoustic microscope (SAM) but lower resolution than X-ray imaging systems. Although terahertz can be used for inspection of packaged objects, it suffers from low resolution for fine inspections. X-ray image and terahertz images of an electronic chip are brought in the figure on the right.[83] Obviously the resolution of X-ray is higher than terahertz image, but X-ray is ionizing and can be impose harmful effects on certain objects such as semiconductors and live tissues.[citation needed]

To overcome low resolution of the terahertz systems near-field terahertz imaging systems are under development.[84][85] In nearfield imaging the detector needs to be located very close to the surface of the plane and thus imaging of the thick packaged objects may not be feasible. In another attempt to increase the resolution, laser beams with frequencies higher than terahertz are used to excite the p-n junctions in semiconductor objects, the excited junctions generate terahertz radiation as a result as long as their contacts are unbroken and in this way damaged devices can be detected.[86] In this approach, since the absorption increases exponentially with the frequency, again inspection of the thick packaged semiconductors may not be doable. Consequently, a tradeoff between the achievable resolution and the thickness of the penetration of the beam in the packaging material should be considered.[citation needed]

THz gap research

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Ongoing investigation has resulted in improved emitters (sources) and detectors, and research in this area has intensified. However, drawbacks remain that include the substantial size of emitters, incompatible frequency ranges, and undesirable operating temperatures, as well as component, device, and detector requirements that are somewhere between solid state electronics and photonic technologies.[87][88][89]

Free-electron lasers can generate a wide range of stimulated emission of electromagnetic radiation from microwaves, through terahertz radiation to X-ray. However, they are bulky, expensive and not suitable for applications that require critical timing (such as wireless communications). Other sources of terahertz radiation which are actively being researched include solid state oscillators (through frequency multiplication), backward wave oscillators (BWOs), quantum cascade lasers, and gyrotrons.

Safety

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The terahertz region is between the radio frequency region and the laser optical region. Both the IEEE C95.1–2005 RF safety standard[90] and the ANSI Z136.1–2007 Laser safety standard[91] have limits into the terahertz region, but both safety limits are based on extrapolation. It is expected that effects on biological tissues are thermal in nature and, therefore, predictable by conventional thermal models [citation needed]. Research is underway to collect data to populate this region of the spectrum and validate safety limits. [citation needed]

A theoretical study published in 2010 and conducted by Alexandrov et al at the Center for Nonlinear Studies at Los Alamos National Laboratory in New Mexico[92] created mathematical models predicting how terahertz radiation would interact with double-stranded DNA, showing that, even though involved forces seem to be tiny, nonlinear resonances (although much less likely to form than less-powerful common resonances) could allow terahertz waves to "unzip double-stranded DNA, creating bubbles in the double strand that could significantly interfere with processes such as gene expression and DNA replication".[93] Experimental verification of this simulation was not done. Swanson's 2010 theoretical treatment of the Alexandrov study concludes that the DNA bubbles do not occur under reasonable physical assumptions or if the effects of temperature are taken into account.[94] A bibliographical study published in 2003 reported that T-ray intensity drops to less than 1% in the first 500 μm of skin but stressed that "there is currently very little information about the optical properties of human tissue at terahertz frequencies".[95]

See also

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Terahertz radiation

View on Grokipedia
from Grokipedia
Terahertz radiation consists of electromagnetic waves with frequencies between 0.1 and 10 THz, corresponding to wavelengths of approximately 30 μm to 3 mm, positioned between microwaves and far-infrared radiation in the . This range, often termed the terahertz gap, has historically been underexplored due to technological challenges in generation and detection, stemming from the lack of natural electronic or photonic resonances at these frequencies. Terahertz waves possess energies too low for efficient interaction with electrons in solids yet sufficient to excite molecular vibrations and rotations, enabling unique applications in and where they penetrate dielectrics like plastics and but are attenuated by and metals. Key advancements include (THz-TDS), which utilizes ultrafast lasers to produce and detect coherent pulses, facilitating high-resolution material characterization without ionizing effects. Notable applications encompass non-destructive testing, biomedical imaging for tissue differentiation, and potential high-bandwidth communications, though atmospheric absorption by limits long-range propagation. Despite progress, persistent challenges such as inefficient sources, sensitive detectors, and integration with existing technologies hinder widespread adoption, driving ongoing research into quantum cascade lasers and metamaterials.

Fundamentals

Definition and Frequency Range

Terahertz radiation consists of electromagnetic waves with frequencies ranging from approximately 0.1 THz to 10 THz (where 1 THz = 10¹² Hz), corresponding to wavelengths between 30 μm and 3 mm. This frequency band lies between the region (typically below 0.1 THz) and the far-infrared region (above 10 THz) of the . The exact boundaries can vary slightly depending on context, with some definitions narrowing to 0.3–3 THz for submillimeter waves, but the broader 0.1–10 THz range is widely accepted in for encompassing terahertz phenomena. This positioning imparts unique properties to terahertz radiation, bridging the gap between electronic () and photonic () technologies, often termed the "terahertz gap" due to historical challenges in generation and detection within this regime. Wavelengths in this range are calculated via the relation λ = c/f, where c is the (3 × 10⁸ m/s) and f is ; for instance, at 1 THz, the wavelength is precisely 300 μm. Terahertz radiation is non-ionizing, with photon energies on the order of 0.4–40 meV, insufficient to break chemical bonds or cause DNA damage unlike higher-frequency or X-rays. Also referred to as T-rays or submillimeter radiation, this band has been delimited by international standards such as those from the (ITU), which designate submillimeter waves from 0.3 to 3 THz, though research applications extend beyond these limits. Empirical measurements confirm transmission characteristics intermediate between radio waves and , enabling penetration of non-conductive materials like or while being absorbed by .

Physical Properties and Interactions

Terahertz radiation consists of electromagnetic waves with frequencies ranging from 0.1 to 10 THz, corresponding to wavelengths between 30 μm and 3 mm. This positions it between millimeter-wave radiation and the far-infrared region of the spectrum. The photon energies span approximately 0.4 to 40 meV, rendering it non-ionizing and incapable of breaking chemical bonds directly. In vacuum, terahertz waves propagate at the with minimal dispersion, behaving similarly to other . However, in the atmosphere, propagation is attenuated by resonant absorption lines from , oxygen, and other molecules, with absorption coefficients reaching several dB/km at certain frequencies around 0.5–1 THz. This limits long-range transmission, particularly in humid conditions. Terahertz radiation exhibits strong penetration into non-polar, low-loss dielectrics such as plastics, ceramics, paper, and fabrics, often with transmission depths exceeding millimeters. In contrast, it is reflected by metals due to free carrier effects and highly absorbed by and hydrated materials, with penetration depths typically on the order of 100–200 μm in liquid at 1 THz owing to reorientation and dynamics. These interactions enable non-destructive imaging of concealed objects but restrict applications in aqueous biological tissues. At the molecular level, terahertz fields couple to low-energy excitations including rotational transitions in gases, intermolecular vibrations in liquids and solids, modes in crystals, and collective plasma oscillations in semiconductors. This selectivity supports spectroscopic identification of material compositions, as distinct absorption fingerprints arise from collective moments rather than single-molecule electronic transitions. In plasmas and semiconductors, terahertz waves can drive transient carrier dynamics, influencing conductivity and via the .

