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Imaging via terahertz electromagnetic waves is a fascinating and rapidly emerging field that blends physics and engineering for practical applications in areas like security, medicine and communications. F or many years, progress in this area was limited by the difficulty of efficiently generating and detecting terahertz radiation, but recent advances have dramatically changed that situation. Terahertz waves occupy the ‘terahertz gap’ between mid-infrared and microwave radiation, sitting at the boundary between photonics (the realm of light and photons) and electronics (the realm of electrons and currents). A one-terahertz signal has a frequency of 1000GHz or 1012 hertz, corresponding to a wavelength of 0.3mm (see Fig.1). The exact frequency range considered as the terahertz band is somewhat flexible and depends on the context. Most researchers and applications define it as spanning roughly 0.1THz to 10THz (100GHz to 10,000GHz), although some extend it down to 0.03THz or up to 30THz depending on the field (eg, astronomy, communications or spectroscopy). For 6G research and future spectrum studies, ITU-R Recommendation SM.2352-1 extends the definition to the range of 100GHz to 10THz (100GHz to 10,000GHz; wavelengths of 3mm to 30µm). The ITU has also identified approximately 137GHz of spectrum space for potential terahertz communications, primarily in the range between 275GHz and 450GHz, allocated for land mobile and fixed service applications. The sub-terahertz range, 100300GHz, is also considered critical for future 6G development due to lower atmospheric absorption and better feasibility with current technology compared to the higher THz frequencies. Frequencies above 3THz are not considered radio waves. ITU Band 13, 3-30THz, is considered part of the optical or infrared spectrum and is free of international radio regulations. According to ISO 20473, frequencies of 0.3-6THz are also considered part of the far-infrared band, while 6-100THz is mid-infrared, 100THz385THz near-infrared and 380-780THz is visible light. Still, the exact boundaries vary according to the field of study. Terahertz radiation has historically been difficult to access until recent times, primarily due to the difficulty of efficiently generating and sensing it. This range is very high compared to regular radio frequencies but very low compared to normal optical frequencies. It is therefore one of the least explored and exploited parts of the electromagnetic spectrum. Terahertz radiation has actual or potential applications in security scanners, medical imaging, 6G telecommunications prototypes and industrial quality control. Early production of terahertz radiation Fig.1: the terahertz band and terahertz gap between microwaves and IR light. 3THz is generally considered the upper limit of radio waves. Although not true terahertz radiation, Sir Jagadish Chandra Bose of India was the first to generate and study extremely high frequency (EHF) waves, approaching the terahertz range. In the 1890s, he produced microwaves with frequencies as high as 60GHz (wavelength ~5mm) using a specialised spark-gap oscillator he designed and built himself. This innovative system generated much higher and more focused frequencies than those achieved by his contemporaries, including Hertz and Lodge. Bose demonstrated 60GHz waves during 1895-1897 – see Fig.2. His oscillator operated by applying a high voltage from an induction coil to a spark gap. A 3mm metal ball (sphere) was mounted on a non-conductive post between two smaller spark balls. Sparks jumped across the gaps, exciting standing waves within the central sphere at a resonant frequency determined by its size and placement. The entire setup was enclosed in a shielded box to minimise interference, with the waves directed outward through a short metal tube, an early form of waveguide. As Bose’s apparatus used a resonant structure to generate the radiation, it Australia's electronics magazine siliconchip.com.au Terahertz waves The International Telecommunication Union (ITU) designates the terahertz range as Band 12 (Tremendously High Frequency or THF), spanning 0.3THz to 3THz (300GHz to 3000GHz; wavelengths of 1mm to 0.1mm). 16 Silicon Chip was narrowband, producing frequencies tightly centred around the cavity’s resonant frequency. Bose’s meticulous engineering extended to other components, including horn antennas, dielectric lenses (made of sulfur or glass) to focus the beam like an optical lens, polarisers and crystal detectors. These enabled pioneering quasi-optical experiments on reflection, refraction and polarisation of radio waves in his small Kolkata (Calcutta) laboratory. How did he measure the frequency? Frequency counters did not yet exist. Bose determined the frequency he was generating using a reflecting diffraction grating made of evenly spaced metal strips (or wires) mounted on a frame. By directing his generated waves at the grating and observing the angles of the reflected diffraction maxima (bright spots), he could apply the standard diffraction grating equation to determine the wavelength, from which he could calculate the frequency. The frequency could also be determined from the size of the resonant oscillator sphere, as well as from interference patterns generated by standing waves. The methods Bose used, which would today be described as microwave optics, were forgotten and not rediscovered until the 1950s. In 1977, Nobel laureate Sir Nevill Mott remarked that Bose was “at least 60 years ahead of his time”. Following Bose’s work on 60GHz waves, the field stagnated for decades due to technological limitations. Bose’s record high of 60GHz was held for nearly 60 years. Discussion of terahertz radiation requires recognition of two types of electromagnetic radiation. In incoherent radiation, all parts of the wave are random in phase and spatially. Coherent radiation is when all parts of the wave are