Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications XVIII

Attoseconds: When intense light interacts with a gas of atoms (or a transparent solid), electron wave packets are released. Attosecond pulse formation exploits the correlated electrons and holes, forcing the electron to return. Without the plasma connection, two of the most important strong-field process that accompany attosecond pulse formation—hot electron formation (inverse Bremsstrahlung) and non-sequential double ionization (collisional ionization)—seemed mysterious. These plasma-like processes lead to laser induced electron diffraction and orbital tomography. THz generation: Terahertz pulse formation by ionization has a similar linage. Using PIC codes to describe azimuthally polarized l=4 mm and 2 mm light interacting with a 150 µm thick jet of helium, we calculate THz pulses reaching 8.5 Tesla. But 10 Tesla is not a limit. 30 THz azimuthally polarized beams can be amplified in high-pressure CO2 reaching isolated magnetic fields of 1-gigagauss.

In recent years, topological phenomena in photonic systems have attracted much attention, with their striking features arising from robust states in the energy gaps of spatially periodic media. However, light waves are entities that extend in space as well as time, such that one may ask whether topological effects can also occur in the temporal domain, or even space-time. Intuitively, systems that are periodic in time may be gapped in momentum, leading to topological states localized at time interfaces. However, time - in contrast to space - exhibits a unique unidirectionality often referred to as the “arrow of time”. Inspired by these features, I will present our most recent experiments on topological states residing at temporal interfaces. Moreover, I will discuss the formation of spacetime-topological events and demonstrate unique features such as their limited collapse under disorder and causality-suppressed coupling.

Quantum technologies promise a change of paradigm for many fields of application, for example in communication systems, in high-performance computing and simulation of quantum systems, as well as in sensor technology. However, the experimental realization of suitable system still poses considerable challenges. Current efforts in photonic quantum science target the implementation of practical devices and scalable systems, where the realization of quantum devices and controlled quantum network structures is key for envisioned future technologies. Here we present our progress on the engineering of integrated photonic systems, which can overcome current limitations for the realization of scalable photonic systems. Specifically, our research currently focuses on three different but complementary topics: integrated devices based on lithium niobate circuits, engineering and harnessing the temporal-spectral structure of quantum states of light, and photonic quantum computation.

Chaos supremacy, which is a function that can only be realized in chaos, is new concept. Highly efficient THz wave using chaotically oscillating laser diode is inves-tigated. Compared it to conventional continuous wave multi-mode semiconductor laser excitation system, about ten times output power is increased because of chaos supremacy in real system.

This work presents a Terahertz (THz) Spoof Surface Plasmon Polariton (SSPP) Bandpass Filter (BPF) based on a Coplanar Stripline (CPS)-SSPP structure featuring internal grooves and central split rings. The proposed BPF achieves high-frequency rejection through its low-pass SSPP characteristics and blocks low frequencies using gaps within the structure. The upper and lower cut-off frequencies can be adjusted by modifying the design geometries. For instance, a BPF with a center frequency of approximately 1 THz was simulated with about 5dB insertion loss considering all of the loss mechanisms, along with the lower and upper 3 dB cut-off frequencies at approximately 0.8 THz and 1.2 THz respectively.

A coupled-cavity mini-array VCSEL is reported in this work as a novel laser source for CW THz generation. Simply tuned by current up to 9.5 mA, the laser provides optical beats up to 300 GHz, which are converted to corresponding THz frequencies in the homodyne detection setup with two photoconductive antennae. The influences of the laser current on the optical spectrum of the laser, and thus on the THz signal are also investigated. With its simpleness, compactness and extremely low cost, the mini-array VCSEL has the potential to replace other laser sources to promote CW THz applications in industry.

In this study, we designed a focusing setup for an electron vacuum accelerator consisting of reflaxicons and a paraboloid ring. The electric field distribution in the focal region was determined by the Stratton–Chu vector diffraction method. Describing the exact characteristics of the electric field of a focused pulse in the focal area is a key point when an accelerator is designed. Parabolic mirrors with high reflectivity are excellent candidates for focusing. By focusing radially polarized 1 ps THz pulses with 3 mJ pulse energy and 50 mm focal length of the paraboloid, calculations predicted longitudinal electric field strength of ~ 91 MV/cm. Such focused pulses are well suited for particle acceleration applications (especially for direct acceleration).

We report on the development of a millimeter-wave spectrometer for sensing radiation in the 140-190 GHz band. The instrument relies on the upconversion of radio waves to optical domain using a broadband lithium-niobate electro-optic modulator, and on an arrayed waveguide grating (AWG) photonic integrated circuit (PIC) for the spectral analysis of the resulting modulation sideband. We achieve 1 GHz resolution over 50 GHz bandwidth by packing 7 meters of waveguides in a PIC having an area less than 2 square centimeters. An array of germanium photo-detectors integrated on the PIC convert the optical power in each 1 GHz-wide spectral bin to electronic signals ready for digitization and subsequent processing. We present the component and system design, fabrication and assembly, and the results of experimental testing.

Heterodyne receivers based on plasmonic photomixers have demonstrated high sensitivity terahertz detection with broad terahertz bandwidth and high spectral resolution. We report an ultra-low-noise, room-temperature terahertz receiver based on a plasmonic photomixer that is optimized for operation in 650-700 GHz range for ground-based astronomy observation. By modulating the received signal and utilizing a digital lock-in detection technique, the background noise is significantly reduced and quantum-level sensitivity levels are achieved. Furthermore, by using an optical phase-locked loop for the stabilization of the terahertz beat frequency, a narrow IF linewidth and high spectral resolution are achieved. Additionally, mechanical stabilization of the optical LO alignment relative to the photomixer active region, offers long-term stability with longer than 1-hour Allan time for high-sensitivity astrophysics observations.

We developed a system that enables detection of THz-wave even when the optical path length of the THz-wave changes, by using a pulse train pump beam in THz parametric detection. First, we investigated the appropriate pulse interval of the pulse train pump beam and the tolerance of the incident timing of the THz-wave. Based on the above, we produced a pulse train pump beam with four pulses at 1.5 ns intervals, and confirmed that continuous THz-wave detection is possible without mechanical operation even when the optical path length of the THz-wave changes about 180 cm.

A range of applications requires THz pulses to be recorded in one shot, and at high acquisition rates. This concerns TDS of rapidly evolving samples, and diagnostics in accelerator-based light sources (synchrotron radiation facilities and free-electron lasers). Although demonstrations of MHz+ single-shot TDS have been made, the application range remained very limited because of the high cost of the equipments needed for the readout (typically oscilloscopes with 8-20 GHz bandwidth). We present here the design of a single-shot time-stretch THz digitizer (or time-domain spectrometer), that uses mid-IR laser probes. As a result, low-bandwidth readout oscilloscopes can be now used (down to 1 GHz), i.e., with costs that are one order of magnitude lower. We present the performances of this novel TDS versus the state-of-art. The new design is expected to extend considerably the application range of high repetition rate single-shot THz recorders in accelerator physics, and in TDS spectroscopy.

