Electronics Guide

Electronic THz Sources

Electronic approaches extend microwave techniques to higher frequencies:

Frequency multipliers: Starting from microwave sources (typically 10 to 30 GHz), chains of frequency multipliers using Schottky diodes can generate signals up to about 2 THz. EMC considerations include:

• Subharmonic leakage from intermediate multiplication stages
• Broadband noise from diode shot noise and thermal effects
• Power supply noise modulating the output frequency
• Spurious outputs from imperfect multiplication

• Resonant tunneling diodes (RTDs): These devices can oscillate at frequencies above 1 THz. They produce relatively clean spectra but at low power levels (microwatts to milliwatts). The main EMC concern is susceptibility of these sensitive devices to external interference.
• IMPATT and Gunn diodes: At the lower end of the THz range, reaching about 300 GHz with harmonic extraction, these oscillator technologies provide useful power, although the output falls steeply with frequency and is measured in milliwatts at the top of their range. Both carry significant phase noise, which raises the noise floor seen by a nearby receiver rather than producing a discrete spurious line.

Photonic THz Sources

Optical approaches generate THz radiation through nonlinear optical processes or semiconductor physics:

Photoconductive switches: Ultrafast laser pulses excite carriers in semiconductor switches, generating broadband THz pulses spanning from about 0.1 to 5 THz. EMC characteristics include:

• Broadband spectrum covers wide frequency range simultaneously
• Pulse repetition rate creates a comb of spectral lines
• Average power is typically low (microwatts to milliwatts)
• Associated femtosecond laser is a potential EMI source

• Photomixing: Two optical frequencies beat in a photoconductive or electro-optic material to generate THz radiation at their difference frequency. This approach offers tunable, narrowband output with EMC considerations similar to other oscillators plus the added complexity of optical system interference.
• Quantum cascade lasers (QCLs): These semiconductor lasers directly emit THz radiation with relatively high power (milliwatts to watts) in a narrow spectral line. Cryogenic operation (typically below 100 K) introduces its own EMC challenges similar to those in quantum computing systems.

THz Generation from Accelerators

High-power THz generation uses accelerator-based techniques:

• Free-electron lasers (FELs): Provide tunable, high-power (kilowatts peak) THz radiation. The accelerator infrastructure generates substantial electromagnetic interference from RF cavities, pulsed magnets, and high-voltage systems.
• Synchrotron radiation: Electron storage rings produce broadband THz radiation. The facility infrastructure dominates EMC considerations.
• Coherent transition radiation: High-power pulsed THz from electron bunches. EMC is dominated by the accelerator drive systems.

For these large-facility sources, EMC is primarily about protecting sensitive THz detectors from the electromagnetic environment of the accelerator, rather than managing emissions from the THz radiation itself.

Atmospheric Absorption

The atmosphere is highly absorptive at most THz frequencies due to rotational transitions of water vapor and oxygen:

• Absorption windows: Relatively transparent regions exist around 0.35, 0.45, 0.65, 0.85, and 1.0 THz. Attenuation in dry air is on the order of tens of decibels per kilometer in the lowest of these windows and rises steeply from one window to the next. On the strong water lines between them, attenuation exceeds 1000 dB/km, which is more than 1 dB for every meter of path.
• Humidity dependence: Water vapor is the primary absorber, so attenuation tracks absolute humidity rather than relative humidity. Within a transmission window it can change by an order of magnitude between cold dry air and warm humid air, which means a link budget that is comfortable on a clear winter day can fail at the same range in summer.
• EMC implications: The strong atmospheric absorption provides natural isolation between THz systems, limiting interference range to tens or hundreds of meters in most terrestrial applications. However, this also means that unwanted THz emissions are unlikely to cause far-field interference problems, shifting EMC concerns to near-field effects and direct system interactions.