synchronised in phase and space, like a laser beam. The radiation Bose produced was coherent. Thermal (incoherent) blackbody terahertz emissions were detected in the 1960s, eg, using bolometers for astronomy. These absorb radiation, causing a temperature rise, which changes the resistance of a sensing element. Still, THz waves were not generated then. Bose’s 60GHz coherent radiation was not significantly surpassed until electronic sources like backward-wave oscillators (BWOs, we will discuss siliconchip.com.au Fig.2: Sir Jagadish Bose’s spark gap oscillator, waveguide and galena point contact crystal rectifier. Bose patented the galena rectifier in 1901. Source: https://w.wiki/7DAY later) were invented in 1951 by Rudolf Kompfner at Bell Labs in the United States (the O-type BWO). The Soviets quickly advanced this technology, with early BWOs (also known as carcinotrons) reaching millimetre waves (up to about 300-500GHz) in the 1950s and 1960s for military and scientific applications. In the 1970s, Soviet submillimeter BWOs extended to about 0.5-1.0THz, supporting spectroscopy and plasma diagnostics. The first phase-locked loop submillimeter BWO was developed in the USSR around 1970. By the late 1970s and 1980s, the highest reported Soviet BWO frequencies approached 1-2THz in laboratory setups, with papers from the Institute of General Physics (USSR Academy of Sciences) describing BWO-based spectrometers covering 0.03-1.5THz (30-1500GHz) by the mid-1980s. Post1990s, Russian BWOs (eg, from Istok) continued to offer reliable commercial sources up to ~1.25THz, but the 1-2THz experimental frontier was largely achieved during the Soviet era. The Soviets were also interested in military applications of THz waves but there are no reliable reports that any eventuated, primarily due to the low powers and high atmospheric absorption of THz waves. Broadband and narrowband THz sources Soviet BWOs produced narrowband, tuneable continuous-wave THz Australia's electronics magazine radiation. This was excellent for spectroscopy in the frequency domain at specific frequencies (see the panel overleaf) but provided limited spectral coverage. For time-domain spectroscopy, imaging and sensing, what was needed was broadband THz generation that could produce a spectrum of frequencies in a single pulse with no single dominant frequency. A major leap into coherent, broadband THz waves came in the mid1980s to early 1990s with laser-based methods. David H. Auston and collaborators at Bell Labs/AT&T (and later at Columbia University) demonstrated the first practical, coherent pulsed THz radiation in 1984-1988, using photoconductive antennas (see later) excited by femtosecond (10-15s) laser pulses. This produced broadband THz pulses (typically 0.1-3THz or higher) with picosecond durations, marking the breakthrough that opened the modern THz era. The pulsed nature of the radiation produced by Auston enabled time-­ domain spectroscopy (THz-TDS), measuring both the amplitude and phase of the THz field in the time domain, which provides rich information about material properties such as refractive index, absorption and complex dielectric function in a single measurement. This technique has applications in materials science, imaging and non-destructive testing. August 2026  17 Apart from generating THz radiation, Auston’s photoconductive antenna could also detect the THz pulse coherently (time-gated sampling), allowing full reconstruction of the waveform, a key enabler for modern THz systems. Auston’s innovation was practical because it used relatively compact femtosecond lasers (then emerging) rather than the large vacuum tubes, magnetic fields and high voltages required by BWOs, making THz waves more accessible to labs worldwide. Today, broadband THz sources (modern photoconductive antennas, optical rectification, or air plasma generation) produce a continuous spectrum of frequencies in a single pulse (over 0.1-10THz or more), with no single dominant frequency. Auston opened the door to coherent broadband pulsed THz, which became the foundation for the THz time-­ domain spectroscopy, imaging and sensing that dominates the field today. Terahertz analysis in the time and frequency domains Terahertz radiation interacts with molecules in materials by exciting lowenergy resonances of large molecular structures, forming the basis of THz spectroscopy. Alternatively, THz waves can pass directly through a material for imaging. Spectroscopy and imaging can be performed in either the time or frequency domain. In time-domain spectroscopy, extremely short broadband pulses of THz radiation are used to record a waveform (electric field intensity as a function of time) that contains all frequencies simultaneously. Applying a Fourier transform to this time-domain data yields the frequency-domain spectrum (see p14 of the January 2026 article for more on Fourier transforms). Time-domain spectroscopy offers moderate spectral resolution and is commonly used for imaging, material characterisation and thickness measurements in quality control. In frequency-domain spectroscopy, a continuous-wave (CW) THz source is tuned or swept across frequencies, producing very high-resolution spectral data. This approach is very useful for gas sensing, chemical analysis and precise identification of ‘molecular fingerprints’. There are several reasons why terahertz radiation has been difficult to generate and therefore was a virtual scientific and technological no-go zone. However, these difficulties are being overcome. Conventional electronic devices like transistors have difficulty operating at terahertz frequencies because electron transit times (drift and diffusion) through the semiconductor become too long. Capacitive effects and gate resistance dominate, limiting the oscillation frequency. Some specialised transistors such as InP (indium phosphide), HBTs (heterojunction bipolar transistors) and HEMTs (high electron mobility transistors) can reach