In this work, a suspended MEMS emitter was designed and fabricated using a CMOS-compatible approach for environmental monitoring. This emitter consists of a phosphorus-doped silicon micro-hotplate perforated with a 2D honeycomb hole array. Emission characteristics were studied in-depth by tuning the period and hole size. The membrane structure is compact in size, does not require substrate emission, and reduces heat dissipation, which is conducive to the miniaturization of gas sensing; the suspended emitter has highly selective emission in the 2.5-6.5μm band, which can realize real-time environmental monitoring of a variety of gases.

Traditionally, CMOS was not considered a light emitting device, nor was it a microwave generating device. Similar to laser, LED, and microwave diodes, CMOS has the capability of generating lights and microwaves at the same time and from the same chip - which is much more efficient than wire bonded SoC (Systems on Chip). In this presentation, we will demonstrate an array of devices comprising CMOS VCSEL (Vertical Cavity Surface Emitting Laser) and millimeter wave CMOS transistors. Interactions between the CMOS VCSELs and Microwave CMOS within the specially designed optoelectronic array generate digitally programmed and modulated photonics, light waves and microwaves. This customized PIC (Photonics Integrated Circuit) may include photonic nonvolatile vertical NAND FLASH for data storage, and photonic SRAM for low power optical computing. The microwave PIC may also include high voltage photonic CMOS transistors to regular high power operations critical for RF ASICs. Wireless Ultra Large Scale Integration (Wireless ULSI) has become a reality - most of the micro metal wires within an ULSI circuit are superseded with localized microwave communications.

High frequency photodiodes provide the starting point for calibrating the phase response versus microwave frequency of measurement instruments up to 110 GHz. The limitations of banded microwave connectors prevent this approach from extending directly to new sub-THz 6G applications (<300 GHz) for amplifiers, transceivers, and instrumentation. To solve this challenge, we demonstrate new optoelectronic primary phase standards. The solution consists of uni-traveling carrier photodiodes connected to a mode-locked fiber laser, coplanar waveguide (CPW) transmission lines, and pads for on-wafer probing. We use dual-comb electro-optic (EO) sampling at a fixed location along the length of the CPW to measure the picosecond voltage pulses generated by the photodiodes when a termination wafer probe is attached to the pads of the CPW. The EO sampling and a suite of S-parameter characterizations up to 220 GHz combine to yield a corrected voltage pulse on-wafer that can be translated virtually to any reference plane. In this way, we achieve a phase reference up to 220 GHz that can unlock time domain and nonlinear measurements beyond the state-of-the-art (<110 GHz) for emerging sub-THz electronics.

We demonstrate novel microwave photonic RF comb generators consisting of an ultra-low phase noise cutting edge fs-pulse laser and an InP-based high-speed photodetector subassemblies with rectangular RF waveguide interface. By combining the components, we generate broadband microwave comb spectra by optical-to-microwave downconversion with 1 GHz spacing in the D- and J-band. The single microwave tones obtain a high spectral purity with a measured spectral width below 1.5 Hz and high temporal stability. These properties can be exploited for test and measurement, wireless communication and RADAR applications.

We report on the development of a compact tunable optical paired source (TOPS) that can be used to generate RF signals at frequencies between 5 and 110 GHz using free-space optics. Like in the fiber-based versions of this system, this compact TOPS utilizes an optoelectronic scheme based on sideband-injection-locked lasers to generate the carriers. By replacing fiber-based components with their free-space counterparts, we are able to achieve lower insertion loss through the system. Fiber-based systems are also susceptible to acoustic noise which is eliminated by switching to free space optical components. Here, it is discussed the component and system design, fabrication and assembly, along with results from experimental testing.

In 6G wireless communication, higher millimeter-wave (mm-wave) frequencies are crucial for their multiple GHz bandwidth and higher data rates. However, these higher frequencies suffer from significant free space path loss and molecular attenuation, complicating long-distance transmission. To overcome this, we propose an RF-photonics approach using photonic generation and distribution of signals. This method leverages low-loss optical fiber and high-bandwidth optical components for high-speed, high-frequency microwave communications. We introduce a Holographic Multiple Input Multiple Output (HMIMO) distributed phased array system, deployed through a fiber network to support multiple users simultaneously. HMIMO creates adaptable beam patterns like optical holograms by applying precise phase and amplitude weights to individual transmit elements. We will demonstrate HMIMO with a distributed antenna array, achieving specific beam patterns for multiple user scenarios. Our setup includes key subsystems like the Tunable Optically Paired Source (TOPS) and high-power photodiodes, enhancing performance, scalability, and capabilities for next-generation wireless systems.

We designed and fabricated a compact high-gain integrated 60-GHz photoreceiver for a low-phase noise optical-electrical oscillator, which is composed of hybrid-integration using a high-speed PIN-photodetector and a quantum dot semiconductor amplifier to be expected for high-temperature stability as well. In the responsivity with the QD-SOA driving currents in the range 0–400 mA, a high responsivity of 14–25 A/W could be observed at 1532 nm. Moreover, the 3-dB bandwidth of 62 GHz could be confirmed in the frequency response measurement result at -2 V.

Highly stable and low-noise terahertz waves (0.1–10 THz) are crucial for advanced applications like 6G communication, terahertz spectroscopy, and radar. We generate these waves at remote sites by transferring comb-rooted optical waves over long distances. Two optical frequencies, extracted from a comb stabilized to a ULE cavity, are transferred through optical links and directed into a photomixer. Stability and performance are evaluated using the beat signal of the transferred terahertz wave with a reference terahertz wave. We measure and analyze the impact of noise from the optical link on the terahertz wave. Our research aims to advance the terahertz industry by providing a foundation for 6G communications and the transfer of terahertz standards to remote sites.

Off-axis parabolic mirror systems are widely used in THz spectroscopy applications but typically suffer from poor optical performance for extended sources or off-axis beams. Here we report that the correct optical orientation of off-axis parabolic mirrors can minimize geometric aberrations and lead to diffraction-limited optical performance. We performed detailed optical simulations to find the best optical arrangement, and derive simple design rules that can be used to construct experiments with good optical performance. We show experimentally that minimizing the degree of wavefront error can significantly boost the bandwidth of pulsed THz radiation, by creating shorter THz pulses that have suffered from smaller variations in path length throughout the spectrometer.