Scattering and Diffraction

With wavelengths from tens of micrometers to a few millimeters, THz radiation interacts with objects in ways different from both microwaves and optical radiation:

• Mie scattering: Particles comparable to THz wavelengths (dust, fog droplets, small debris) cause significant scattering. This is more pronounced than at microwave frequencies but less than for visible light.
• Diffraction: THz beams diffract around obstacles with aperture dimensions comparable to the wavelength. A 1 THz beam (wavelength 300 micrometers) passing through a 1 cm aperture experiences significant diffraction effects.
• Surface roughness: Surfaces that appear smooth at microwave frequencies may cause significant scattering at THz frequencies. Surface roughness on the order of wavelength (hundreds of micrometers) affects reflection and scattering behavior.

Guided Wave Propagation

Waveguides and transmission lines for THz frequencies present unique challenges:

• Rectangular waveguide: WR-1.0 waveguide, whose recommended band runs from about 0.75 to 1.1 THz, has an internal cross section of 254 by 127 micrometers, which places its TE₁₀ cutoff near 0.59 THz. Manufacturing tolerances of a few micrometers are a material fraction of that dimension. Loss is severe: the conductor attenuation computed from the surface resistance of copper is on the order of 80 dB/m at 1 THz, and a real guide whose surface roughness is comparable to the skin depth does worse still. Practical runs are measured in centimeters rather than meters, which is why THz systems turn quasi-optical as soon as they can.
• Dielectric waveguides: Plastic or silicon waveguides can guide THz with lower loss than metal, but coupling to free space is challenging. Mode confinement decreases at lower frequencies.
• Planar transmission lines: Coplanar waveguide and microstrip can operate in the THz range but with increasing loss. Radiation losses become significant as wavelength approaches guide dimensions.
• EMC considerations: The difficulty of guiding THz radiation means that unintended radiation from transmission line discontinuities is common. Careful attention to transitions and terminations is required to minimize spurious emissions.

Quasi-Optical Propagation

Much THz system design uses quasi-optical techniques borrowed from infrared and optical systems:

• Gaussian beam propagation: THz beams are typically Gaussian, with waist size and divergence governed by the wavelength. A 1 THz beam focused to a waist radius of 1 mm has a Rayleigh range of about 10 mm, so by 100 mm from the waist it has spread to roughly 20 mm across. Such a beam diverges far faster than an optical one, and an enclosure that would contain a laser will not contain it.
• Lens and mirror systems: Focusing and collimating optics use similar principles to optical systems but at larger scales. Common materials include TPX, HDPE, and silicon for lenses; metal mirrors for reflective optics.
• Standing waves and resonances: The long wavelengths make standing wave formation common in quasi-optical setups. Optical table dimensions often resonate with THz wavelengths, requiring absorptive baffling.

Dielectric Properties

The dielectric properties of materials in the THz range are often dramatically different from lower frequencies:

• Polar liquids: Water is highly absorptive in the THz range due to hydrogen bond relaxation. The absorption coefficient of liquid water is approximately 200 cm⁻¹ at 1 THz, so intensity falls by a factor of e in about 50 micrometers and a water film a fraction of a millimeter thick is effectively opaque.
• Polymers: Many plastics are relatively transparent to THz radiation. HDPE, TPX, and Teflon have absorption coefficients below 1 cm⁻¹ over much of the band, making them useful for windows and lenses. However, some polymers show strong absorption bands from vibrational modes.
• Ceramics: High-purity alumina and silicon are relatively transparent, while common ceramics with impurities may be lossy. Dielectric resonator antennas and filters exploit these properties.
• Semiconductors: High-resistivity silicon is nearly transparent to THz and is widely used for optics. Doped semiconductors are absorptive due to free carrier absorption that increases with doping and temperature.

Conductor Properties

Metal behavior at THz frequencies deviates from ideal conductors:

• Skin depth: At 1 THz, the skin depth in copper is about 70 nm. This means that surface quality dominates conductor loss, and thin metallic coatings can provide effective shielding.
• Surface impedance: Within the Hagen-Rubens regime, the surface resistance scales as the square root of frequency; for copper at 1 THz this yields roughly 0.2 to 0.3 ohms per square, on the order of a hundred times the value at 100 MHz and a major contributor to conductor loss. Real surfaces, where roughness exceeds the few-tens-of-nanometers skin depth, perform worse still.
• Anomalous skin effect: At THz frequencies, the electron mean free path can exceed the skin depth, leading to increased surface impedance and modified reflection behavior.
• Surface plasmons: Metal surfaces can support surface plasmon polaritons at THz frequencies, enabling subwavelength confinement but also creating additional coupling paths for interference.

