Novel Applications of Terahertz Quantum Cascade Lasers: Gas Spectroscopy, Active Control & Nearfield Imaging

Since the advent of Terahertz (THz) quantum cascade lasers (QCLs) in 2002, they have seen significant improvement in output power, operating temperature, and spectral coverage as well as having been widely applied to astronomical applications, imaging, and spectroscopy because of their high output power and narrow spectral linewidth. In this work, three techniques, self-mixing gas spectroscopy, THz amplitude modulation and THz nearfield imaging, will be experimentally studied and demonstrated. These techniques can either expand or increase the efficiency of their corresponding applications using other techniques. The dissertation starts with a brief introduction of the basic knowledge of THz QCLs and some characterization methods for a THz system, as well as a summary of their main applications and their current capabilities. When the lasing of a THz QCL is suppressed, it becomes a quantum cascade amplifier (QCA), which compared with a THz QCL can enhance the sensitivity in both spectroscopy and imaging applications. A THz QCA has been achieved by adhering an antireflection-coated Si lens to the QCL facet and is then used as the source for THz gas spectroscopy based on a self-mixing effect, in which the THz QCL functions as both the light source and the detector. This offers an approach to achieving a compact spectroscopy system with a fast response. For spectroscopy, communication and astronomical applications, the ability to actively control the amplitude of the THz radiation is desired. This has been demonstrated with a graphene loaded metamaterial device, with which and a PID feedback loop, the output power fluctuation of a THz QCL is reduced from 1.52% to 0.043% of the total power. An amplitude stabilised THz source is also essential for high resolution THz imaging, where amplitude fluctuations will distort the acquired THz images. A scattering type nearfield microscope (s-SNOM) with a better than λ/1000 resolution has been demonstrated with a tuning fork based atomic force microscope (AFM) and a partially suppressed THz QCL. The detection scheme is also harnessing the self-mixing effect. Compared with conventional detection methods, this can give a fast response and high sensitivity, which are essential for high-speed high-resolution imaging. The performance of this home-built THz s-SNOM has then been significantly improved by vibration isolation and electronic noise reduction and it has been used to examine a variety of samples. It is able to reveal the plasmonic resonance of resonant structures, spatially map the electric field distribution on a metamaterial device and image subsurface plasmons. By using tips of different materials, it has also been found that a gold coating can improve the THz sensitivity of the system. Afterwards, to optimise the design of metamaterials, a special metasurface has been designed to study the influence of the geometric parameters on the optical performance of it. This can be achieved by probing the electric field distribution of the metasurface with the THz s-SNOM. The dissertation is then concluded with all the results obtained and a brief overview of what can be done in the future in related research fields

Effects of particle size and content of RDX on burning stability of RDX-based propellants

Abstract Particle size and content of RDX are the two main factors that affect the burning stability of RDX-based propellants. However, these effects and the corresponding mechanisms are still controversial. In this work, we investigated the physicochemical processes during burning and the corresponding mechanisms through the technologies of structure compactness analysis on the base of voidage measurement and theoretical interfacial area estimation, apparent burning rate measurement using closed vessel (CV) and extinguished burning surface characterization relying on interrupted closed vessel (ICV) and scanning electron microscope (SEM). The results indicate that the voidage increased with the increase of RDX content and particle size due to the increasing interfacial area and increasing interface gap size, respectively. The apparent burning rate increased with the increase of RDX particle size because of the decreasing RDX specific surface area on the burning surface, which could decrease the heat absorbing rates of the melting and evaporation processes of RDX in the condensed phase. Similarly, the apparent burning rate decreased with the increase of RDX content at pressures lower than around 55 MPa due to the increasing RDX specific surface area. Whereas, an opposite trend could be observed at pressures higher than around 55 MPa, which was attributed to the increasing heat feedback from the gas phase as the result of the increasing propellant energy. For propellants containing very coarse RDX particles, such as 97.8 and 199.4 μm average size, the apparent burning rate increased stably with a flat extinguished surface at pressures lower than around 30 MPa, while increased sharply above around 30 MPa with the extinguished surface becoming more and more rugged as the pressure increased. In addition, the turning degree of u-p curve increased with the increase of coarse RDX content and particle size, and could be reduced by improving the structure compactness

Monitoring of atopic dermatitis using leaky coaxial cable

In our daily life, inadvertent scratching may increase the severity of skin diseases (such as atopic dermatitis, etc.). However, people rarely pay attention to this matter, so the known measurement behavior of the movement is also very little. Nevertheless, the behavior and frequency of scratching represent the degree of itching, and the analysis of scratching frequency is helpful to the doctor's clinical dosage. In this paper, a novel system is proposed to monitor the scratching motion of a sleeping human body at night. The core device of the system are just a Leaky coaxial cable (LCX) and a router. Commonly, LCX is used in the blind field or semi blind field in wireless communication. The new idea is that the leaky cable is placed on the bed, then the state information of physical layer of wireless communication channels is acquired to identify the scratching motion and other small body movements in the human sleep process. The results show that it can be used to detect the movement and its duration. Channel state information (CSI) packet is collected by card installed in the computer based on the 802.11n protocol. The characterization of the scratch motion in the collected channel state information is unique, so it can be distinguished from the wireless channel amplitude variation trend