Electromagnetic simulations of tunable terahertz metamaterial infiltrated with highly birefringent nematic liquid crystal

In this article electromagnetic simulations of tunable terahertz metamaterial infiltrated with nematic liquid crystal are described. A full-wave analysis technique based on the finite-difference time-domain (FDTD) method was done by employing QuickWave 3D electromagnetic solver. Scattering parameter analysis shows a resonant frequency shift of 0.8 THz obtained by reorienting the layered highly birefringent nematic liquid crystal. Effective refractive index for operation frequency varies from negative to positive values.     

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

Using DFT/B3LYP/6-311++G** method, the molecular structure and absorption spectra in terahertz (THz) range of liquid crystal 5CB are investigated. In a frequency range 0–15 THz, an assignment of the vibrational modes corresponding to absorption frequencies is performed using potential energy distribution for the first time. It is found that the cyano group radical (–CN) do actively take part in the strongest THz absorption of 1.743, 3.942, 5.169 and 14.769 THz in different vibration modes. The results suggest that the strong polar group should be avoided in designing liquid crystal molecule and mixtures in order to reduce the absorption intensity in THz range.     

Terahertz properties of fluorinated liquid crystals

In this article, optical properties of four fluoro-substituted 4-propyl-4′-[(4-ethylphenyl)ethynyl] biphenyls and liquid crystal mixture A are presented in the terahertz (THz) range. Birefringence, refractive indices and absorption coefficients for ordinary and extraordinary ray of liquid crystals are described in the range of 0.3 to 3.0 THz. It shows that the measured parameters are dependent on the number and placement of fluorine atoms in the molecules. Measurements have been performed using time-pulsed spectroscopy.     

Simulations of some physical parameters of homologous series of nBT and nCHBT at 0.3–20.0 THz

ABSTRACT

In this work, the terahertz (THz) absorption of homologous series of isothiocyanobiphenyls (nBT) and 4-(trans-4?-n-alkylcyclohexyl)isothiocyanato-benzene (nCHBT) are simulated by using the density functional theory (DFT) method. Numerical results show that absorption of the tested nematic liquid crystals (NLCs) varies up to 0.04 in the broad range of 0.3–20.0 THz and takes maximum value in the range of 9–14 THz.

In addition, the values of molecular polarisabilities (α_o, α_e) increase with the increase in the number of carbon atoms in the alkyl chain, while refractive indices (n_o, n_e) decreases. The calculated birefringence (Δn) values for nCHBT are approximately twice as large as the birefringence value for nBT. It is crucial to examine these series of nematic liquid crystals at THz frequencies due to the role of soft matter in various switchable THz components.     

β-丙氨酸的THz时域光谱研究

运用THz时域光谱测试技术(THz-TDS)与理论模拟相结合的方法, 研究了β-丙氨酸在THz波段的光谱特性. 在室温氮气环境中, 得到了样品在0.2～2.4 THz波段的吸收谱和折射率谱, 表明其特征吸收峰位于2.11 THz处, 平均折射率为1.96. 同时利用Gaussian 03软件的半经验理论计算了该分子在0.1～10.0 THz的振动吸收谱, 其在0.2～2.4 THz波段的吸收峰与实验相互对应, 且峰位符合较好. 研究了分子低频的振动模式, 并给出了分子的构象参数.     

不同地区及仿真和田玉太赫兹光谱分析

利用太赫兹时域光谱技术对不同地区和田玉及其仿真品进行测量与分析,为不同地区的和田玉及真伪鉴别提供技术支撑。实验选取了新疆、韩国和青海三个不同产地的和田玉样品,同时选取了三种仿和田玉样品,在0.1~2.5THz频段进行太赫兹光谱数据采集。不同地区和田玉在太赫兹频段具有不同的特征吸收峰,并且每个样品在相对高频和低频的吸收量不同,随频率的增加,样品在相对低频处的折射率呈增加趋势,在高频处呈缓慢下降趋势,不同样品的变化趋势不同。仿真样品与和田玉在0.1~2.5 THz波段的吸收系数、折射率、相对介电常数相差很大。实验结果初步表明,用太赫兹波段的吸收系数和折射率来鉴别和田玉及仿真品是可行的,能够区分不同地区和田玉及仿真样品,该方法快速、便捷,能为市场监管提供有力技术支撑。     

THz spectroscopic investigation of 2,4-dinitrotoluene

We have investigated the terahertz (THz) spectrum of 2,4-DNT by using Fourier transform infrared spectroscopy in the 0.2–19.5 THz region. We also examined low-frequency intermolecular or phonon modes between 0.2 and 1.8 THz via THz time-domain spectroscopy. The extracted absorption coefficient and refractive index of an intermolecular band at 1.08 THz are 110 cm⁻¹ and 1.67, respectively. Density functional theory (DFT) was applied to obtain structure and vibrational frequencies of 2,4-DNT. The calculated results are in agreement with the experimental data. Observed vibrational frequencies have been interpreted using DFT. Two intermolecular or phonon modes were identified at 1.08 and 2.52 THz.     

