This PDF file contains the front matter associated with SPIE Proceedings Volume 12683, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Topological materials have rapidly gained interest as contenders for development of coherent, controllable terahertz emitters. Possessing Weyl nodes either at the surface or within the bulk, they host spin-polarised, helicity-dependent currents that offer possibility to control the emitted THz pulse by changing the polarization of the optical pulses generating the radiation. Here, we show that upon near-infrared excitation at oblique incidence, multi-cycle pulses are generated with a narrow bandwidth of ∼0.4 THz for cadmium arsenide bulk crystals and nanowire ensembles. Both the bandwidth and peak emission frequency of the generated THz radiation can be tuned by respectively varying the photon helicity and angle of incidence of the photoexcitation light.

The time-domain waveforms of terahertz (THz) emission spectroscopy (TES) can tell us the ultrafast nature of the material photoresponse, and TES is becoming an essential tool to explore the advanced material functionalities and disclose the dynamics of the photocarriers. However, universal applications can not be reached without understanding physics in more detail over the broad dimensional range in the time-domain. We have applied TES and LTEM to Si-based materials and devices and proven that one can estimate various parameters, such as surface potential, work function, impurity doping density, defects density in passivation layers, surface state density, and so on, semi-quantitatively and non-contactly by just observing the THz radiation. In the present work, we review the TES application to wide bandgap semiconductors.

A multipixel photoconductive emitter which consisted of an array of interdigitated electrodes was used to electrically control the spatial profile of a THz beam. We demonstrated that by varying the applied voltage levels, it is possible to spatially modulate the THz beam, with shapes tunable from gaussian to arbitrary non-gaussian shapes, such as a top hat. We characterized the THz beam profile at different off-axis transverse positions to validate the beam steering capability of our device. The spatial resolution of the approach was determined for different THz frequencies and the diffraction-limited performance of the system was established by comparison with the Abb´e and Sparrow criteria. We further discuss the scalability of this technology by demonstrating design variations that provide a multilevel THz beam functionality. By varying the emitter’s geometry to adjacent horizontal and vertical pixels, THz beams with either azimuthal, radial, or linear polarization states can be generated that circumvent the need for mechanical polarization optics. The device can be integrated into a compact fibre-based system to realize the fast measurement of both s- and p- polarization states relevant for in-vivo skin diagnostics.

Signal loss due to Fresnel reflections on optical elements is a common issue in the terahertz frequency region, particularly in materials with high refractive indices, such as silicon and sapphire. Moth-eye structures are an effective solution to this problem, as they serve as broad-spectrum antireflection methods. However, creating sub-millimeter-sized protrusions for terahertz waves has been a challenge using conventional methods. Recently, ultrashort-pulsed laser processing has emerged as a precise technique for 3D microstructure production. We have been developing this technology and have achieved moth-eye structures with sub-wavelength precision, specifically tailored to cater to terahertz wavelengths. This presentation will focus on fabricating broadband terahertz anti-reflection moth-eye structures using ultrashort-pulsed lasers. Our design demonstrates almost total transmittance near 100 GHz and maintains this over 1 THz. We have achieved success in using this method to create large moth-eye structures with a diameter of 300mm, which have been utilized for radio astronomy purposes. These laser-fabricated microstructures hold significant promise in terahertz wave control.

Using a single-molecule switch, we study atom-scale light-matter interaction. First, we utilize atomically confined terahertz transients to measure and control molecular motion in real space. Based on atomically precise lightwave-driven scanning tunneling microscopy (STM), we unravel how light pulses can act as sub-picosecond atomic forces on key atoms of a molecular switch to coherently steer structural dynamics. This allows us to control a frustrated structural rotation that modulates the molecule’s switching probability. Second, we investigate near-field waveforms on extremely sub-wavelength volumes. As atomic light-matter interaction crucially depends on both the temporal evolution and the absolute strength of local fields, a parameter-free method to directly measure and calibrate atom-scale waveforms has been highly desirable. Calibrating the electric near field with a single-molecule switch, we quantitatively measure the temporal shape and amplitude of atomically confined light-field transients inside the tunneling gap of the scanning tunneling microscope.

We present original work on non-destructive testing with an imaging solution combining a sensor that can measure the millimeter-wave radiation of a black body at temperatures between 290-400°K and a portable solution based on augmented reality with a smartphone. This handy portable solution makes it possible to do away with mechanical scanning systems which are heavy and slow, and therefore may be suitable for civil engineering detection or imaging applications, or in the field of aeronautics.