Historical Development

Early Discoveries and Observations

In the late , the first systematic observations of radiation in the terahertz frequency range emerged from efforts to extend to longer wavelengths using sensitive thermal detectors. In 1897, American physicist Edward F. Nichols advanced technology to measure energy flux in the far- spectrum, detecting at wavelengths exceeding 50 micrometers, which correspond to frequencies below 6 THz. This work confirmed the presence of substantial thermal energy in the spectral region between conventional and nascent electrical oscillations, bridging optical and radio domains. German physicist Heinrich Rubens, collaborating with Nichols and others, further explored this "gap" in 1897 through joint experiments on "heat rays of great wavelength." Using residual radiation from heated sources passed through rock salt prisms and detected via bolometers, they quantified radiation up to wavelengths of approximately 1 millimeter (around 300 GHz), observing transmission properties distinct from shorter infrared rays and noting their polarization akin to light. These measurements, published in Physical Review, represented the initial explicit acknowledgment of a technological divide in the electromagnetic spectrum, where optical methods faltered and electronic generation was rudimentary. Rubens pioneered the Reststrahlen (residual rays) method around 1900, exploiting resonances in ionic s like and to generate and selectively reflect far-infrared radiation. By heating s to excite lattice vibrations, he produced narrowband emission in the 1–3 THz range, enabling early spectroscopic observations of material dispersion and absorption. Interferometric techniques developed by Rubens in the early 1900s resolved wavelengths down to 20 micrometers (15 THz), revealing anomalous dispersion near crystal reststrahlen bands and supporting continuity. These incoherent, thermal-based observations laid foundational empirical data on terahertz interactions with matter, though limited by detector sensitivity and source brightness.

Key Technological Milestones

The development of practical terahertz (THz) sources began in the with the demonstration of continuous-wave molecular gas , such as the HCN laser operating near 891 GHz, which provided early spectroscopic capabilities in the submillimeter range. These electrically pumped systems, pioneered by researchers like H.A. Gebbie, overcame limitations of earlier oscillators by enabling tunable, narrow-linewidth emission suitable for high-resolution measurements. A significant advance in THz imaging occurred in 1976, when T.S. Hartwick and colleagues recorded the first THz images using an optically pumped molecular gas source, demonstrating the potential for non-ionizing of concealed objects. This was followed in 1995 by B.B. Hu and M.C. Nuss, who introduced the first THz time-domain (TDS) system based on laser-pumped photoconductive antennas, achieving generation and detection for . By 1996, raster-scan-free THz-TDS was realized using electro-optic sampling with a ZnTe crystal and CCD camera, reducing acquisition times and enabling real-time applications. The invention of the terahertz quantum cascade laser (QCL) in 2002 by R. Köhler and team represented a breakthrough in compact, solid-state sources, operating via intersubband transitions in superlattices to produce milliwatt-level continuous-wave output up to several THz. This addressed prior reliance on bulky cryogenic systems, facilitating integration into portable devices despite initial needs for low temperatures. Subsequent refinements, including metal-metal waveguides, extended operation toward by the late . In 2008, engineers at Harvard University achieved room-temperature emission of coherent THz radiation using a semiconductor source. In 2013, researchers at the Georgia Institute of Technology and the Polytechnic University of Catalonia developed a method to create graphene antennas for emitting radio waves in the THz frequency range. These milestones collectively narrowed the "THz gap" by improving source efficiency, coherence, and detectivity, paving the way for applications in and communications.

Sources and Generation

Natural Sources

Terahertz radiation occurs naturally as part of the from objects with temperatures exceeding approximately 10 , where the Planck distribution tail extends into the THz range, though peak emission for terrestrial temperatures (around 300 ) lies in the . This contributes to background levels detectable in controlled environments, but its intensity in the 0.1–10 THz band diminishes rapidly with decreasing due to the exponential falloff in the Rayleigh-Jeans tail. In astronomical contexts, THz emission arises from atomic and molecular transitions in , including fine-structure lines of ions like [C II] at 1.9 THz and [O I] at 4.7 THz, which trace ionized regions and zones around young stars. Molecular rotational lines, such as those from CO and H2O, emit in the submillimeter to THz regime from cold molecular clouds (10–50 K), enabling mapping of star-forming regions and protoplanetary disks via facilities like the Atacama Large Millimeter/submillimeter Array (ALMA). Dust continuum emission from interstellar grains, peaking around 1–3 THz for typical dust temperatures of 20–30 K, provides insights into mass distributions in galaxies and the cosmic far-infrared background. Solar activity generates THz radiation during flares, with observations of rising-frequency sub-THz emission from flare ribbons attributed to gyrosynchrotron processes involving non-thermal electrons accelerated in events, as detected by instruments like the KOSMA telescope in events peaking near 0.7 THz. Atmospheric natural sources are minimal, as and oxygen absorption lines dominate propagation losses rather than emission, though transient events like may produce weak broadband THz pulses via transient plasma discharges, though empirical detection remains limited.

Artificial Sources and Methods

Artificial sources of terahertz (THz) radiation encompass semiconductor devices, nonlinear optical processes driven by ultrafast lasers, vacuum electronic oscillators, and large-scale accelerators, each exploiting distinct physical mechanisms to produce electromagnetic waves in the 0.1–10 THz range. These methods address the THz gap by extending toward higher frequencies or adapting techniques to longer wavelengths, though challenges persist in achieving high power, efficiency, and room-temperature operation. Semiconductor-based sources like quantum cascade lasers (QCLs) enable compact, coherent emission via engineered intersubband transitions in quantum wells, typically operating at frequencies from 1–5 THz with output powers reaching milliwatts to watts in pulsed or continuous-wave modes, though cryogenic cooling is often required for optimal performance. Photoconductive antennas (PCAs), fabricated from semiconductors like low-temperature-grown GaAs, generate broadband THz pulses through the acceleration of photocarriers excited by optical pulses, yielding single-cycle waveforms with bandwidths spanning up to an (e.g., 0.1–3 THz) and pulse energies scalable with antenna size and intensity. These devices support both pulsed and continuous-wave operation via optical techniques, offering tunability by adjusting the optical pump difference, but require precise alignment with ultrafast lasers, limiting average powers to microwatts without amplification. Nonlinear optical methods, such as optical rectification in electro-optic crystals (e.g., ZnTe or LiNbO3) or gases, produce THz radiation via second-order nonlinear polarization induced by intense femtosecond laser pulses, generating broadband emission from DC to several THz with higher peak fields possible in plasma-based schemes involving . These approaches achieve pulse energies in the microjoule range but suffer from low conversion efficiencies (typically <1%) and beam divergence, necessitating velocity-matching crystals or tilted pulse fronts for optimization. Vacuum electronic sources include backward-wave oscillators (BWOs), which use electron beams interacting with slow-wave structures to amplify backward-propagating waves at frequencies up to 1.5 THz with powers in the milliwatt range, and gyrotrons employing cyclotron resonance in relativistic electron beams for higher-power output at sub-THz frequencies. These provide continuous-wave operation suitable for spectroscopy but require high voltages and bulky magnet systems. Free-electron lasers (FELs) and synchrotrons, leveraging undulator radiation from accelerated electron bunches, deliver megawatt-level peak powers across the THz band for research applications, though their scale confines them to facilities like those at Jefferson Lab or European XFEL. Emerging trends include room-temperature QCLs with improved wall-plug efficiencies approaching 1% at 3 THz and hybrid integration of PCAs with metasurfaces for enhanced directivity, driven by demands in portable imaging and communications. Despite progress, electronic sources generally yield lower powers at higher frequencies due to carrier transit-time limitations, while optical methods trade coherence for bandwidth, underscoring ongoing efforts in materials like graphene for unified high-performance generation.

