Specific features of studying anisotropic media by methods of time-domain terahertz spectroscopy

Wide-band time-domain terahertz (THz) spectroscopy is a popular method of studying optical properties of various materials. This method is considered as applied to polarization-sensitive measurements of properties of anisotropic materials and media possessing significant dichroism and birefringence. It is important to understand that the development of the elemental base for the terahertz range, in particular, for THz polarizers, falls behind the achievements of polarization optics for the visible range. In studying anisotropic materials, this may lead to simultaneous detection of orthogonal components of the THz field and to formation of artifacts in the spectral range in calculations of properties of examined samples. A unique specific feature of time-domain terahertz spectroscopy, namely, a possibility of capturing the time-dependent shape of the THz pulse, allows separation of the orthogonal components of the field in the time domain. Thus, it is possible to detect the signal from radiation with the orthogonal polarization and eliminate it by means of appropriate adjustment of polarization elements of the spectroscopy system. This method is demonstrated by an example of studying the terahertz properties along the optical axis z of an anisotropic ferroelectric crystal of lead germanate in which significant birefringence and dichroism arise as the crystal is heated to the phase transition temperature.     

A Computer-Controlled Magnetic Resonance Spectrometer with a Pulsed Magnetic Field

An automatic magnetic resonance spectrometer with a pulsed magnetic field is described. It has the following characteristics: a field range of 0–100 kOe, a frequency band of 25–140 GHz, a temperature range of 4.2–300 K, and a pulse duration of 12.6 ms. The spectrometer is automated to the CAMAC standard using standard modules. To record a magnetic-resonance spectrum, the following procedure is applied: the resonance-absorption signal is determined by a 10-bit analog-to-digital converter, and the magnetic filed is scanned by simulating a current pulse in a discharge circuit of the solenoid using a single input parameter—the initial charge voltage of the capacitor bank. In calibration measurements performed with spherical yttrium–iron-garnet samples, the average measurement error was 0.15%.     

LED magneto-optical ellipsometer with the switching of orthogonal polarization states

A light-emitting diode (LED) magneto-optical ellipsometer with the switching of orthogonal polarization states is described. The distinctive features of the ellipsometer are the implementation of the maximally deep azimuthal modulation, application of two-channel detector, use of a set of LEDs ensuring a high signal-to-noise ratio, and the absence of moving polarization elements. These features have substantially increased the precision of measurements of ellipsometric parameters ψ and Δ for ferromagnetic materials and their changes (δψ and δΔ) in the magnetic field, which are connected for the transverse Kerr configuration by simple relations with magneto-optical parameters. At wavelengths of 365, 372, 390, 405, 420, and 465 nm, the precision of ψ-δψ and Δ-δΔ measurements of metal films is about 0.0003° and 0.001°, respectively. The spectral resolution is 4 nm; the minimal measurement time of ψ and Δ in the continuous spectral range of 270–1000 nm is 20 s; the magnetic field is varied from–2500 to +2500 Oe; and the step of the magnetic field setting is 0.5 Oe.     

Broadband electron spin resonance at low frequency without resonant cavity

We have developed a nonconventional broadband electron spin resonance (ESR) spectrometer operating continuously in the frequency range from 0.5 to 9 GHz. Dual antenna structure and the microwave absorbing environment differentiate the setup from the conventional one and enable broadband operation with any combination of frequency or magnetic field modulation and frequency or magnetic field sweeping. Its performance has been tested with the measurements on a 1,1-diphenyl-2-picrylhydrazyl (DPPH) sample and with the measurements on the single molecular magnet, V6, in solid state at low temperature.     

Compact terahertz passive spectrometer with wideband superconductor-insulator-superconductor mixer

We developed a compact terahertz (THz) spectrometer with a superconductor-insulator-superconductor (SIS) mixer, aiming to realize a portable and highly sensitive spectrometer to detect dangerous gases at disaster sites. The receiver cryostat which incorporates the SIS mixer and a small cryocooler except for a helium compressor has a weight of 27 kg and dimensions of 200 mm × 270 mm × 690 mm. In spite of the small cooling capacity of the cryocooler, the SIS mixer is successfully cooled lower than 4 K, and the temperature variation is suppressed for the sensitive measurement. By adopting a frequency sweeping system using photonic local oscillator, we demonstrated a spectroscopic measurement of CH(3)CN gas in 0.2-0.5 THz range.     

A unified terahertz radiation source based on a backward-wave tube

A unified terahertz radiation source based on backward-wave tubes is described. The device is intended for performing spectroscopic studies of condensed media, gases, and plasma in the frequency range 0.18–1.50 THz. The advantage of this design is the actual development of a terahertz radiation source with continuously tuned oscillation frequency in a wide range. No analogues of this instrument exist. Its schematic design is presented and individual functional units are described. The output radiation power is 0.5 mW–0.5 W, and the frequency range is 0.18–1.50 THz.     

