Terahertz Channel Characterization using a Silicon-based Picosecond Pulse Source

Terahertz (THz)-band (0.1-10 THz) communication has been envisioned as a key technology to enable wireless Terabit-per-second (Tbps) links. At THz frequencies, the path-loss is governed by the spreading loss and the molecular absorption loss. The latter also determines the available transmission bandwidth, which drastically shrinks with distance. As a result, traditional modulation schemes cannot fully take advantage the THz channel, and new modulation schemes are needed if THz channel communications are going to reach their full potential. Several solutions have been presented including Hierarchical Bandwidth Modulations (HBM), which is the only presented work that not only compensates for molecular absorption losses but leverages those losses to improve the capabilities of the system. The focus of this thesis is two-fold. First the design of HBM is formalized, exploring the trade-offs and achievable performance as a function of system parameters. Secondly, these trade-offs and performance metrics are verified using a one-of-a-kind experimental testbed for ultrabroadband communication networks. The results show that with proper design HBM successfully achieves its goal of exploiting the distance-dependent characteristics of the THz channel.--Author's abstract

Stochastic geometry analysis of interference and coverage in Terahertz networks

Nowadays, many wireless communication applications and services demand high capacity and bandwidth. Experts expect that in next few years, communications markets need to use high data rates up to 40 Gbps. For meeting the requirements of high data rate, bands up to 3 THz hold promise as they can achieve data rates up to a few tens of Gbps. The bands ranging from 275 to 3,000 GHz are known as terahertz (THz) bands. THz bands are not assigned yet globally for dedicated services and communication applications. The implementation of THz communication requires various kinds of evaluations and effort in order to assess different aspects related to channel characterizations, THz system designs, propagation, algorithms and techniques for signal processing, platform configurations, in addition to security and privacy issues. The evaluation and assessment should also consider the statistical parameters and measurements of the channels. This chapter introduces the key aspects of THz channel specifications and related considerations. In addition, a comparison between mm-waves and THz channel characteristics is made. The chapter also discusses the theoretical concepts of THz channel characterizations, limitations, requirements, and methodologies for measurements including a comparison between different channel analyses. Moreover, the chapter provides a brief description of THz channel characterization in nano communications.

We report a homogeneous quantum cascade laser (QCL) emitting at terahertz (THz) frequencies, with a total spectral emission of about 0.6 THz, centered around 3.3 THz, a current density dynamic range Jdr = 1.53, and a continuous wave output power of 7 mW. The analysis of the intermode beatnote unveils that the devised laser operates as an optical frequency comb (FC) synthesizer over the whole laser operational regime, with up to 36 optically active laser modes delivering ∼200 µW of optical power per optical mode, a power level unreached so far in any THz QCL FC. A stable and narrow single beatnote, reaching a minimum linewidth of about 500 Hz, is observed over a current density range of 240 A/cm2 and even across the negative differential resistance region. We further prove that the QCL FC can be injection locked with moderate radio frequency power at the intermode beatnote frequency, covering a locking range of 1.2 MHz. The demonstration of stable FC operation, in a QCL, over the full current density dynamic range, and without any external dispersion compensation mechanism, makes our proposed homogenous THz QCL an ideal tool for metrological applications requiring mode-hop electrical tunability and a tight control of the frequency and phase jitter.