Detection and Measurement

Principles of THz Detection

Detection of terahertz (THz) radiation faces inherent challenges due to the low photon energy (approximately 0.4 to 40 meV), which falls between typical electronic bandgaps of semiconductors and microwave photon detectors, necessitating specialized mechanisms that exploit thermal, photoconductive, or electro-optic effects rather than direct photoelectric absorption as in higher-frequency regimes. Principles of THz detection generally divide into incoherent direct detection, which measures power or intensity via thermal or resistive changes, and coherent detection, which captures the electric field amplitude and phase for spectroscopic applications. Thermal detection relies on the absorption of THz waves causing a measurable temperature rise in a sensitive material, while non-thermal methods leverage field-induced modulation of charge carriers or optical properties. In thermal detectors, such as bolometers, incoming THz radiation is absorbed by a low-heat-capacity element, leading to a temperature-dependent change in electrical resistance; for microbolometers operating at room temperature, vanadium oxide or amorphous silicon films exhibit a temperature coefficient of resistance around -2% to -3% per Kelvin, enabling noise-equivalent powers (NEPs) as low as 10^{-9} W/√Hz. Pyroelectric detectors, alternatively, exploit the spontaneous polarization in materials like lithium tantalate, where THz-induced heating modulates the surface charge, producing a voltage proportional to the rate of temperature change; these achieve responsivities of 10^4 to 10^5 V/W but require mechanical chopping for DC signals due to their AC-only response. Golay cells, a pneumatic variant, detect THz-induced gas expansion in a gas-filled chamber via membrane deflection, offering broadband sensitivity from 0.1 to 10 THz with NEPs near 10^{-9} W/√Hz, though limited by slower response times on the order of milliseconds. Photoconductive detection operates on the principle that the THz electric field accelerates photoexcited carriers in a biased semiconductor gap, such as low-temperature-grown GaAs, modulating photocurrent; this effect, enhanced by antenna structures like bow-tie or log-periodic designs, supports ultrafast sampling with bandwidths exceeding 5 THz when gated by femtosecond optical pulses. In field-effect transistor (FET) detectors, the THz wave induces a photoresponse via plasma-wave rectification or overdamped cyclotron resonance in the two-dimensional electron gas channel, yielding voltage responsivities up to 10^3 V/W at room temperature for silicon or graphene-based devices. Electro-optic sampling provides coherent detection by measuring the THz-induced birefringence in nonlinear crystals like ZnTe or GaP via the Pockels effect, where the field alters the refractive index anisotropy, modulating a near-infrared probe beam's polarization; this technique resolves field transients with sub-picosecond precision and bandwidths up to 10 THz, limited primarily by phonon resonances in the crystal (e.g., 5 THz reststrahlen band in GaAs). For continuous-wave applications, Schottky barrier diodes detect via nonlinear rectification of the THz voltage across a metal-semiconductor junction, achieving cut-off frequencies beyond 5 THz with video-mode sensitivities down to 10^{-12} W. Heterodyne principles, often combining these with a local oscillator, enable phase-sensitive mixing to down-convert THz signals to IF bands, improving signal-to-noise ratios by 20-30 dB over direct detection in noisy environments. These mechanisms collectively address the THz gap by bridging microwave electronics and infrared photonics, though room-temperature operation remains constrained by Johnson noise and thermal fluctuations, with cryogenic cooling enhancing sensitivity by factors of 10-100 in bolometric systems.

Common Detectors and Systems

Thermal detectors, which operate by sensing temperature changes induced by absorbed THz radiation, are among the most established for room-temperature operation. Bolometers measure resistance variations due to heating, achieving noise-equivalent powers (NEP) around 10^{-12} W/Hz^{1/2} at 300 K with responsivities of 10^5–10^6 V/W, and cover frequencies from 0.2–2 THz when antenna-coupled. They offer high sensitivity across broad spectra but suffer from response times of about 10 ms, limiting use in high-speed applications. Golay cells detect via gas expansion in a chamber that displaces a membrane, yielding NEP values near 10^{-10} W/Hz^{1/2} and responsivities up to 1.5 × 10^5 V/W at 1 THz, effective from 0.5–4 THz. Their flat response and sub-nanowatt sensitivity make them suitable for low-power measurements, though fragility, bulkiness, and 15 ms response times constrain practicality. Pyroelectric detectors exploit temperature-dependent polarization changes in materials like lithium tantalate, providing NEP ~10^{-9} W/Hz^{1/2} and responsivities ~10^5 V/W over 0.1–30 THz. They enable broad bandwidth detection without bias but exhibit millisecond-scale responses and wavelength-dependent efficiency drops. Semiconductor detectors, particularly Schottky diodes, enable faster, coherent detection through rectification of THz fields at metal-semiconductor junctions. Zero-bias Schottky diodes fabricated from GaAs or similar materials detect up to 5.56 THz at room temperature, leveraging high cutoff frequencies (>1 THz) and low for broadband response from 0.1–10 THz. Their advantages include simplicity, maturity in fabrication, and compatibility with integrated circuits, though performance degrades at elevated temperatures due to increased leakage currents. Thermopiles, using Seebeck-effect voltage from thermal gradients across junctions, offer NEP ~13 pW/Hz^{1/2} and responsivities ~28 V/W across 0.1–30 THz, prized for reliability and low cost but with inferior sensitivity to bolometers. Detection systems typically classify as direct (incoherent) or (coherent). Direct systems, common in and power measurement, pair thermal detectors like bolometers or pyroelectrics with for , amplitude-sensitive operation without local oscillators. systems, prevalent in , integrate Schottky diodes as mixers with a to downconvert THz signals to intermediate frequencies (1–30 GHz), enabling phase-sensitive detection, higher resolution, and noise reduction via low-noise amplifiers. These setups often incorporate focal plane arrays for multi-pixel , with thermal detectors dominating room-temperature direct systems and diodes suiting compact receivers. Performance across both relies on weak isolation for sensitivity but faces the THz gap's challenges in at ambient conditions.

