Novel Pre-Compensation Schemes for Low-Cost Nonlinear Tera-Hertz Transmitters

Owing to the unique characteristics of terahertz (THz) waves and their interaction with matter, THz imaging has significant potential in diverse fields, including life sciences, non-destructive quality control, and security. However, today’s THz technology faces major obstacles in providing the cost, compactness, and functional scope that is required to facilitate a wide-reaching use outside the laboratory. Therefore, the general aim of this thesis is to increase the societal impact of THz technology by exploring novel THz imaging modalities with integrated circuits (ICs) based on conventional silicon technolgy. Previous research in THz ICs mostly focused on building compact integrated source and detector components for far-field imaging systems that exploit the ability of THz waves to penetrate through diverse dielectric materials. One part of this thesis joins this research and demonstrates low-cost volumetric THz imaging based on computed tomography with silicon components for the first time. To this end, a high-power 0.43 THz source in 0.13 μm silicon-germanium heterojunction bipolar transistor (SiGe HBT) technology is developed in this thesis. However, the resolution of THz far-field imaging is limited by diffraction to the millimeter range, whereas some of the most promising THz applications in life sciences, such as intraoperative imaging of cancerous tissue, are in need for a microscopic resolution to resolve material properties on the cellular level. This thesis presents the realization of microscopic THz imaging with a silicon-based sensor system. The central contribution of this work is the development and analysis of a 128-pixel THz near-field sensor System-on-a-Chip (SoC) operating at 0.55 THz and showing a spatial resolution around 10 μm. The sensor exploits the capacitive near-field interaction between split-ring-resonator probes and imaging objects, giving an imaging contrast based on the dielectric permittivity. To simultaneously enable real-time image acquisition, high sensor sensitivity, and an integration level that is comparable to conventional consumer electronics, the full integration capabilities of a high-speed 0.13 μm SiGe bipolar CMOS (SiGe-BiCMOS) technology are exploited. In particular, the presented SoC employs cointegration of a chip-scale one-dimensional THz sensor front-end, analog signal processing, and digital circuitry for controlling the chip and external communication. The achieved results in terms of imaging speed, system cost, and integration level reach well beyond the state of the art in THz near-field imaging. The ability to rapidly acquire THz images with micrometer-scale resolution will be of benefit to fundamental research into material properties in the THz range and lays the foundation for the exploration of THz bioimaging applications on the cellular level. In particular, the sensor enables the conduction of large-scale clinical studies on the relevance of microscopic THz imaging to ex vivo tumor margin assessment in breast cancer surgeries for the first time.

Focused THz wave from a spintronic terahertz Fresnel Zone Plate emitter

High modulation depth fiber-based modulators working at terahertz (THz) wavelength are of paramount importance for the application of THz communication technology by taking advantages of the low loss, high interference resistance and strong adaptability THz fiber network. Here, we demonstrate a THz modulator based on side-polished THz fiber combined with GaAs film/Au-nanoparticles (Au-Nps). The Au-Nps and the remaining thickness of the side-polishing are careful designed to achieve target resonance wavelengths. The side-polishing region of fiber provides a flat platform for integrating nanostructures to enhanced light-matter interactions which greatly enhances the modulation performance. The experimental results show that this new fiber modulator can achieve the THz wave amplitude modulation under a relatively low irradiation of an external 808 nm laser, with a tunable frequency range of 0.366-1.093 THz and a maximum modulation depth of 99.24%. Besides, resonance frequency can be adjusted by changing the remaining thickness of the device. Moreover, the area of this modulator is only 9 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> which can be easily integrated into THz systems. Therefore, we believe our work will contribute to the developments of high-performance fiber-based THz modulator for applications in THz communication system.

Actively tunable ultrafast broadband terahertz (THz) polarimetry using a reconfigurable phase‐change material holds great potentials and prospects for the achievement of next‐generation versatile integrated THz components and systems in a variety of THz applications. Here, an optically tunable ultrafast broadband THz polarimetric device (THz‐PoD) composed of a phase‐change material Ge2Sb2Te5 (GST) and a thin mica substrate is demonstrated. This proposed novel THz‐PoD is verified for a frequency range of 0.1–2.5 THz, exhibiting broadband and ultrafast determination of polarization states for linearly polarized THz waves at polarization angles from −90° to 90°. It is shown that optical excitation with ns pulses allows easy and efficient control of the polarimetric properties of such THz‐PoD. The essential role of the GST film in switching the phase transition between the amorphous and crystalline phases is emphasized by the theoretical investigation of the optically tunable ultrafast polarimetric mechanism of the device. This phase transition allows optically changing the THz‐PoD's properties by ns‐pulsed laser in a controlled way to achieve THz polarimetry for linearly polarized THz waves. The combined advantages of this strategy can open up a new and promising way for realizing versatile reconfigurable and integrated THz devices, which may further promote the development of novel THz systems and applications.

