Terahertz scanning tunneling microscopy (THz-STM) is an emerging technique that provides simultaneous ultrafast temporal and Angstrom spatial resolution through lightwave control of an atomic tunnel junction. THz-STM can further accesses extreme tunneling regimes thanks to the low duty cycle and ultrafast duration of the THz bias. Here, we use low-temperature, ultrahigh vacuum THz-STM to explore the local density of electronic states in 7-atom-wide armchair graphene nanoribbons (7AGNRs) at tip heights that are inaccessible to conventional STM. Using a new procedure for THz scanning tunneling spectroscopy (THz-STS), we determine the differential conductance above the 7AGNR with atomic resolution in three dimensions.

Terahertz (THz) emission from the silicon (Si) surface by the femtosecond laser illumination shows a lot of information, known as the THz emission spectroscopy (TES) or the laser-induced THz emission microscope (LTEM), which seems to be the promising method for evaluating the surface/interface properties of Si-based devices, such as Si metal-oxide-semiconductor (Si MOS) structure. To enlarge the application and explore silicon electronics, it is necessary to build a theoretical model, which can accurately and simply describe the relationship between the THz emission amplitude and the external bias voltage on the Si MOS structure. Here, we focus on the THz emission spectrum from the p-type Si MOS structure and discuss the THz emission field under different DC bias conditions. The theoretical model is derived from Poisson’s equation and shows great consistency with the experiment results. Besides that, the flat-band voltage, interface trap states, and photo-Dember effect have been discussed to modify the model.

We present investigations of the fin-shaped GaN/AlGaN field effect transistors with two lateral Schottky barrier gates exactly placed on the edges of the fin-shaped transistor channel. We call this kind of FinFET modification the EdgeFET. It allowed us to efficiently control the current flow in two-dimensional electron gas conduction channel. We present experimental data of sub-THz detection by EdgeFETs. Control of the side gates allows changing the width of two-dimensional electron gas and forming a wire, as we expect should be beneficial for observation of terahertz plasma wave resonances. This paves the way towards future terahertz optopair using high-quality factor plasma wave resonances, for which it is necessary to eliminate oblique modes. We report also on the high-voltage, noise, and radio frequency (RF) performances of aluminium gallium nitride/gallium nitride (AlGaN/GaN) on silicon carbide (SiC) devices without any GaN buffer. Such a GaN–SiC hybrid material was developed in order to improve thermal management and to reduce trapping effects should be beneficial for observation of resonant emission.

We present our recent development of a broadband and low-noise THz-wave detector called the Fermi-level managed barrier (FMB) diode. The FMB diode has a very low barrier height of around 100 meV, which is essential for attaining low noise-equivalent power (NEP) under a zero-biased condition. The fabricated module with a pre-amplifier could detect signals in a frequency range from 160 GHz to 1.4 THz with a very low NEP of 3 x 10^-12 W/√Hz at 300 GHz in the direct detection mode. In the mixing (heterodyne) detection mode, an FMB diode module integrating a broadband transimpedance amplifier exhibited an intermediate frequency bandwidth of 36 GHz and an extremely low NEP of 3 x 10^-19 W/Hz at around 300 GHz with a local oscillator power of only 70 μW. We also developed linear detector arrays consisting of 100 and 128 FMB diodes for practical THz-wave imaging.

Terahertz devices provide ample avenues for high-frequency information processing. Bulky THz detectors and generators are required to be synchronized with the presently established semiconductor industry. Settling terahertz native to GaAs and Silicon would enable on-chip high-frequency operations, THz imaging, THz Generation-Detection. This work demonstrates the formation of Rapid Thermal Annealing (RTA) based hole arrays in AuGe deposited thin film over GaAs substrate for terahertz detection mechanism. Ultrafast transient spectroscopic excitation around the wavelength matching with the size of holes creates time-domain oscillations in the transient relaxation spectrum that lies in the THz frequency domain. This behavior depicts that the structure may be responsive to the THz detection, where incident radiation is mediated through the AuGe hole arrays; the charge density is excited and relaxed with phonon-phonon interaction in the THz frequency domain. By varying the hole diameter and thickness of the film, we can tune the THz frequency response over GaAs. The free charge carrier density on the GaAs surface is influenced with the hole-spacing and distribution depending on the incident radiation. AuGe-GaAs interfacial properties are much influenced by thermal annealing, which induces intermixing of materials. The size of holes is not uniform throughout the film, which provides the broader response in the THz spectrum with higher pumping wavelengths. This method of THz detector fabrication is less complex and much feasible with batch processing. The THz response is available for tuning by changing the film thickness and RTA recipe. This work will contribute to a more accessible and efficient THz detector demonstration.