or exceed 1THz, but often with low power due to thermal limits and low efficiency. Typically, they can only handle less than 10mW, although over 100mW has been demonstrated with an InP HEMT power module at 670GHz. Diodes have difficulties at such frequencies due to the RC time constant of the device, the skin effect, ohmic losses at high frequencies, thermal management, uneven field distribution and other effects. Optical methods such as lasers have difficulties in the same range due to material absorption and phase-­ matching within non-linear crystals. A non-linear crystal is a crystal that, when hit with a higher-frequency optical laser, generates lower-frequency terahertz waves as the laser travels through the crystal. As the laser beam travels through the crystal, terahertz waves are further generated at every point along the laser’s path. For these individual sources to add up constructively, to generate maximum power, the peaks and troughs of the waves must be matched through phase-matching by appropriate adjustment of the crystal Fig.3: a photoconductive antenna. “j” is the photocurrent and “V” the bias voltage. Source: Paul C. Gow Fig.4: generating THz radiation via optical rectification in a non-linear crystal (ω is frequency). Original source: Sonal Saxena Generating terahertz radiation 18 Silicon Chip Australia's electronics magazine properties, such as through orientation of the crystal or laser pulse. Examples of such crystals include complicated organic crystals like DAST Z (4 -dimethy lami no- N-­ methyl-4-stillbazolium tosylate), OH1 (2-(3-(4-hydroxystyryl)-5,5-­ dimethylcyclohex-2-enylidene) malononitrile) and inorganic crystals like GaAs (gallium arsenide) and ZnTe (zinc telluride). There are no known natural materials that resonate at these frequencies. For example, microwaves (ITU Band 9, 300MHz to 300GHz) interact with large-scale rotational modes of molecules in materials like water, enabling the cooking of food. Microwave generation is also simple, as electrons in a magnetic field gyrate at microwave frequencies (cavity magnetron). Infrared radiation (ITU Band 13 and 14, 3-300THz) interacts with the vibrational modes of molecular bonds, making infrared a powerful tool for spectroscopy. Terahertz waves sit in an awkward middle ground: too high for efficient electronic oscillators like transistors or siliconchip.com.au Table 1 – key characteristics of terahertz generation methods Method Type Main advantage Main limitation Photoconductive antenna Pulsed (broadband) High signal-to-noise ratio (SNR), table-top Low average power Optical rectification Pulsed (broadband) High peak power Requires complex laser Spintronic emitters Pulsed (broadband) Extremely high bandwidth, inexpensive Emerging technology Air plasma generation Pulsed (broadband) Ultra-broadband and high intensity Low efficiency, complex setup, needs expensive laser Photomixing CW (narrowband) Tuneable, high-precision Low power output Resonant tunneling diodes CW (narrowband) Compact, room temperature Low power, low frequency IMPATT diode Pulsed or CW (narrowband) Up to 10THz; high power up to 100-200GHz High noise, low efficiency, thermal problems, limited tuneability Quantum cascade lasers CW (narrowband) High power, compact Cryogenic cooling Gyratron Pulsed or CW (narrowband) High power and coherent Large, heavy, needs superconducting magnets Backward wave oscillator CW (narrowband but tuneable) Tuneable, high power, low noise Low efficiency, complex fabrication, high voltage Frequency multiplier – schottky diode CW (narrow to moderate bandwidth) Room temperature, reliable, compact, solid state, up to 2THz, suitable for space and handheld Low power at high frequencies, limited power handling Free-electron laser (FEL) Pulsed (broadband), coherent Very high power Large and expensive Synchrotron Pulsed (broadband), less coherent than FEL High power Large and expensive magnetrons, too low for most molecular vibrations, and without natural material-specific resonances to exploit. We will now look at the production of THz radiation by optoelectronics, electronics, frequency multipliers and other methods. Table 1 summarises the key characteristics of the various methods. Optoelectronic methods – laser-based Photoconductive antennas (Fig.3), also known as “Auston switches” after the person who invented them, generate broadband terahertz waves by directing femtosecond laser pulses onto the gap of a biased semiconductor. The gap is the narrow space (typically a few µm wide) between the two metal electrodes on the semiconductor surface where the bias voltage creates a strong electric field. The intense light pulse creates electron-­hole pairs (photogeneration). Under the influence of an applied DC bias voltage, these charge carriers accelerate, producing a rapid transient photocurrent. This short-lived current siliconchip.com.au acts as a Hertzian dipole radiator, a small oscillating dipole that efficiently radiates electromagnetic waves, here in the terahertz range. It emits broadband THz radiation (typically 0.1-3THz, or higher) in a single pulse. The current then decays rapidly as the carriers recombine, ending the emission for that laser pulse. Optical rectification is one of the most common methods for generating broadband terahertz waves. An intense femtosecond laser pulse is passed through a non-linear crystal (such as ZnTe, GaP, LiNbO3, or organic crystals like DAST, OH1 or BNA [N-benzyl-­2methyl-4-nitroaniline]). A non-linear crystal emits light of a different frequency or polarisation from the input. The strong electric field of the pulse induces a non-linear polarisation in the crystal that follows the pulse’s intensity envelope. Because the broadband femtosecond pulse contains a broad range of optical frequencies, different frequency components combine to form new frequencies (beat frequencies) within its spectrum, generating new coherent oscillations at terahertz frequencies, typically