Spectroscopic Signatures

Many molecules have characteristic absorption or emission lines in the THz range:

• Rotational transitions: Small polar molecules like water, ammonia, and hydrogen cyanide have strong rotational lines throughout the THz range. These transitions provide both interference hazards and sensing opportunities.
• Phonon modes: Crystalline solids have phonon resonances in the THz range that appear as absorption features. These are highly material-specific and enable spectroscopic identification.
• Intermolecular vibrations: Weak bonds in molecular crystals and biological materials vibrate at THz frequencies, providing fingerprint spectra for materials identification.
• EMC implications: The spectroscopic richness of the THz range means that material composition significantly affects propagation and shielding. A package that provides good shielding for electronics at one THz frequency may be transparent at another due to material resonances.

Metamaterials and Engineered Surfaces

The relatively long wavelengths make THz frequencies accessible to metamaterial engineering:

• Frequency-selective surfaces: Periodic metallic patterns on dielectric substrates can create bandpass or bandstop filters with controllable spectral response. Applications include THz-transparent windows with EMC filtering.
• Absorbing structures: Metamaterial absorbers can achieve near-perfect absorption at specific frequencies, useful for eliminating standing waves and stray reflections.
• Beam-steering surfaces: Gradient-index metasurfaces can steer THz beams without mechanical motion, enabling novel antenna and scanner designs.

Power Measurement

Measuring THz power accurately is challenging due to low signal levels and lack of calibrated standards:

• Thermal detectors: Pyroelectric detectors and bolometers provide absolute power measurement but with limited bandwidth and sensitivity. Calibration traceability to primary standards is still being developed.
• Detector nonlinearity: At higher power levels, detector saturation and thermal effects cause nonlinear response that must be characterized and corrected.
• Beam characterization: Power measurements require knowledge of beam spatial distribution. Non-uniform beams give misleading results if the detector aperture does not capture all the power.
• Atmospheric effects: Absorption by atmospheric water vapor can cause significant measurement errors, especially for frequencies near absorption lines. Purging with dry nitrogen or operating in vacuum improves accuracy.

Spectrum Analysis

Characterizing THz spectra requires techniques beyond conventional spectrum analyzers:

• Heterodyne receivers: Mixing with a local oscillator downconverts THz signals to IF for analysis. This approach provides high spectral resolution but requires tunable THz local oscillators that may themselves be EMI sources.
• Fourier transform spectroscopy: Broadband THz sources can be analyzed using scanning Michelson interferometers. Resolution depends on mirror travel, and mechanical stability requirements are stringent to avoid spectral artifacts.
• Time-domain spectroscopy: Pulsed THz systems measure the electric field as a function of time, with Fourier transformation yielding spectral information. This approach provides both amplitude and phase but requires femtosecond laser synchronization.
• Dynamic range limitations: THz measurement systems offer far less dynamic range than microwave spectrum analyzers, and in a time-domain system the usable range narrows sharply toward the high-frequency end of the pulse spectrum, where the source has least power and the detector least response. This limits the ability to measure weak spurious signals in the presence of intended emissions, so a reported spurious level should always be quoted together with the measurement floor that applies at that frequency.

Field Mapping

Characterizing THz field distributions for EMC purposes requires specialized scanning techniques:

• Near-field scanning: Electro-optic sampling or scanned photoconductive probes can map THz fields with sub-wavelength resolution. These techniques reveal coupling mechanisms not apparent from far-field measurements.
• Imaging arrays: Focal-plane arrays of THz detectors enable real-time imaging but with lower sensitivity than single-detector systems. Array non-uniformity must be corrected for quantitative measurements.
• Beam profiling: Knife-edge scanning or direct camera imaging characterizes beam spatial distribution. THz camera technology is rapidly advancing but still offers lower resolution than optical cameras.

