Interaction Features of Stepped-Impedance Stripline Resonators in Comb Filters

Mixed coefficients of coupling between the closely spaced stepped-impedance resonators in comb filters of stripline design have been investigated. Transmission zeros at frequencies f _zi correspond to mixed coupling coefficients k _i . These zeros can be moved with respect to the filter passband central frequency f₀ by modifying the shape of resonators. It was proved that the reduction of gap between resonators made it possible to locate frequencies f _z and f₀ closer to one another. The existing restrictions on the minimal value of gap between resonators limit the degree of proximity between f _z and f₀. The N-resonator stripline comb filters with mixed coupling can have N?1 transmission zeros. The absence of cross-coupling links in stripline filters simplifies their construction. It has been established that the thickness of central conductors of stripline resonators affects the positive and negative mixed coupling coefficients. The paper presents measurement data of miniature stripline three-resonator comb filter having an enhanced selectivity at the expense of two transmission zeros. The central frequency of filter is f₀ = 1850 MHz, the bandwidth BW = 100 MHz. The filter having dimensions 5.8×4.2×2 mm was implemented by connecting two ceramic substrates having relative dielectric permittivity ε _r = 92 and the metallized patterns deposited on them.     

Microwave Dielectric Properties of BiCu<Subscript>2</Subscript>PO<Subscript>6</Subscript> Ceramics with Low Sintering Temperature

A BiCu₂PO₆ microwave dielectric ceramic was prepared using a solid-state reaction method. As the sintering temperature increased from 800°C to 880°C, the bulk density of BiCu₂PO₆ ceramic increased from 6.299 g/cm³ to 6.366 g/cm³; the optimal temperature was 860°C. The best microwave dielectric properties [permittivity (? _r ) = ～16, a quality factor (Q × f) = ～39,110 GHz and a temperature coefficient of resonant frequency (τ _f ) = ～?59 ppm/°C] were obtained in the ceramic sintered at 860°C for 2 h. Then, TiO₂ with a positive τ _f (～+400 ppm/°C) was added to compensate the τ _f value. The composite material was found to have a near-zero τ _f (+2.7 ppm/°C) and desirable microwave properties (? _r  = 19.9, Q × f = 24,885 GHz) when synthesized at a sintering temperature of 880°C. This system could potentially be used for low-temperature co-fired ceramics technology applications.     

Electron transport and detection of terahertz radiation in a GaN/AlGaN submicrometer field-effect transistor

Electron transport and photoresponse in the terahertz range in a GaN/AlGaN field-effect transistor with the submicrometer gate (0.25 μm) and two-dimensional electron gas in the channel (the electron concentration n _s = 5 × 10¹² cm^?2) were studied at 4.2 K. The charge-carrier mobility in the transistor’s channel μ = 3500 cm²/(V s) was determined from the dependence of the conductance on magnetic field. It is found that the dependence of photovoltage at the radiation frequency f = 574 GHz on the gate voltage (i.e., on the concentration of two-dimensional electrons) features a characteristic maximum, which is related to a resonance response of the subgate plasma in the transistor channel.     

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.     

Functionalized Multiwalled Carbon Nanotube Electrochemical Sensor for Determination of Anticancer Drug Flutamide