The Terahertz Gap

Origins of the Gap

The terahertz gap, spanning approximately 0.1 to 10 THz, originated from the technological impasse between electronics and photonics, where neither paradigm efficiently generates or detects coherent radiation in this intermediate regime. devices, such as Gunn oscillators and backward-wave oscillators, achieve high powers up to about 0.3 THz but suffer exponential declines in output due to electron transit-time limitations: at THz frequencies, the oscillation period (on the order of 1 ) matches or exceeds the time for carriers to traverse active regions, typically microns in size, given saturation velocities around 10^7 cm/s in materials like . Parasitic effects, including series resistance and junction capacitances, further degrade performance, while quantum tunneling and introduce noise and instability. From the photonic perspective, down-conversion techniques like optical rectification or difference-frequency generation in nonlinear crystals—pioneered in the 1970s—yield low average powers (microwatts to milliwatts) because phase-matching bandwidths narrow at longer wavelengths, and low energies reduce conversion efficiencies compared to visible or near-infrared regimes where lasers routinely exceed watts. Early quantum cascade lasers, proposed in 1971 but not demonstrated until 1990s prototypes, faced intersubband absorption losses and thermal management issues that confined reliable operation above 1.2 THz initially. Atmospheric absorption, peaking between 0.5 and 2 THz, compounds propagation challenges but stems from rather than the core technological origins. This duality of limitations traces to post-World War II developments: and solid-state tech matured by the 1960s for frequencies below 100 GHz, while invention in 1960 spurred advancements above 30 THz, leaving the THz band reliant on incoherent thermal sources like heated tubes or mercury arcs, which offered broadband but low-intensity emission unsuitable for or . The term "gap" gained prominence in the as computational demands highlighted untapped potential in and material characterization, yet component immaturity—exemplified by detector sensitivities dropping orders of magnitude from Schottky diodes to photodiodes—perpetuated underdevelopment.

Advances in Closing the Gap

Significant progress in closing the terahertz gap has been achieved through the development of room-temperature quantum cascade lasers (QCLs), particularly those employing difference-frequency generation (DFG) within mid-infrared QCLs, enabling electrically pumped, monolithic sources operable without cryogenic cooling. These devices cover frequencies from 1 to 5 THz with continuous-wave output powers exceeding 1 mW at , as demonstrated in strain-balanced designs reported in 2016. Further enhancements include operating temperatures up to 261 K in optimized structures by 2023, reducing reliance on and facilitating practical integration into compact systems. Advances in detection have leveraged two-dimensional materials like graphene, whose high carrier mobility and tunable conductivity enable broadband, uncooled THz responsivity. Graphene field-effect transistors (FETs) have achieved noise-equivalent powers below 1 pW/√Hz at room temperature across 0.1–10 THz, with photothermoelectric and bolometric mechanisms enhancing sensitivity. Hybrid graphene-gold metasurfaces integrated with machine learning have improved detection limits for biosensing, yielding responsivities up to 279 V/W in zero-bias configurations extending to 0.3 THz as of 2025. Multigate graphene nanostructures further amplify signals by factors of up to 6 through asymmetric grating gates that create potential barriers for efficient carrier collection. Metamaterials and metasurfaces have addressed manipulation challenges by enabling reconfigurable control and enhanced light-matter interactions in the THz regime. Active tunable designs responsive to electrical, , or optical stimuli allow dynamic modulation, with recent 3D rolled-up resonators providing compact, high-Q factors for efficient absorption and emission. Integration of into THz metasurfaces has boosted sensor performance, as seen in 2025 reviews highlighting hybrids for high-specificity biosensing with sub-wavelength resolution. Numerical simulations have accelerated these developments by modeling novel absorbers and modulators, bridging empirical gaps between and technologies.

Applications

Imaging and Spectroscopy

Terahertz leverages the partial transparency of THz radiation to non-conductive materials such as , , plastics, and ceramics, allowing for the detection of concealed metallic or dense objects without . This capability has been demonstrated in applications, where THz systems identify hidden weapons or explosives under , as evidenced by systems achieving detection ranges up to several meters with resolutions on the order of millimeters. Empirical tests show THz outperforming millimeter-wave alternatives in distinguishing materials based on contrasts, though atmospheric absorption limits standoff distances to under 10 meters in humid conditions. In non-destructive testing, THz imaging reveals subsurface defects like delaminations or voids in composite materials, critical for structures; for instance, studies on carbon-fiber composites have detected flaws as small as 0.5 mm deep using time-domain reflectometry. Biomedical applications include screening, where THz pulses differentiate healthy tissue (reflection coefficient ~0.1) from malignant areas (up to 0.3) due to water content variations, with clinical trials reporting sensitivity exceeding 80% for . Recent advancements, such as UCLA's 2024 real-time 3D multi-spectral THz array, enable video-rate imaging at 0.1-1 THz with sub-wavelength resolution via computational reconstruction. Terahertz spectroscopy measures absorption and dispersion in the 0.1-10 THz range, corresponding to intermolecular vibrations, rotations, and modes not resolvable by or techniques. Time-domain THz (TDS), dominant since the 1990s, uses lasers to generate and detect pulses, yielding both and phase spectra for quantitative analysis of material properties like (typically 1.5-3 for organics). In pharmaceuticals, it enables non-destructive identification of polymorphs and hydration states; for example, THz spectra distinguish from monohydrate forms of drugs like via distinct peaks at 1.8 THz and 2.2 THz, supporting quality control without sample preparation. Applications extend to solid-state characterization, where THz-TDS quantifies tablet coating thickness (accuracy ~1 μm) and porosity via effective medium models, correlating spectral features to density variations in wet-granulated formulations. For explosives detection, spectroscopic fingerprints—such as RDX's absorption at 1.6 THz—allow standoff identification, with field trials confirming specificity over interferents like fabrics. Limitations include low signal-to-noise in aqueous samples due to strong water absorption (~200 cm⁻¹ at 1 THz), necessitating dry or thin-sample configurations. Ongoing research integrates metamaterials to enhance sensitivity, achieving detection limits below 1% concentration for biomolecules.

Communications

Terahertz (THz) communications leverage the frequency range of approximately 0.1 to 10 THz to enable ultra-high data rates, addressing spectrum scarcity in future wireless networks such as 6G. This band offers vast contiguous bandwidths, theoretically supporting terabit-per-second (Tbps) transmission speeds due to the inverse relationship between frequency and achievable Shannon capacity limits under fixed power constraints. Primary applications include short-range indoor links for data centers, wireless backhaul in urban environments, and integrated sensing-communication systems, where THz waves facilitate both data transfer and environmental mapping. Significant challenges arise from THz wave propagation characteristics, including severe atmospheric attenuation—primarily from absorption peaks—and high scaling with the square of . These factors limit practical ranges to tens of meters without line-of-sight, necessitating advanced with highly directional antennas and precise alignment to mitigate spreading losses. Hardware constraints, such as the lack of efficient THz transceivers and amplifiers, further complicate deployment; silicon-based technologies struggle with power efficiency above GHz, often requiring hybrid photonic-electronic approaches. Experimental demonstrations have validated THz feasibility for high-speed links. In 2024, a 0.22 THz achieved 84 Gbps over 1.26 km for uncompressed 8K video transmission, employing advanced modulation and error correction. Other prototypes have exceeded 100 Gbps per channel at 300 GHz using thin-film technologies for signal generation. Research emphasizes channel modeling to account for molecular absorption and multipath effects in indoor scenarios, alongside for dynamic resource allocation and beam tracking. For , sub-THz bands (90–300 GHz) are prioritized for initial rollout, with full THz integration expected to enhance capacity in dense networks by 2030. Ongoing efforts focus on reconfigurable intelligent surfaces to extend coverage and hybrid to balance with mobility.