The ever-growing number of interconnected devices and the emergence of high-bandwidth technologies such as holographic communications in mixed reality are driving an exponential growth in the demand for terabits-per-second (Tbps) data rates with sub-milliseconds latency. For this reason, the sixth-generation (6G) wireless communications are moving towards the terahertz (THz) frequency band because of its wider spectral bandwidth availability to enable higher channel capacity and data rates according to the Shannon-Hartley theorem. However, the efficiency of wireless THz signal transmission is reduced by the increased path loss and atmospheric attenuation due to water vapor absorption at THz frequencies, which significantly decreases the signal-to-noise ratio and the spectral efficiency of THz wireless communications. The realization of THz band communications is further impeded by the limited power output of existing compact THz sources due to the extremely low conversion efficiencies. Therefore, it is crucial to develop broadband THz integrated circuits with waveguiding and beamforming functionalities for supporting high data-rate on-chip communications and to provide sufficient gains to counteract the path loss and limited power of THz wireless transmissions. This thesis aims to improve the spectral efficiency of 6G wireless communications via the spatiotemporal shaping of THz waves using silicon photonics. Spatiotemporal light shaping is the basis of nearly all THz photonic devices, ranging from THz wavefront manipulation using metasurfaces to THz planar waveguiding using photonic crystal waveguides. Silicon photonics is a promising material platform for THz wave shaping due to the low-loss propagation of THz waves in high-resistivity silicon and the prospect of a monolithic integrated circuit due to the complementary metal-oxide semiconductor (CMOS) fabrication process compatibility. Silicon metasurfaces allow the wavefront modulation of incident THz radiation by exploiting the generalized Snell’s law of refraction and reflection to beamform highly directional THz waves in a dynamic multi-user communication environment for enhancing the spectral efficiency. Likewise, broadband silicon photonic crystal waveguides allow the in-plane and lossless transport of THz waves to support on-chip high data-rate THz communications that can also seamlessly integrate multiple THz photonic devices in a THz integrated circuit. In addition, silicon electron emitters and silicon gratings potentially allow the on-chip generation of highly intense and broadband THz radiation to counteract the significant THz attenuation. In this thesis, spatiotemporal wave shaping techniques using silicon photonics for enhancing the spectral efficiency of THz band communications is presented. First, a self-adaptive and deep reinforcement learning model for THz beamforming is proposed, which can beamform multiple highly directional THz radiation beam in real-time to serve a dynamically changing multi-user communication environment and improve the signal-to-noise ratio of THz wireless transmission while potentially reducing the communication latency. Second, a broadband THz topological silicon waveguide designed based on valley photonic crystals is proposed for the robust and lossless transport of broadband THz waves for the construction of THz integrated circuits with broadband channel capacity for enabling higher data-rate on-chip communication. Last, the versatility of free-electron radiation from silicon gratings is investigated for the generation of highly intense spatiotemporal wave packets with user-designed group velocities and tunable intensity profile, with the potential to realize integrated high-power THz sources. The deep learning beamforming model, the broadband topological waveguide designs, and the free-electron radiation intensity shaping techniques presented in this thesis extends beyond the THz regime and are scalable across the entire electromagnetic spectrum. The CMOS process compatibility of silicon metasurfaces, silicon planar waveguides, along with silicon electron emitters and silicon gratings offers the opportunity to realize a broadband THz photonic integrated circuit with waveguiding, wave generation and beamforming functionalities for enhancing the spectral efficiency of THz communications in 6G wireless networks.

In recent years, the field of Terahertz (THz) science and technology has entered a completely new phase of unprecedented expansion that is generating ever-growing levels of broadbased international attention. Indeed, the plethora of activities that have arisen recently in both the technology and scientific arenas associated with the THz frequency domain i.e., between 1 millimeter (300 GHz) and 100 micrometers (3 THz), suggest that the field might be attempting to undergo a dramatic transition that could lead to long-awaited payoffs in a number of application areas. The inherent advantages and potential payoffs of the THz regime for military & security as well as industry relevant applications have long stood as an important driver of interest in this science and technology area. This extremely expansive and spectrally unique portion of the EM spectrum had initial application in space-based communications, upper atmospheric sensing and potentially for short-range terrestrial communications and non-intrusive package screening. However, the very rapid growth in more recent years is arguably most closely linked to the potential payoffs of THz sensing and imaging for an array of military, security and industrial applications. These applications include the spectroscopic-based detection identification and characterization of chemical and biological agents and materials, remote and standoff early-warning for chemicalbiological warfare threats, and imaging of concealed weapons and explosives, just to name a few. In addition, THz-regime finds its application possibilities in industry and private-sector areas as food-industry process control, pharmaceutical industry, biological science, medical diagnostics and security screening. Systems for rapidly emerging applications at THz frequencies thus require reliable highpower sources. In the last few years, the development of suitable sources for this frequency regime is being extensively explored worldwide. There are broadly two technology roadmaps for THz semiconductor devices. Approaching from the lower frequency range in the THz regime, electronic devices such as, Gunn diode, Resonant Tunneling diode (RTD) and nanometer Field Effect Transistors (FET) based on plasma wave have been widely investigated for THz frequency generation. From higher portion of the THz frequency spectrum, the photonics-based device Quantum Cascade Laser (QCL) extends the emission 7

