Terahertz Absorption Spectroscopy in Bicrystal Josephson Junctions Formed from Mutually Tilted c-Axes YBa2Cu3O7−x Films

Frequency combs, spectra of phase-coherent equidistant lines, have revolutionized time and frequency metrology. The recently developed quantum cascade laser (QCL) comb has exhibits great potential with high power and broadband spectrum. However, in the terahertz (THz) range, cryogenic cooling has to be applied for THz QCL combs. We report a room temperature THz frequency comb at 3.0 THz based on difference-frequency generation from a mid-IR QCL comb. A largely detuned distributed-feedback grating is integrated into the QCL cavity to provide the single mode operation as well as enhanced spatial hole-burning effect for multimode comb operation. Multiheterodyne spectroscopy with multiple equally spaced lines by beating it with a reference Fabry-Pérot comb confirms the THz comb operation. This type of THz comb provides a new solution to chip-based high-speed high-resolution THz spectroscopy with compact size at room temperature.

Terahertz (THz) quantum cascade laser sources based on optical nonlinearity are the only electrically pumped monolithic semiconductor sources operable at room temperature in the 0.6–6 THz range. We investigated the temperature dependence of the imaging characteristics of a broadband THz nonlinear quantum cascade laser and evaluated several important properties: the spectrum, far-field pattern and THz imaging results. Consequently, we found that the far-field patterns were single-lobed Gaussian-like, and THz images were well-resolved despite the lower operating temperature of the device. The stable temperature-performance indicates that this broadband THz source is promising for THz imaging applications.

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.

Terahertz (THz) range holds between infrared light and millimeter wave or microwave radiation. Moreover, THz waves is highly attenuated by the metal object or sensitive to an inter-molecular binding force. Therefore, imaging using THz range is attracted much attention for security, manufacturing, chemical imaging, and so on. In our research, the THz detector composed of Indium arsenide (InAs) high electron mobility transistor (HEMT) and one-sided directional slot antenna on a chip will be developed. In this paper, we focused on the antenna on a chip. The proposed antenna has three layers, namely, top antenna metal, dielectric substrate (BCB, benzocyclobutene) and bottom floating metal layer. There are a coplanar (CPW) feed lines and slots on the top antenna metal. By optimizing the size of the bottom floating metal layer, the radiation toward the back side is suppressed. The CPW feed line is connected the gate electrode on the InAs HEMT. In order to maximize the receiving THz signal form the antenna to InAs HEMT, antenna and gate input impedance is characterized by using the 3D electromagnetic simulator. It has been found that when the input impedance of the gate electrode changes from 10 ohms to 50 ohms, the voltage generated at the gate electrodes is tripled. The antenna was fabricated by the conventional photolithography process. The size of the radiation metal is 290 μm x 210 μm on the top metal with probe pads. The measured antenna gain is 5.57 dBi at 0.93 THz compared with the 5.96 dBi antenna gain at 1 THz from the simulation.

Quantum cascade laser (QCL) is a semiconductor based laser in a superlattice structure based on intersubband transitions. Although QCLs have been achieved with high performance such as Watt-level emission, it is always highly desired to further improve the device performance with multiple functions , for example, achieving high beam collimation, arbitrary polarization control, and high speed modulation. It is also desired that those performance could be achieved through an integrated approach for miniaturalization, easy alignment and reducing cost. In this presentation, we will use Terahertz (THz) QCL as a demonstration example to obtain high collimated THz QCLs through plasmonic collimation designs with a record beam divergence, electrically tunable THz polarizations by designing integrated THz metasurfaces on a hybrid dielectric-plasmonic waveguides, and broadband graphene-based integrated THz modulators with a fast modulating speed.