Australia's electronics magazine in the range of 0.1-5THz (up to 10THz or more with optimised crystals). These oscillations radiate broadband THz pulses directly from the crystal (see Fig.4). Most laboratory-­ based terahertz time-domain spectroscopy (THz-TDS) systems rely on this technique due to its simplicity, coherence and broad spectral coverage. A spintronic terahertz emitter converts ultra-short flashes of laser light into terahertz waves by harnessing the spin (a magnetic property) of electrons. It works in three main steps (see Figs.5 & 6): 1. A femtosecond laser pulse strikes a thin bilayer structure consisting of a ferromagnetic metal (eg, iron, cobalt, or nickel) and a heavy metal with strong ‘spin-orbit coupling’ (eg, platinum). Spin-orbit coupling is a magnetic interaction where an electron’s internal spin is locked to its physical movement, causing it to turn sideways when it flows through certain metals. The laser excites electrons in the ferromagnetic layer to higher energy levels. 2. In the magnetic layer, electrons with one spin direction are more August 2026  19 Fig.5: the process of THz emission from a spintronic bilayer after a laser pulse. js – spin current in the z direction; jc – charge current in the y direction; M – magnetic field; FM – ferromagnetic; NM – non-magnetic. Source: www. degruyterbrill.com/document/doi/10.1515/nanoph-2020-0563/html Fig.6 a commercial spintronics Terahertz emitter from TeraSpinTec. Source: https://teraspintec.com/en/ products mobile and preferentially diffuse into the heavy-metal layer. This movement creates a flow of spin (a ‘spin current’) that carries over into the heavy metal layer. 3. In the heavy metal, the inverse spin Hall effect makes the moving electrons suddenly deflect sideways, turning the spin current into a regular electric current. Any fast-moving electric charge produces electromagnetic radiation, so this sudden sideways motion emits broadband terahertz waves. The emitted radiation is broadband (typically 0.1-30THz) and its polarisation can be easily controlled by rotating an external magnet. These emitters are simple to fabricate, using only common metals in layers a few nanometres thick, making them inexpensive and scalable for practical applications. Air plasma generation of terahertz waves works by focusing powerful femtosecond laser pulses into (targeting data rates of 100Gbps or higher), high-resolution spectroscopy and terahertz imaging systems. air, ionising the gas to form a plasma filament. The plasma’s free electrons oscillate under the laser’s electric field and radiate broadband terahertz waves as they accelerate, typically spanning 0.1-10THz or more. A continuous-wave (CW) terahertz source can be produced by photomixing (or optical heterodyne mixing). Two single-mode lasers operating at slightly different frequencies are combined to create a difference frequency (beat frequency) in the terahertz range. These two laser beams are directed onto a high-speed semiconductor (typically a photoconductive material like InGaAs or GaAs), where the beat frequency induces a modulated photocurrent oscillating at the THz difference frequency – see Fig.7. An integrated antenna on the semiconductor radiates this oscillating current as a continuous terahertz wave. The main applications of this method include proposed 6G communications Electronic methods – oscillator-based A resonant tunnelling diode (RTD) is a specialised high-speed semiconductor device that uses quantum mechanics to generate and detect terahertz radiation. Technically, it is a ‘double-barrier quantum well’ (DBQW). Imagine a microscopic structure with two thin insulating ‘walls’ (barriers) and a narrow ‘well’ of semiconductor between them. In classical physics, electrons could not cross the barriers. However, thanks to quantum mechanics we know that if an electron’s energy matches one of the specific energy levels in the well, it can tunnel through the barriers with a high probability. This is called resonant tunnelling. Fig.9: the structure of an RTD transmitter chip. Original source: www.mdpi.com/2076-3417/12/8/3822 20 Silicon Chip Australia's electronics magazine siliconchip.com.au When a voltage is applied across the device, the current initially increases as the electron energy aligns with the well’s energy level. As the voltage increases further, the alignment is lost, and the current drops sharply, creating a region of negative differential resistance (NDR), where more voltage causes less current. This negative resistance allows the device to sustain oscillations when connected to a resonant circuit (such as an antenna or cavity), producing THz waves. RTD THz emission relies on the NDR region of the I-V curve, where the device provides gain to sustain oscillations in a resonant circuit – see Fig.8. While weak or induced radiation can occur elsewhere on the curve, practical, high-performance THz generation and oscillation are effectively limited to the NDR region. Tunnelling occurs predominantly at the discrete energy levels E1 and E2. The current peaks are sharp and localised because resonance only happens when there is good energy matching with one of those discrete states. State-of-the-art RTDs can achieve frequencies up to about 2THz in the milliwatt range. The structure of a practical device is shown in Fig.9. An IMPATT diode (IMPact ionisation Avalanche Transit-Time diode) generates terahertz radiation by exploiting two key phenomena (see Figs.10 & 11): a. High-frequency avalanche breakdown (also called impact ionisation), where energetic charge carriers collide with atoms in the semiconductor lattice, creating additional electron-hole pairs in a rapid cascade. b. Transit-time phase delay, where the time it takes these carriers to travel across the diode’s active region is carefully tuned to be approximately half a cycle of the oscillation frequency. Fig.10: the structure of IMPATT diode. P+ is heavily doped P-type material, N is