Temperature-Dependent Refractive Index of Quartz at Terahertz Frequencies

Characterisation of materials often requires the use of a substrate to support the sample being investigated. For optical characterisation at terahertz frequencies, quartz is commonly used owing to its high transmission and low absorption at these frequencies. Knowledge of the complex refractive index of quartz is required for analysis of time-domain terahertz spectroscopy and optical pump terahertz probe spectroscopy for samples on a quartz substrate. Here, we present the refractive index and extinction coefficient for α-quartz between 0.5 THz and 5.5 THz (17–183 cm^??1) taken at 10, 40, 80, 120, 160, 200 and 300 K. Quartz shows excellent transmission and is thus an ideal optical substrate over the THz band, apart from the region 3.9 ± 0.1 THz owing to a spectral feature originating from the lowest energy optical phonon modes. We also present the experimentally measured polariton dispersion of α-quartz over this frequency range.     

Optical Properties of K<Subscript>2</Subscript>O-Li<Subscript>2</Subscript>O-WO<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> Glasses: Evidence of Mixed Alkali Effect

Glass with compositions xK₂O-(30 ? x)Li₂O-10WO₃-60B₂O₃ for 0 ≤ x ≤ 30 mol.% have been prepared using the normal melt quenching technique. The optical reflection and absorption spectra were recorded at room temperature in the wavelength range 300–800 nm. From the absorption edge studies, the values of the optical band gap (E _opt) and Urbach energy (ΔE) have been evaluated. The values of E _opt and ΔE vary non-linearly with composition parameter, showing the mixed alkali effect. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple Di-Domenico model.     

基于双开口环型的太赫兹超材料传感器设计

设计了一款可用于检测材料折射率及厚度的双开口环型太赫兹超材料传感器,其结构由双开口方环与圆环嵌套的超材料结构和聚酰亚胺衬底两部分构成.当太赫兹波垂直入射超材料表面时,该传感器结构在0.8～1.8 THz范围内形成三个高Q值谐振峰(中心频率分别为f1,f2和f3).通过探讨超材料结构表面电流分布与三个谐振峰形成的关系,观察到超材料结构对入射太赫兹波的不同响应特性导致产生不同的表面电流分布.此外,还对该传感器在折射率传感和厚度传感方面的应用进行了探究.在待测物厚度一定的情况下,该传感器在谐振频率f1,f2和f3处的传感灵敏度分别可达170,103和119 GHz/RIU,均具有优越的传感特性,可利用其多谐振峰进行高灵敏度折射率传感.这种高灵敏度的多谐振峰折射率传感器可以检测到待测分析物的微小变化,在生物化学检测领域具有广阔的应用前景.     

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.     

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.     

First-Principles Study of Structural,Electronic, Optical,and Thermal Properties of BeSiSb<Subscript>2</Subscript> and MgSiSb<Subscript>2</Subscript>

Structural, electronic, optical, and thermal properties of ternary II–IV–V₂ (BeSiSb₂ and MgSiSb₂) chalcopyrite semiconductors have been calculated using the full-potential linearized augmented plane wave scheme?in the generalized gradient approximation. The optimized equilibrium structural parameters (a, c, and u) are in good agreement with theoretical results obtained using other methods. The band structure and density of states reveal that BeSiSb₂ has an indirect (Γ–Z) bandgap of about 0.61 eV, whereas MgSiSb₂ has a direct (Γ–Γ) bandgap of 0.80 eV. The dielectric function, refractive index, and extinction coefficient were calculated to investigate the optical properties, revealing that BeSiSb₂ and MgSiSb₂ present very weak birefringence. The temperature dependence of the volume, bulk modulus, Debye temperature, and heat capacities (C _v and C _p) was predicted using the quasiharmonic Debye model at different pressures. Significant differences in properties are observed at high pressure and high temperature. We predict that, at 300 K and 0 GPa, the heat capacity at constant volume C _v, heat capacity at constant pressure C _P, Debye temperature θ _D, and Grüneisen parameter γ will be about 94.91 J/mol K, 98.52 J/mol K, 301.30 K, and 2.11 for BeSiSb₂ and about 96.08 J/mol K, 100.47 J/mol K, 261.38 K, and 2.20 for MgSiSb₂, respectively.     

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.     