含硫氨基酸的太赫兹光谱

利用太赫兹时域光谱(THz-TDS)技术研究室温条件下多晶含硫氨基酸L-蛋氨酸(Met)和L-半胱氨酸(Cys)的光谱特性, 得到相应的吸收谱和折射率谱, 表明含硫氨基酸在THz波段具有区别于其它氨基酸的显著特征. 在实验测量的有效光谱范围0.2～2.8 THz内, L-蛋氨酸的THz吸收峰分别位于1.06, 1.88和2.70 THz; L-半胱氨酸的吸收峰分别位于1.40, 1.70, 2.33和2.61 THz, 两种氨基酸的平均折射率均为1.44. 利用GAUSSIAN 03软件包中的Hartree-Fock理论计算了蛋氨酸双分子的低频振动谱, 表明了与蛋氨酸各吸收峰对应的分子微观振动模式, 并对实验光谱进行了解析讨论.     

Dynamic properties of blue-phase mixtures

The dynamics of electric-field-induced lattice distortions and phase transitions in the BPI, BPII and BPIII (fog) phases of blue-phase mixtures, which include low-molar-mass and/or polymeric components, have been studied. Several electric-field-induced effects have been observed in the blue phases, and the electro-optic characteristics of the driven and non-driven responses of each are considered. Anomalous behaviour in BPIII, where driven response times were measured to be slower than the relaxation responses, is described and considered with respect to relaxation processes previously observed in BPII. Finally, we present data for a mixture containing 40 per cent by weight of a side-chain-polymer liquid crystal. In this case only BPI was observed, and it appears that the increased viscosity had little effect on the response times of the system. These data are discussed.     

Dynamic properties of blue-phase mixtures

The dynamics of electric-field-induced lattice distortions and phase transitions in the BPI, BPII and BPIII (fog) phases of blue-phase mixtures, which include low-molar-mass and/or polymeric components, have been studied. Several electric-field-induced effects have been observed in the blue phases, and the electro-optic characteristics of the driven and non-driven responses of each are considered. Anomalous behaviour in BPIII, where driven response times were measured to be slower than the relaxation responses, is described and considered with respect to relaxation processes previously observed in BPII. Finally, we present data for a mixture containing 40 per cent by weight of a side-chain-polymer liquid crystal. In this case only BPI was observed, and it appears that the increased viscosity had little effect on the response times of the system. These data are discussed.     

Active control of terahertz radiation using a metamaterial loaded with a nematic liquid crystal

We experimentally demonstrate an effective method of manipulating terahertz radiation using a metamaterial loaded with a liquid crystal. Active control of the terahertz beam was performed by in-plane electrical switching of the nematic liquid crystal in the metamaterial device. By changing the magnitude of the AC bias voltage from 0 to 100 V, a reversible transmittance shift of up to 10% was achieved for an incident beam with horizontal electric field polarisation and 27% for vertical polarisation with frequency at around 0.7 THz. Metamaterials with electrically tunable characteristics have potential applications in transmission, modulation and switching components and devices for controlling the intensity and phase of terahertz radiation.     

Terahertz properties of liquid crystals doped with ferroelectric BaTiO3 nanoparticles

In this paper, we present the results of the terahertz measurements of liquid crystal (LC) ferroelectric BaTiO₃ nanoparticles (nps) suspensions in the range of frequency from 0.3 up to 3.0 THz. Two different sol-gel methods and the harvesting technique were used to fabricate the nanoparticles. Five LC materials served as hosts for the suspensions: two single compounds: 6CHBT and 2,3′,5′-trifluoro-4-(4-pentylcyclohexyl)-4′-(trifluoromethoxy)-1,1′-biphenyl, and three mixtures: 1867, 2037 and 2020. We characterise, for the first time, the refractive indices and absorption parameters of suspensions with harvested nps in the terahertz range and show how the process of the nps’ preparation affects their response. We observed the increase of birefringence for few LC suspensions in comparison with the pure LCs. The highest increase of birefringence was for 2020 suspension with one kind of ferroelectric nps. On the other hand in most cases the addition of ferroelectric nps to LC causes the increase of its absorption in the THz range. The measurements of LCs terahertz properties by using time-pulsed spectrometer were performed.     