N-type material, I is intrinsic undoped material, and N+ is heavily doped N-type material. siliconchip.com.au Fig.7: the process of generating terahertz waves using photomixing; PCS stands for photoconductive switch. Original source: https://w. wiki/HyHN Fig.8: the structure of a typical RTD. The energy levels during which tunnelling can occur are around E1 and E2. THz radiation is emitted in the blue region of the plot, while the device can act as a receiver in both the blue and tan regions. Original source: www.mdpi.com/2076-3417/12/8/3822 Together, these effects produce negative differential resistance, a region where an increase in voltage causes a decrease in current, which sustains oscillations when the diode is placed in a resonant cavity (a tuned electromagnetic structure that reinforces the desired frequency, similar to how a guitar string resonates at a specific pitch). A quantum cascade laser (QCL) is a semiconductor laser designed to generate light in the terahertz range, typically 1-5THz, through a cascading process of electrons in an engineered multi-layer quantum structure. In a conventional semiconductor laser, an electron transitioning from a higher to a lower energy level emits a single photon. In a QCL, the active region consists of many repeating stages (often 30-100 or more). An electron emits one photon per stage as it drops to a lower energy level, then tunnels through a thin barrier into the next stage to repeat the process – see Fig.12. Thus, a single electron can generate 30-100 or more photons as it cascades through the entire structure. Most THz QCLs require cryogenic cooling (liquid nitrogen temperatures) to operate efficiently, although recent advances have achieved room-­temperature performance at lower frequencies. These devices produce narrowband, highly monochromatic terahertz radiation with excellent spectral purity, making them ideal for spectroscopy, sensing and high-resolution applications. Work on QCLs is being undertaken in Australia at the Uni of Qld. Fig.11: the assembly of an IMPATT diode into a cavity resonator and then into a power supply/heatsink module. Source: https://terasense.com/ terahertz-technology/impatt-diodes Australia's electronics magazine August 2026  21 Fig.14: the atmospheric absorption of terahertz waves as a spectrum. In this case, the ITU extended definition of Band 12 is used: 100GHz to 10THz. Source: www.researchgate.net/figure/fig3_337266839 A gyrotron (see Fig.15) is a highpower vacuum tube device invented in 1964 in the USSR that generates THz radiation by the ‘cyclotron resonance’ of electrons in a strong magnetic field. Output frequencies range up to about 500GHz, with output powers from tens of kilowatts to 1-2MW. Electrons are emitted from an electron gun and accelerated by a high-voltage DC anode. They then enter a resonant cavity immersed in a strong axial magnetic field produced by a superconducting magnet. This field forces the electrons of the beam to spiral helically around the magnetic field lines at the cyclotron frequency. As the electrons gyrate, they emit electromagnetic radiation (cyclotron emission) at frequencies determined primarily by the magnetic field strength (the cyclotron frequency), with relativistic effects introducing additional spectral broadening. When the cyclotron frequency matches a resonant mode of the cavity, standing waves build up coherently, amplifying the radiation. The resulting high-power THz beam (often in the 0.1-1.5THz range, with some devices reaching 2-3THz) is emitted from the cavity, then converted by a mode converter (where the pattern of the wave is converted to one that can be more efficiently used) to a suitable output mode, reflected by a mirror and directed through a vacuum window into a waveguide or free space. The electron beam is absorbed by a collector. Backward-wave oscillators (BWOs) are vacuum-tube devices that generate high-frequency, narrowband, tuneable terahertz (or sub-terahertz) radiation, either continuous-wave or pulsed. They work by firing a high-velocity electron beam through a slow-wave structure (such as a helix, folded waveguide or grating), which reduces the phase velocity of the electromagnetic wave (the speed of its crests) to match the electron beam velocity – see Fig.13. The electrons interact with this backward-propagating wave, losing energy to amplify it through bunching and collective effects. Because the wave travels opposite to the electron flow, it provides internal feedback that sustains self-oscillations at terahertz frequencies (typically up to ~1-2THz in advanced designs). The output is inherently coherent, Australia's electronics magazine siliconchip.com.au Fig.12: an energy level diagram showing emission of four photons from one electron. Original source: www.teamwavelength.com/using-a-laserdiode-or-quantum-cascade-laser-dontforget-the-electronics Fig.13: a BWO; K = cathode, G = grid, A1 = first anode, A2 = second anode. Original source: https://w.wiki/HyHQ 22 Silicon Chip stable and well-suited for applications such as spectroscopy, interferometry and as local oscillators. Frequency multipliers Schottky diodes can act as a frequency multiplier to produce THz waves by taking a lower frequency signal and converting it into a higher one. This is because the schottky diode is a strongly non-linear device, meaning it distorts the input signal, creating many higher harmonics at odd multiples of the input signal frequency. The fundamental and other harmonics can be filtered to isolate the desired THz signal. Efficiency is reduced as higher harmonics are used, so chains of diodes are often cascaded to produce the desired frequency. Schottky diodes are preferred because, as majority-­carrier devices, they exhibit negligible reverse recovery charge, along with very low junction capacitance and strong non-linearity at high frequencies. Other methods Free electron lasers (FELs) can produce terahertz radiation using a high-energy