In the development of terahertz (THz) communication systems, the nanoantenna is the most significant component. Especially, the focus is to design highly directive antennas, because it enhances the performance of the overall system by compensating the large path loss at THz and thus improves the signal-to-noise ratio. This paper presents suitable material for nanoantenna design and the advancement in their performance for THz communications. Copper, Graphene, and carbon nanotube materials are used as promising candidates for nanoantenna design. The performance of nanoantennas is carried out by analyzing the properties and behavior of the material at THz. Results show that the Graphene nanoantenna provides better performance in terms of miniaturization, directivity, and radiation efficiency. Further, the performance enhancement of the nanoantenna at THz is studied by dynamically adjusting the surface conductivity via the chemical potential of Graphene using the electric field effect. The performance of the nanoantenna is enhanced in terms of high miniaturization, high directivity, low reflection, frequency reconfiguration, and stable impedance. The THz nanoantennas using Graphene have the potential to be used for THz communication systems. In view of the smart THz wireless environment; this paper finally presents a THz Hypersurface using Graphene meta-atoms. The user-side Graphene nanoantennas and environment-side Graphene Hypersurface can build a promising smart THz wireless environment.

Recent research progress of terahertz (THz) radiation generation based on the ultrafast spin dynamics is reviewed. The transient spin-charge conversion based on the inverse spin Hall effect and the Rashba-Edelstein effect is introduced, and it is pointed out that the ferromagnetic/non-magnetic heterostructure has been used to design a low-cost and high-efficiency THz radiation source. The efficiency and bandwidth of a spintronics-based THz emitter can be improved by optimizing layer thickness, growth conditions, substrates, and construction. The applications of the THz emission spectrum are outlined in the study of the ultrafast formation dynamics of the spin Seebeck effect. © 2019, Chinese Lasers Press. All right reserved.

The area of Terahertz (THz) is one of the fastest growing research fields in recent years. THz antennas based upon photoconduction techniques are the most common devices in THz systems. However, the radiated THz power from these devices and the efficiency are very low. Also, different antenna excitation and current generation process in THz antennas, as compared to microwave antennas, requires new analyses approaches. Therefore, the motivation of this thesis is to theoretically establish why the THz antenna is inefficient; from which, general methods to improve the performance of such antennas are explored and validated. These investigations are essential to gain a better understanding of THz photoconductive antenna performance. In this research a new equation for the source conductance of a THz antenna is firstly developed. This is a prerequisite for further antenna radiated power analysis. Next, a new equivalent circuit, modelling the underlying physical behaviour of the device through the use of a lumped-element network, is developed. Through this model, various factors which affect the radiated power and efficiency of the THz photoconductive antenna are examined and compared with measurement results. This model can be applied to maximize the optical-to-THz conversion efficiency. Also, temporal voltage behaviour of the antenna can be predicted more realistically. Furthermore, a computational simulation procedure, solving both optoelectronic and electromagnetic problems, is proposed and validated by measurement results. This approach facilitates prediction of THz photoconductive antenna performance before antenna fabrication. In addition, considering the requirement of high THz power and good SNR devices for various THz applications, a new top loaded THz antenna embedded on a conical horn with the trapezoidal photomixer is proposed. The generation of THz photocurrent, impedance matching and coupling of the THz wave to air (the necessary factors for power enhancement) are improved. Moreover, the new trapezoidal photomixer is examined and the measurement results show that it has better radiated THz power and SNR than the bare gap and rectangular photomixers.

Quasi-optics are essential components of terahertz (THz) systems, used to direct and manipulate THz radiation. High-resolution additive manufacturing (AM) techniques, such as vat photopolymerization (VP), enable the low-cost production of complex quasi-optical components with optically smooth surfaces and fine geometric features with dimensions comparable to wavelengths in the THz region. However, VP-produced quasi-optics are often inefficient owing to significant absorption losses, with commercially available photopolymers exhibiting absorption coefficients ranging between 19 – 27 cm –1 at 1 THz. Polytetrafluoroethylene (PTFE) is highly transparent in the THz region, making it a promising filler material to reduce the absorption of photopolymers. However, no previous studies have reported the development of a PTFE composite suitable for processing via VP. In this study, PTFE powder-filled composites were formulated containing PTFE-to-photocurable-resin ratios ranging from 1:10–4:10, corresponding to approximately 9.0 % and 28.5 % by weight, respectively. Samples were manufactured via VP and characterised from 0.5 to 2.0 THz using a THz time-domain spectroscopy system. The results demonstrated a correlation between higher PTFE ratios and a reduction in the absorption coefficient of the samples from 0.5 to 2.0 THz, with the 4:10 sample having an absorption coefficient of 15.3 ± 0.3 cm –1 at 1 THz, 27.8 % lower than that of the neat resin (21.2 ± 0.4 cm –1 ). Additionally, experimental results were shown to agree well with a Maxwell Garnett effective medium approximation-based calculation of the THz optical properties of AM composites. These findings highlight the potential of non-polar polymer fillers to enhance the transparency of photopolymers to THz radiation, potentially supporting the future development of more efficient VP-produced quasi-optical components. • PTFE-filled photopolymer composites were developed for stereolithography systems. • The ∼30 wt% PTFE composite had 27.8 % less absorption at 1 THz than the base resin. • The Maxwell Garnett model matched experimental results with high accuracy. • Non-polar fillers can enhance the THz transparency of photopolymers. • Improves the efficiency of THz optics made via stereolithography.