A frequency-tunable fractal antenna is proposed for use with a stacked dielectric layer in a resonant tunneling diode (RTD) based THz transceiver. The antenna’s fractal geometry allows frequency tuning through discrete switching of RTDs, adjusting the resonance by altering the antenna’s electrical length. The antenna design incorporates iterations of the Minkowski fractal operator, which increases the current path and causes a leftward frequency shift as the antenna's side length enlarges with each iteration. In the first iteration, the resonance frequency tunes from 0.204 THz to 0.416 THz by changing fractal depth and width. The second iteration achieves a stable resonance shift from 0.413 THz to 0.578 THz with varying geometries. The maximum tunable bandwidths are approximately 0.2 THz and 0.16 THz for the first and second iterations, respectively. This antenna can be integrated with RTD arrays for high-power THz radiation, supporting 6G communication, sensing, biomedical, and security applications.

The increasing complexity of electronic devices and circuits, especially VLSI, presents increasing challenges for testing and fault diagnostics. Traditional AC and DC electrical testing methods are expensive, do not guarantee complete fault identification, and pose security risks, as they can be circumvented by designing counterfeit circuits that evade detection by conventional testing techniques. The detection of counterfeit VLSI, often referred to as “trojan hardware,” is a crucially important and growing problem. I will review a novel testing approach using terahertz (THz) testing for transistors, MMICs, and VLSI and present application examples for testing Si VLSI, AlGaAs/GaAs and AlGaN/GaN HEMTs, and AlGaAs/GaAs MMICs. THz testing measures circuit responses to the impinging THz or sub-THz radiation at the pins or input/output leads of transistors and VLSI (even packaged devices) and compares them with reference responses. THz testing can also be extended for fault identification and predictions of lifetime and reliability.

We have fabricated doubly clamped MEMS beam resonators using p-type modulation-doped AlGaAs/GaAs heterostructures and characterized their performance for detecting terahertz (THz) radiation by measuring resonance frequency shift of the MEMS beams by THz radiation heating. Previously, we used the piezocapacitive effect of n-type AlGaAs/GaAs heterostructures to detect the radio-frequency (rf) signal of the MEMS beam vibration. However, the rf signals were less than 1 μV because of stray capacitance of readout cables. To overcome this problem, we have developed p-type MEMS beam resonators to detect the rf signal using their large piezoresistive effect in the valence band. The p-type MEMS resonators exhibited rf signal as large as 1 mV. Furthermore, we characterized the noise performance and found that the frequency noise is in the order of 10 mHz/√Hz, when the MEMS resonance frequency was around 220 kHz, indicating low-noise performance.

In this work, we present an uncooled lead-Schottky PbSe/CdSe mid-wave infrared (MWIR) photodetector that integrates photovoltaic (PV) and photoconductive (PC) properties. Fabricated using a vapor-phase deposition (VPD) system, this hybrid photodetector demonstrates significant improvements in surface morphology and a reduced 1/f noise profile under low bias conditions, compared to conventional chemically bath deposited (CBD) PbSe detectors. The device's multi-stacked nanostructure, featuring a tunable cutoff wavelength, offers promising potential for applications in military, security, environmental monitoring, and medical diagnostics, requiring compact, efficient, and sensitive MWIR detection systems.

We report a lock-in detection technique that maintains the spectral information of the detected signals. It involves the separation of different frequency components of the captured signal, followed by demodulation and filtering applied to each frequency component to form the lock-in detected spectrum. We specifically develop and apply this technique to signals captured by plasmonic heterodyne terahertz receivers and demonstrate significant signal-to-noise ratio enhancement with a 10 dB per decade reduction in the noise as a function of the integration time. Therefore, the introduced lock-in detection technique has broad applicability to astronomy, cosmology, atmospheric studies, and gas sensing.

Terahertz (THz) sensing and imaging is a promising non-ionizing and non-invasive technique for epithelial cancer detection which constitute more than 80% of all adult cancers. THz absorption is highly sensitive to water content, which can help differentiate between normal tissues and cancerous tissues that have a higher water content. Measuring the hydration levels in the skin by either THz Transmission Time-Domain Spectroscopy (TDS) or continuous wave terahertz spectroscopy could determine the lateral spread of basal cell carcinoma. The nanoparticle-enabled terahertz imaging of cancer cells uses the surface plasmon resonances to enhance the detection sensitivity and achieves the resolution of a few microns. Applying artificial intelligence and machine learning techniques to the analysis of THz imaging improve accuracy of cancer detection and provides a more accurate diagnostic. In particular, it could be used to analyze cancer images combining absorption with differential absorption data applying frequency modulation technique. This new approach has potential to improve skin cancer diagnostics and accurately determine the skin cancer spread.

We have developed a terahertz focal plane array (THz-FPA) using plasmonic nanoantennas, which function as light concentrators in the infrared band and as terahertz antennas in the terahertz band. This 64-pixel detector array can scan a 5 cm linewidth at high speed. By offering rapid scanning with a large field of view, the THz-FPA has the potential to transform terahertz pulsed imaging systems from specialized metrology tools into high-throughput instruments suitable for a wide range of industrial non-destructive evaluation applications.

In this work, we present a terahertz (THz) metasurface designed for chemical sensing, using fractal-based structures to achieve a multiband response in the 1-10 THz range. The fractal unit cells allow for multiple resonant modes, providing high flexibility in identifying chemical compounds through their unique absorption features. Compared to conventional designs, this fractal metasurface offers enhanced sensitivity and accuracy, making it highly effective for detecting trace quantities of chemicals. This technology has significant potential for advanced THz sensing applications, such as environmental monitoring, security screening, and biomedical diagnostics.

Carbon dioxide(CO2) is a greenhouse gas that not only cause impact to climate but could also affect health when sick building syndrome(SBS) kicks in. Here, we sense CO2 using a pyroelectric-based non-dispersive infrared(NDIR) gas sensor. The pyroelectric sensing layer used is aluminum nitride(AlN). To understand the behaviour of AlN-based pyroelectric detectors on CO2 gas sensing, we vary the AlN film thickness and operating frequency of the pyroelectric detector. We note ~50% change in signal response when AlN film thickness reduces. The results will provide more understanding on characteristics of AlN-based pyroelectric detectors and their behaviours in NDIR gas sensing.