electron beam that passes through a series of magnetic ‘undulators’ or ‘wigglers’. These alternating magnetic fields force the relativistic electrons to travel in a sinusoidal (wiggling) path, causing them to emit electromagnetic radiation at a frequency determined mainly by the beam energy and the magnetic period – see Fig.16. The emitted radiation is coherent and can reach very high power levels (watts to kilowatts average, or megawatts peak in pulsed systems). Synchrotrons can also produce terahertz radiation in a manner somewhat similar to free-electron lasers (FELs). A relativistic electron beam passes through an undulator or wiggler, causing it to oscillate and emit radiation at the desired wavelength. However, in a synchrotron, the electrons are accelerated and stored in a circular storage ring rather than a linear accelerator, and the radiation is generated primarily from the bending magnets or edge effects. This results in broadband, incoherent THz emission, unlike the narrowband, coherent output of FELs. For further information on synchrotrons, see our article in the May 2012 issue (siliconchip.au/ Article/671). Atmospheric absorption of terahertz waves Fig.14 shows atmospheric attenuation of electromagnetic waves between 10GHz (microwaves) and 1000THz (UV light) for a range of atmospheric conditions. It includes the absorption peaks of atmospheric species such as water vapour, oxygen and carbon dioxide. Based on atmospheric absorption characteristics, the best bands for communications applications are 275320GHz and 335-360GHz, both with losses under 10dB/km (see Table 3). A comparison of the relative advantages and disadvantages of various communication frequencies from microwaves up is shown in Table 2. Detecting terahertz radiation Detectors need to be sensitive to the low photon energies of terahertz waves in an environment where there is high noise and atmospheric absorption, especially by water vapour. Direct detection Bolometers (Fig.17) are broadband thermal detectors that sense THz radiation by measuring resistance changes caused by heating in an absorbing element. They are often paired with antennas to improve radiation coupling or use thin absorbing membranes in MEMS (micro-electromechanical systems) structures to reduce thermal mass and increase sensitivity. Fig.16: a free electron laser. Original source: https://w.wiki/HyHR electrically conductive pillar membrane thermo resistor material thermal insulation using narrow beams ("arms") electrically conductive pillar metallic reflector film Fig.15: the structure of a gyrotron. Original source: https://w.wiki/ HyHP siliconchip.com.au substrate Fig.17: one pixel of a microbolometer array imaging system (MAIS) for medical applications to detect 1-5THz signals on the body’s surface. Source: https://w.wiki/HyHS Australia's electronics magazine August 2026  23 Cryogenic cooling is essential for the highest-performance bolometers, enabling low-noise operation and detection of very weak signals. Bolometers can be assembled in arrays for medical imaging. Pyroelectric detectors generate a voltage when electromagnetic radiation, such as THz waves, impinges on them, as the absorbed energy causes a temperature change that alters the material’s polarisation – see Fig.18. They are a common choice for THz detection because they operate at room temperature, are broadband and require no cooling. Golay cells are highly sensitive, uncooled detectors that operate on a pneumatic principle and are capable of detecting terahertz radiation. THz radiation enters through a window transparent to these wavelengths and heats a small quantity of gas inside the cell, causing it to expand. The expanding gas increases the pressure, which deforms a flexible membrane – see Fig.19. This membrane movement is then detected and measured optically (eg, via a reflected light beam). Golay cells can operate across a broad frequency range up to about 20THz. They offer very high sensitivity and require no cooling, but they are highly susceptible to vibration and mechanical noise. They are commonly used in spectroscopy, astronomy and precise power measurements. Rectifying detectors such as schottky barrier diodes and special field-effect transistors (TeraFETs) can be used to rectify and convert THz signals to DC voltages for detection and measurement. Other detectors Extrinsic detectors use impurities (dopants) added to a semiconductor to create extra energy levels, allowing the material to absorb THz photons and produce a measurable signal. Josephson junction detectors are based on superconducting Josephson junctions. These extremely sensitive devices detect THz radiation by measuring tiny changes in the junction’s current-voltage characteristics caused by the incoming THz field. They offer very high sensitivity but require cryogenic cooling. Heterodyne detection involves mixing the incoming THz signal with a local oscillator (another THz source) to produce a lower-frequency difference signal that is easier to measure. It provides precise amplitude and phase information, similar to the photomixing generation method described earlier, and is widely used in high-­ resolution spectroscopy. Terahertz applications Some of the applications for terahertz laser imaging include: Astronomy Due to atmospheric absorption of THz waves, observatories need to be on the highest mountains or in balloons, high-altitude aircraft or spacecraft. Terahertz radiation is emitted by astronomical objects above about 1.7K and the frequency depends on the temperature. At frequencies where the most thermal emission occurs, 0.1THz corresponds to a temperature of about ~1.7K (-271.5°C), 1THz to ~17K (-256°C) and 10THz to ~170K (-103°C). This enables the observation of the ‘cold universe’, regions too cool to emit strong infrared but too warm to be dominated by radio emission. Terahertz waves are able to penetrate interstellar dust and gas, allowing the following observations to be