Terahertz (THz) wave science and technology have been found countless applications in biomedical imaging, security screening, and non-destructive testing as they approach maturity. However, due to the challenge of high ambient moisture absorption, the development of remote open-air broadband THz spectroscopy technology is lagging behind the compelling need that exists in homeland security, astronomy and environmental monitoring. Furthermore, the underlying physical mechanisms behind the interaction between the THz wave and laserinduced plasma which responds strongly to electromagnetic waves have not been fully understood. This review aims to explain the light-plasma interaction at THz frequencies within a semiclassical framework along with experimental study of the femtosecond-laserinduced nitrogen plasma fluorescence under the illumination of single-cycle THz pulses. The results indicate that THz-radiation-enhanced-emission-of-fluorescence (THz-REEF) is dominated by electron kinetics in the THz field and the electron-impact excitation of gas molecules/ions. The information of the time-dependent THz field can be recovered from the measured time-resolved THz-REEF from single-color laser induced plasma with the help of the bias as local oscillator. The calculations and experimental verification lead to complete understanding of the science behind these effects and push forward to extend their capabilities in related applications such as remote THz sensing, plasma diagnostics and ultrafast photoluminescence modulation. Systematic studies in selected gases including neon, argon, krypton, xenon, methane (CH4), ethane (C2H6), propane (C3H8), and n-butane (C4H10) gases were performed to obtain an improved understanding of the THz-REEF. The dependences of the enhanced fluorescence on the THz field, laser excitation intensity, gas pressure, and intrinsic atomic properties were experimentally characterized. Both narrow line emission and broad continuum emission of the gas plasma were enhanced by the THz field. Their fluorescence enhancement ratios and time-resolved enhanced fluorescence were largely dependent on the scattering cross section and ionization potential of atoms. For the first time, we demonstrated a novel ‘all-optical’ technique of broadband THz wave remote sensing by coherently manipulating the fluorescence emission from asymmetrically ionized gas plasma that interacted with THz waves. By studying the ultrafast electron dynamics under the single cycle THz radiation, we found that the fluorescence emission from laser-induced air plasma was highly dependent on the THz electric field and the symmetry of the electron drift velocity distribution created by two-color laser fields. The time-resolved THz-REEF can be tailored by switching the relative two-color phase and laser polarizations. Owing to the high atmospheric transparency and omni-directional emission pattern of fluorescence, this technique can be used to measure THz pulses at standoff distances with minimal water vapor absorption and unlimited directionality for optical signal collection. The coherent THz wave detection at a distance of 10 m had been demonstrated. The combination of this method and previously demonstrated remote THz generation would eventually make remote THz spectroscopy available. We also introduced a unique plasma diagnostic method utilizing the THz-wave-enhanced fluorescence emission from the excited atoms or molecules. The electron relaxation time and plasma density were deduced through applying the electron impact excitation/ionization and electron-ion recombination processes to the measured time-delay-dependent enhanced fluorescence. The electron collision dynamics of nitrogen plasma excited at different gas pressures and laser pulse energies were systematically investigated. This plasma diagnostic method offers picosecond temporal resolution and is capable of omnidirectional optical signal collection. The ultrafast quenching dynamics of laser-pulse-induced photoluminescence in semiconductors under the radiation of single-cycle THz pulses was studied. It was found that the quenching in both cadmium telluride (CdTe) and gallium arsenide (GaAs) was linearly proportional to the intensity of incident THz waves and reaches up to 17% and 4% respectively at the peak intensity of 13 MW/cm2. The THz-wave-induced heating of the carriers and lattice and the subsequent decreased efficiency of photocarrier generation and recombination were most likely to be responsible for the quenching. This is potentially useful for the applications of a non-invasive ultrafast light modulator for photoluminescence devices with picoseconds switching time in the fields of the light-emitting devices and optical communication.