Measurement and analysis of the diffuse reflectance of powdered samples at terahertz frequencies using a quantum cascade laser

We report terahertz (THz) diffuse reflectance measurements of bulk powdered samples at a frequency of 2.83 THz using a narrowband quantum cascade laser. Samples studied comprise polydisperse powders with absorption coefficients extending over two orders of magnitude from ～3 cm(-1) to >200 cm(-1). Diffuse reflectance measurements are used to obtain the effective absorption coefficient of these samples from the backscattering cross-section, predicted under the quasi-crystalline approximation (QCA) in the T-matrix formulation and in conjunction with the Percus-Yevick pair distribution function. Results are compared with effective absorption coefficients obtained from THz time-domain spectroscopy measurements on pressed pellet samples, and show good agreement over the range of effective absorption coefficients studied. We observe that the backscattering cross-section predicted under the QCA is strongly dependent on both the real and imaginary components of the complex permittivity of the sample, and we show that reliable determination of the absorption coefficient from diffuse reflectance measurements therefore requires knowledge of the sample's refractive index. This work demonstrates the applicability of diffuse reflectance measurements, using a THz frequency quantum cascade laser, to the high-resolution spectroscopic analysis of bulk powdered samples at THz frequencies.     

MWNTs/HDPE复合体系在太赫兹波段的光电性质

利用太赫兹时域光谱技术和电导测量研究了多壁碳纳米管/高密度聚乙烯(MWNTs/HDPE)复合体系的光电性质. 直流电导率结果表明, 复合体系的逾渗阈值在5%左右; 交流电导率则随频率的变化出现一个拐点, 拐点的位置与颗粒浓度以及导电颗粒的存在状态密切相关. 加入碳纳米管后, 复合体系在太赫兹波段的吸收系数和折射率均有很大的提高. 通过假定在此波段导电颗粒在聚合物中的介电行为类似于偶极子的弛豫, 利用Cole-Cole理论对复合材料的介电性质进行了解释.     

The magnitude and temperature dependence of the Kerr constant in liquid crystal blue phases and the dark conglomerate phase

A new method of evaluating the Kerr constant in liquid crystals (LCs) is used to determine the temperature dependence of the Kerr effect in blue phases I and II (BPI and BPII) and to investigate the Kerr constant of the isotropic dark conglomerate (DC) phase. This method employs relatively small driving voltages and a vertical field switching (VFS) device geometry. An unusually large Kerr constant, K, is determined in the BPs of a non-polymer-stabilised material, ～3×10^?9 mV^?2 (BPI). The large value of K is attributed to significant pre-transitional values of the dielectric anisotropy and birefringence. K follows an inverse dependence on temperature and we consequently suggest that BPI demonstrates properties best suited to electro-optic devices. The new methodology has the advantage of revealing the dispersion of K in a single measurement. It is also possible to deconvolute the influence of the Kerr effect from measurements of electrostriction of the BP lattice. Finally, the Kerr effect has been measured for the first time in the DC phase of an oxadiazole bent-core liquid crystalline material, and is found to take rather low values, ～1×10^?11 mV^?2, which can be understood in the context of the physical properties of the material.     

Cure monitoring of two-component epoxy adhesives by terahertz time-domain spectroscopy

We report nondestructive measurements of the properties of two-component epoxy adhesives at terahertz wavelengths using a transmissive time-domain spectroscopy system. The results show that the different epoxies have measurably different THz properties, that the changes which occur during the curing process can be monitored by measurements of their refractive index and absorption at terahertz wavelengths, and that this technique is sensitive enough to record changes in the optical material parameters during the postcuring process.     

Observation of blue phases in chiral networks

Abstract

We report the first observation of cholesteric blue phases in chiral anisotropic polymer networks. In two-component mixtures of a chiral and a non-chiral diacrylate, we observed typical textures of BPI, BPII and BPIII phases. By photopolymerization of these materials at constant temperature we obtained blue phase networks. After polymerization, the blue phases were stored, which enabled us to further study them without any temperature control.     

The birefringence and extinction coefficient of positive and negative liquid crystals in the terahertz range

Birefringence and extinction coefficients of positive nematic liquid crystal (NLC) MLC-2142 and negative NLC BHR28000-400 are measured by terahertz time-domain spectroscopy system (THz-TDS). Frequency ranging from 0.1 to 0.6 THz, the birefringence of positive NLC MLC-2142 increases with the increase of frequency, and keeps larger than 0.23, which exhibits application potential in tunable broadband LC THz device where a large birefringence is required. In contrast, the birefringence of BHR28000-400 decreases with the increased frequency, which shows a completely different optical property from the positive NLC. The extinction coefficient of the above two kinds of liquid crystals are also investigated.