Manufacturing and Materials Processing

Terahertz radiation finds application in manufacturing primarily through non-destructive testing (NDT) and process monitoring, leveraging its ability to penetrate non-conductive materials such as polymers, composites, ceramics, and coatings while detecting subsurface defects, voids, delaminations, and thickness variations without physical contact. In industrial settings, THz systems enable in sectors including automotive, , pharmaceuticals, and , where they inspect multilayer structures, verify coating uniformity, and assess material integrity in components like solar cells and semiconductors. For instance, THz time-domain (TDS) measures coating thicknesses and locates defects in composites used in , providing high-resolution that outperforms traditional methods for non-metallic materials. In materials processing, THz techniques support evaluation and optimization of fabrication steps, such as assessing sinterability in ceramics and monitoring curing or drying in polymers. THz-TDS has been applied to analyze the and absorption of pottery bodies to predict behavior, correlating spectral features with and post-firing. Similarly, in polymer processing, THz tracks hydration and film formation during the drying of emulsions, revealing phase transitions and dynamics that influence final mechanical . For dental composites, THz-TDS monitors light-curing processes by measuring changes in and absorption, enabling real-time assessment of completeness. These applications capitalize on THz's sensitivity to molecular vibrations and intermolecular interactions, offering advantages over X-rays by avoiding risks and over ultrasonics by eliminating couplant needs, though penetration is limited to a few millimeters in dense materials. Recent advancements, as of 2025, include integration of THz imaging with drying systems like fluidized beds for in-line monitoring of material moisture and defects during production. Despite these benefits, adoption remains constrained by equipment costs and the need for controlled environments to mitigate atmospheric absorption.

Research and Emerging Developments

Biomedical and Biological Applications

Terahertz (THz) radiation enables non-invasive biomedical by exploiting differences in absorption and refractive indices between healthy and diseased tissues, primarily due to its sensitivity to and biomolecular vibrations, with penetration depths typically limited to superficial layers such as (up to 1-2 mm). In cancer diagnostics, THz reflection and transmission have differentiated malignant tissues from normal ones in excised samples, achieving contrasts based on elevated concentrations in tumors; for instance, studies on specimens reported accurate boundary delineation with spatial resolutions of approximately 0.5 mm. Similarly, THz systems have identified cancers like and ex vivo, with sensitivity exceeding 90% in some prototypes by detecting spectral signatures in the 0.1-3 THz range. For head and neck pathologies, THz spectroscopy and have shown efficacy in detecting oral and laryngeal lesions, leveraging endogenous contrast from tissue hydration and properties without risks associated with X-rays. A 2023 review highlighted THz's role in non-invasive for mucosal abnormalities, where frequency-domain analysis revealed contrasts correlating with grades. In dental applications, THz pulses have imaged enamel demineralization and caries with sub-millimeter resolution, outperforming near-infrared in depth selectivity due to reduced in the THz band. Biologically, THz spectroscopy probes low-frequency vibrational modes of macromolecules, enabling label-free analysis of protein folding dynamics and DNA hydration shells; experiments at intensities below 1 mW/cm² have quantified collective motions in biomolecules, aiding drug binding studies. Furthermore, THz waves interact with biomacromolecules by exciting their rotational and vibrational energy levels, leading to alterations in the structure and function of proteins, DNA, and RNA; these interactions can cause changes in molecular conformation, influencing processes such as transcription inhibition and membrane phase transitions. In neuroscience, low-power THz exposure (0.1-1 THz) has modulated neuronal excitability and morphology in vitro, suggesting potential for targeted neuromodulation, though applications remain exploratory with observed effects on membrane potential and gene expression limited to cellular models. Despite preclinical promise, clinical translation is constrained by limited penetration and the need for enhanced signal-to-noise ratios, as evidenced by ongoing developments in metamaterial-enhanced detectors as of 2023.

High-Energy Physics and Accelerators

Terahertz radiation plays a significant role in high-energy physics, particularly in particle accelerators, where it is generated through mechanisms like coherent synchrotron radiation (CSR) and coherent transition radiation (CTR). In storage rings such as the Advanced Light Source (ALS), CSR arises from ultrashort electron bunches with lengths comparable to the THz wavelength, producing intense, coherent pulses that reveal beam instabilities and collective effects. These emissions enable precise characterization of bunch profiles, with measured spectra extending to several THz and pulse energies reaching microjoules. Coherent THz sources from accelerators also facilitate advanced beam diagnostics, offering non-intercepting methods to assess longitudinal bunch properties. At facilities like CERN's CLEAR linac, sub-THz radiation from electron bunches has been used to diagnose beam dynamics, with detected powers scaling quadratically with bunch charge due to coherence enhancement. Such diagnostics support (FEL) operations by measuring emittance and energy spread, critical for optimizing high-brightness beams in high-energy experiments. Beyond diagnostics, THz radiation drives novel schemes, promising compact alternatives to conventional radiofrequency systems. Dielectric-loaded structures excited by THz pulses have achieved gradients exceeding 100 MV/m, as demonstrated in experiments accelerating over millimeter-scale distances. Multistage THz accelerators, using plasma or segmented waveguides, have shown stable electron acceleration with energies up to 10-30 keV per stage, paving the way for table-top devices in high-energy physics research. These developments, tested at SLAC and other labs since 2020, leverage laser-generated THz pulses with peak fields of 1-10 GV/m to manipulate relativistic beams.

Recent Innovations (2020–2025)

In 2021, researchers outlined seven defining features of terahertz (THz) systems, emphasizing their potential for ultra-high data rates exceeding 100 Gbps over short distances, supported by channel modeling and techniques to overcome severe and molecular absorption in the 0.1–10 THz band. This framework highlighted innovations in THz transceivers using and III-V semiconductors, enabling prototypes for backhaul and indoor networks with demonstrated bandwidths up to 10 GHz. Advancements in nanoengineered THz generation and detection emerged prominently, with metasurface-based sources achieving tunable emission in the 0.1–1 THz range through of nanostructures like or quantum wells, offering compact alternatives to traditional quantum cascade lasers. By 2023, polarization-independent nano-antennas, optimized via algorithms, extended reception angles to over 60 degrees, facilitating THz harvesting for energy-efficient sensors. In biomedical applications, THz radiation demonstrated effects in 2022, where 30–45 THz photons resonated with molecules, enhancing synaptic transmission in neural models without damage, as evidenced by increased calcium influx in cell cultures. Further, 2024 studies on 2D materials like enabled room-temperature THz detectors with responsivities above 100 V/W and response times under 1 ps, advancing non-invasive imaging and wound assessment by exploiting contrasts in biological tissues. By September 2025, a layered confined THz light to nanoscale dimensions below 100 nm, leveraging phonon-polaritons for subwavelength waveguides, potentially revolutionizing on-chip THz interconnects and surpassing limits in platforms. Concurrently, topological THz metadevices introduced reconfigurable wave manipulation, using on-chip structures to achieve robust edge-state propagation immune to defects, with applications in fault-tolerant quantum THz systems. These developments underscore progress in overcoming the THz gap through and device innovations, though remains constrained by fabrication yields below 80% in prototypes.