The use of fundamental modelocking to generate short terahertz (THz) pulses and THz frequency combs from semiconductor lasers has become a routine affair, using quantum cascade lasers (QCLs) as a gain medium. Here, using time-resolved THz techniques, we show the first of demonstration harmonic injection and mode- locking in which THz QCLs are modulated at the harmonics of the round-trip frequency. This generates multiple THz pulses per round trip in both active and self-starting harmonic regimes. This behaviour is supported by time-resolved Maxwell-Bloch simulations of induced gain and loss in the system. This work exploits the inherent ultrafast dynamics of QCLs and opens up new avenues in THz pulse generation.

Temperature dependent refractive index and dielectric modulation of paramagnetic magneto-optic PrF3 crystal with terahertz time-domain spectroscopy

The article presents a rectangular core photonic crystal fiber (RCPCF) structure to detect the major components of blood. We used Topas as bulk fiber material for its extraordinary properties in the terahertz (THz) frequency region and the investigation is accomplished from 0.8 THz to 1.8 THz. The RCPCF sensor provides enriched relative sensitivity (almost 94.38%, 93.72%, 92.94%, 92.14%, and 90.8% for red blood cells (RBCs), hemoglobin (HB), white blood cells (WBCs), plasma, and water respectively at 1.8 THz frequency) with negligible confinement loss (only 2.75 × 10−13 cm−1, 2.87 × 10−13 cm−1, 3.06 × 10−13 cm−1, 3.27 × 10−13 cm−1, and 3.48 × 10−13 cm−1 for RBCs, HB, WBCs, plasma, and water respectively at 1.8 THz frequency). Besides, the proposed sensor offers a large effective area (approximately 3.382 × 105 μm2, 3.436 × 105 μm2, 3.493 × 105 μm2, 3.551 × 105 μm2, and 3.613 × 105 μm2for RBCs, HB, WBCs, plasma, and water respectivelyat 1.8 THz frequency). Also, this RCPCF sensor structure can be fabricated by utilizing the current fabrication techniques.

Saturable absorbers (SA) operating at terahertz (THz) frequencies can open new frontiers in the development of passively mode-locked THz micro-sources. Here we report the fabrication of THz SAs by transfer coating and inkjet printing single and few-layer graphene films prepared by liquid phase exfoliation of graphite. Open-aperture z-scan measurements with a 3.5 THz quantum cascade laser show a transparency modulation ∼80%, almost one order of magnitude larger than that reported to date at THz frequencies. Fourier-transform infrared spectroscopy provides evidence of intraband-controlled absorption bleaching. These results pave the way to the integration of graphene-based SA with electrically pumped THz semiconductor micro-sources, with prospects for applications where excitation of specific transitions on short time scales is essential, such as time-of-flight tomography, coherent manipulation of quantum systems, time-resolved spectroscopy of gases, complex molecules and cold samples and ultra-high speed communications, providing unprecedented compactness and resolution.