Half Cycle Terahertz Pulse Generation by Prism-Coupled Cherenkov Phase-Matching Method

Nonlinear optical terahertz wave generation is a promising method for realizing a practical source with wide frequency range and high peak power. Unfortunately, many nonlinear crystals have a strong absorption in the terahertz frequency region. This limits efficient and widely tunable terahertz wave generation. The Cherenkov phase-matching method is one of the most promising techniques for overcoming these problems. We propose a prism-coupled Cherenkov phase-matching method, in which a prism with a suitable refractive index at terahertz frequencies is coupled to a nonlinear crystal. We demonstrate prism-coupled Cherenkov phase-matching terahertz generation using the DAST and LiNbO₃ crystals. With a DAST crystal, we obtain a spectral flat tunability up to 10 THz by difference frequency generation. With a LiNbO₃ crystal, we observe a spectral flat broadband terahertz pulse generation up to 5 THz pumped by a femto second fiber laser. The obtained temporal waveform is an ideal half cycle pulse suitable for reflection terahertz tomography.     

Characteristics of Gadolinium Oxide Nanoparticles as Contrast Agents for Terahertz Imaging

For the application of gadolinium oxide (Gd₂O₃) nanoparticles as terahertz contrast agents, their optical properties in a solvent were studied using terahertz time-domain spectroscopy. The power absorption and refractive index of the samples were measured with various concentrations of nanoparticles. The power absorption was extremely large, as much as three orders of magnitude higher than that of water, so that a few ppms of Gd₂O₃ nanoparticles were distinguished in terms of their power absorption capacity. The results show that the interaction between the terahertz electromagnetic waves and the Gd₂O₃ nanoparticles is strong enough to allow their exploitation as contrast agents for terahertz medical imaging.     

Features of the field of a cylindrical wave scattered and focused by an isolated perfect Veselago lens with finite dimensions or by a system of three lenses

The results of solving a scattering problem for the field of a cylindrical wave incident onto an isolated Veselago lens with finite dimensions or onto a system of three lenses are presented for the lens refractive index n _r = ?1 and loss parameter ν. The influence of geometric dimensions, the loss in the lens medium, and the position of the source of cylindrical wave on the field structure near the focusing region are studied. It is shown that the resolution of this system cannot be increased via an increase in the geometric dimensions of the lens or the number of lenses.     

A kind of graphene film metamaterial for terahertz absorbers

A kind of functional graphene thin film metamaterial on a metal-plane separated by a thick dielectric layer is designed for terahertz (THz) absorbers. We investigate the properties of the graphene metamaterial with different interlayers in the 0–3 THz range. The simulation results show that the absorption rate reaches up to 99.9% at the frequency of 1.917 THz. Changing the period to 80 μm×18 μm can get a narrow-band high quality factor (Q) absorber. We present a novel theoretical interpretation based on the standing wave field theory, which shows that the coherent superposition of incident and reflection rays produces standing waves, and the field energy is localized inside the thick spacers and dissipates through the metal-planes.     

The Relation Between the Refractive Index of the Equatorial Ionospheric F2 Region and Long-Term Solar Indices

Ionospheric refractive index is especially important in the reflection and propagation of HF (3–30 MHz) waves from the ionosphere. For this reason, in this study, the relation between the real parts (μ\(_{0}^{2}\), μ\(_{x}^{2}\) and μ\(_{p}^{2}\)) of the refractive index computed as based on the direction of the Earth’s magnetic field for 300 km altitude in the equatorial ionospheric F2 region and the long-term solar indices (Sunspot Number-R12, Solar Flux at 10.7 cm -F10.7, Coronal Mass Ejection-CME) has been examined by using the multiple regression model. As a result of the examinations, it has been determined that there is a very strong relation between the three refractive index values and solar indices. While it was determined that the R12 and F10.7 indices have a very strong relation, it was also determined that CME did not have a statistically significant relation. This insignificant situation may only be explained with the magnetic field of the Earth acting like a shield.     

Structural transformations and optical properties of As<Subscript>2</Subscript>S<Subscript>3</Subscript> chalcogenide glasses

The effects of melt temperature T _i and quenching rate V _i on the structure and optical properties of As₂S₃ glasses is studied. It is found that the glass band gap increases with T _i and V _i , whereas a decrease is observed in the glass density, refractive index (from 2.71 to 2.48), and two-photon absorption coefficient (from 0.37 to 0.15 cm/MW), which is accompanied by an increase in the optical-breakdown damage threshold.     