Safety and Biological Effects

Exposure Guidelines and Standards

International standards for terahertz (THz) radiation exposure are established by organizations such as the International Commission on Protection (ICNIRP) and the Institute of Electrical and Electronics Engineers (IEEE), focusing on preventing effects from non-ionizing radiation due to its shallow skin of approximately 0.1–1 mm. These guidelines apply primarily to frequencies up to 300 GHz (0.3 THz), covering the lower THz range, with limits derived from biophysical models limiting tissue temperature rise to below 5°C for occupational exposure and 1–2°C for general public. Higher THz frequencies (above 300 GHz) lack dedicated RF-style limits and may be assessed under standards like ANSI Z136.1, which extend conservatively into the THz regime for extended sources. Under the ICNIRP 2020 guidelines for 100 kHz–300 GHz, basic restrictions for localized exposure above 6 GHz emphasize absorbed power density (Sab) rather than specific absorption rate (SAR), as energy deposition is surface-limited. For occupational exposure averaged over 6 minutes, Sab is restricted to 100 W m−2 over 4 cm2 or 200 W m−2 over 1 cm2 for frequencies above 30 GHz; general public limits are one-fifth of these values (20 W m−2 and 40 W m−2, respectively). Corresponding reference levels for incident power density (Sinc) at 300 GHz are 100 W m−2 occupational and 20 W m−2 general public, averaged over 6 minutes, with adjustments for lower frequencies via formulas like Sinc = 275 / fG0.177 W m−2 (where fG is frequency in GHz). For exposures shorter than 6 minutes, restrictions shift to absorbed energy density (Uab), such as 72 kJ m−2 occupational over 1 cm2. The IEEE C95.1-2019 standard aligns closely with ICNIRP, specifying safety levels from 0 Hz to 300 GHz with whole-body and localized exposure limits to avert neural stimulation and thermal damage, including caps of 10 W m−2 for uncontrolled environments ( equivalent) above 50 GHz. For THz applications, both standards note that blink reflex and averaging over small areas mitigate risks, with empirical thresholds for skin pain around 12–13 kW m−2 for brief pulses at 94 GHz informing conservative margins of 10–50-fold below damage levels. In the United States, the (FCC) enforces RF exposure limits up to 100 GHz under 47 CFR §1.1310, adopting IEEE-derived values like 10 W m−2 for general population over 6 minutes above 6 GHz, with ongoing proceedings to extend to higher frequencies including THz bands for considerations. (OSHA) guidelines under 29 CFR 1910.97 reference 100 W m−2 (10 mW cm−2) for 10 MHz–100 GHz, often extrapolated to THz systems. Emerging THz devices, such as imaging systems, typically operate below 1–10 mW cm−2 to comply, though long-term non-thermal effects remain unaddressed in standards due to insufficient empirical data. No major updates to THz-specific limits occurred between 2020 and 2025, reflecting the technology's limited deployment.

Empirical Evidence of Effects

Empirical studies on the biological effects of terahertz (THz) radiation have primarily utilized cell cultures, tissues, and limited animal models, revealing both thermal and potential non-thermal influences at power densities ranging from microwatts to tens of milliwatts per square centimeter. For instance, exposure of fibroblasts to 2.52 THz radiation at 21 mW/cm² for 10 minutes induced DNA double-strand breaks and increased reactive oxygen species production, effects persisting up to 24 hours post-exposure, as measured by comet assays and fluorescence microscopy. Similarly, in rat retinal pigment epithelial cells irradiated with 1.7 THz pulses at average powers of 25–50 mW/cm² for 30 minutes, researchers observed elevated markers of , including caspase-3 activation and Bax upregulation, alongside ultrastructural changes like mitochondrial swelling via . At lower intensities, non-thermal effects have been documented in neuronal systems. Exposure of hippocampal neurons to 0.17 THz continuous waves at 0.1–1.0 mW/cm² modulated density and synaptic protein expression (e.g., PSD-95), with dose-dependent increases in spine formation observed after 20-minute sessions, quantified through and confocal imaging; these changes correlated with altered calcium influx via patch-clamp . In contrast, higher fluences (e.g., 10 J/cm² at 0.1–10 THz) on human neuroblastoma cells disrupted polymerization and induced G2/M cell cycle arrest, as evidenced by and immunofluorescence, suggesting interference with cytoskeletal dynamics independent of bulk heating below 1°C rise. Animal studies provide limited in vivo evidence, often focusing on short-term exposures. In mice exposed to 0.3 THz at 10 mW/cm² for 30 minutes daily over 7 days, skin tissue exhibited increased inflammatory cytokine levels (IL-6, TNF-α) and epidermal hyperplasia, confirmed by histology and ELISA assays, without detectable thermal damage via infrared thermography. Ocular exposure experiments in rabbits using 0.22 THz at 1–5 mW/cm² for 2 minutes led to transient corneal endothelial cell loss and altered aqueous humor protein profiles, detected through slit-lamp biomicroscopy and proteomics, raising concerns for repeated dosing. However, replication challenges persist, with some studies reporting no significant genotoxicity or mutagenesis in bacterial and mammalian assays at fluences up to 100 J/cm², highlighting variability due to parameters like pulse duration, frequency resonance with biomolecular vibrations (e.g., DNA phonons at 1–3 THz), and exposure geometry. Long-term human data remain scarce, confined to occupational monitoring rather than controlled trials, with no conclusive of or systemic from ambient THz levels in emerging applications like security scanners. Thresholds for adverse effects appear below those causing detectable heating (e.g., <10 mW/cm² for non-thermal cellular perturbations), but causal mechanisms—potentially involving resonant absorption by clusters or vibrational modes in proteins—require further dosimetry-standardized investigations to distinguish artifactual from physiological responses.

