Metamaterial Demonstrates Both a High Refractive Index and Extremely Low Reflection in the 0.3-THz Band

Communication and imaging in the terahertz waveband have advanced rapidly in offering industrial applications. However, optical elements such as collimated lenses in the terahertz waveband are bulky compared with the wavelength due to the lack of naturally occurring substances with a high refractive index and low loss. It is essential to miniaturize optical elements in the terahertz waveband for industrial application. Metamaterials consisting of subwavelength structures can arbitrarily control permittivity and permeability and provide a range of refractive indices. Here, we demonstrate a metamaterial with both a high refractive index and extremely low reflection consisting of symmetrically aligned paired cut metal wires with 18,800 units on the front and back surfaces of a dielectric substrate. Measurements by terahertz time-domain spectroscopy (THz-TDS) confirm a high effective refractive index of 6.66 + j0.123, extremely low reflection power of 1.16%, and the unprecedented high figure of merit (FOM = |n _real/n _imag|) of above 300 in the 0.3-THz band. Components with such specifications would enable miniature, high-performance optical elements in the terahertz waveband such as ultrathin flat antennas with high directivity. Further, the concept of the metamaterial with both a high refractive index and extremely low reflection potentially offers a wide range of attractive applications such as solid immersion lenses and cloaking devices.     

Observation of Ultrafast Magnon Dynamics in Antiferromagnetic Nickel Oxide by Optical Pump-Probe and Terahertz Time-Domain Spectroscopies

We have studied the ultrafast magnon dynamics in an antiferromagnetic 3d-transition-metal monoxide, nickel oxide (NiO), using optical pump-probe spectroscopy and terahertz time-domain spectroscopy (THz-TDS). THz damped magnon oscillations were observed in the Faraday rotation signal and in the transmitted THz electric field via optical pump-probe spectroscopy and THz-TDS, respectively. The magnon signals were observed in both the optical pump-probe spectroscopy and THz-TDS experiments, which shows that both Raman- and infrared-active modes are included in the NiO magnon modes. The magnon relaxation rate observed using THz-TDS was found to be almost constant up to the Néel temperature T _N (= 523 K) and to increase abruptly near that temperature. This shows that temperature-independent spin-spin relaxation dominates up to T _N . In our experiment, softening of the magnon frequency near T _N was clearly observed. This result shows that the optical pump-probe spectroscopy and THz-TDS have high frequency resolution and a high signal to noise ratio in the THz region. We discuss the observed temperature dependence of the magnon frequencies using three different molecular field theories. The experimental results suggest that the biquadratic contribution of the exchange interaction plays an important role in the temperature dependence of the sublattice magnetization and the magnon frequency in cubic antiferromagnetic oxides.     

A Modified 8<Emphasis Type="Italic">f</Emphasis> Geometry with Reduced Optical Aberrations for Improved Time Domain Terahertz Spectroscopy

We present a modified 8f geometry for time domain terahertz (THz) spectroscopy (TDTS) experiments. We show, through simulations and data, that a simple rearranging of the off-axis parabolic mirrors, which are typically used to focus and direct THz radiation in TDTS experiments, results in a nearly 40 % reduction in the THz focal spot diameter. This effect stems from significant reduction of the principle optical aberrations which are enhanced in the conventional 8f geometry but partially compensated in the modified 8f experimental setup. We compare data from our home-built TDTS spectrometer in the modified 8f geometry to that of previous iterations that were designed in the conventional 8f geometry to demonstrate the effect.     

THz Induced Nonlinear Effects in Materials at Intensities above 26 GW/cm<Superscript>2</Superscript>

Nonlinear refractive index and absorption coefficient are measured for common semiconductor material such as silicon and organic molecule such as lactose in the terahertz (THz) spectral regime extending from 0.1 to 3 THz. Terahertz pulses with field strengths in excess of 4.4 MV/cm have been employed. Transmittance and the transmitted spectrum were measured with Z-scan and single shot noncollinear electro-optic pump-probe techniques. The THz-induced change in the refractive index (Δn) shows frequency-dependence and a maximum change of \(-~0.128\) at 1.37 THz in lactose and up to \(+~0.169\) at 0.15 THz in silicon was measured for a peak incident THz intensity of 26 GW/cm². Furthermore, the refractive index variation shows a quadratic dependence on the incident THz field, implying the dominance of third-order nonlinearity.     