A 3D metamaterial absorber based on an advanced structure with a simple fabrication process is proposed to enhance tunability and refractive index sensitivity in mid and far-infrared regimes. The metamaterial comprises a nano-disk with a nano-wall array. As the refractive indices of a dielectric overlayer deposited on the metamaterial are shifted from 1.0 to 2.0, the tunability of a metamaterial absorber based on the nanodisks with nanowalls structures can be 621 % times larger than without nano-wall structures. Therefore, the metamaterial absorber based on the nanodisks with nanowalls structures has a high refractive index sensitivity of 3847 nm/RIU. The metamaterial absorber based on the nano-disk with nano-wall structures has a high tunability and refractive index sensitivity due to the nano-walls generating high surface plasmon fields. Therefore, the metamaterial absorber based on the nano-disk with nano-wall structures has a huge potential to develop biosensors and infrared filters in mid and far-infrared regimes.

Terahertz (THz) radiation, between microwave and infrared frequencies, offers groundbreaking solutions for 6G communications and non-destructive inspection of semiconductors and battery electrodes. Its ability to penetrate materials and provide detailed insights is crucial for high-precision assessments. Frequency combs, recognized by the 2005 Nobel Prize, have enabled precise manipulation of THz waves. THz wave synthesizers based on optical frequency combs offer unrivaled stability, pushing measurement precision to sub-micrometer and nanometer scales. This technological advancement enhances THz measurement accuracy and expands applications in material science, semiconductor inspection, and 6G communications, addressing key challenges in high-precision metrology. Acknowledgments This work was funded by the National Research Foundation of Republic of Korea (NRF-2021R1A4A103166013, NRF-2020R1A2C210233813, and NRF-2022M1A3C2069728, from the Ministry of Small and Medium-sized Enterprises (SMEs) and Startups under grant RCMS-S3207602, from the Commercialization Promotion Agency for R&D Outcomes (COMPA) under grant RS-2023-00260002.

To address the limitations of existing terahertz time-domain imaging systems, we present a new terahertz source array based on plasmonic nanoantenna arrays that offer efficient optical-to-terahertz conversion. As a first proof-of-concept, we use a terahertz time-domain spectroscopy system with the terahertz source array and a single pixel detector to capture multi-pixel hyperspectral terahertz images without requiring raster scanning. We demonstrate that the imaging system can capture both amplitude and phase information by acquiring far-field images of different phase and amplitude objects. The introduced terahertz source array has the potential to realize high-throughput, high-SNR and high-resolution imaging and inspection systems, benefiting from the unique features of terahertz waves.

We present a new generation of terahertz optoelectronics based on quantum well structures that can be monolithically integrated with other active and passive optical components on the same chip. As a first proof-of-concept, we demonstrate monolithically-integrated, frequency-tunable terahertz transmitters and receivers operating over 140-500 GHz at room temperature, for the first time, with better or similar performance compared to the state-of-the-art terahertz transmitters and receivers that are pumped by free-space-coupled lasers. The introduced monolithically-integrated terahertz optoelectronics platform paves the way for compact and scalable terahertz systems for imaging, spectroscopy and communication applications.

Terahertz (THz) wave is expected to be utilized as wireless carrier waves in next-generation (6G) mobile communications, where low phase noise is crucial for high-speed, high-capacity transmission. Photomixing for near-infrared optical beat signals with uni-travelling carrier photodiode (UTC-PD) is a promising method to generate THz wave. By employing micro-cavity optical comb for near-infrared optical beat signals, ultralow phase noise characteristics in THz wave can be achieved over that of electrical THz generation methods such as frequency multiplication. We have previously demonstrated THz wireless communication at 560 GHz with 1-GBaud modulation of OOK, BPSK, QPSK, or 16QAM by a square-law detection using a Schottky barrier diode (SBD). In this study, we further enhanced data transmission speed up to 10 GBaud by using a sub-harmonic mixier (SHM) in place of SBD for heterodyned detection of THz wave.

We show that multi-layered heterogenous structures can increase damage thresholds and improve terahertz output from organic nonlinear optical materials. We even show that these layered structures can even “heal” the surface for nonlinear optical crystals whose surface has degraded, significantly improving the terahertz output.

All-silicon metasurfaces based on bound states in the continuum (BICs) have gained attention for their high quality (Q) factors at terahertz (THz) frequencies. This study simulates all-silicon BIC metasurfaces using various shapes of holes for comparison. Circular hole structures achieve resonance by varying hole position or radius, while other shapes achieve it by altering side length and position. Our results show high-Q resonances for both configurations, with differences in resonance characteristics. These high-Q metasurfaces have potential applications in THz modulators, biosensors, and other photonic devices, advancing THz photonics and enabling novel applications in spectroscopy and wireless communication.

The applications of THz time-domain spectroscopy are rapidly growing in non-destructive testing and biomedical imaging. In many such applications, air gaps can naturally occur in a multi-layer sample geometry, which can alter the measured reflection spectra and thus influence the characterization of the sample. Here, we present an iterative computational algorithm based on the Hilbert transform and cross correlation to precisely calculate the distance of the air gaps. Subsequently, by artificially removing the air gaps from the time-domain data, the optical parameters of the sample can be extracted.

Terahertz Time-Domain Spectroscopy (THz-TDS) is an excellent candidate for the non-invasive, precise and rapid characterisation of the electrical properties of large-area graphene. However, THz-TDS parameter extraction is a non-trivial task especially with challenging samples. We present the use of artificial neural networks trained on simulated data to extract the electrical parameters from transmission measurements of large-area graphene layers. We demonstrate improvement on iterative fitting methods when applied to real-world measurements, particularly when the full frequency range is used. We additionally present our investigation into the use of experimental data for training as well as extracting parameters directly from the time-domain.

This study presents an advanced terahertz continuous-wave spectroscopy method using an optical frequency comb to enhance testing reliability of non-destructive testing. By employing two infrared lasers in a photomixer, we generate terahertz waves and achieve extensive frequency control over a bandwidth exceeding 2 THz at a rate of over 10 THz/s. The frequency comb ensures precise spectral definition, with frequency instabilities tens of MHz at a single sweep, which can be further reduced by increasing averaging times. This precision allows accurate measurement of the physical properties and geometry of tested objects, benefiting the semiconductor and battery industries by improving product quality.

In this work we describe the use of a THz quantum cascade laser (QCL) to perform bi-directional reflectance distribution (BRDF) measurements on highly absorbing ultra-black coatings. Characterizing the long wavelength ( λ > 25 µm) properties of ultra-black coatings is particularly relevant in emerging applications that rely on their absorption properties such as in detector technologies, blackbodies, or in stray light suppression. For all of these applications, understanding how much radiation reflection is occurring (directional-hemispherical reflectance (DHR)), and the angular distribution of reflected radiation are important in device-design criteria and in reducing reflection-measurement uncertainties. A fully-characterized BRDF can be used to determine the reflection characteristics, which is ideal for assessing and optimizing ultra-black coatings. In this work we will demonstrate our capacity to measure a THz BRDF on a small-area (1 mm) vertically aligned carbon nanotube detector for earth outgoing-radiation measurements and present results.