Debates and Uncertainties

While terahertz (THz) radiation is non-ionizing and primarily absorbed superficially by biological tissues due to , debates persist regarding the existence of non-thermal biological effects, such as alterations in or protein conformation without measurable temperature increases. Some studies report THz exposure inducing changes in transcription regulators and macromolecular structures, potentially leading to shifts in gene activity, as observed in keratinocyte models where specific frequencies modulated inflammatory pathways. However, critics argue these findings may stem from undetected thermal stress or experimental artifacts, with mathematical models predicting minimal DNA interaction at typical intensities, and direct experimental verification of non-thermal protein disruption in aqueous solutions remaining elusive. Empirical inconsistencies across studies fuel uncertainty, with reports of THz influencing , , or stress responses in lymphocytes and stem cells contrasting with others showing no impact on morphology, viability, or genomic integrity even at intensities up to several mW/cm². For frequencies above 0.15 THz, investigations into genotoxic potential are sparse, leaving gaps in understanding chronic low-level exposure risks, particularly for or ocular tissues where absorption is highest. Reviews highlight challenges due to variations in pulse duration, , and biological endpoints, with some effects attributed to secondary heating rather than direct THz-tissue interactions. Safety standards, such as those from ICNIRP, rely on thermal limits extrapolated from lower frequencies, but debates question their adequacy for THz-specific mechanisms like resonant vibrational excitations in biomolecules, which could amplify effects at non-heating fluences. Emerging associations with mm-wave technologies amplify concerns, though peer-reviewed syntheses emphasize that while surface-level bioeffects are plausible, systemic or carcinogenic risks lack robust evidence, underscoring the need for standardized, long-term studies to resolve these uncertainties.