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

As the communication band gradually approaches the terahertz (THz) range, there is an urgent need to explore materials with ideal dielectric properties for THz communication devices. Nevertheless, most polymers present a low dielectric constant (Dk), and the regulation of their dielectric properties in the THz range has rarely been reported. In this work, the isotactic polypropylene (iPP)/α-Al2O3 whisker composites were synthesized and their THz dielectric parameters were optimized. The Dk values increased from 2.23 to 3.13 with filler (α-Al2O3 whisker) concentration, ranging from 0 to 20 vol%, but were almost independent of the test frequency. The loss tangent (Df) values presented an increasing tendency along with both filler concentrations and test frequency. All composites exhibited Df values of less than 4.0 × 10−3. Particularly, the dielectric properties of composites can be further regulated by adjusting the orientation direction of the whisker fillers. The orientation of the whisker fillers was adjusted via the injection molding method. Along the direction of the whisker orientation distribution, the composites exhibit higher Dk values and lower Df values. This work presented a schematic to design polymer composites with higher Dk but controlled Df in the THz range and is significant for the development of advanced materials-based THz devices.

Room-temperature terahertz (THz) quantum cascade laser sources with intracavity difference-frequency nonlinear mixing are electrically pumped monolithic semiconductor laser sources operating in the 0.6–6 THz spectral range. We report widely tunable, low-frequency THz quantum cascade laser sources using a lens-coupled Cherenkov waveguide scheme. Based on a watt-class high-power, λ ∼ 13.7 μm quantum cascade laser, the monolithic THz source is strongly coupled with a high-resistivity silicon lens, which causes a major increase in the THz coupling efficiency and demonstrates significant performance improvements. A room-temperature 1.5 THz device produces a 0.2 mW peak output power with a high-quality beam pattern. Improved THz outcoupling efficiency using the lens-coupled scheme enabled the demonstration of a high-performance external-cavity semiconductor THz source that is tunable from 420 GHz to 2 THz. The external-cavity, lens-coupled device configuration can technically be assembled into a butterfly-style package for a thumb-sized, widely frequency tunable THz semiconductor source.

We report our development of terahertz (THz) quantum cascade lasers (QCLs), in which the depopulation of the lower radiative level is achieved through resonant longitudinal optical (LO) phonon scattering. This depopulation mechanism, similar to that implemented in all the QCLs operating at mid-infrared frequencies, is robust at high temperatures and high injection levels. The unique feature of resonant LO-phonon scattering in our THz QCL structures allows a highly selective depopulation of the lower radiative level with a sub-picosecond lifetime, while maintaining a relatively long upper level lifetime (more than 5 ps) that is due to upper-to-ground-state scattering. The first QCL based on this mechanism achieved lasing at 3.4 THz (lambda approximately 87 microm) up to 87 K for pulsed operations, with peak power levels exceeding 10 mW at ca. 40 K. Using a novel double-sided metal waveguide for mode confinement, which yields a unity mode confinement factor and therefore a low total cavity loss at THz frequencies, we have also achieved lasing at wavelengths longer than 100 microm.

Multifunctional active terahertz metasurface with electromagnetically induced transparency, perfect absorption, and circular dichroism

Molecular vibration mode assignment of nematic liquid crystal 5CB on Terahertz spectra

The infrared region is a range of the electromagnetic spectrum split into three parts, the near-infrared (NIR), the mid-infrared (MIR) and the far-infrared (FIR). Previously, it was usual to refer to the terahertz (THz) range as the range of frequencies below FIR. Nowadays, the THz range is attributed to the electromagnetic spectrum which spans the FIR to the millimeter wave. This THz range of the electromagnetic spectrum starts at 1000 µm and finishes at 50 µm (0.3 THz to 6 THz). So, these wavelengths are commonly named THz, far-infrared or submillimeter wave. THz waves are located between the optical region (infrared) and the microwaves. Only a few compact, easy to use, room temperature sources are available.

Research in the area of terahertz (THz) waveguides has seen a rapid progress recently and has led to demonstration of THz waveguides with transmission losses comparable to losses in air. We will discuss dielectric-lined hollow metallic waveguides, in which THz waves propagate with attenuation as low as 1 dB/m and dispersion of 6 ps/(THz·m), and compare them to other low-loss THz waveguides. As a key technique for THz waveguide research, we will discuss the application of THz near-field microscopy in combination with THz time-domain spectroscopy for waveguide characterization. This technique allows us to map spatial profiles of normal modes and to measure transmission loss and dispersion spectra for individual modes within the THz range.

Ultra-broadband shielding of cellulose nanofiber commingled biocarbon functional constructs: A paradigm shift towards sustainable terahertz absorbers

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.