The applications of Terahertz time-domain spectroscopy in non-destructive testing and biomedical imaging have significantly increased during recent years. There are numerous scenarios where the sample of interest has a multilayered structure. Evaluation of thin layers in such samples are challenging due to the limitations arising from the pulse width of the THz wave. In this work, we will show the usage of super-resolution sparse deconvolution in extracting the thickness of coating materials on interwoven fibers using polarized THz images. Additionally, the accuracy of ASOPS and ECOPS sampling techniques in thickness measurement will be compared.

Optical microscopy critically links nanoscale dynamics to applications of condensed matter physics. We exploit atomic nonlinearities in tip-confined evanescent fields to achieve angstrom-scale spatial and true subcycle temporal resolution. This captures the ultrafast quantum flow of electrons through atomic-size junctions, offering unprecedented insights into light-matter interaction at ultimate spatiotemporal scales.

Dynamically reconfigurable metasurface antennas and terminals represent an excellent match for the demands of modern communications and applications that have a growing need for wireless communication and sensing. In contrast to conventional phased arrays and electronically steered antennas, beam-steering metasurfaces scale extremely well with aperture size in terms of cost, complexity, power consumption, and many other key metrics. Initially commercialized for applications at microwave frequencies, the metasurface concept also scales well across the electromagnetic spectrum, enabling new beam steering paradigms that now include millimeter and infrared wavelengths. We discuss the metasurface architecture, its advantages and challenges, and recent progress.

We have integrated an electrically switchable THz metamaterial into the interior of a leaky parallel-plate waveguide to create an electrically switchable leaky-wave THz emitter. Leaky-wave antennas and active metamaterials have been used in the THz range to provide functionality in wireless systems. They have yet to be integrated together. We focus on the use of a planar metamaterial integrated into a THz leaky-wave antenna to vary the waveguide’s internal boundary conditions and realize electronic beam steering. Applying reverse DC bias voltage to the metamaterial opens capacitive gaps, putting the device in the ‘on’ state. This shifts the wave vector of the guided wave and thus changes the properties of the emitted radiation, enabling angular tuning of the frequency-dependent emission angle.

Passive millimeter-wave (mmW) imaging has a broad range of applications including degraded visual environment (DVE) mitigation, stand-off security screening and next generation high speed communication systems. Key limitations in the deployment of these systems have been cost, size and weight. We have previously reported on the development of optical up-conversion and distributed array apertures. These include demonstrations of these systems in relevant test environments. In this work, a system is presented in which the miniaturization of the sensor is prioritized, demonstrating a man-portable, 64-element, distributed aperture array which operates at 86 GHz.

Porosity of pharmaceutical tablets is a crucial parameter determining their deformation properties, mass transport, and disintegration time. Terahertz waves offer great functionalities for tablet porosity measurement because they can penetrate through pharmaceutical tablet excipients with low scattering loss and the tablet porosity can be determined by the effective terahertz refractive index and density of the tablet. However, previously-demonstrated porosity measurement techniques are limited in their dynamic range and require a separate step to mechanically measure the tablet thickness. We demonstrate a high-speed terahertz time-domain imaging system in reflection configuration for pharmaceutical tablet porosity measurement. The system utilizes plasmonic photoconductive terahertz sources and detectors to offer significantly higher dynamic range compared with conventional systems. In addition, the physical thickness of the sample is directly determined from the captured terahertz radiation reflected from the sample, enabling extracting the refractive index, density, and porosity of the tablet in real-time.

We introduce the architecture of a sub-THz band transmission technology based on photonics that accommodates future-proof next-generation mobile services. Depending on data modulation and detection scheme, the sub-THz band transmission technology is classified into three structures. They are comprised of 1) intensity modulation using external optical modulator and envelope detection, 2) intensity modulation using directly modulated laser and envelope detection, 3) optical IQ modulation and coherent detection. The technical characteristics of key elements required for each structure and the required performances for the realization of the sub-THz transmission technology were reviewed. Several experimental demonstrations were performed, and the transmission performances were measured and technically analyzed. Finally, a few technical challenges and future works that must be overcome for the practical deployment of photonics-based sub-THz band communication systems are listed and discussed. Acknowledgments This research was supported by Electronics and Telecommunications Research Institute (ETRI), South Korea grant funded by the Korean government [24ZH1100, Study on 3D communication technology for hyper-connectivity].

We achieved to demonstrate sinusoidal signal up to 4.5 GHz superimposed on sub-terahertz carrier waves in two JPE devices. One device radiates at 840–890 GHz with the maximum FM bandwidth of 40 GHz [4] and the other radiates at 400-500 GHz with high- and low bias regions. The results verify that the instantaneous JPE frequency follows the gigahertz-modulated bias voltage. The on-chip FM terahertz generation shows a sharp contrast to the mode-lock frequency comb constructed by highly sophisticated optics on a bench. [4] M. Miyamoto, R. Kobayashi, G. Kuwano, M. Tsujimoto, and I. Kakeya, Wide-Band Frequency Modulation of a Terahertz Intrinsic Josephson Junction Emitter of a Cuprate Superconductor, Nat Photonics 18, 267 (2024).

A palmtop-integrated backward THz-wave parametric oscillator has been developed, which is mountable on a robot. The THz-wave output above 10 W at peak was obtained at 0.33 THz with a 60 GHz-band tunability. THz-wave reflection imaging was performed, demonstrating the stable measurements in operation ready for real-world THz-wave sensing applications.

Thanks to extensive research and development efforts over the years, terahertz technology has advanced to the stage of practical industrial applications. Recently, various compact, high-performance, and cost-effective technologies, which are crucial for industrialization, have been developed, leading to more active efforts to apply these technologies in industry. The growing demand for photonics-based ultra-broadband terahertz technology and advancements in real-time high-resolution imaging are enabling differentiated performance in major application areas such as communication, spectroscopy, and imaging—areas where other technologies fall short. Terahertz research, which began with the development of integrated semiconductor photonic sources, has expanded to include various projects such as the development of low-cost photonics-based imaging systems and imaging systems for scanning and acquiring images inside shoes using MMIC array chips. This invited talk will discuss the characteristics of the systems developed thus far and provide future development prospects for systems expected to be applied in the increasingly in-demand field of scanning.