The compound terahertz (THz) channel model has been widely adopted and recognized as a potent tool to capture the unique behaviors of wireless THz channels and analyze various performance of communication systems operating over THz spectra. However, accurate parameter estimation for the compound THz channel model still remains an open problem. In this letter, we propose to employ particle swarm optimization (PSO) to enable accurate yet efficient channel parameter estimation for the compound THz channel model. Based on the maximum likelihood (ML) estimation criterion, simulation results illustrate that the proposed PSO based parameter estimation technique is capable of achieving near-optimal estimation outcomes and possesses high robustness across multiple application scenarios. The proposed PSO based parameter estimation technique provides an easy-to-adapt mechanism for balancing the trade-off between estimation accuracy and computational efficiency.

In this paper, we compare the performance of two main MIMO techniques, beamforming and multiplexing, in the Terahertz (THz) band. The main problem with the THz band is its huge propagation loss, which is caused by the tremendous signal attenuation due to molecule absorption of the electro-magnetic wave. To overcome the path loss issue, massive MIMO has been suggested to be employed in the network and is expected to provide Tbps for a distance within a few meters. In this context, beamforming is studied recently as the main technique to take advantage of MIMO in THz and overcome the very high path loss with the assumption that the THz communication channel is Line-of- Sight (LoS) and there are not significant multipath rays. On the other hand, recent studies also showed that the well- known absorbed energy by molecules can be re-radiated immediately in the same frequency. Such re-radiated signal is correlated with the main signal and can provide rich scattering paths for the communication channel. This means that a significant MIMO multiplexing gain can be achieved even in a LoS scenario for the THz band. Our simulation results reveal a surprising observation that the MIMO multiplexing could be a better choice than the MIMO beamforming under certain conditions in THz communications.

Researchers are interested in unmanned aerial vehicles (UAVs) because they have many uses in current 5G and future 6G networks and are safer than human-operated vehicles. Terahertz (THz)-band communications are a possible option in 6G for fast communication. THz wireless communication between UAVs is taken into consideration in this work. Different fading channels, which are significant influencing factors in THz communication channel modeling, are used to analyze the performance of UAV communications in THz networks. Consideration must be made to the structure, wireless channel parameters, and transmission characteristics when evaluating the performance of wireless technology. Nakagami-m, Rician, Weibull, and Rayleigh fading channels are all taken into consideration, along with log-normal fading. Moreover, an optimization algorithm for the THz channel is presented, which is meant to minimize transmission power by optimizing the trajectory of uplink and downlink transmissions between the UAV and users. The equations of the UAV locations and the transmission power optimization of each user are derived. Analytical formulations regarding capacity, outage probability, and bit error rate (BER) are generated when performance-influencing factors are taken into account. The analytical analysis is supported by the numerical results that are provided.

The terahertz (THz) frequency range is key to studying collective excitations in many crystals and organic molecules. However, due to the large wavelength of THz radiation, the local probing of these excitations in smaller crystalline structures or few-molecule arrangements requires sophisticated methods to confine THz light down to the nanometer length scale, as well as to manipulate such a confined radiation. For this purpose, in recent years, taking advantage of hyperbolic phonon polaritons (HPhPs) in highly anisotropic van der Waals (vdW) materials has emerged as a promising approach, offering a multitude of manipulation options, such as control over the wavefront shape and propagation direction. Here, we demonstrate the THz application of twist-angle-induced HPhP manipulation, designing the propagation of confined THz radiation between 8.39 and 8.98 THz in the vdW material α-molybdenum trioxide (α-MoO3), hence extending twistoptics to this intriguing frequency range. Our images, recorded by near-field optical microscopy, show the frequency- and twist-angle-dependent changes between hyperbolic and elliptic polariton propagation, revealing a polaritonic transition at THz frequencies. As a result, we are able to allocate canalization (highly collimated propagation) of confined THz radiation by carefully adjusting these two parameters, i.e. frequency and twist angle. Specifically, we report polariton canalization in α-MoO3 at 8.67 THz for a twist angle of 50°. Our results demonstrate the precise control and manipulation of confined collective excitations at THz frequencies, particularly offering possibilities for nanophotonic applications.