Calibration and Standards

The THz range lacks the mature calibration infrastructure of microwave or optical frequencies:

• Power standards: Primary standards for THz power are being developed by national metrology institutes but are not yet widely available. Traceability is typically through thermal transfer standards.
• Frequency standards: THz frequencies can be referenced to optical frequency combs, providing excellent accuracy. However, this requires sophisticated equipment not available in typical EMC labs.
• Antenna standards: Standard gain antennas for THz are not commercially available. Antenna gain is typically determined through measurement comparisons or simulation.
• Material standards: Reference materials with known THz properties are needed for validation of dielectric measurements. Silicon and HDPE are commonly used as secondary standards.

Tissue Interaction Mechanisms

THz radiation interacts with biological tissue primarily through thermal mechanisms:

• Water absorption: Biological tissue is approximately 70% water, and the strong absorption by water dominates THz tissue interaction. Measured penetration depths in living skin fall between roughly 0.1 and 0.5 mm across the band, with the deepest penetration reported near 0.5 THz, so the energy is deposited in the stratum corneum and the epidermis rather than in the vascular dermis beneath. Penetration shortens as frequency rises, which is why THz imaging of tissue is confined to the surface and near-surface layers.
• Thermal effects: Absorbed THz energy heats tissue, and because it is deposited in a layer only a few hundred micrometers thick, the surface temperature responds quickly. With a thermal diffusivity near 1 × 10⁻⁷ m²/s, heat diffuses across 200 micrometers in roughly a tenth of a second, so for exposures longer than about a second the tissue settles into a quasi-steady state in which blood perfusion in the dermis carries the heat away. Incident power density, rather than accumulated energy, then governs the temperature rise, which is why exposure guidance in this regime is written as a power density averaged over a small area and a few minutes rather than as an energy dose.
• Non-thermal effects: Possible non-thermal effects of THz radiation on DNA, proteins, and cell membranes remain under investigation. THz photon energies are far below the threshold for ionization or for breaking a covalent bond, and the prevailing view is that thermal effects account for what is observed at practical exposure levels, but the question is not closed.

Eye and Skin Exposure

The primary tissues at risk from THz exposure are the eyes and skin:

• Corneal exposure: The cornea is the most exposed ocular tissue, because it is avascular, which limits heat dissipation, and because its high water content makes it strongly absorbing. Absorption in the tear film and cornea is complete within a few hundred micrometers, so no THz energy reaches the lens or the retina, and the retinal hazards that dominate laser safety at visible and near-infrared wavelengths do not arise. Concern focuses instead on corneal surface heating and on disturbance of the tear film.
• Skin exposure: THz absorption occurs in the stratum corneum and the epidermis. It is sometimes argued that thermal injury could occur before any pain is felt, on the grounds that the nerve endings lie deeper than the absorbing layer. That argument does not hold at the bottom of the band: nociceptive free nerve endings terminate within the epidermis, and the millimeter-wave active denial system, which operates at 95 GHz, works precisely by heating them to produce an immediate and intense heat pain. Whether the warning response stays as reliable higher in the THz band, where deposition is shallower and the surface heats faster, is not well characterized, and prudent practice does not rely on it.
• Exposure limits: The THz band sits where two ICNIRP frameworks meet. The 2020 radiofrequency guidelines run from 100 kHz to 300 GHz, and above 6 GHz they express local restrictions as absorbed power density rather than as specific absorption rate, because the energy no longer penetrates far enough for a mass-averaged quantity to be meaningful. The basic restriction is 100 W/m² for occupational exposure and 20 W/m² for the general public, averaged over 6 minutes and over a square 4 cm² of body surface. Above 30 GHz, which takes in the whole of the lower THz band, a further constraint holds the exposure averaged over a square 1 cm² to twice the 4 cm² figure, so that a tightly focused beam cannot be averaged away. Both restrictions derive from an operational threshold of 200 W/m² over 4 cm², reduced by a factor of 2 for occupational exposure and 10 for the general public. Above 300 GHz the ICNIRP laser guidelines take over: issued in 2013, they cover wavelengths from 180 nm to 1,000 micrometers, and 1,000 micrometers is exactly 300 GHz, so for a coherent source the two frameworks abut without a gap. They do not join smoothly, because they restrict different quantities over different areas and different averaging times. Broadband incoherent THz sources above 300 GHz fall between them, since ICNIRP guidance for incoherent optical radiation covers the visible and near-infrared and does not reach submillimeter wavelengths. No guideline is written for the THz band as such.