Synthesis,Optical and Photoluminescence Properties of Cu-Doped Zno Nano-Fibers Thin Films: Nonlinear Optics

Different concentrations of copper-doped zinc oxide thin films were coated on a glass substrate by sol–gel/spin-coating technique. The structural properties of pure and Cu-doped ZnO films were characterized by different techniques, i.e., atomic force microscopy (AFM), photoluminescence and UV–Vis-NIR spectroscopy. The AFM study revealed that pure and doped ZnO films are formed as nano-fibers with a granular structure. The photoluminescence spectra of these films showed a strong ultraviolet emission peak centered at 392 nm and a strong blue emission peak cantered at 450 nm. The optical band gap of the pure and copper-doped ZnO thin films calculated from optical transmission spectra (3.29–3.23 eV) were found to be increasing with increasing copper doping concentration. The refractive index dispersion curve of pure and Cu-doped ZnO film obeyed the single-oscillator model. The optical dispersion parameters such as E _o , E _d , and \(n_{{\infty }}^{2}\) were calculated. Further, the nonlinear refractive index and nonlinear optical susceptibility were also calculated and interpreted.     

氧化镁单晶在太赫兹波段的介电特性

利用太赫兹时域光谱系统,在0.5~9.5 THz范围内对氧化镁单晶基片的介电特性进行了研究,并获得折射率、吸收系数以及复介电函数信息。实验数据表明,在低频（ 2 THz）范围内,氧化镁单晶透过性较好,折射率在3.12~3.15之间。折射率和吸收系数均随频率增加而增大,且在3.16 THz和8.11 THz两处存在明显的吸收峰。通过经典的赝简谐振动理论很好地拟合了实验结果,分析了晶体中的横向光学声子振动模式,为氧化镁单晶在宽带太赫兹波段的应用提供了有益参考。     

Temperature Dependence of Crystal Structure and THz Absorption Spectra of Organic Nonlinear Optical Stilbazolium Material for High-Output THz-Wave Generation

A stilbazolium material comprising 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate (DAST), which has a large nonlinear optical susceptibility, was studied for application in terahertz (THz)-wave generation. The temperature-dependent structure of the DAST crystal was measured by using powder X-ray diffraction from ?100 to 200 °C, indicating a volume expansion of 4.6 %. The lattice constants show anisotropic thermal expansion. Also, the temperature dependence of THz absorption spectra was measured by terahertz time-domain spectroscopy (THz-TDS) in the temperature range varying from ?80 to 88.1 °C. A strong absorption peak was found at around 1 THz, shifting slightly toward a lower frequency with increasing temperature. The temperature dependence of the THz spectra was compared with that of X-ray diffraction. The shifting of THz-vibrational frequencies of the DAST crystal suggests that the change in its lattice structure is temperature dependent.     

Experimental studies of the effects of atomic ordering in epitaxial Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>1 – <Emphasis Type="Italic">x</Emphasis></Subscript>P alloys on their optical properties

The properties of epitaxial Ga _x In_{1 – x} P alloys with an ordered arrangement of atoms in the crystal lattice are studied by a number of spectroscopic methods. The alloys are grown by metal-organic chemical vapor deposition onto single-crystal GaAs(100) substrates. It is shown that, under conditions of the coherent growth of an ordered Ga _x In_{1 – x} P alloy on a GaAs(100) substrate, atomic ordering results in radical modifications of the optical properties of the semiconductor compared to the properties of disordered alloys. Among these modifications are a decrease in the band gap and an increase in the luminescence intensity. From the data of dispersion analysis of the infrared dispersion spectra and from ultraviolet spectroscopy data obtained in the transmittance–reflection mode of measurements, the basic optical characteristics, specifically, the dispersion of the refractive index and the high-frequency permittivity of Ga _x In_{1 – x} P alloys with ordering are determined. All of the experimental data are in good agreement with the developed theoretical concepts.     