An electrochemical sensor based on functionalized multiwalled carbon nanotubes (MWCNT_f) has been developed and applied for determination of anticancer drug flutamide in pharmaceutical and artificial urine samples. The electrode was prepared by modifying a glassy carbon electrode with MWCNT_f, denoted herein as MWCNT_f/GCE. The MWCNT_f/GCE electrode exhibited high catalytic activity, high sensitivity, and high stability and was applicable over a wide concentration range for flutamide. The effects of the scan rate, pH, and nature of the electrolyte on the electrochemical behavior of flutamide on the MWCNT_f/GCE were investigated. The results showed that this electrode presented the best square-wave voltammetric response to flutamide in Britton–Robinson buffer solution at pH 5.0 at frequency of 50 Hz and amplitude of 0.06 V. The proposed sensor presents a wide linear response range from concentration of 0.1 μmol L^?1 up to 1000 μmol L^?1 (or 27.6 μg L^?1 up to 0.27 g L^?1), with limit of detection, limit of quantification, and sensitivity of 0.03 μmol L^?1, 0.1 μmol L^?1, and 0.30 μA μmol^?1 L, respectively. The MWCNT_f/GCE electrode was successfully applied for determination of flutamide in pharmaceutical formulations and artificial urine samples, giving results in agreement with those obtained by a comparative method described in literature. A paired Student’s t-test revealed no statistical difference between the reference and proposed method at 95% confidence level. The average recovery for fortified samples was 101 ± 1%.     

Low-frequency noise in gallium nitride epitaxial layers with different degrees of order of mosaic structure

The correlation between the noise level 1/f and the degree of mosaic-structure order in gallium nitride epitaxial layers was studied for the first time. Samples with a doping level of N _d ?N _a ≈8×10¹⁶ cm^?3 and a relatively high degree of order were characterized by the Hooge parameter α≈1.5×10^?3. This value is unprecedently low for thin GaN epitaxial films. The Hooge parameter was significantly higher for samples with N _d ?N _a ≈1.1×10¹⁸ cm^?3 and a low degree of order despite the fact that α generally decreases with increasing doping level at the same degree of order. Thus, the degree of mosaic-structure order affects not only the optical and electrical characteristics but also the fluctuation parameters of GaN epitaxial layers.     

Detection of terahertz radiation by resonant tunneling nanoheterostructures

The phenomenon of terahertz radiation detection by resonant tunneling structures (RTSs) has been studied. The calculations of the changes ΔI₀ in the direct current (DC) component I₀ under the action of an alternating electric field were carried out by the solution of a nonstationary Schrodinger equation with a time-periodic electric field based on the Floquet mode expansion of the wave functions. The dependences of the DC component I₀ in resonant tunneling structures on the frequency ν and AC signal amplitude V_ac have been built. It is shown that the ΔI₀ value in triple-barrier RTSs at resonance frequency hv ≈ E_r2–E_r1 (E_r1 and E_r2 are the energies of the size-quantized levels) can exceed a low-frequency value by more than an order of magnitude. The parameters of the structures have been optmized, in order to use them in the terahertz radiation detectors in the anbsence of an external bias. The possibility of tuning the resonance frequency in the terahertz range by changing the DC bias has been shown.     

Graphene-Based\ Waveguide Terahertz Wave Attenuator

We design an electrically controllable terahertz wave attenuator by using graphene. We show that terahertz wave can be confined and propagate on S-shaped graphene waveguide with little radiation losses, and the confined terahertz wave is further manipulated and controlled via external applied voltage bias. The simulated results show that, when chemical potential changes from 0.03 into 0.05 eV, the extinction ratio of the terahertz wave attenuator can be tuned from 1.28 to 39.42 dB. Besides the simplicity, this novel terahertz wave attenuator has advantages of small size (24?×?30 μm²), a low insertion loss, and good controllability. It has a potential application for forthcoming planar terahertz wave integrated circuit fields.     

Special features of radiation-defect annealing in silicon <Emphasis Type="Italic">p-n</Emphasis> structures: The role of Fe impurity atoms

Deep-level transient spectroscopy is used to study the formation of complexes that consist of a radiation defect and a residual impurity atom in silicon. It is established that heat treatment of the diffused Si p⁺-n junctions irradiated with fast electrons lead to the activation of a residual Fe impurity and the formation of the FeVO (E_0.36 trap) and FeV₂ (H_0.18 trap) complexes. The formation of these traps is accompanied by the early (100–175°C) stage of annealing of the main vacancy-related radiation defects: the A centers (VO) and divacancies (V₂). The observed complexes are electrically active and introduce new electron (E_0.36: E _t ^e =E _c -0.365 eV, σ _n =6.8×10^?15 cm²) and hole (H_0.18: E _t ^h =E _v +0.184 eV, σ _p =3.0×10^?15 cm²) levels into the silicon band gap and have a high thermal stability. It is believed that the complex FeVO corresponds to the previously observed and unidentified defects that have an ionization energy of E _t ^e =E _c ?(0.34–0.37) eV and appear as a result of heat treatment of irradiated diffused Si p⁺-n junctions.     