Occupational Safety Considerations

Workers in THz research and industrial applications require appropriate safety measures:

• Beam enclosure: Where possible, THz beams should be enclosed to prevent exposure. The enclosure materials that contain a visible or infrared laser cannot be assumed to contain a THz beam: opaque plastic panels are largely transparent at these wavelengths, so an enclosure must be metallic or metallized, and its joints and viewports treated as apertures.
• Power monitoring: Real-time power monitoring can alert operators to beam escape or elevated exposure conditions.
• Training: Personnel working with THz systems should understand the unique propagation characteristics that make THz beams behave differently from visible light.
• PPE limitations: Standard laser safety eyewear is designed for optical wavelengths and provides no attenuation at THz frequencies, and ordinary spectacle lenses and most plastics are effectively transparent. A thin conductive coating does attenuate THz, since the skin depth in metal is only tens of nanometers, but a conductive mesh must have apertures far smaller than the wavelength to work, and at a few hundred micrometers that is a demanding requirement for anything worn over the eyes. THz-specific protective equipment is not yet commercially established.

Medical and Security Screening Considerations

Several THz applications involve intentional human exposure:

• Security screening: The full-body scanners deployed at airports are millimeter-wave systems rather than true terahertz ones, despite being loosely called terahertz scanners in popular accounts, and they radiate at levels far below any thermal effect threshold. Systems working higher in the THz band remain largely developmental. In both cases the practical obstacle is public perception rather than dose, and it is met by clear communication about what the system emits.
• Medical imaging: THz imaging for cancer margin assessment and wound and burn assessment trades diagnostic value against exposure, and the shallow penetration confines it to the skin and to exposed surfaces during surgery. Its diagnostic contrast comes largely from differences in tissue water content, which is also what limits the depth.
• Regulatory approval: Medical and security devices require regulatory approval that considers both efficacy and safety. The developing state of THz safety standards creates uncertainty in the approval process.

6G and Beyond Wireless

Future wireless systems are expected to use frequencies above 100 GHz:

• Available bandwidth: The THz spectrum offers tens of gigahertz of contiguous bandwidth, enabling data rates that cannot be reached at lower frequencies at any practical spectral efficiency. IEEE 802.15.3d, published in 2017, was the first standard to define a wireless physical layer in this region, covering 252 to 321 GHz with channel bandwidths built from 2.16 GHz units up to about 69 GHz, and targeting 100 gigabits per second.
• Atmospheric windows: Communication links will use the atmospheric transmission windows, principally those near 0.35, 0.45, and 0.65 THz, together with the region from roughly 0.25 to 0.32 THz at the low edge of the band that the WRC-19 identifications and IEEE 802.15.3d occupy. Absorption there is manageable over short ranges, and it grows steeply in the higher windows.
• Link distances: Atmospheric absorption and free-space path loss limit outdoor THz links to tens of meters in typical conditions, and to less than that in humid air. Indoor links escape the weather but not the path loss, and demonstrated indoor ranges are measured in meters to tens of meters rather than in hundreds. Extending the range means raising antenna gain, which narrows the beam and makes alignment the limiting practical problem.
• Beam steering: High-gain antennas are required to overcome path loss, but these narrow beams require precise pointing. Beam-steering systems must maintain alignment while avoiding interference with other links.