Optoelectrical Behavior of Ferroelectric Lithium Rubidium Sulfate Crystals

Single crystals of lithium rubidium sulfate with good transparency have been grown from aqueous solution by a slow evaporation technique. Single-crystal x-ray diffraction results revealed that, at room temperature, the crystal belongs to the monoclinic system in space group P2₁/n. From the transmittance and reflectance spectra, various optoelectrical constants such as the refractive index, extinction coefficient, optical bandgap for direct transition, optical conductivity, electronic polarizability, high-frequency dielectric constant, and optical electronegativity were calculated for the investigated lithium rubidium sulfate crystal. A correlation between the various optical parameters was also observed. The refractive index was fit to a three-term Cauchy dispersion relationship. The single-oscillator Wemple–DiDomenico model was applied to discuss the dispersion of the refractive index. The optical transmittance study revealed transparency of the crystal in the entire visible region with wide optical bandgap. The electronic polarizability values calculated from the Clausius–Mossotti equation and from bandgap analysis were found to be in good agreement.     

Design of broadband polarization converter for terahertz waves

A broadband reflective polarization converter is proposed. The unit cell of polarization converter is composed of a single-split resonant ring, a double-split resonant ring, a dielectric substrate and a metallic ground. The simulated results show that the polarization converter can convert x-polarized waves into y-polarized waves and obtain a broadband polarization conversion from 0.501 0 THz to 1.390 0 THz with the polarization conversion ratio (PCR) beyond 80% at normal incidence. Moreover, the surface current distributions are investigated to explain the polarization conversion mechanism. Finally, a good agreement is achieved between simulated and measured results. The polarization converter can be applied in terahertz imaging, communication and stealthy technology.     

Injection-induced terahertz electroluminescence from silicon <Emphasis Type="Italic">p–n</Emphasis> structures

Injection-induced terahertz electroluminescence from silicon p ⁺–n structures is observed at helium temperatures. Structures fabricated by the diffusion of boron into a phosphorus-doped n-Si substrate are studied. Relatively narrow luminescence lines are observed in the luminescence spectra against a broad smooth background. The spectral position of a number of emission lines corresponds to optical transitions between excited donor states and the ground state of phosphorus donors. The intracenter optical transitions of electrons at phosphorus donors are excited as a result of recombination processes occurring in the n-type region of the structure under the injection of nonequilibrium holes. A number of other lines in the terahertz emission spectra are associated with intracenter transitions at acceptor centers, which are also excited as a result of injection. The structureless background in the electroluminescence spectra may be due to terahertz emission upon the intraband energy relaxation of “hot” carriers having an effective temperature exceeding the lattice temperature. These “hot” carriers appear in the structure under injection conditions.     

Multi-Cycle Terahertz Emission from <Emphasis Type="Italic">β</Emphasis>-Barium Borate

We report the observation of strong narrowband terahertz emission at 10.6 THz from the common nonlinear second harmonic generation crystal β-barium borate crystal excited by 40 fs laser pulses at a 800 nm center wavelength. The emitted THz field amplitude is linear with incident pump energy with no saturation observed up to 2 mJ of pump pulse energy, and exhibits a crystal azimuthal angular dependence consistent with an optical rectification process. The narrowband emission can be explained by the large dispersion caused by optical phonons in the vicinity of the emission energy.     

Topology-Based Prediction of Pathway Dysregulation Induced by Intense Terahertz Pulses in Human Skin Tissue Models

The strong interaction between terahertz (THz) radiation and biological systems has motivated the development of several biomedical technologies, including imaging and spectroscopy applications with promising potential for improved disease diagnosis. This interaction mechanism also implies that external excitation with intense pulses of THz energy could couple to important biological structures and induce significant downstream phenotypic effects. In this study, we expose human skin tissue models to a prolonged train of high-intensity THz pulses and measure the resulting global differential gene expression. From these data, signal pathway perturbation analysis identified pathways that are predicted to be significantly dysregulated, including the cytokine-cytokine receptor interaction and glioma pathways, and further identified the gene-level mechanisms predominantly responsible. These results indicate that induction of an inflammatory-like response and suppression of division/differentiation in cancer are possible. These effects could be further explored and characterized in different types of normal and cancerous tissues to determine potential novel clinical applicability of intense THz pulses.     

Method for narrowing the directional pattern of an InGaAs/GaAs/AlGaAs multiwell heterolaser

A semiconductor laser with a new waveguide is developed. It allows significant narrowing of the directional pattern (to 4° in the plane perpendicular to the p–n junction). In the used waveguide, the minimum excess of the effective refractive index n_eff of the excitation mode over the substrate refractive index n_s (n_eff–n_s ? 1) is provided by selecting the thickness of Al_0.3Ga_0.7As confinement layers, which significantly increases the waveguide mode size and leads to directional-pattern narrowing.     