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.     

Features of the infrared reflection spectra of SmS semiconductor in the homogeneity range

Infrared reflection spectra in the range of 5200–380 cm^?1 were measured for polycrystalline samarium monosulfide samples in the semiconductor phase with compositions Sm_{1 + x} S lying within the homogeneity range (0 ≤ x ≤ 0.17) in the temperature range of 300–600 K. Five peaks with energies in the range of 1150–880 cm^?1, whose positions depend weakly on the composition and temperature. It was shown that the reflection peaks are associated with ⁷ F ₀–⁷ F ₂ transitions of 4f electrons of Sm²⁺ ions.     

基于频率梳的太赫兹辐射强度测量

为实现太赫兹辐射特性精准认知,开展太赫兹辐射绝对强度测量研究。通过光学频率梳产生太赫兹频率梳,利用太赫兹频率梳实现太赫兹辐射源空间强度测量。本文利用电光采样和光电导探测两种方式,实现了100 GHz辐射源空间辐射强度测量;将100 GHz辐射总功率溯源到标准太赫兹功率计,实现太赫兹辐射强度绝对测量。分析比较了利用800 nm空间光进行电光采样和利用1550 nm光纤激光进行光电导探测的测量结果。本文在不同距离下,对太赫兹辐射源的空间辐射绝对强度进行了测量,实验揭示了太赫兹辐射传输的空间演化特性。     

Terahertz absorption and emission upon the photoionization of acceptors in uniaxially stressed silicon

Experimental data on the spontaneous emission and absorption modulation in boron-doped silicon under CO₂ laser excitation depending on the uniaxial stress applied along the [001] and [011] crystallographic directions are presented. Room-temperature radiation is used as the probe radiation. Low stress (less than 0.5 kbar) is shown to reduce losses in the terahertz region by 20%. The main contribution to absorption modulation at zero and low stress is made by A⁺ centers. Intersubband free hole transitions additionally contribute to terahertz absorption at higher stress. These contributions can be minimized by compensation.     

Interphase interactions and features of structural relaxation in TiB<Subscript>x</Subscript>-<Emphasis Type="Italic">n</Emphasis>-GaAs (InP,GaP, 6<Emphasis Type="Italic">H</Emphasis>-SiC) contacts subjected to active treatment

The results of experimental studies of interphase interactions in TiB_x-n-GaAs (GaP, InP, 6H-SiC) contacts stimulated by external effects are described. These effects are rapid thermal annealing at temperatures as high as 1000°C, microwave treatment at f=2.5 GHz, and ⁶⁰Co γ radiation in the range of doses 10⁵–10⁷ rad. Possible thermal and athermal relaxation mechanisms of internal stresses are considered. It is shown that thermally stable TiB_x-n-GaAs (GaP, InP, 6H-SiC) interfaces can be formed.     

Terahertz luminescence of GaAs-based heterostructures with quantum wells under the optical excitation of donors

Spontaneous emission from selectively doped GaAs/InGaAs:Si and GaAs/InGaAsP:Si heterostructures is studied in the frequency range of ～3–3.5 THz for transitions between the states of the two-dimensional subband and donor center (Si) under the condition of excitation with a CO₂ laser at liquid-helium temperature. It is shown that the population inversion and amplification in an active layer of 100–300 cm^?1 in multilayered structures with quantum wells (50 periods) and a concentration of doping centers N _D ≈ 10¹¹ cm^?2 can be attained under the excitation-flux density 10²³ photons/(cm² s).     