The terahertz (THz) regime of the electromagnetic spectrum is rich with the emerging possibilities in imaging applications with unique characteristics to screening for weapons, explosives and bio-hazards, imaging of concealed objects, water content, and skin, and these advantages can be harnessed by using the effective THz sources and detectors. In THz imaging systems, the pulsed THz sources and detectors find unique applications and thus we have emphasized on re-visiting these kinds of systems. Several novel imaging techniques which exploit the distinctive properties of the THz systems have been presented. Moreover, the THz antenna is one of the most important components of a THz imaging system as it plays a significant role in both impedance matching and power source. Therefore, the recent developments in THz antenna design for imaging applications are reviewed and the potential challenges of such THz systems are investigated. The photoconductive antennas form the basis of many THz imaging and spectroscopy systems and finds promising applications in various scientific fields. However, for the imaging applications, there is a requirement of planar and compact THz antenna sources with on-chip fabrication and high directivity in order to achieve large depth-of-field for better image resolution. Therefore, the key modalities of improving photoconductive dipole antennas performance are identified for imaging applications. Also, the ways to improve the directivity of the photoconductive dipole antenna are discussed. The main purpose of this review is to provide an assortment of all relevant literature to bring researchers up-to-date on the current state-of-the-art and potential challenges of THz antenna technology for imaging applications.

Due to the difficulties faced in fabricating robust Terahertz (THz) optical components with low Fresnel reflection loss, the need to increase the efficiency of THz system with reduced cost is still considered as one of the most essential tasks. In this report, a new low cost THz polarizer with robust structure is proposed and demonstrated. This new THz wire grid polarizer was based on an anti-reflection (AR) layer fabricated with low temperature metal bonding and deep reactive ion etching (DRIE). After patterning Cu wire gratings and the corresponding In/Sn solder ring on the individual silicon wafers, the inner gratings were sealed by wafer-level Cu to In/Sn guard ring bonding, providing the protection against humidity oxidation and corrosion. With the low eutectic melting point of In/Sn solder, wafers could be bonded face to face below 150°C. Two anti-reflection layers on both outward surfaces were fabricated by DRIE. With the mixing of empty holes and silicon, the effective refractive index was designed to be the square root of the silicon refractive index. The central frequency of the anti-reflection layers was designed between 0.5THz to 2THz with an approximate bandwidth of 0.5THz. The samples were measured with a commercial free-standing wire grid polarizer by a THz time domain spectroscopy (THz-TDS) from 0.2THz to 2.2THz. The power transmittance is close to 100% at central frequency. Extinction ratio of the polarizer is between 20dB to 40dB depending on the frequency. The advantages of this new polarizer include high transmittance, robust structure and low cost with no precision optical alignment required.

Advances in infrared (IR) detector technologies over the last decade have led to compact low-cost thermal imaging systems that have become almost ubiquitous. They are now used in such market applications as automotive, security and construction. Terahertz (THz) imagers can take advantage of the state-of-the-art in the infrared domain to reduce their size and cost. Such an example is the IRXCAM-THz-384 Terahertz camera whose electronics core is based on the IRXCAM camera core and whose detector has been specifically designed and optimized for the THz. The 384 x 288 35- micron-sized pixel detectors of both cameras are uncooled microbolometers. A micro-electronics core is currently being developed for both platforms that will yield ultra-compact IR and THz cameras. While IR systems are passive and thus do not require an illumination source, the THz system does. Thus, the THz source must be included when talking about overall imaging system size and cost. There are a wide variety of THz sources, from quantum cascade lasers on the optical side of the radiation spectrum to different types of diodes on the electromagnetic micro-wave side. When considering a source for a given application, the output wavelength, output power, size, weight and cost are primary factors that must be taken into account. This paper presents a description of a compact real-time imaging system at 750 μm wavelength. An overview of the motivation for the wavelength choice is discussed, a description of the imaging components is given and finally image results are presented.