Resonant features of the terahertz generation in semiconductor nanowires

The paper presents the results of experimental studies of the generation of terahertz radiation in periodic arrays of GaAs nanowires via excitation by ultrashort optical pulses. It is found that the generation of THz radiation exhibits resonant behavior due to the resonant excitation of cylindrical modes in the nanowires. At the optimal geometric parameters of the nanowire array, the generation efficiency is found to be higher than that for bulk p-InAs, which is one of the most effective coherent terahertz emitters.     

太赫兹宽谱聚合物波导及其传感应用

利用太赫兹时域光谱系统对三种反谐振波导的太赫兹谱进行了研究, 这三种波导分别是PMMA管型波导, PMMA自支撑结构波导和SiO2管型波导.该研究验证了三种波导的太赫兹传输基于反谐振理论, 并在实验上实现了PMMA自支撑结构波导超过2 THz的宽光谱传输.同时, 对基于反谐振原理的太赫兹物质传感进行了理论和实验分析, 为物质探测提供了一种可行的手段.     

Bright MM-Wave and THz Luminescence by Down-Conversion of Near-IR Amplified-Spontaneous-Emission

We report the measurement of exceptionally bright, incoherent radiation in the THz region by frequency down-conversion of amplified spontaneous emission around 775-nm wavelength. The down-conversion technique is optical mixing in an interdigital photoconductive capacitor made from ultra fast ErAs:GaAs. The brightness temperature into a single spatial mode is approximately 1.1×10⁵ K, making the new radiation at least 70 times more intense than common incandescent sources in the THz region.     

石英玻璃太赫兹光学参数测量的误差分析

测量液体或气体样品的太赫兹光学参数时,常采用石英玻璃作为容器,因此在计算样品的折射率和吸收率时需要考虑石英玻璃太赫兹光学参数的影响。文中利用太赫兹时域谱技术测量了石英玻璃的太赫兹时域光谱,根据测量模型计算了太赫兹波段的折射率和吸收率,并利用误差传播理论分析了这些光学参数的误差。结果发现折射率误差在0.2~2.0 THz范围内基本保持不变,而吸收率误差则随频率增大而呈指数增大。这对于提高测量石英玻璃容器中的气体及液体样品太赫兹光学参数的准确性具有重要价值。     

Insights on the Side Panels of the <Emphasis Type="Italic">Franciscan Triptych</Emphasis> by Fra Angelico Using Terahertz Time-Domain Imaging (THz-TDI)

The side panels of the Franciscan Triptych (St. Jerome, St. John the Baptist, and the Archangel Gabriel and St. Francis, St. Onofrio, and the Virgin Annunciate, by Fra Angelico, before 1429) were scanned by means of terahertz time-domain imaging (THz-TDI). THz analysis supplied information on the stratigraphy of the panel paintings and the associated construction, “gessoing” and gilding techniques. Furthermore, THz-TDI provided information regarding the location of restoration materials within the painting stratigraphy on St. Jerome, St. John the Baptist, and the Archangel Gabriel, as well as on the extension and nature of subsurface cracks in the panel painting of St. Francis, St. Onofrio, and the Virgin Annunciate.     

3D-Printed Broadband Dielectric Tube Terahertz Waveguide with Anti-Reflection Structure

We demonstrate broadband, low loss, and close-to-zero dispersion guidance of terahertz (THz) radiation in a dielectric tube with an anti-reflection structure (AR-tube waveguide) in the frequency range from 0.2 to 1.0 THz. The anti-reflection structure (ARS) consists of close-packed cones in a hexagonal lattice arranged on the outer surface of the tube cladding. The feature size of the ARS is in the order of the wavelength between 0.2 and 1.0 THz. The waveguides are fabricated with the versatile and cost efficient 3D-printing method. Terahertz time-domain spectroscopy (THz-TDS) measurements as well as 3D finite-difference time-domain simulations (FDTD) are performed to extensively characterize the AR-tube waveguides. Spectrograms, attenuation spectra, effective phase refractive indices, and the group-velocity dispersion parameters β ₂ of the AR-tube waveguides are presented. Both the experimental and numerical results confirm the extended bandwidth and smaller group-velocity dispersion of the AR-tube waveguide compared to a low loss plain dielectric tube THz waveguide. The AR-tube waveguide prototypes show an attenuation spectrum close to the theoretical limit given by the infinite cladding tube waveguide.