We report on a security enhancement method of the wireless communication in the terahertz (THz)-band, which utilizes an optical exclusive OR (XOR) circuit. An optical 2 x 2 directional coupler functions as a lightwave interference-based XOR circuit when two input optical signals to the coupler are coherent. Encoded data by the XOR operation can only be restored to its original form by another XOR operation using the same key. We produced an encoded optical 20 Gbit/s on-off keying (OOK) signal with the coupler-based optical XOR circuit, and then converted the encoded optical signal into a signal in the 300 GHz-band with high-speed photo-mixing. Thus, we carried out 20 Gbit/s OOK communication in the 300 GHz-band, whose security was enhanced with the high-speed and low-power consumption XOR operation.

Microwave photonic link (MPL) allows high-fidelity transmission of broadband analog signals over long distances using fibers. In this paper, we present a balanced MPL using a low-Vπ thin film lithium niobate (TFLN) modulator and a high-power modified uni-traveling carrier balanced photodiode. Due to relative intensity noise (RIN) suppression from the balanced detection scheme, shot noise limited link performance is theoretically modeled and experimentally characterized at >40mA photocurrent. Using this link, we also demonstrated positive link gain, <15dB link NF and ~20GHz bandwidth.

Infrared devices have an important role in many fields. Although germanium has been widely used as an optical material in infrared applications, concerns related to its supply has led to an increase in demand for chalcogenide glass. For this reason, we develop unique chalcogenide glass materials and manufacture optical elements using them. Thus, our strongpoint is the provision from materials to design and development of optical elements. In our company, we develop optical elements suitable for applications in various fields including aerospace, medical, and spectral imaging. We also focus on material development to meet various application requirements. In this presentation, we discuss some cases of optical elements developed using our chalcogenide glass materials.

This paper presents up-conversion module for the applications to the photonic-assisted passive millimeter wave (pmmW) imagery by leveraging thin-film Lithium Niobate (TFLN) technology. Compared with the bulk LN modulators, TFLN modulators offer stronger electro-optic interaction due to tighter RF and optical mode sizes, thereby demonstrating significant modulation enhancement, >50×, and higher index contrast for smaller waveguide bending allows single-end optical feed from one side and RF from the other side. Furthermore, capacity loaded electrodes are developed for better RF and optical index match, and lower RF loss for better conversion efficiency in W band. The fabricated TFLN modulators are integrated with the RF module, and the packaged module was experimentally verified that the TFLN-based module demonstrates significantly improved performance, > 8 dB in EO conversion efficiency, ~ 50% reduction in packaging size when packaged as a multiple channel blade.

The meta-absorber with silver-SiO2-silver structure operating at terahertz frequencies is proposed, simulated, and subsequently evaluated using a terahertz time-domain spectroscopy (THz-TDS) system. The meta-array of the proposed absorber possesses a polarization-insensitive unit cell structure with fourfold symmetry. The simulation result demonstrated several near-perfect absorption peaks within the 3-6 THz range. The proposed multiband near-perfect terahertz absorber undoubtedly plays a qualified role in terahertz wave sensing. Specifically, the simulation had pinpointed near-perfect absorption peaks at 3.12, 4.06, and 5.08 THz. The simulation also revealed that the absorption peak at 4.46 THz, although not as highly absorbed as the former three, became rather conspicuously pronounced in absorption as confirmed by a later experimental observation. Furthermore, the aforementioned high absorption peaks all appeared to be blue-shifted by an average of 0.4 THz. These high absorption peaks at the specified THz frequencies would certainly find themselves useful in distinct THz applications.

In this paper, we demonstrated the use of k-nearest neighbors (KNN), support vector machines (SVM), and random forests (RF) machine learning (ML) algorithms to classify multiple-shape object detection using mm-wave radar. By generating a 1 GHz – 4 GHz bandwidth radar signal at a frequency range of 77 GHz-81 GHz, different types of objects are detected and training and test datasets are prepared. Through the analysis, the KNN and SVM provided about 70.59% accuracy and RF provided about 64.71% accuracy in recognizing different types of objects. The presented approach with a lower bandwidth radar signal, can be performed with a photonically generated high bandwidth radar signal which provides finer resolution in object detection which is under investigation. It is anticipated that with photonically generated high-bandwidth radar signal (> 8 GHz), an accuracy percentage as close to 90% can be obtained.

An intelligent reflecting surface (IRS)-aided generalized spatial modulation (GenSM) millimeter-wave (mmWave) massive multiple-input multiple-output (MIMO) system offers an excellent tradeoff between the achievable sum-rate (ASR) and energy efficiency (EE). This article focuses on the integration design of the IRS beamforming and the GenSM hybrid precoding mechanism for the mmWave massive MIMO system. To improve the ASR significantly, a Gram-Schmidt (GS)-based scheme is proposed to determine the array response vectors and their corresponding path gains. Finally, simulation results demonstrate that the proposed IRS-assisted GenSM mmWave massive MIMO system with the aid of the GS method is capable of achieving a substantial improvement in EE.

In modern Radar systems, FMCW transmitters with large frequency sweep range in microwave domain have been developed successfully by microwave-photonic technology based on optoelectronic devices, like laser diodes and high-speed photodetectors, rather than a pure electronic technology. In this paper a laser diode under the condition of optical injection is modeled by solving a set of rate equations of semiconductor lasers by the method of 4th-order Runge-Kutta. Four different nonlinear dynamic states, in which the period-one oscillation state is the most interesting phenomenon in this paper, are observed under different parameters of optical injection and the injected laser. In a FMCW Radar system for monitoring people, the period-one oscillation state could be utilized to FMCW microwave generation if the frequency sweep range had to reach up to 15 GHz at high repetitive rate which was a difficult goal to reach in a pure electronic system. However, by the simulation in this paper the goal was proved to be able to reach if the relaxation resonant frequency of the injected laser could be increased by some effect, like as detuned-loading effect in semiconductor lasers.

Terahertz time-domain spectroscopy of eggshells was performed. With good transmission around 0.65 THz, other (lower) transmission peaks at around 2.6, 2.9 and 3.2 THz were also observed. There was no noticeable difference in transmitted signals between the white eggshells with and without the membrane. Thermally treated eggshells showed similar characteristics. Comparative measurements using the brown shell eggs were also performed.

Terahertz (THz) radiation, operating within a low-energy frequency range, plays a crucial role in non-destructive imaging applications like material inspection, security screening, and medical diagnostics. Current THz imaging faces challenges balancing resolution and dynamic range. Direct detection methods offer high resolution but limited dynamic range, while heterodyne imaging, using dual THz sources, boosts dynamic range at the expense of complexity and cost. Conversely, homodyne imaging, based on Mach-Zehnder interferometry, simplifies setup while preserving phase information and achieving high resolution. This method enhances sensitivity to subtle optical material changes, ideal for imaging low-absorption targets. However, previous implementations lacked polarization-sensitive detection and the ability for simultaneous reflection and transmission measurements. This study aims to advance THz imaging by integrating C-shaped metalenses, enhancing homodyne detection capabilities for polarization-resolved feature extraction, improved imaging of low-absorption samples, and simultaneous detection in both transmission and reflection geometries.