Terahertz (THz) waves, occupying a unique position bridging microwave and infrared wavelengths, exhibit promising potential across diverse domains, particularly in medicine. With attributes such as non-ionizing nature and notable resolution capabilities, THz waves offer avenues for non-invasive medical applications. In cancer diagnosis, THz spectroscopy emerges as a pivotal tool for qualitative and quantitative analysis of biomarkers, enabling swift and accurate differentiation of substances based on molecular structure. Despite challenges such as limited penetration depth in tissue, THz imaging proves invaluable in discriminating cancerous tissues from normal ones during surgery. In this paper we have summarized recent research results on practical implementation of THz waves in medicine. Recent advancements in in vivo imaging showcase promising results, although challenges persist in human-based studies due to tissue complexity. Integration of advanced algorithms enhances the accuracy of cancerous tissue identification. Additionally, THz spectroscopy finds versatile applications in pharmaceuticals and life sciences, offering insights into molecular interactions critical for drug efficacy and understanding protein conformations. Further advancements in THz systems, including enhanced light sources and detectors, are crucial to propel biomedical research forward. Establishment of a comprehensive THz spectrum database will augment existing data, emphasizing the potential of THz waves to revolutionize medical diagnostics and treatment modalities.

To support extremely high data rates in 6G wireless networks, reconfigurable intelligent surface (RIS)-assisted terahertz (THz) communications have gained much attention in recent years. By manipulating the phase shifts of reflecting elements, the RIS can proactively adjust the wireless propagation environment of THz systems, thereby enhancing the overall throughput significantly. To realize the full potential of RIS-assisted THz systems, an acquisition of accurate channel information is of great importance. However, since the wavefront of the THz electromagnetic signal is spherical, the conventional channel estimation techniques using the planar wavefront assumption suffer from severe performance degradation in the near-field RIS-assisted THz systems. An aim of this work is to propose an efficient channel estimation technique for near-field RIS-assisted wideband THz systems. Key idea of the proposed polar-domain frequency-dependent RIS-assisted channel estimation (PF-RCE) scheme is to estimate the sparse multipath components (i.e., angles, distances, and path gains) of the near-field THz channel by exploiting the polar-domain sparsity and common support properties. We demonstrate from the numerical evaluations that PF-RCE achieves a significant performance gain over the conventional THz channel estimation schemes in terms of the normalized mean square error (NMSE).

The rapid evolution of terahertz (THz) technology has opened new frontiers in high-speed wireless communication, imaging, and sensing. However, the practical deployment of THz systems, particularly in remote or space-constrained environments, is often limited by power constraints and rigid antenna structures. A reconfigurable THz antenna system that is powered by solar energy is suggested. By leveraging real-time energy harvesting, the antenna system maintains operation without external power sources, making it ideal for deployment in remote IoT nodes, CubeSats, or energy-constrained sensor networks. Material used in antenna design is lead glass while gold is used as a radiator, the MIMO antenna loaded with graphene with chemical potential of graphene= 0 works from 2.4–2.5 THz, 3.87–3.98 THz, and 5.17–5.3 THz. The MIMO antenna with graphene chemical potential= 1 eV works from 2.54–2.63 THz, 3.96–4.1 THz, and 5.27–5.45 THz. The MIMO antenna with graphene chemical potential= 2 eV works from 2.6–2.68 THz, 4–4.2 THz, and 5.3–5.55 THz. Gain of antenna at 2.45, 3.95, and 5.3 THz maximum gain obtained at these frequencies is 6, 5 and 7 dBi, respectively. By integrating real-time solar energy harvesting with a tunable THz antenna structure, the design enables self-sustained, high-efficiency operation for remote IoT nodes, CubeSats, and energy-constrained sensor networks, merging green energy and next-generation THz communication in a compact platform. Advantages of the proposed design are self-powered operation, reconfigurable multi-band tuning, high gain, and suitability for diverse THz applications.OPEN ACCESS Received: 23/06/2025 Accepted: 17/09/2025 Published: 23/01/2026

In this work, we experimentally demonstrate a bifunctional terahertz (THz) metasurface that can achieve broadband linear-to-circular polarization conversion (LCPC) and linear polarization conversion (LPC) in two different bands, respectively. The measured 3-dB axial ratio relative bandwidth is 34.2% for the LCPC in the lower band from 0.34 THz to 0.48 THz, and the measured polarization conversion ratio is over 80% for the LPC in the higher band from 0.53 THz to 0.85 THz. The physical mechanism of the LCPC and LPC is explained by the surface current distribution and is further discussed by employing the multiple interference model. This bifunctional THz metasurface polarization converter has promising applications in THz systems, communications, spectroscopy, etc., and the design strategy can also be extended to microwave and millimeter bands.