Electronics Guide

Electronic THz Sources

Electronic approaches extend microwave techniques to higher frequencies:

Frequency multipliers: Starting from microwave sources (typically 10-30 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 (up to about 300 GHz), these oscillator technologies provide moderate power (milliwatts to watts) but with significant phase noise that can cause interference in nearby receivers.

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, with attenuation of tens of dB per kilometer in dry conditions. Between these windows, attenuation can exceed 1000 dB/km.

Humidity dependence: Water vapor is the primary absorber. Attenuation varies dramatically with humidity, from about 10 dB/km at low humidity to over 100 dB/km at high humidity for frequencies around 0.5 THz.

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: Fundamental mode cutoff for WR-1.0 waveguide (used around 1 THz) is about 0.5 THz. Manufacturing tolerances of a few micrometers affect performance. Insertion loss is typically 10-20 dB/m due to surface roughness and finite conductivity.

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 beam waist and divergence governed by wavelength. A 1 THz Gaussian beam focused to a 1 mm waist diverges to about 20 mm diameter at 100 mm distance.

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, meaning significant attenuation over submillimeter paths.

Polymers: Many plastics are relatively transparent to THz radiation. HDPE, TPX, and Teflon have absorption coefficients below 1 cm^-1, 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: The Hagen-Rubens relation for surface resistance gives values around 0.1 ohms per square for copper at 1 THz, significantly higher than at microwave frequencies.

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 typically have dynamic range of 40-60 dB, less than microwave spectrum analyzers. This limits the ability to measure weak spurious signals in the presence of intended emissions.

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. Most THz energy is absorbed in the first few hundred micrometers of tissue.

Thermal effects: Absorbed THz energy heats tissue. The thermal relaxation time of skin is on the order of milliseconds, so continuous exposure at intensities above about 10 mW/cm^2 can cause measurable temperature rise.

Non-thermal effects: Research continues into possible non-thermal effects of THz radiation on DNA, proteins, and cell membranes. Current consensus is that thermal effects dominate at practical exposure levels, but research continues.

Eye and Skin Exposure

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

Corneal exposure: The cornea is the most sensitive tissue due to its avascularity (limiting heat dissipation) and high water content. THz radiation does not penetrate to the retina, so concerns focus on corneal damage thresholds.

Skin exposure: THz absorption occurs primarily in the epidermis. Pain receptors are typically deeper than the absorption depth, so thermal damage could potentially occur before pain is felt at high intensities.

Exposure limits: ICNIRP guidelines for infrared (which extend into the THz range) suggest maximum permissible exposure on the order of 100 mW/cm^2 for brief exposures, decreasing for longer durations. Specific THz guidelines are still under development.

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. Unlike optical systems, the infrared-absorbing enclosures used for visible lasers may be transparent to THz.

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 not designed for THz frequencies and may provide no protection. THz-specific protective equipment is not yet commercially established.

Medical and Security Screening Considerations

Several THz applications involve intentional human exposure:

Security screening: Full-body THz scanners for airport security expose individuals to low-level THz radiation. Exposure levels are typically far below thermal effect thresholds, but public perception requires clear communication of safety.

Medical imaging: THz imaging for cancer detection and wound assessment requires balancing diagnostic capability against exposure. The low penetration depth limits THz to surface and near-surface imaging.

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 achieved at lower frequencies regardless of spectral efficiency.

Atmospheric windows: Communication links will likely use the atmospheric transmission windows around 0.3, 0.45, and 0.65 THz, where atmospheric absorption is manageable for short-range links.

Link distances: Atmospheric absorption and free-space path loss limit outdoor THz links to tens of meters in typical conditions. Indoor applications may achieve hundreds of meters in controlled environments.

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: As THz systems proliferate, spectrum sharing and coexistence rules will need to be developed. The current regulatory vacuum will eventually require resolution.

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: Several frequency ranges are protected for radio astronomy observations of molecular lines. These include bands around 275-323 GHz and various specific frequencies throughout the THz range.

Passive sensing services: Earth exploration satellites and meteorological services use THz frequencies for passive sensing of atmospheric constituents. These receive interference protection.

Active services: There are currently few allocated bands for active THz services. The World Radiocommunication Conferences are beginning to address THz spectrum allocation for future services.

Emerging Regulations

Regulatory bodies are beginning to develop THz-specific rules:

FCC: The US FCC has opened spectrum above 95 GHz for unlicensed use under Part 15 rules, with power limits to prevent interference. Experimental licenses are available for higher-power THz research.

ETSI: European standards bodies are developing standards for THz short-range devices, including emissions limits and measurement methods.

ITU: The International Telecommunication Union is studying THz spectrum needs for future services and developing recommendations for sharing between services.

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 has published guidelines that extend into the lower THz range, based primarily on thermal effects.

IEEE/ICES: IEEE exposure standards (C95.1) cover frequencies up to 300 GHz, with work underway to extend to higher frequencies.

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.