References

  1. https://pmc.ncbi.nlm.nih.gov/articles/PMC8683584/
  2. https://spj.science.org/doi/10.34133/research.0586
  3. https://www.ccny.cuny.edu/sites/default/files/2020-04/2011_Pavel%2520Shymyatsky%2520-%2520Terahertz%2520sources%2520-%2520Journal%2520of%2520Biomedical%2520Optics%2520-%2520Volume%252016%2520No%25203%2520Page%2520033001.pdf
  4. https://thz.yale.edu/sites/default/files/tutorial.pdf
  5. https://www.nature.com/articles/s41377-023-01278-0
  6. https://pmc.ncbi.nlm.nih.gov/articles/PMC8837828/
  7. https://link.springer.com/article/10.1007/s10762-022-00902-1
  8. https://spj.science.org/doi/10.34133/2022/9852503
  9. https://www.rp-photonics.com/terahertz_radiation.html
  10. https://www.toptica.com/technology/technical-tutorials/terahertz/terahertz-properties
  11. https://www.findlight.net/blog/terahertz-sources-advanced-guide/
  12. https://www.sciencedirect.com/science/article/pii/S2589004223011379
  13. https://terasense.com/terahertz-technology/
  14. https://das-nano.com/resources/library/terahertz-waves/
  15. https://www.bfs.de/SharedDocs/Glossareintraege/EN/T/terahertz-radiation.html?view=renderHelp
  16. https://www.edinst.com/resource/application-note-terahertz-molecular-lasers-introduction-applications/
  17. http://depts.washington.edu/cmditr/mediawiki/index.php?title=Terahertz_Radiation
  18. https://sites.brown.edu/mittleman/files/2024/07/JansenJIMT.pdf
  19. https://www.bu.edu/cas/researchers-at-bu-mit-make-breakthrough-in-study-of-terahertz-radiation/
  20. https://pmc.ncbi.nlm.nih.gov/articles/PMC7881098/
  21. https://pmc.ncbi.nlm.nih.gov/articles/PMC10341590/
  22. https://pubmed.ncbi.nlm.nih.gov/11580188/
  23. https://ieeexplore.ieee.org/iel5/5/4337826/04337844.pdf
  24. https://pmc.ncbi.nlm.nih.gov/articles/PMC11642474/
  25. https://www.cambridge.org/core/journals/journal-of-plasma-physics/article/terahertz-radiation-in-the-interaction-of-a-focused-laser-pulse-with-plasma/6704EA866178E46F56B3F67866974EE2
  26. https://pmc.ncbi.nlm.nih.gov/articles/PMC3456412/
  27. https://royalsocietypublishing.org/doi/10.1098/rsta.2023.0378
  28. https://link.springer.com/article/10.1140/epjh/s13129-022-00044-x
  29. https://pubs.aip.org/aip/jap/article/122/23/230901/281616/Perspective-Terahertz-science-and-technology
  30. https://pmc.ncbi.nlm.nih.gov/articles/PMC8232131/
  31. https://www.mdpi.com/2076-3417/11/1/71
  32. https://www.sciencedirect.com/science/article/pii/S0079672721000483
  33. https://phys.org/news/2008-05-room-temperature-semiconductor-source-coherent-terahertz.html
  34. https://news.gatech.edu/news/2013/03/07/graphene-based-terabit-transceivers-could-lead-tiny-machines
  35. https://www.sciencedirect.com/science/article/abs/pii/S0975761913000264
  36. https://physicstoday.aip.org/features/astrochemistry-in-the-terahertz-gap
  37. https://pmc.ncbi.nlm.nih.gov/articles/PMC11751206/
  38. https://www.aanda.org/articles/aa/pdf/2018/12/aa34124-18.pdf
  39. https://www.nature.com/articles/s41598-023-47586-8
  40. https://www.rp-photonics.com/terahertz_sources.html
  41. https://apps.dtic.mil/sti/trecms/pdf/AD1056826.pdf
  42. https://www.rp-photonics.com/terahertz_detectors.html
  43. https://pmc.ncbi.nlm.nih.gov/articles/PMC11548466/
  44. https://advanced.onlinelibrary.wiley.com/doi/full/10.1002/apxr.202400094
  45. http://antonirogalski.com/wp-content/uploads/2011/12/THz-detectors-and-FPAs-Opto-Electron-Rev-193-346-404-2011.pdf
  46. https://arxiv.org/pdf/2308.08503
  47. https://www.mdpi.com/1424-8220/23/7/3469
  48. https://www.sciencedirect.com/science/article/abs/pii/S1350449524003517
  49. https://ietresearch.onlinelibrary.wiley.com/doi/full/10.1049/cje.2021.00.302
  50. https://www.sciencedirect.com/science/article/abs/pii/S007967271000025X
  51. https://physicsworld.com/a/navigating-the-terahertz-gap/
  52. https://www.chemistryworld.com/features/the-terahertz-gap-into-the-dead-zone/3004857.article
  53. https://iopscience.iop.org/book/mono/978-0-7503-3171-5/chapter/bk978-0-7503-3171-5ch1
  54. https://www.researchgate.net/publication/332613193_Brief_history_of_THz_and_IR_technologies
  55. https://www.hamamatsu.com/jp/en/our-company/business-domain/central-research-laboratory/optical-materials/qcl.html
  56. https://www.nature.com/articles/srep23595
  57. https://pubs.aip.org/aip/apl/article/122/16/161101/2882855/Enhanced-operating-temperature-in-terahertz
  58. https://pubs.acs.org/doi/10.1021/acsaelm.3c01511
  59. https://advanced.onlinelibrary.wiley.com/doi/10.1002/adom.202500748?af=R
  60. https://pmc.ncbi.nlm.nih.gov/articles/PMC11501572/
  61. https://www.nature.com/articles/srep43334
  62. https://pmc.ncbi.nlm.nih.gov/articles/PMC12395562/
  63. https://www.photonics.com/Articles/Bridging-the-Terahertz-Gap-with-Simulation/a68064
  64. https://www.nist.gov/programs-projects/terahertz-imaging-and-sources
  65. https://opg.optica.org/fulltext.cfm?uri=ao-47-9-1286
  66. https://ieeexplore.ieee.org/document/10134245/
  67. https://pmc.ncbi.nlm.nih.gov/articles/PMC10722018/
  68. https://cnsi.ucla.edu/january-16-2024-ucla-engineers-develop-terahertz-imaging-system-capable-of-capturing-real-time-3d-multi-spectral-images-for-the-first-time/
  69. https://pubs.acs.org/doi/10.1021/ac200907z
  70. https://www.omicsonline.org/open-access/terahertz-spectroscopy-principles-applications-and-future-directions-133969.html
  71. https://pmc.ncbi.nlm.nih.gov/articles/PMC10393043/
  72. https://pmc.ncbi.nlm.nih.gov/articles/PMC10386691/
  73. https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2022.869537/full
  74. https://www.sciencedirect.com/topics/medicine-and-dentistry/terahertz-spectroscopy
  75. https://www.rohde-schwarz.com/us/solutions/wireless-communications-testing/wireless-standards/6g/thz-communication/thz-communication_257042.html
  76. https://arxiv.org/abs/2207.11021
  77. https://ieeexplore.ieee.org/document/10494372/
  78. https://opg.optica.org/abstract.cfm?uri=optcon-2-10-2154
  79. https://ieeexplore.ieee.org/document/7765354
  80. https://www.sciencedirect.com/science/article/pii/S2405844025017773
  81. https://pmc.ncbi.nlm.nih.gov/articles/PMC11399333/
  82. https://www.facebook.com/IEEE.Spectrum/posts/the-potential-of-terahertz-data-links-is-being-realized-with-thin-film-technolog/1208578124647534/
  83. https://ieeexplore.ieee.org/document/10620553/
  84. https://www.ericsson.com/en/6g/spectrum/sub-thz
  85. https://arxiv.org/html/2407.01957v1
  86. https://www.menlosystems.com/applications/thz-non-destructive-testing/
  87. https://pmc.ncbi.nlm.nih.gov/articles/PMC6806174/
  88. https://www.industryweek.com/none/article/21954596/terahertz-technology-poised-for-manufacturing
  89. https://lunainc.com/solution/non-destructive-test-ndt-terahertz
  90. https://www.sciencedirect.com/science/article/abs/pii/S0963869521000724
  91. https://raybloc.com/terahertz-imaging-in-non-destructive-testing/
  92. https://www.tandfonline.com/doi/full/10.1080/21870764.2018.1439610
  93. https://pubs.acs.org/doi/10.1021/acs.langmuir.4c03103
  94. https://pubmed.ncbi.nlm.nih.gov/23162722/
  95. https://pmc.ncbi.nlm.nih.gov/articles/PMC7039230/
  96. https://www.sciencedirect.com/science/article/pii/S0260877424002498
  97. https://pmc.ncbi.nlm.nih.gov/articles/PMC7304303/
  98. https://pubmed.ncbi.nlm.nih.gov/33583155/
  99. https://pmc.ncbi.nlm.nih.gov/articles/PMC10521734/
  100. https://mmrjournal.biomedcentral.com/articles/10.1186/s40779-021-00321-8
  101. https://pmc.ncbi.nlm.nih.gov/articles/PMC8070290/
  102. https://www.nature.com/articles/s41598-025-03869-w
  103. https://spj.science.org/doi/10.34133/research.1028
  104. https://pmc.ncbi.nlm.nih.gov/articles/PMC10968323/
  105. https://qims.amegroups.org/article/view/15326/html
  106. https://www2.als.lbl.gov/als_physics/website/coherent.html
  107. https://iopscience.iop.org/article/10.1088/0256-307X/26/12/124101
  108. https://link.aps.org/doi/10.1103/PhysRevAccelBeams.22.020402
  109. https://www2.als.lbl.gov/als_physics/Fernando/USPASJan08AllLecturesPDF/Introduction.pdf
  110. https://scitechdaily.com/invention-using-terahertz-radiation-could-make-particle-accelerators-10-times-smaller/
  111. https://link.aps.org/doi/10.1103/PhysRevLett.127.074801
  112. https://www6.slac.stanford.edu/news/2020-03-05-terahertz-radiation-technique-opens-new-door-studying-atomic-behavior
  113. https://arxiv.org/pdf/2102.07668
  114. https://spj.science.org/doi/10.34133/research.0562
  115. https://www.nature.com/articles/s41598-023-43709-3
  116. https://spj.science.org/doi/10.34133/research.0010
  117. https://www.liebertpub.com/doi/10.1089/photob.2024.0079
  118. https://phys.org/news/2025-09-layered-material-successfully-confines-terahertz.html
  119. https://pmc.ncbi.nlm.nih.gov/articles/PMC12385967/
  120. https://www.nature.com/articles/s41467-024-44750-0
  121. https://www.icnirp.org/cms/upload/publications/ICNIRPrfgdl2020.pdf
  122. https://terahertz.fr/thz/is-terahertz-radiation-dangerous/
  123. https://ieeexplore.ieee.org/document/6528026/
  124. https://www.fcc.gov/general/radio-frequency-safety-0
  125. https://docs.fcc.gov/public/attachments/fcc-19-126a1.pdf
  126. https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.97
  127. https://pubs.aip.org/lia/jla/article/15/3/192/220446/Do-in-vivo-terahertz-imaging-systems-comply-with
  128. https://www.researchgate.net/publication/33037722_Do_in_vivo_terahertz_imaging_systems_comply_with_safety_guidelines
  129. https://www.icnirp.org/en/activities/news/news-article/rf-guidelines-2020-published.html
  130. https://pmc.ncbi.nlm.nih.gov/articles/PMC10523010/
  131. https://www.nature.com/articles/s41378-023-00612-1
  132. https://www.sciencedirect.com/science/article/pii/S2589004221015182
  133. https://pmc.ncbi.nlm.nih.gov/articles/PMC11772664/
  134. https://www.frontiersin.org/journals/bioengineering-and-biotechnology/articles/10.3389/fbioe.2023.1147684/full
  135. https://www.mdpi.com/2076-3425/14/3/279
  136. https://journal.hep.com.cn/foe/EN/10.1007/s12200-024-00146-y
  137. https://pmc.ncbi.nlm.nih.gov/articles/PMC11354197/
  138. https://www.nature.com/articles/s41598-021-01774-6
  139. https://onlinelibrary.wiley.com/doi/full/10.1002/pssa.202200263
  140. https://pmc.ncbi.nlm.nih.gov/articles/PMC3184876/
  141. https://meridian.allenpress.com/radiation-research/article/179/1/38/191730/Terahertz-Radiation-at-0-380-THz-and-2-520-THz
  142. https://link.springer.com/article/10.1007/s10762-011-9794-5
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