made: 1 The interstellar medium: the cold, dense molecular gas and dust clouds Fig.18: an experimental pyroelectric sensor for THz radiation with circuit diagram. Original source: www.mdpi. com/2076-3417/14/10/3967 24 Silicon Chip Australia's electronics magazine within the Milky Way and beyond 1 Star and planet formation, mapping the dense cores of gas and dust clouds and the detection of protoplanetary disks 1 Astrochemistry, to identify complex organic molecules for understanding the possible origins of life 1 High-redshift galaxies; those moving away from us fast enough that their infrared radiation is shifted down into the terahertz range 1 Direct detection of exotic ions such as helium hydride from the early universe 1 Black hole environments, such as the event horizon, using the Event Horizon Telescope (EHT) (86345GHz), a global network of linked telescopes, including imaging M87 1 Mapping magnetic fields in space by measuring the polarisation of waves caused by dust particles aligned with magnetic fields The following facilities routinely operate in the terahertz/submillimeter regime, making them critical for studying cold astrophysical phenomena that are invisible at other wavelengths: 1 The ALMA (Atacama Large Millimeter/submillimeter Array, 31.3950GHz) in Chile 1 The James Clerk Maxwell Telescope (86-690GHz) in Hawaii 1 The Submillimeter Array (SMA, 180-418GHz) in Hawaii Fig.20 shows polarised images from ALMA, VLBA and EHT images of the M87 black hole environment, revealing the magnetic fields. Communications As wireless data traffic continues to increase, the demand for higher frequencies has driven spectrum use upward for greater bandwidth and capacity. Current systems operate at the highest practical radio frequencies; Fig.19: a Golay cell. Original source: www.mdpi.com/1424-8220/24/21/6784 siliconchip.com.au Table 2 – wireless communications candidates Technology Microwaves Terahertz Near-infrared Visible light Ultraviolet Data rate Up to 10Gbps Up to 100Gbps Up to 10Gbps Up to 10Gbps A few Gbps Range Short Short-medium Short-long Short Short Power consumption Medium Medium Relatively low Relatively low Expected to be low Network topology Point to multipoint Point to multipoint Point to point Point to point Point to multipoint Noise source Thermal noise Thermal noise Sunlight + ambient light Sunlight + ambient light Sunlight + ambient light Weather conditions Robust Resistant to fog, dust, turbulence and drizzle but not rain Sensitive – Sensitive Security Medium High High High To be determined Wi-Fi 6E and Wi-Fi 7 use the 6GHz band (5.925-7.125GHz), while 5G’s Frequency Range 2 (FR2/mm-wave) operates over 24-71GHz, with the most common bands being 26GHz, 28GHz and 39GHz. Satellite communications reach up to the Ka-band (26.5-40GHz), with experimental V-band trials in the 40-75GHz range. Free-space optical communications such as Li-Fi (visible/near-infrared light) and Starlink’s inter-satellite laser links (operating around 1550nm wavelength, or ~193THz) already achieve massive data rates, but remain limited to line-of-sight, short-range or space environments. In the future, 6G is expected to span a wide range of frequencies, from sub6GHz legacy bands through new centimetric bands (7-15GHz), up to sub-­ terahertz and terahertz candidates (90300GHz or higher). The deployment of THz waves is constrained by two major challenges: the relatively low power levels that can be practically generated, and severe atmospheric absorption that limits range and reliability, although specific frequency ranges have been identified that are less prone to atmospheric absorption, as mentioned earlier. Path losses increase with both the square of the frequency and the distance, making long-distance comms at higher THz frequencies even more difficult. Terahertz 6G connectivity is expected to be complementary and used as a data rate booster if close enough to a transmitter site, rather than exclusive use due to the range limitations of THz comms. For siliconchip.com.au longer-­ range comms with a lower data rate, the lower bands will still be used. Beamforming will be necessary to enhance the range by concentrating the power of THz signals, as is already done for other cellular frequencies (see our article on 5G Mobile Networks from September 2020 – siliconchip. au/Article/14572). The high atmospheric absorption of certain frequencies in the THz band can be put to good use with “whisper radio”. This is a concept intended for ultra-short, ultra-secure comms with ranges from centimetres to metres that cannot be easily intercepted. Frequencies around 183GHz, 325GHz, 380GHz and 450GHz have been identified as suitable. Terahertz imaging for security As mentioned above, the major breakthrough for THz imaging came in the late 1980s to early 1990s with David Auston’s development of pulsed broadband THz radiation, enabling time-domain spectroscopy (THz-TDS) and sparking widespread interest in the field. Table 3 – loss vs frequency Frequency Bandwidth Loss 275-320GHz 45GHz <10dB/km 335-360GHz 25GHz <10dB/km 275-370GHz 95GHz <100dB/km 380-445GHz 65GHz <100dB/km 455-525GHz 70GHz <100dB/km 625-725GHz 100GHz <100dB/km 780-910GHz 130GHz <100dB/km Fig.20: images of the environment around the M87 black hole, taken with the Hubble Space Telescope (HST) in visible light, the ALMA telescope at 230GHz, the VLBA (Very Long Baseline Array) at 43GHz and the Event Horizon Telescope (EHT) at 230GHz. Source: www.eso.org/public/ images/eso2105c/ Australia's electronics magazine August 2026  25 Passive THz imaging was first demonstrated in 2002-2004. For example, MIT’s Lincoln Lab showed passive imaging of concealed objects using microbolometer arrays or Golay cells, followed by portable security cameras around 2004-2008 from Thruvision or Microsemi. Since the 2010s, compact THz sources such as quantum cascade lasers (QCLs) have become available. THz imaging is particularly attractive for human applications because