Quantum cascade laser (QCL) is becoming the leading laser source in the mid-infrared and terahertz range due to its rapid development in power, efficiency, and spectral covering range. Owing to its unique intersubband transition and fast carrier lifetime, QCL possesses strong nonlinear susceptibilities that makes it the ideal platform for a variety of nonlinear optical generations. Among this, terahertz (THz) source based on difference-frequency generation (DFG) and frequency comb based on four wave mixing effect are the most exciting phenomena which could potentially revolutionize spectroscopy in mid-infrared (mid-IR) and THz spectral range. In this paper, we will briefly discuss the recent progress of our research. This includes high power high efficiency QCLs, high power room temperature THz sources based on DFG-QCL, room temperature THz frequency comb, and injection locking of high-power QCL frequency combs. The developed QCLs are great candidates as next generation mid-infrared source for spectroscopy and sensing.

In the past decades, terahertz (THz) large area emitters (LAEs) have been used for broadband THz generation. To achieve broadband operation, the ultrafast photoconductor of THz LAE is mainly based on III-V semiconductor materials, which is designed to offer decent quantum efficiency, high carrier mobility, and low carrier lifetime simultaneously. However, the sophisticated epitaxial process constrains the application scopes of conventional THz LAEs. Here, we present a cost-effective, CMOS-compatible, bias-free GeSn THz LAE on a Si substrate. The GeSn thin film is grown on undoped Si substrate with Ge buffer layer by reduced pressure chemical vapor deposition (RPCVD). Since the charge neutrality level is close to the top of the valence band, a built-in electric field at the surface of GeSn is created by the fermi-level pinning. As the optical absorption coefficient of the GeSn ultrafast photoconductor is higher than 7000 cm^-1 in the 1500 – 1700 nm wavelength range, it can generate a comparable amount of photocarriers as InGaAs-based ultrafast photoconductor. To investigate the broadband operation capability of the bias-free GeSn THz LAE prototypes, we further characterize the carrier dynamics of the epitaxial GeSn thin film. At the optical pump power of 400 mW, the bias-free GeSn THz LAE generates broadband THz radiation with a pulse width of 500 fs full width at half maximum. The GeSn THz LAE can be monolithically integrated on Si photonic platform with the bias-free operation. It paves the way toward THz system-on-chip (SoC) for many on-site applications in non-destructive evaluation, biomedical imaging, and industrial inspections.

We analytically and experimentally investigate the radiated terahertz fields from a stack of two spintronic Fe/Pt terahertz emitters that are aligned back to back with the Pt-surfaces facing each other. We experimentally and theoretically study the dependence of the emitted terahertz fields from the stack on the relative orientation of the individual emitters. For collinear alignment in the same direction, we determined an increase of the maximal emission amplitude by a factor of 1.57 in comparison with a single emitter. We also evaluated the cavity effects that originate from the air gap between the individual emitters in theory and experiment.

The challenge of Unipolar Quantum Optoelectronics (UQO) is to bring reliable technology in the mid-infrared and terahertz domains with dozens of GHz bandwidth and room-temperature operation. The semiconductor devices based on this novel technology rely on two-dimensional electronic states localized in the conduction band, which implies that electrons are the only charge carriers involved. Though UQO technology has been proven useful for emission (quantum cascade lasers) and detection (quantum cascade detectors), it is still underdeveloped for other applications, like high-speed modulation. In this paper, we will review our recent results with a full transmission system UQO in the 8 to 14 µm atmospheric window, composed of a quantum cascade (QC) laser, an external modulator and a QC detector, all optimized for operation at 33 THz optical wavelength. Dynamics down to a few dozens of picoseconds are observed, which allow us demonstrating data rate transmission of 10 Gbps without any signal processing. In addition, the paper aims at discussing further applications of UQO in particular for radio over free-space. The basic principle for producing microwave carriers is based on an optical heterodyne beating technique taking advantage of the high-bandwidth potential of QC detectors. Then, the microwave signal is transmitted through a point-to-point wireless link by using radiofrequency antennas. With UQO, microwave signals of dozens of GHz can be achieved. To sum, this paper highlights the importance of using UQO devices operating at a few dozens of THz optical wavelength for both free-space optics and microwave photonics targeting 100 GHz radiofrequencies.

THz imaging proved to be an efficient and powerful NDE method for the inspection of materials and detection of defects. Here, we present the development of a Low-Terahertz Imaging Radar(L-TIR) which is specifically designed for the evaluation of FRP composites. L-TIR will be a compact active radar with a laser pulse transmitter and a receiver which operates at the lower THz band (1THz-2THz). L-TIR will probe the structure and sub-layers of the FRP composites which makes it a suitable and fast tool to detect the cracks, breaks and deamination at sub-mm resolution.

Record data rate in wireless networks, autonomous cars, imaging of cosmic radiation, and subatomic particle physics colliders, are among the applications that will be boosted by the emergence of a new class of millimeter-wave oscillators lying between the microwaves and the infrared light (100 GHz to a few THz). This emergence relies on how efficient and low noise those oscillators can be implemented, preferably in mass-production form factor. We have unlocked the limitations in terms of spectral purity that were, up until now, ruled by RF and microwave technologies. Here, a millimeter-wave at 300 GHz has been referenced to an optical signal for the first time enabling it to overcome constraints imposed by microwave references. The core of a novel oscillator, that we propose, consists in a dissipative Kerr soliton comb that is generated from a Silicon-nitride-chip-based microresonator. We demonstrate low phase noise 300 GHz wave generation through optical frequency division of an optically-carried multi-THz reference through an integrated dissipative Kerr soliton. The obtained phase noise at 10 kHz Fourier frequency, measured with a devised system, is -100 dBc/Hz.