Data Center and Backhaul

High-capacity fixed links for data center interconnects and wireless backhaul are early THz applications:

• Point-to-point links: Fixed THz links between buildings or across data center floors can provide fiber-like data rates without the installation complexity of fiber.
• Interference isolation: The high directivity and atmospheric absorption that limit THz range also provide natural isolation between links, potentially allowing frequency reuse at short distances.
• Weather effects: Rain, fog, and humidity significantly affect THz link reliability. Fade margins and backup paths must be designed into critical infrastructure.

EMC for THz Communications

THz communication systems face unique EMC challenges:

• Transmitter emissions: Achieving spectrally pure THz output is challenging. Spurious emissions from multipliers, oscillators, and modulators can interfere with nearby THz receivers.
• Receiver susceptibility: THz receivers are often cryogenically cooled for sensitivity, making them vulnerable to interference. Front-end filtering at THz frequencies is difficult due to component losses.
• Multipath: Reflections from surfaces and objects create multipath that can cause interference in directional links. Unlike at microwave frequencies, surface roughness effects are significant.
• Coexistence: The identifications made above 275 GHz are deliberately light on conditions, on the reasoning that highly directional short-range links interfere little with one another. That reasoning holds while deployments are sparse. As THz systems proliferate, sharing and coexistence rules will have to be worked out, and the passive services that already observe this spectrum will be the first constraint on them.

Active Imaging Systems

Active THz imaging illuminates the target with THz radiation and detects reflections or transmission:

• Continuous-wave systems: CW sources provide high power for strong signals but measure only amplitude unless interferometric detection is used. EMC considerations include source spectral purity and detector susceptibility to ambient THz.
• Pulsed systems: Time-domain imaging uses short THz pulses, providing depth information and spectroscopic data. The broadband nature raises EMC concerns about interference to and from narrowband systems.
• Scanning configurations: Most THz imagers mechanically scan the beam or object. Scan mechanisms can generate vibration and electromagnetic noise that affects measurement quality.

Passive Imaging

Passive THz imaging detects naturally emitted or reflected thermal radiation:

• Radiometric sensitivity: Passive imagers require very sensitive receivers to detect the weak thermal emission. These receivers are susceptible to interference from any THz sources in the environment.
• Background radiation: The thermal background from the environment, enclosure, and optical components contributes noise. Careful thermal design minimizes this background.
• EMC for standoff imaging: Security screening systems that image from a distance must cope with reflections and emissions from the environment, requiring sophisticated background subtraction or modulated operation.

Resolution and Image Quality

THz imaging resolution and quality depend on wavelength and system design:

• Diffraction limit: The fundamental resolution limit is approximately half the wavelength, ranging from about 150 micrometers at 1 THz to 1.5 mm at 0.1 THz. This is much coarser than optical imaging but finer than microwave.
• Near-field enhancement: Various near-field techniques can achieve sub-wavelength resolution by probing the evanescent fields close to the surface, enabling resolution of tens of micrometers.
• Image artifacts: Interference from standing waves, reflections, and multipath creates artifacts that can be mistaken for target features. Understanding and minimizing these EMC-related artifacts is essential for reliable imaging.

Current Spectrum Allocations

International spectrum allocations in the THz range are limited:

• Radio astronomy protection: The Radio Regulations list bands above 275 GHz that administrations are urged to protect for radio astronomy observations of molecular lines, beginning with 275 to 323 GHz, alongside further bands identified for passive Earth exploration and space research. These are urgings rather than exclusive allocations, but they shape what active services may do in the same spectrum.
• Passive sensing services: Earth exploration satellites and meteorological services use THz frequencies for passive sensing of atmospheric constituents. These receive interference protection.
• Active services: The 2019 World Radiocommunication Conference opened the region above 275 GHz to active use. Radio Regulations No. 5.564A now identifies 275 to 296, 306 to 313, 318 to 333, and 356 to 450 GHz, 137 GHz in total, for land mobile and fixed service applications, with no conditions attached to protect passive services. The intervening bands, 296 to 306, 313 to 318, and 333 to 356 GHz, may be used by those services only where conditions protecting Earth exploration-satellite passive applications have been determined under Resolution 731. These are identifications rather than exclusive allocations, and nothing above 450 GHz has been identified for active use.