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.     

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.     

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.     

High Efficiency and Wideband 300 GHz Frequency Doubler Based on Six Schottky Diodes

A high efficiency and wideband 300 GHz frequency doubler based on six Schottky diodes is presented in this paper. This balanced doubler features a compact and robust circuit on a 5-μm-thick, 0.36-mm-wide, and 1-mm-long GaAs membrane, fabricated by LERMA-C2N Schottky process. The conversion efficiency is mainly better than 16% across the wide bandwidth of 266–336 GHz (3 dB fractional bandwidth of 24%) when pumping with 20–60 mW input power (P _in) at the room temperature. A peak output power of 14.75 mW at 332 GHz with a 61.18 mW P _in, an excellent peak efficiency of 30.5% at 314 GHz with 43.86 mW P _in and several frequency points with outstanding efficiency of higher than 25% are delivered. This doubler served as the second stage of the 600 GHz frequency multiplier chain is designed, fabricated, and measured. The performance of this 300 GHz doubler is highlighted comparing to the state-of-art terahertz frequency doublers.     

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。     

Structural and Electrical Properties of Ag/<Emphasis Type="Italic">n</Emphasis>-TiO<Subscript>2</Subscript>/<Emphasis Type="Italic">p</Emphasis>-Si/Al Heterostructure Fabricated by Pulsed Laser Deposition Technique

We have investigated the structural and electrical characteristics of the Ag/n-TiO₂/p-Si/Al heterostructure. Thin films of pure TiO₂ were deposited on p-type silicon (100) by optimized pulsed laser ablation with a KrF-excimer laser in an oxygen-controlled environment. X-ray diffraction analysis showed the formation of crystalline TiO₂ film having a tetragonal texture with a strong (210) plane as the preferred direction. High purity aluminium and silver metals were deposited to obtain ohmic contacts on p-Si and n-TiO₂, respectively. The current–voltage (I–V) characteristics of the fabricated heterostructure were studied by using thermionic emission diffusion mechanism over the temperature range of 80–300 K. Parameters such as barrier height and ideality factor were derived from the measured I–V data of the heterostructure. The detailed analysis of I–V measurements revealed good rectifying behavior in the inhomogeneous Ag/n-TiO₂/p-Si(100)/Al heterostructure. The variations of barrier height and ideality factor with temperature and the non-linearity of the activation energy plot confirmed that barrier heights at the interface follow Gaussian distributions. The value of Richardson’s constant was found to be 6.73 × 10⁵ Am^?2 K^?2, which is of the order of the theoretical value 3.2 × 10⁵ Am^?2 K^?2. The capacitance–voltage (C–V) measurements of the heterostructure were investigated as a function of temperature. The frequency dependence (Mott–Schottky plot) of the C–V characteristics was also studied. These measurements indicate the occurrence of a built-in barrier and impurity concentration in TiO₂ film. The optical studies were also performed using a UV–Vis spectrophotometer. The optical band gap energy of TiO₂ films was found to be 3.60 eV.     

Real-Time Determination of Absolute Frequency in Continuous-Wave Terahertz Radiation with a Photocarrier Terahertz Frequency Comb Induced by an Unstabilized Femtosecond Laser

A practical method for the absolute frequency measurement of continuous-wave terahertz (CW-THz) radiation uses a photocarrier terahertz frequency comb (PC-THz comb) because of its ability to realize real-time, precise measurement without the need for cryogenic cooling. However, the requirement for precise stabilization of the repetition frequency (f _rep) and/or use of dual femtosecond lasers hinders its practical use. In this article, based on the fact that an equal interval between PC-THz comb modes is always maintained regardless of the fluctuation in f _rep, the PC-THz comb induced by an unstabilized laser was used to determine the absolute frequency f _THz of CW-THz radiation. Using an f _rep-free-running PC-THz comb, the f _THz of the frequency-fixed or frequency-fluctuated active frequency multiplier chain CW-THz source was determined at a measurement rate of 10 Hz with a relative accuracy of 8.2?×?10^?13 and a relative precision of 8.8?×?10^?12 to a rubidium frequency standard. Furthermore, f _THz was correctly determined even when fluctuating over a range of 20 GHz. The proposed method enables the use of any commercial femtosecond laser for the absolute frequency measurement of CW-THz radiation.