We present InP-based Triple Barrier Resonant Tunneling Diodes monolithically integrated with an on-wafer resonant or broad band antenna. Biasing these diodes in the negative differential resistance regime provides a fundamental mode oscillation of preliminary 90 μW at f₀ = 260 GHz. At 280 GHz an estimated high zero-bias resonant responsivity of 50.000 V/W is modeled. A broad band average responsivity of 900 V·W^-1 was determined in the frequency range from 230 … 330 GHz along with a minimum Noise Equivalent Power of 1 pW·Hz^-0.5. This concept is expected to provide very high sensitivities at frequencies up to f ≥ 1 THz.

Herein, we are making a step forward dealing with the novel theoretic and computational determination of the effective permittivity of the composite biological media. The presented methodology stands for as a perfect tool allowing to evaluate the permittivity tensor of the sample analytically with no needs of human intervention by performing an experimental analysis to measure the parameters of the sample. The distribution of the cancerous cells is taken into account. Doing so, we end up with the determination of the tensor components for random multi-phase composites. The former provides a fertile ground aiming to detect and treat cancer. It has been concluded that the increase of the concentration of the cancerous cells and their distribution in the sample makes a dramatic impact on the obtained numerical results.

Weyl semimetals are defined by massless, chiral quasiparticles that derive from electronic band-crossings split by either spatial inversion or time-reversal symmetry breaking. These nodal points in the bulk band structure serve as sources and sinks of “topological charge” that are responsible for their phenomenology, including, e.g., Fermi arc surface states, and the chiral anomaly. Here we describe measurements of laser driven currents in both the bulk and on the Fermi arc surface states of structurally chiral Weyl semimetals as measured through the THz frequency radiation they emit upon photoexcitation, and describe their significance in the context of topological ordering.

We experimentally demonstrate, for the first time, the activation of long-range attractive electrodynamic forces between proteins by means of THz spectroscopy. Our experimental work provides a proof of principle of out-of-equilibrium collective oscillations and the related activation of dipole-dipole electrodynamic intermolecular forces. It paves the way for exploring the potential role of electrodynamic intermolecular forces in living matter.

We review recent progress in the development of grating-gate plasmonic THz detectors based on InGaAs-channel high-electron- mobility transistors for fast, highly sensitive, room-temperature-operating THz detection. First, a 2D diffraction grating structure, instead of the conventional 1D grating structure, was proposed for realization of polarization-independent detection and was experimentally demonstrated to alter the polarization characteristic of output photoresponse drastically. Second, it was demonstrated that a new way to read the output photovoltage from the gate of a detector simultaneously enables the scaling of the photovoltage with the active area size and the impedance matching with 50-Ω interconnection systems, which were the main issues in the conventional drain-readout configuration. Third, a significant enhancement of the photovoltage was observed at a positive gate bias voltage application in the gate-readout configuration, due to the “3D rectification effect" originating from the heterobarrier between the InGaAs channel layer and InAlAs spacer/carrier-supply/ barrier layers.

In this paper, we address the state-of-the-art of CMOS-based electronic sources and detectors developed for the THz frequency range. In particular, we present a system operating at 250 GHz exhibiting input power-related signal-to-noise ratio (SNR) exceeding 70 dB in the direct detection regime for one Hz equivalent noise bandwidth. It combines the state-of-the-art detector based on CMOS field-effect-transistors (FET) and a voltage-controlled oscillator (VCO) employing SiGe bipolar transistors provided by the BiCMOS process. The manuscript presents different emitter–detector pair operation modalities, including data transmission, spectroscopy, and imaging.

Terahertz time-domain spectroscopy is a powerful tool for characterizing material properties that were experimentally inaccessible until recently. For multilayered systems, the signal presents echoes with characteristics related to the optical properties of each corresponding layer. However, if the layers are very thin, echoes in the time domain may overlap and more complex and specific methods of analysis should be used to calculate the optical properties of the samples. In this work, we implement four different reported methods for analyzing thin layered samples and we compare them to evaluate their capabilities and results.

Recent advances with THz technologies have been growing significantly. The study and development of THz technology require having different components such as reflectors, lenses, splitters, and waveguides. As THz waves can penetrate a wide variety of non-conductive materials such as polymers, 3D printing can generate these components quickly and inexpensively. In this work, we present the characterization of some thermoplastic materials commonly used in 3D printing with respect to different printing specifications of infill density and layer height. We characterize the refractive index and absorption coefficient of the samples in the THz range of 0.2 to 1 THz using a THz time-domain spectrometer.

This PDF file contains the front matter associated with SPIE Proceedings Volume 12230 including the Title Page, Copyright information, Table of Contents, and Conference Committee Page.