Optical constants of CuO and ZnO particles in the terahertz frequency range

The optical constants (complex dielectric function) of CuO and ZnO particles in polyethylene pellets are measured by terahertz time-domain spectroscopy. The determination of the dielectric properties of these materials is of interest for energy-storage applications, for instance. Maxwell-Garnett theory is used to extract the contribution to the frequency-dependent optical constants of the oxide powders. The validity of the assumptions of Maxwell-Garnett theory are experimentally verified and self-consistency of the results of the model confirmed. On this basis, experimental complex permittivity values for isotropic CuO and ZnO oxide powders are reported in the 100 GHz-3 THz range.     

太赫兹量子级联激光器中的光电调控：综述

太赫兹量子级联激光器作为目前产生太赫兹激光的最有效手段之一,如何提升其性能表现一直是科学界所关注的重点。本篇综述将从光电调控的角度,阐述目前太赫兹量子级联激光器的性能进展。从激光器有源区设计原理开始,介绍几种新的有源区设计,再从谐振腔的角度介绍一系列新的结构,并展示他们对于功率和光束质量的提升。最后,阐述了太赫兹量子级联激光器在偏振调控和频率调谐的最新进展。     

太赫兹应用分析和展望

以6G需求为抓手引入了太赫兹技术,介绍了太赫兹的研究背景和国内外研究现状。之后结合太赫兹的频谱范围,对太赫兹频段和毫米波以及红外光进行了详细对比分析,发现了太赫兹特有的一些频谱特性,包括宽带性、低能性、电磁特性和特征谱等,并进一步挖掘基于这些频谱特性的应用,研究表明,太赫兹未来可应用于军事、医疗、检测和通信等诸多领域,对于仍存在的挑战和瓶颈,中国联通将联合产业界一同推动解决,最大化发挥太赫兹应用潜力。     

Optically tuneable broadband terahertz metamaterials using photosensitive semiconductor material

We present optically tuneable broadband terahertz metamaterials consisting of photosensitive semiconductor material. The characteristics of semiconductor materials are very important for terahertz functional devices, which is strongly dependent on the photoexcited carriers in semiconductor materials. Based on the model of the generation of the carriers in semiconductor materials, the tuneability of the resonance frequency and resonance strength in terahertz metamaterials under the femtosecond laser pulse has been analysed. The experimental results of the transmissivities of several bare semiconductor materials show that GaAs semiconductor could be a potential candidate for broadband terahertz functional devices design. Moreover, the simulation results of the resonance frequency increase with the increase of the laser fluence, whereas the resonance strength near the resonance frequency is significantly weakened with the increase of the laser fluence. Meanwhile, the modulation depth can be reached approximately 90% at 1.24 THz. Our proposed broadband tuneable terahertz metamaterials have numerous potential applications, including terahertz modulator, absorber and switches.     

Photo-excited broadband tunable terahertz metamaterial absorber

We demonstrate a photo-excited broadband tunable metamaterial absorber for use in the terahertz region. The metamaterial absorber consists of a hybrid metal-semiconductor square split ring and a metallic ground plane that are separated by a dielectric resonator spacer. The conductivity of the silicon used to fill the gap in the metallic resonator is tuned actively as a function of the incident pump power, which results in frequency modulation of the resonance absorption peak. Broadband tunable metamaterial absorbers are produced by suitable placement of the photoconductive silicon in different critical regions of the metallic resonator. In addition, the proposed method is applicable to a concentric rings-based metallic resonator. The proposed photo-excited broadband tunable metamaterial absorber has numerous potential applications, including uses as terahertz modulators and switches.     

The intensive terahertz electroluminescence induced by Bloch oscillations in SiC natural superlattices

We report on efficient terahertz (THz) emission from high-electric-field-biased SiC structures with a natural superlattice at liquid helium temperatures. The emission spectrum demonstrates a single line, the maximum of which shifts linearly with increases in bias field. We attribute this emission to steady-state Bloch oscillations of electrons in the SiC natural superlattice. The properties of the THz emission agree fairly with the parameters of the Bloch oscillator regime, which have been proven by high-field electron transport studies of SiC structures with natural superlattices.     

Material and process considerations for terahertz planar nanodevices

Using a two-dimensional ensemble Monte Carlo method, we have studied the capacitances in a planar nano-diode, called the self-switching device (SSD) that is based on an asymmetric nanochannel. We show that the terahertz (THz) response of the SSD can be affected by the dielectric constant and width of the insulating trenches that confine the nanochannel of the diode. The simulations reveal that the capacitive coupling over the insulating trenches has little influence on the device RF properties below 100 GHz, but significantly affects the device performance around and beyond 1 THz. We show that the capacitance is more sensitive to the dielectric constant of the substrate but is relatively less sensitive to the width of the insulating trench or the dielectric constant of the infill material in the insulating trenches. This differs significantly from a conventional capacitor with two parallel plates. The findings in this work provide useful implications in optimizing the material and lithography parameters of such and similar nanodevices for THz applications.     

Low Noise Hot-Electron Bolometer Mixers for Terahertz Frequencies

Hot-electron bolometer (HEB) mixers are used in many low noise heterodyne radio astronomical receivers. Their noise temperature is at the level of 10–15 times the quantum limit. However, their gain bandwidth is a serious limiting factor. Here we review the state of the art of the HEB mixers gain bandwidth for different materials and substrates. We compare the gain bandwidth of HEB mixers made on bulk substrates and thin membranes. Finally, results for MgB₂ thin films for broadband HEB mixers are discussed.      

Nondestructive evaluation of forced delamination in glass fiber-reinforced composites by terahertz and ultrasonic waves

Glass fiber-reinforced composite laminates in polyetherimide resin have been studied via terahertz imaging and ultrasonic C-scans. The forced delamination is created by inserting Teflon film between various layers inside the samples prior to consolidating the laminates. Using reflective pulsed terahertz imaging, we find high-resolution, low-artifact terahertz C-scan and B-scan images locating and sizing the delamination in three dimensions. Furthermore, terahertz imaging enables us to determine the thicknesses of the delamination and of the layers constituting the laminate. Ultrasonic C-scan images are also successfully obtained; however, in our samples with small thickness-to-wavelength ratio, detailed ultrasonic B-scan images providing quantitative information in depth cannot be obtained by 5 MHz or 10 MHz focused transducers. Comparative analysis between terahertz imaging and ultrasonic C-scans with regard to spatial resolution is carried out demonstrating that terahertz imaging provides higher spatial resolution for imaging, and can be regarded as an alternative or complementary modality to ultrasonic C-scans for this class of glass fiber-reinforced composites.     

太赫兹波在烟尘气溶胶中的传播特性

随着太赫兹波技术的发展,对其在各种大气条件下的传播特性的研究是很有必要的。本文利 用扩散限制凝聚模型(DLA 方法)模拟了组成烟尘气溶胶的簇团粒子,并利用广义Mie 理论(GMM)方法 计算了不同基本粒子个数的单个簇团粒子对波长分别为30μm、35μm 和40μm 的太赫兹波的散射特性。 另外本文还利用蒙特卡罗方法模拟计算了浓度较大的烟尘气溶胶对太赫兹波的衰减特性并将结果与波 长为6.2μm 的红外波段的结果进行比较,结果显示,太赫兹波在烟尘气溶胶中传播的衰减要小的多, 这也说明太赫兹波更加适合在烟尘气溶胶中传播。