The challenges of THz electronics in CMOS technologies for THz sensing and communication applications are addressed in this work, focusing on device-level design to system integration approaches. State-of-the-art THz circuits, components, design methodologies, and system demonstration were proposed, including a 40-nm-CMOS transistor layout design using an electromagnetic modeling approach, a 340-GHz higher-order-mode high-gain dielectric resonator antenna, a 340-GHz CMOS heterodyne receiver, a 340-GHz heterogeneously-integrated THz transmitter with 2×25 antennas, a THz heterogeneously-integrated platform with low-loss single-band, dual-band, and broadband interconnects, and a THz transmissive imaging system with a spatial resolution of 1.4 mm at 336 GHz.

We review recent progress in the development of the UTC-PD-integrated HEMT carrier frequency down-converter as an optical-to-sub-terahertz (THz)/THz carrier frequency down-converter for use in the future beyond 5G wireless networks. First, we investigate the effect of the scaling of the UTC-PD active area on the double-mixing conversion gain. We show that the conversion gain monotonically increases as the active-area size is reduced, due to the suppression of the in-plane diffusion of photoelectrons in the UTC-PD absorption layer. Second, we examine the employment of intensified, low-noise subcarrier signal input to the device enabled by an optical injection-locking technique.

THz photoconductive antennas (PCAs) have found widespread use in THz generation, detection, and various applications such as sensing, imaging, and communication. For achieving ultrafast operation, most commercially available THz PCAs rely on III-V epitaxial materials due to their high mobility and ultrafast response. However, launching the entire device fabrication process through IC foundries presents significant challenges, thereby limiting the capability of device mass production. In this study, we propose the use of GeSn alloys as the photoconductive material for THz generation. Furthermore, the use of GeSn alloys can potentially offer additional advantages such as cost-effectiveness, scalability, and improved performance.

GeS and GeSe are 2D semiconductors with band gaps in the near infrared and predicted high carrier mobility. We find that excitation with 800 nm pulses results in long-lived free photocarriers, persisting for hundreds of picoseconds, in GeS and GeSe noribbons. We also demonstrate that zerovalent Cu intercalation is an effective tool for tuning the photoconductive response. Intercalation of ~ 3 atomic % of zerovalent Cu reduces the carrier lifetime in GeSe and GeS. In GeS, it also shortens the photoconductivity rise and improves carrier mobility.

Aperture near-field microscopy and spectroscopy (a-SNOM) enables the direct experimental investigation of subwavelength-sized resonators by sampling highly confined local evanescent fields on the sample surface. Despite its success, the versatility and applicability of a-SNOM is limited by the sensitivity of the aperture probe, as well as the power and versatility of THz sources used to excite samples. Recently, perfectly absorbing photoconductive metasurfaces have been integrated into THz photoconductive antenna detectors, enhancing their efficiency and enabling high signal-to-noise ratio THz detection at significantly reduced optical pump powers. Here, we discuss how this technology can be applied to aperture near-field probes to improve both the sensitivity and potentially spatial resolution of a-SNOM systems. In addition, we explore the application of photoconductive metasurfaces also as near-field THz sources, providing the possibility of tailoring the beam profile, polarity and phase of THz excitation. Photoconductive metasurfaces therefore have the potential to broaden the application scope of aperture near-field microscopy to samples and material systems which currently require improved spatial resolution, signal-to-noise ratio, or more complex excitation conditions.

This paper reviews recent advances in the research and development of graphene-based plasmonic metamaterials for terahertz laser transistors, particularly focusing on the topic of the mechanisms of graphene plasmonic instability which includes a new type called “Coulomb-drag instability” that the author’s group has recently theoretically discovered.

This paper reviews recent advancements in the research of THz detection by an asymmetric dual-grating gate structure epitaxial-graphene-channel field effect transistor (ADGG-EG-FETs). We designed and fabricated ADGG-EGFET for plasmonic (PL) detection, and it performed a high sensitivity and fast response to irradiated THz with 0.95 THz. The behavior of measured dependence on gate bias voltage cannot be explained only by the PL effect. We found such a phenomenon as a new current-driven phototermoelectric (PTE) detection assisted by electrostatic carrier drift/diffusion under the application of DC drain biases. Furthermore, we analyze the response speed of our fabricated detector to reveal the transition point between PL and PTE detection mechanisms. The minimum output pulse width was ~190 ps when one ADGG bias was at the Dirac voltage (i.e., charge neutrality point) to promote the PL detection, whereas the pulse width was ~200 ps when both ADGG biases were at well-doped levels to promote the PTE detection. Compared with the input pulse width of 155 ps, the intrinsic response time of the detector was estimated to be 10 ps for the PL and 20 ps for the PTE detection. This can be quantitatively explained by the characteristic relaxation times of the momentum relaxation for the PL, and the energy relaxation of the hot electrons by optical-phonon emission for the PTE detection. These results indicate that the ADGG-EG-FETs THz detectors are promising for applications in 6G to 7G-class THz wireless communication systems.