Emerging Regulations

Regulatory bodies are beginning to develop THz-specific rules:

• FCC: Under its 2019 Spectrum Horizons order, the US FCC designated 21.2 GHz of spectrum above 95 GHz (in the 116 to 123, 174.8 to 182, 185 to 190, and 244 to 246 GHz bands) for unlicensed use under Part 15-style rules, with power limits and a non-interference obligation toward passive services. The same order created a new experimental-license category spanning 95 GHz to 3 THz, with terms up to ten years, to support higher-power THz research and market trials.
• ETSI: European standards bodies are developing standards for THz short-range devices, including emissions limits and measurement methods.
• ITU: Beyond the identifications made at WRC-19, the ITU-R study groups continue to work on sharing and compatibility above 275 GHz, including the conditions under which active services may enter the bands that Resolution 731 still constrains, and on the propagation models that any such sharing study depends on.

EMC Testing Standards

EMC testing at THz frequencies lacks the mature standards infrastructure of lower frequencies:

• Test facilities: Anechoic chambers for EMC testing typically are not characterized above a few tens of gigahertz. THz EMC testing requires quasi-optical test setups or waveguide-based measurements.
• Measurement equipment: Standard EMC receivers and spectrum analyzers do not cover THz frequencies. Specialized THz instrumentation is required, with calibration traceability still developing.
• Limits and requirements: There are currently no established emissions or immunity limits specific to THz frequencies. Applying traditional EMC concepts at THz requires adaptation for the different propagation and coupling characteristics.

Safety Standards

Human exposure standards for THz radiation are being developed:

• ICNIRP: The International Commission on Non-Ionizing Radiation Protection issued its current radiofrequency guidelines in 2020, covering 100 kHz to 300 GHz on a thermal basis, and its laser guidelines in 2013, covering wavelengths from 180 nm to 1,000 micrometers and so reaching down to 300 GHz. The two meet at the bottom of the THz band rather than leaving a gap, but neither is written for the THz band as such, and the quantities they restrict are not the same.
• IEEE/ICES: The IEEE exposure standard C95.1-2019 covers 0 Hz to 300 GHz, expressing limits above 6 GHz as epithelial power density to reflect the shift to surface heating. Extending coordinated limits above 300 GHz, into the THz band proper, remains a subject of active study.
• National standards: Various countries are developing national exposure standards that may differ in limits and measurement procedures.

Personnel Screening

THz imaging can detect concealed objects without ionizing radiation:

• Clothing penetration: THz radiation passes through most clothing materials, enabling detection of concealed objects. However, some materials (wet fabric, metallic threads) can block or attenuate THz.
• Body scanners: Active and passive THz scanners can image the body surface through clothing. Resolution is sufficient to detect objects of centimeter scale but not to identify fine details.
• Privacy considerations: The ability to image through clothing raises privacy concerns that affect system design and deployment. Image processing to represent detected threats rather than body details addresses some concerns.

Package and Mail Inspection

THz imaging can inspect packages without opening them:

• Penetration capabilities: Cardboard, paper, plastic, and fabric packaging is generally THz-transparent. Metal and water-containing materials block THz, limiting inspection of some package types.
• Spectroscopic identification: The THz spectra of explosives, drugs, and other contraband can potentially enable identification of materials within packages. However, packaging materials and mixtures complicate spectral analysis.
• Throughput limitations: Current THz imaging systems are relatively slow compared to X-ray scanners, limiting application to high-value or high-suspicion items rather than bulk screening.

Standoff Detection

Detection of threats at a distance is a key security application:

• Range limitations: Atmospheric absorption limits standoff detection range. In humid conditions, reliable detection beyond a few meters is challenging. Dry environments allow detection at tens of meters.
• Material identification: Spectroscopic identification at standoff ranges requires high-power sources and sensitive receivers. Current technology enables detection but not always confident identification at practical ranges.
• Countermeasures: Knowledge of THz detection capabilities enables development of countermeasures. This drives an ongoing cycle of detection improvement and countermeasure development.