Optically Tunable Ultrafast Broadband Terahertz Polarimetric Device Using Nonvolatile Phase‐Change Material

Focused THz wave from a spintronic terahertz Fresnel Zone Plate emitter

With the fast development of terahertz (THz) techniques, THz absorbers have a variety of applications. However, current designs have a series of shortcomings, such as low absorption rate and a fixed and narrow absorption bandwidth. To solve such problems, in this paper, we design a tunable broadband THz absorber, which consists of two layers of graphene structures: the top layer is arrayed with the graphene concentric hexagonal rings and the bottom layer is arrayed with regular graphene hexagon rings; therefore, both layers support hybridization and stacking effects. The coexistence of both effects achieves the absorption rate of over 90% in a broad band from 0.93 THz to 1.80 THz. Moreover, the absorption spectrum can be tuned by adjusting the graphene chemical potential, which is insensitive to both incident angle and polarization. Considering advantages such as a tunable broad absorption band, a high absorption rate and insensitive incident angle and polarization, the proposed dual-layer graphene based tunable broadband THz absorber can be a useful reference for absorber design even in other electromagnetic wavebands.

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.

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.

Tunable broadband terahertz absorber based on graphene metamaterials and VO2

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.

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).

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.

Tunable broadband THz absorber using vanadium dioxide metamaterials

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.

The efficient generation and active modulation of terahertz (THz) waves are strongly required for the development of various THz applications such as THz imaging/spectroscopy and THz communication. In addition, due to the increasing degree of integration for the THz optoelectronic devices, miniaturizing the complex THz system into a compact unit is also important and necessary. Today, integrating the THz source with the modulator to develop a powerful, easy-to-adjust, and scalable or on-chip THz emitter is still a challenge. As a new type of THz emitter, a spintronic THz emitter has attracted a great deal of attention due to its advantages of high efficiency, ultrawide band, low cost, and easy integration. In this study, we have proposed a multifield-modulated spintronic THz emitter based on the VO2/Ni/Pt multilayer film structure with a wide band region of 0-3 THz. Because of the pronounced phase transition of the integrated VO2 layer, the fabricated THz emitter can be efficiently modulated via thermal or electric stimuli with a modulation depth of about one order of magnitude; the modulation depths under thermal stimulation and electrical stimulation were 91.8% and 97.3%, respectively. It is believed that this multifield modulated spintronic THz emitter will provide various possibilities for the integration of next-generation on-chip THz sources and THz modulators.

Terahertz (THz) imaging and sensing technologies are increasingly being used at international airports for security screening purposes and at majo r medical centers for cancer and burn diagnos is. The emergence of new THz applications has directly resulted in an increased interest regarding the biological effects associated with this frequency range. Knowledge of THz biological effects is al so desired for the safe use of THz systems, identification of health hazards, and development of empirically-based safety standards. In this study, we developed a state-of-the-art exposure chamber that allowed for highly controlled and reproducible studies of THz bi ological effects. This inno vative system incorporated an industry grade cell incubator system that permitted a highly controlled exposure environmen t, where temperatures could be maintained at 37 °C ± 0.1 °C, carbon dioxide (CO 2 ) levels at 5% ± 0.1%, and relative humidity (RH) levels at 95% ± 1%. To maximize the THz power transmitted to the cell culture region inside the humid incubator, a secondary custom micro-chamber was fabricated and incorp orated into the system. This micro-chamber shields the THz beam from the incubator environment and could be nitrogen-purged to eliminate water absorption effects. Additionally, a microscope that allowed for real-time visualization of the live cells before, during, and after THz exposure was integrated into the exposure system. Keywords: Terahertz, exposure, in vitro , biological effects, incubator, enclosur e, cell homeostasis, real-time monitoring *Cesario.z.cerna.ctr@mail.mil; phone 1 210 539-8130; fax 1 210 539-7945

Tera-Hertz (THz) transmission can offer several attractive applications, yet developing low-cost energy-efficient THz devices is at an early stage. The most promising low-cost THz transmitter architecture in the literature is the so-called frequency-multiplier-last architecture but it is incapable of transmitting quadrature amplitude modulation (QAM) due to the architecture's inherent nonlinear distortions. We study such nonlinear THz communication systems by incorporating the nonlinearity aspects of the low- cost THz devices into the signal model. Then we propose novel pre-compensation schemes to compensate the nonlinearity effect, thus enabling QAM-capable frequency-multiplier-last architecture for THz systems. Numerical results show that the proposed pre-compensation schemes overcome the prominent problems experienced in the existing scheme, namely severe nonlinear distortions of the modulation symbols as well as spectral spreading and/or large spectrum sidelobes.

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.

Despite significant advancements in terahertz (THz) generation and detection technologies, the practical deployment of portable THz systems remains constrained by existing modulator limitations, particularly stringent temperature requirements, inefficient thermal management, and high power consumption. Here, we demonstrate a breakthrough in magnetically controlled THz modulation using a CoFe2O4/MgO/F-Mica composite structure. The tunable optical response and dielectric properties of the composite under varying magnetic fields were systematically investigated via THz time-domain spectroscopy. Experimental results revealed a magnetic field-dependent THz transmission attenuation, with systematic suppression of transmission spectra proportional to the applied magnetic field intensity. A theoretical model accounting for external magnetic field variations was proposed, which agrees well with the experimental results regarding the imaginary component. Remarkably, the composite material realizes phase modulation simultaneously under magnetic field. This exceptional magnetic responsiveness significantly broadens the application potential of ferrites in the THz regime. These findings provide critical insights for designing tunable multifunctional THz magnetic devices in 6G communications, medical diagnostics, and nondestructive testing applications.