We report on various metasurfaces for the purpose of THz driven electron field emission and subsequent detection using vacuum electronics. The underlying principle is based on strong localised field enhancement at metal and semimetal emission points, which bends the vacuum potential temporarily to allow for field emission of electrons from the parent material. The structures are investigated for varying electric field strength using electron time-of-flight measurements as well as electron multiplication and visualisation on a phosphor screen. Measured properties include the emitted electron energy, their count, and the emission threshold. From the recorded data, the local field enhancement for each structure is extracted and compared to simulated values. Subsequently, optimised metasurfaces are implemented into handheld devices that serve as easy-to-use THz detectors. These devices include photomultiplier tubes which operate at frequencies from THz to infrared, as well as live imaging devices with kilohertz framerates. The investigated metallic structures include standard dipole antennas, double split-ring resonators, bow-tie designs, hybrid split-ring and dipole designs, and logarithmic spirals. Semimetallic structures are based on structured and unstructured graphene, which show different emission characteristics. All samples are investigated using strong-field THz radiation generated using lithiumniobate tilted pulse front setup, as well as commercial THz-TDS instruments. In conclusion, we present a holistic overview of the current state-of-the-art THz-PMTs and image intensifiers.

Terahertz spectral measurements of crystalline materials are known to be particularly sensitive to both the material’s long-range order and lattice dynamics. Here we explore a range of materials including common crystalline organic materials such as α-lactose monohydrate and l-cysteine through to engineered materials including topological insulators (Bi₂(Te_(1−x)Se_x)₃) where structure and disorder can be more finely controlled. By comparing variable temperature THz spectral measurements to ab-initio simulations using a range of methods and structures we can begin to unpick the origins of these spectra, and how they are influenced by dynamics and disorder across the lattice.

This research addresses the complex issues encountered when studying biological and chemical samples in the terahertz (THz) range, mainly due to water's high absorption properties. Using THz-Attenuated Total Reflectance (ATR) spectroscopy, we investigate water and NaCl solutions and provide both experimental data and a comprehensive interpretation of the optical phenomena observed. . Our focus lies in understanding the changes in ATR responses in the THz range for solutions in both bulk and thin film forms, using a non-linear regression analysis of effective optical functions. Moving beyond the traditional low-absorption limit, our study highlights the need for precise equations. We further reveal the significant impact on the optical constants n and k of solutions when NaCl is introduced, offering valuable insights for interpreting spectral data for lossy samples using ATR. Our work seeks to deepen understanding and encourage further research of biological and chemical solutions at THz frequencies.

Commensurate with the push toward next-generation (6G) wireless communications and sensing, branches of terahertz research have begun shifting from material, device, and simulation studies to more system-level, real-world demonstrations. Commercial mm-wave and terahertz instruments are beginning to reach a point where measurements can be made with realistic waveforms for validating channel models, signal processing schemes, and the performance of devices such as intelligent reflecting surfaces or beam manipulators, all of which are critically involved in communications, sensing, or their joint combination. Nevertheless, it remains challenging to implement functional systems that support broad bandwidths and terahertz carrier frequencies, simultaneously. Using a combination of commercial-off-the-shelf components, we present an integrated platform for studying terahertz applications, such as 6G communications and remote sensing, enabling up to 25 GHz of instantaneous bandwidth with any carrier frequency in the range of 75-500 GHz. The platform can generate arbitrary waveforms to accommodate a variety of applications and further contains a diverse suite of amplifiers, lenses, reflectors, and antennas to enable studies of long-distance terahertz communication, terahertz beam steering and shaping devices, and channel phenomenology. For two examples, we first present a demonstration of 20 Gbit/second wireless communication at 130 GHz between 50-180 m. Second, we present a demonstration of broadband terahertz communications at 300 GHz with up to 19 Gbit/second data rate to validate an artificial-dielectric-based beam scanning device.