We report on the spectrally narrow thermal terahertz (THz) emission from the n-GaAs/GaAs structures equipped Ti/Au or n-GaAs metasurfaces comprised of squares or rectangles metacells. In such structures resonant magnetic polariton modes could be excited in GaAs squeezed between metasurface and conducting n-GaAs layer. Reflection and thermal emission experiments revealed the narrow spectral features due to magnetic polaritons, which also tailors the broad spectra of thermal emission. Frequency dependence on polarization was revealed for rectangle-shaped metacells. Up to fifth harmonic of magnetic polariton excitation was observed. The demonstrated emitters promise the possibility of integration with GaAs THz photonics devices.

Tunable fishnet gradient metamaterials (TGFMMs) based on liquid crystals (LC) have shown great potential for controlling electromagnetic properties in the Terahertz (THz) range. These metamaterials, using sub-wavelength structure, gain tunability by re-orienting loquid crystals under external simuli like electric fields, temperature, and light. Early designs allowed negative refraction but were static and complicated to fabricate. Subsequent versions include tunability but struggles with unit cell optimization and dielectric breakdown risks. This new TGFMM integrates polymer dispersed liquid crystal (PDLC) into a fishnet-patterned metal structure, enabling significant refractive index modulation when an electric field is applied. The design reduces dielectric breakdown risks through circular electrodes, and each unit cell can be controlled independently. Simulations and experiments confirmed the TGFMM’s ability to dynamically steer beams and manipulate wavefronts. Beam deflection reached a maximum of 7.91° under 20 Volts, with further tests underway to explore its lensing capabilities.

This report explores the design and assembly of nonparaxial THz imaging systems using high-resistivity silicon diffractive optics elements (DOEs), such as Fresnel zone plates, Fibonacci lenses, Bessel axicons, and Airy zone plates. Fabricated by femtosecond laser ablation from 500 μm thick silicon, these DOEs enable efficient THz light illumination and collection at 600 GHz. The study evaluates imaging performance through metrics like contrast, resolution, and focus, highlighting the decoupling of optimal modulation transfer from image irradiance. It underscores the importance of structuring illumination and collection schemes for compact THz imaging.

Terahertz (THz) radiation is non-ionizing and highly transmissive through many materials, making it promising for hyperspectral imaging applications. While THz generation techniques continue to improve, there are significant limitations in characterizing complete spatial beam profiles in time and frequency domains. Current THz imaging methods are time consuming, large, and expensive. We propose a cost-effective and compact electro-optic THz camera that uses a stack of optical elements followed by a small and readily accessible camera.

3D imaging technology based on frequency modulated continuous wave (FMCW) LiDAR systems has been developed significantly in aspects of high resolution, high accuracy and fast frame rate. In this talk, the methods and configurations of FMCW light sources are reviewed and the dominated limiting mechanism of frequency sweep slope of FMCW light generators is analyzed and discussed. The simulation about improvement of frequency linearly swept rate by utilizing the detuned-loading effect in a direct modulation laser diode, via a set of modified rate-equations, is reported for the purpose of 3D imaging at high frame rate by FMCW LiDAR technology.

We developed RECILS WG technology for fabricating hollow waveguides (WGs) in the 200–400 GHz band using our proprietary ultravoilet (UV)-curable resin 3D printer (RECILS) and electroless plating. Conventional metal machining struggles with THz-band WGs because of their submillimeter scale and 3D complexity. Our technology successfully creates low-loss straight, bandpass filters, bent, twisted, spiral, and Y-branch WGs, achieving minimal propagation losses of 0.5–0.8 dB/inch. We also achieved conversion from TE10 mode electromagnetic waves in WR-3.4 rectangular WGs to TE02 mode in circular WGs (azimuthal polarization) with an insertion loss below 1.5 dB. This technology enables the creation of virtually unrestricted 3D freeform WGs, paving the way for the realization of 3D THz integrated circuits.

We experimentally showcase a coaxial-W1-fed omnidirectional dielectric resonator antenna (DRA) operating above 100 GHz, specifically designed for characterizing 6G antenna systems. This antenna overcomes the challenges posed by large antenna-under-test ranges, enabling indoor far-field conditions at terahertz frequencies. The designed DRA-based probe antenna is realized by silicon micromachining featuring a precise alignment and an easy hand assembly concept. Functionality as a compact radiator at 107 GHz is confirmed, and a demonstration of near-field scanning to confirm its viability as a mm-Wave (millimeter-wave) compact and cost-effective probe antenna.

Integrated photonic circuits are in high demand for the upcoming THz communications. This study demonstrates experimentally a 3D-printed 3-channel demultiplexer that uses grating-assisted, contra-directional couplers for integrated photonic circuits in THz communications. The integrated circuits were optimized for operation in the 120-165 GHz frequency range, with ~5 GHz individual channel bandwidths and ~3 GHz inter-channel spectral spacing. Suspended-in-air integrated terahertz circuits demonstrated in this work show strong potential for developing linear optic transformers required for energy-efficient analog processing in upcoming terahertz communications.

In this study we investigated the response of dielectric materials for high frequency laminates such as rogers and quartz substrates. Free space frequency-domain non-destructive technique using CW THz source is employed for these experiments. We get insight into the losses induced by these materials in THz domain (0.1 to 3.5 THz). We studied atomic polarization and orientation polarization effect induced in these materials. The results on broadband response of material is helpful to design novel frequency selective passive components at a glance which will be used for packaging and integration of THz detectors.

Optical sensors are essential for detecting physical, biological and chemical properties through light-matter interactions. Recent developments emphasize the use of terahertz (THz)-based absorbers, due to their low photon energy and strong penetration capabilities. These sensors utilize engineered materials to achieve near-complete absorption of THz radiation, enhancing sensitivity. Particularly, graphene-based absorbers are preferred for their high sensitivity. This paper presents a novel multi-band square ring resonator-based perfect absorber composed of graphene stripes on the top, SiO2 as substrate, and a gold layer at the bottom. The designed absorber achieves three absorption peaks having above 90% absorption in the frequency range of 5 to 10 THz. Its tunable chemical potential allows for dynamic adjustments of absorption spectra, while its symmetrical design ensures angular and polarization stability. Additionally, the absorber shows potential as a refractive index sensor, making it well-suited for high-sensitivity biomedical applications

We propose and demonstrate a reservoir computing-based recurrent neural network (RC-RNN) for mitigating interference in a photonics radar operating in a multi-radar environment. With the grid search method, hyperparameters of RC-RNN are optimized. Trained with interference signals of varying parameters, RC-RNN achieves over 92% mean R squared and less than 0.1 mean square error (MSE) values during the test stage, demonstrating effective suppression of multi-radar interference.