Ce@Au<Subscript>14</Subscript>: A Bimetallic Superatom Cluster with 18-Electron Rule

Doping of gold clusters and nanoparticles has received substantial attention due to their ability to encapsulate atoms and molecules. Here, the geometric and electronic properties of the cerium-encapsulated nanocage Ce@Au₁₄ are reported using density functional theory. Calculated results show that its ground electronic state is a singlet state and conforms to the superatomic 18-electron configuration of 1S ²1P ⁶1D ¹⁰ jellium state, both primarily involving the bonding interaction between s- and d-shell atomic orbitals of the Ce atom and superatomic orbitals of the hollow polyhedral Au₁₄ cage. In addition, it should be noted that f electrons in rare earth atoms trend to retain their localized state, and their doping in gold clusters could easily lead to clusters with large magnetic moments. However, in the case of superatom clusters, the f-shell electrons will be the preferential arrangement at the unfilled d-shell to satisfy the superatomic electron structure. Further analysis of the electronic structure also proves that the unoccupied 1F superatomic orbitals mainly originate from the contribution of the 4f-shell. As a consequence, this work provides a theoretical basis for the future design and synthesis of f-elements-encapsulated gold nanoclusters.     

A 4<Emphasis Type="Italic">H</Emphasis>-SiC<Emphasis Type="Italic">p-i-n</Emphasis> diode fabricated by a combination of sublimation epitaxy and CVD

The possibility of fabricating heavily doped (N _a ?N _d ≥ 1 × 10¹⁹ cm_?3) p⁺-4H-SiC layers on CVD-grown lightly doped n-4H-SiC layers by sublimation epitaxy has been demonstrated. It is shown that a Au/Pd/Ti/Pd contact, which combines a low specific contact resistance (～2 × 10^?5 Ω cm²) with high thermal stability (up to 700°C), is the optimal contact to p-4H-SiC. The p-n structures obtained are used to fabricate packaged diodes with a breakdown voltage of up to 1400 V.     

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.     

Analysis of the magnetic properties of (La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>0.93</Subscript>MnO<Subscript>3</Subscript> manganites at high temperatures

For the first time, the Faraday method is used to measure the temperature dependence of paramagnetic susceptibility χ(T) of (La_{1 ? x} Sr _x )_0.93MnO₃ (x = 0.2, 0.25, or 0.3) manganites in the temperature interval 60–850°C. It is demonstrated that the dependences have two kinks and three linear sections. The kink of curve χ^?1(T) is related to polymorphic transformations (Q′ → Q* and Q* → R) that take place in the crystal lattices of the samples. The main magnetic characteristics of the samples are determined with the least-squares processing of curve χ^?1. Is is demonstrated that dependence χ^?1(T) obeys the Curie-Weiss law. The energy state of the magnetoactive manganese atom in the Q′-and Q*-phase samples is close to the energy state of a free Mn²⁺ ion. In the R phase, this state is close to the state of a free Mn³⁺ ion.     

Investigation of the ZnS-CdHgTe interface

The ZnS-Cd_xHg_1?xTe interface was investigated using the capacitance-voltage characteristics of MIS structures in experimental samples. During fabrication of the n⁺-p junctions based on p-Cd_xHg_1?xTe, the density of states within the range N _ss =(1–6)×10¹¹ cm^?2 eV^?1 at T=78 K was obtained. The experiments showed that the conditions in which n⁺-p junctions are fabricated only slightly affect the state of the ZnS-CdHgTe interface. The negative voltages of the at bands V _FB , even if immediately after deposition of the ZnS films V _FB >0, point to the enrichment of the ZnS-p-CdHgTe near-surface layer with majority carriers, specifically, holes. This led to a decrease in the leakage current over the surface. During long-term storage (as long as ～15 years) in air at room temperature, no degradation of differential resistance R _d , current sensitivity S _i , and detectivity D* of such n⁺-p junctions with a ZnS protection film was observed.