We report the generation of 200kW-peak-power coherent radiation at 53 microns from a Raman-suppressed stimulatedpolariton- scattering laser using a thin KTP gain crystal at room temperature. The far-infrared radiation consists of ~500 radiation cycles in an 83-ps pulse width. Such a far-infrared source has a comparable or higher peak power than a nominal THz free-electron laser.

The carrier frequency of the modern telecommunication system had been raised to hundreds of GHz and aimed at the THz regime. Besides, THz waves have been intensively applied in many fields, e.g., spectroscopy, imaging, and communications. However, there is a rarity of available techniques for modulating few-cycle THz waves on picoseconds timescale. Here we report a simple/reliable system without spatial light modulators (SLMs) for generating circularly polarized THz dual pulses with variable helicity, frequency, and interval. These degrees of freedom allow us to arbitrarily control the THz double pulses of interests, which have potential applications in imaging, spectroscopy, and next-generation communications.

Significant progress has been made in the development of table-top terahertz (THz) radiation sources, and the two-color laser-induced air plasma method is leading the way. Its ultra-broad emission bandwidth and intense radiation make it a top contender. In this paper, we report the successful generation of narrowband THz radiation from air plasma induced by a two-color laser pulse train. Using a Michelson interferometer implemented with a delay stage and a wedge pair, we coherently controlled and fine-tuned the primary characteristics of the narrowband THz pulse, including the center wavelength and carrier-envelope phase (CEP), which was achieved by manipulating the delay stage and wedge pair displacement. Our work has demonstrated that air plasma is an exceptional THz emitter with outstanding tunability and has revealed a potential method for actively stabilizing the CEP of narrowband THz radiation.

In this report, we theoretically show that intense multicycle terahertz (THz) pulses can be generated by optical rectification in an artificial periodically poled lithium niobate (PPLN) structure formed by placing a phase-shift mask in front of a large aperture stoichiometric lithium niobate (SLN) crystal. In contrast to the common scheme with a triangular prism-shaped congruent LN crystal, THz generation is studied for a rectangular trapezoid SLN sample having a small angle (≈ 26°) of the inclined surface. A matching Si-prism is attached to the trapezoid base to guide the generated THz wave into free space. It is shown that the number of field oscillations (from nearly single-cycle to many cycles) can be varied by the changing of the pump beam linear size in the crystal. Also, there is a possibility of tuning the generation frequency (in the range of 0.4 - 0.8 THz) by building a mask image in the SLN with various demagnification. According to estimates, the energy of narrowband THz pulses at a frequency of 0.5 THz in SLN crystal at temperature 100 K is 265 μJ at a pump pulse energy of 220 mJ. This corresponds to a pump-to-THz conversion efficiency of 0.12 %.

This study presents a systematic approach for designing and fabricating a new type of Terahertz optical fiber using 3D printing technology. Negative curvature optical fibers with multiple nested-tubes were designed using an FEM-based electromagnetic solver. The number of supported modes was found to increase significantly with utilization of more tubes. The fibers with tube thicknesses of 0.09 mm and core diameters of 3 mm and 8 mm were fabricated using a UV resin-based 3D printer. An imaging setup was built to confirm the geometrical properties. This study demonstrates the feasibility of using 3D printing to fabricate functional Terahertz optical fibers.

In this study, a 3D printing compatible THz chemical sensing platform using negative curvature fibers was numerically investigated. Since the negative curvature design of the fiber allows spectral sensitivity based on the refractive index of the fiber core area, high sensitivities for liquid chemical sensing are observed. The fibers with elliptical tube cladding elements made of UV epoxy resin were designed using a finite element based electromagnetic solver to optimize the confinement and material losses, as well as to control polarization-based sensing by asymmetrical placement of tubes. By analyzing both confinement and material losses for different cladding structures, high sensitivities (>98%) for the detection of ethanol and benzene at an operational frequency of 1 THz are achieved. In order to calculate sensitivity values, the power fraction between the core and cladding areas were computed, and dispersion coefficients were also analyzed in the designed fibers. Using a UV resin-based 3D printer, the designs with a core diameter of 3 mm and tube thicknesses of 0.1 mm were fabricated, and the feasibility of using 3D printing was investigated using image analysis. Overall, the optimized negative curvature fiber design with elliptical cladding elements allowed improved sensitivities for chemical sensing applications. The use of 3D printing technology offers potential for cost-effective and efficient fabrication of THz chemical sensing platforms.