Although high-bandwidth radar signals can be generated with photonics, frequency regulation restriction, and low-bandwidth electronic front-end components necessitate the fusion of narrowband photonic radar signals. In this paper, we propose and demonstrate the long short-term memory recurrent neural network (LSTM-RNN) based fusion technique. For the proof-of-concept demonstration, we used LSTM-RNN to fuse the 2 GHz narrow-band signals generated at > 75GHz frequencies. A dataset with object detection at varying distances between radar and objects is created. In the training stage, the hyperparameters are optimized and the optimum model is used in the test stage. The proposed LSTM-RNN-based model showed a range resolution value of 2.6 cm for fused narrow-band signals, which is close to the theoretical range resolution of 2.5 cm. Additionally, a 10-dB peak-to-sidelobe level (PSL) improvement is observed with LSTM-RNN compared to the performance when not using it for the fused narrowband signals, demonstrating the effectiveness of the proposed method.

The article introduces a novel terahertz-band-operated MIMO antenna structure with four elements, emphasizing high isolation and a wider bandwidth. Multiple iterations were conducted to optimize the circular radiating element's shape, progressing from one to two and, ultimately, four radiating elements arranged orthogonally. A partial ground plane with stubs enhances isolation, while CSRR-based radiating patches improve bandwidth. The structure utilizes polyimide material for construction. Performance analysis includes return loss, radiation pattern, gain response, isolation, and various MIMO diversity parameters such as ECC, TARC, MEG, DG, and CCL. Comparative evaluation against existing designs validates its novelty. The proposed antenna design is suitable for applications in healthcare, explosive detection, multipliers, and WBAN communications.

This study presents a compact 2x2 circular ring-shaped MIMO antenna for terahertz band applications in upcoming 6G wireless communication. The design incorporates graphene due to its high-speed transmission properties. Decoupling techniques such as DGS and NL enhance isolation between the antenna's radiating elements. A parametric study evaluates the effectiveness of these techniques. The HFSS simulator performance analysis shows that the proposed MIMO antenna achieves high gain, significant isolation, and satisfactory diversity performance. The study also includes a comparative analysis with relevant designs from the literature.

The significant frequency disparity poses challenges in maintaining the compact form factor of 5G handsets while supporting mm-wave bands. Our solution is a super-compact 4-port MIMO antenna utilizing an electromagnetic bandgap (EBG) structure inspired by metamaterials. This design effectively addresses the issue by covering a wide frequency range and reducing mutual coupling (MC). Constructed on a Rogers RT duroid substrate, the antenna integrates four planar patch antennas arranged orthogonally at the corners. Each multiband antenna element features a rectangular patch with four circular slots and a partial ground plane. The antenna achieves improved diversity gain (DG), mean effective gain (MEG), total active reflection coefficient (TARC), and envelope correlation coefficient (ECC). Comparative analysis against existing designs validates its performance enhancement, particularly suitable for 5G communication applications in mm-wave bands, instilling confidence in its effectiveness.

To address the growing number of mobile users, the study proposes a high isolation, high gain, and low ECC terahertz-band-operated MIMO antenna array for next-generation wireless applications. Enhancements in gain, return loss, and isolation are achieved through complementary split-ring resonators integrated into the bottom layer of the THz MIMO antenna array. Graphene serves as the conducting material with a polyimide substrate. This configuration ensures excellent radiating efficiency and high gain. The antenna array's performance is evaluated based on properties such as channel capacity loss, diversity gain (DG), isolation, and envelope correlation coefficient (ECC), adhering to standard values. The manuscript includes a comprehensive literature review and comparative analysis. The proposed THz MIMO antenna array holds promise for applications in sensing, video-rate imaging systems, security scanning, medical imaging for cancer detection, and identifying illicit substances.

The article introduces a novel four-port Plasmonic MIMO antenna design tailored for THz communication. A defective ground structure enhances the slotted design to achieve an ultra-wideband response from 5 THz to 50 THz. The compact size of the antenna facilitates easier integration into products. Design parameter optimization significantly boosts the gain and bandwidth of the proposed design. After adjustments, the antenna demonstrates substantial bandwidth, high gain, and effective isolation. The study investigates various MIMO antenna characteristics, such as ECC, TARC, DG, and MEG. The architecture aims at THz 6G communication applications and wireless personal area networks (TWPAN). Comparative analysis is conducted to highlight the advancements of the proposed design.

Terahertz (THz) holography holds a unique position within the family of coherent imaging techniques. In this work we have demonstrated multifrequency or “coloured” digital holography within the range of 1.39-4.25 THz. Recorded holograms were reconstructed by using two- and four-step phase shifting (PS) technique, which removes the unwanted information and improves the quality of holographic images. Phase values corresponding to the PS introduced by the object in the reference beam, allows deeper and more qualitative inspection of the investigated materials. Such technique provides more information about the low-absorbing objects, which can not be obtained using other imaging methods. This property was demonstrated by recording and reconstructing holograms of different amount of graphene layers. Moreover, holographic images, obtained at three different frequencies – 1.39-, 2.52- and 3.11 THz were combined into one „coloured“ image, by employing a methodology analogous to the RGB colour model.

While it has been long accepted that spin-to-charge conversion processes, namely the inverse spin Hall effect (ISHE), are the driving force behind terahertz (THz) generation in spintronic terahertz emitters (STEs), recent works have questioned this, attributing a major portion of the THz signal directly to laser-induced demagnetisation-driven charge dynamics. We probe carrier dynamics in STEs by fabricating Co/Pt/Co multilayers where the symmetry results in the generation of two THz pulses that are out of phase with each other, resulting in a weak signal compared to a Co/Pt bilayer. By varying the thickness of the Pt layer from 2 nm to 5 nm, we observe that the THz signal amplitude increases as a result of lower optical fluence in the second Co layer, which is responsible for generating an ISHE current in the opposite direction to the first Co layer, leading to a weaker ISHE current. By comparing the THz emission characteristics of these samples, we gain insight into the working mechanism of these emitters and prove experimentally that while several mechanisms may be at play behind the generation of THz radiation in magnetic multilayers, the ISHE is indeed the dominant force.