it is harmless and non-ionising, unlike X-rays. Fig.21: the Thruvision security screening system. Source: https:// thruvision.com/case-studies/o2 Passive THz imaging Passive THz imaging detects the natural THz radiation emitted by all living things and objects; thus, no THz radiation source is required. A typical application is security screening, such as Thruvision (https://thruvision. com). It operates around a centre frequency of 250GHz – see Fig.21. Such systems are typically used at airports. Fig.22: a simulated explosive belt imaged by the Terasense Body Scanner 2.0. Source: https://terasense.com/applications/security/ Other applications for this technology include astronomy, non-destructive testing and biomedical imaging. Active THz imaging The Terasense (https://terasense. com) Body Scanner 2.0 system is active and illuminates the target with harmless THz radiation. It can illuminate a target from several meters away and can also do it covertly if desired, from behind any object or wall transparent to THz waves. It can detect objects such as a ceramic knife, which won’t show up on a metal detector, and can view an imaging area of 70×70cm from 3m away with a resolution of 3cm, or a 120×120cm area from 6m away with a 6cm resolution. It operates at 100GHz with six or more sources and has a 3×3mm 32×32 pixel sensor array – see Fig.22. Active imaging can also be performed on items as they pass on a conveyor belt (see Fig.23). Tomography THz radiation can be used for tomography, a technique that creates detailed 3D visualisations of an object’s internal structure. By directing THz waves through an object from multiple angles and measuring how much radiation is transmitted, absorbed, or scattered at each position (often using time-of-flight or phase information), appropriate computational analysis (similar to CT scanning) reconstructs a 3D image. This method is particularly valuable for non-destructive inspection of materials that are opaque to visible light but partially transparent to THz, such as plastics, ceramics, pharmaceuticals, certain composites and medical patients. Quality control and nondestructive testing One application of THz imaging in quality control is to test the integrity of tyres; for example, detecting a missing reinforcing cord. THz imaging can also be used to check the thickness and look for bubbles, defects or inclusions etc. Fig.23: security screening of packages using a THz scanner on a conveyor belt. Source: https://youtu.be/iHOt7Quyduk Medical imaging When surgeons remove a suspected skin cancer or other diseased tissue, they have to excise a margin of healthy tissue around the tumour as it might also be cancerous but not visibly so. Obviously, surgeons wish to minimise Australia's electronics magazine siliconchip.com.au 26 Silicon Chip the amount of healthy tissue they remove. THz imaging has the potential to distinguish healthy areas from unhealthy areas by the different spectral response of the tissues. THz imaging is also being evaluated for applications in dentistry, such as to evaluate the thickness of enamel or examine other parts of the tooth without patient exposure to X-rays. THz imaging is also being evaluated for rapid blood analysis, such as the non-invasive measurement of glucose. Art analysis THz imaging can be used to reveal hidden layers of a painting and can see, or see through, layers that may not be visible to other techniques such as infrared or X-rays. An example is a painting that was thought to be by Francisco Goya but did not have a signature. Fig.24 shows a hidden signature revealed by THz imaging of the painting. Terahertz spectroscopy Terahertz spectroscopy is non-­ destructive and non-ionising. It is used to analyse materials by probing their unique molecular fingerprints through various vibrations, rotations and interactions of molecules within the material under examination. Spectroscopy may be performed in either the time domain using pulses (see Fig.25) or frequency domain using continuous waves, swept in frequency. Terahertz waves can penetrate materials that are non-metallic and non-­ polar such as plastics, clothes and cardboard. Applications include: 1 detecting explosives or illicit drugs in packaging 1 analysing pharmaceutical drugs to detect different crystalline forms 1 checking the quality of food (eg, moisture content) or for contaminants 1 biomedical applications for checking molecular structures, hydration, or disease markers 1 checking plant water content, pesticides or seed quality for agriculture 1 analysing polymers, semiconductors and detecting defects Fig.24: THz imaging of a Goya painting revealing the hidden signature. Source: https://arxiv.org/pdf/1305.3101 Fig.25: the configuration of a time-domain terahertz spectrometer. Source: www2.riken.jp/lab/THz-img/ English/annual_gas.htm distance contact at 30THz (10µm wavelength) of 60m. This frequency is outside of standard radio amateur allocations and is not subject to regulations. The transmitter was a ‘blackbody’ emitter (heated plate) that was mechanically modulated. See siliconchip.au/link/acb5 and the YouTube video at https://youtu. be/6gJtzMLR6T0 for more details. Conclusion Amateur radio In November 2020, Australians The terahertz gap has been signifiAndrew VK3CV/WQ1S and Karl cantly narrowed. Commercial sources VK3LN made a world-record long-­ now exist for 0.3-1.0THz that are in siliconchip.com.au Australia's electronics magazine routine use in laboratories and some industrial systems, for applications like spectroscopy, non-destructive testing and imaging. However, the full 0.1-10THz range is still not fully exploited compared to adjacent bands because of ongoing challenges in developing high-power, tuneable, room-temperature, compact sources and detectors, as well as the atmospheric absorption that limits long-range use. Several different kits of sensors that can detect terahertz band radiation are SC also now available. August 2026  27