A versatile and reconfigurable setup for all-terahertz time-resolved pump-probe spectroscopy

A versatile optical setup for all-terahertz (THz) time resolved pump-probe spectroscopy was designed and tested. By utilizing a dual THz pulse generator emitter module, independent and synchronized THz radiation pump and probe pulses were produced, thus eliminating the need for THz beam splitters and the limitations associated with their implementation. The current THz setup allows for precise control of the electric fields splitting ratio between the THz radiation pump and probe pulses, as well as in-phase, out-of-phase, and polarization dependent pump-probe spectroscopy. Since the present THz pump-probe setup does not require specialized THz radiation optical components, such as phase shifters, polarization rotators, or wide bandwidth beam splitters, it can be easily implemented with minimal alterations to a conventional THz time domain spectroscopy system. The present setup is valuable for studying the time dynamics of THz coherent phenomena in solid-state, chemical, and biological systems.     

Correction of temperature dependency of a standard; case-study – magnetic flux density standard

Temperature dependence of measuring instruments represents one of the most influential error sources in measurement of most physical quantities. There are several methods of reducing or cancelling substantial temperature influence on instruments.

In this paper we are describing a calculative correction method in the field of precision magnetic flux density measurements, which involves mathematical correction of the instrument properties. For dc magnetic fields under 5 mT, nuclear magnetic resonance (NMR) measurements are both impractical and time-consuming and in some environments even impossible. To avoid complex NMR measurements, field coils as magnetic flux density transfer standards are often used. A method for correction of an air-cored field coil’s coil-constant in order to take into account its temperature dependence is presented. As a result, instead of a complex NMR measurement only a simple current and temperature measurement were needed to calculate the generated magnetic flux density with relative uncertainty of 0.025%.     

Precision differential-gated radiopolarimeters of microwave and extremely high frequency bands: Practical implementations and prospects for application

The basic modules of a two-channel precision radiopolarimeter (DGRP) of fully polarized waves with automatic compensation of systematic errors, which allowed us to lower the relative error in the measured Stokes parameters to 0.2%, are described along with the basic units of a radiopolarization complex composed of a precision extremely high (EHF) band (frequency 35.2 GHz) DGRP and a microcomputer and designed for measuring the Stokes parameters of slightly polarized radiation with a relative error of ∼1%. The devices demonstrate high speed of polarization analysis and ensures simultaneous measurement of the Stokes parameters of fully or partially polarized radiation in microwave and EHF bands. Characteristics of the precision DGRP are considered and main factors limiting the accuracy of polarization measurements are analyzed.     

An automatic setup for measuring low-field temperature dependencies of magnetoresistances of film samples

A simple method for magnetoresistance measurements in weak fields based on using a low-frequency alternating magnetic field is proposed. The special feature of this method is the direct measurement of the difference in resistance ΔR of the sample under study in the field and in the absence of the field, which offers a chance to significantly increase measurement sensitivity. The automatic setup for measuring temperature dependencies of magnetoresistance of film metallic samples with a ΔR sensitivity of up to 10^?7 Ω is described. The temperature dependencies of magnetoresistance of thin-film nanocrystalline Co-Cu alloy samples in weak magnetic fields and in a temperature range of 77–300 K are studied.     

Portable x-ray fluorescence spectrometer for coating thickness measurement

A handheld x-ray spectrometer has been realized and tested. The purpose of the device is to measure the thickness of coated samples in the range of 1-1500 nm in an industrial environment. Accuracy of approximately 3% has been achieved in this range with a measurement time of 1 min. Automated software has been implemented to allow utilization by a nonspecialist operator. An automated calibration procedure, based on measurements of reference samples, is used.     

Three different attitude measurements of spinning projectile based on magnetic sensors

Micro-inertial sensors currently could not provide long-time stability attitude information for the high spinning projectile because of drift errors. Meanwhile, the method of the navigation and attitude measurement with respect to the earth’s magnetic field is still auxiliary, and the attitude angle information cannot be got only by measuring the three-axis components of the geomagnetic field. In view of the flying characteristics of high spinning projectile, three different attitude measurements only using magnetic sensors are researched. Through comparative analyses, the calculating principle, system composition, applicable condition and error range of these methods are explained. Meanwhile, the semi-physical experiments are made to prove the effectiveness of the three attitude measurements. The experiment results indicate that only scalar arithmetic operations are required for these angular measurements and they have the day/night and all weather capability. The three different measurements have same angle error range within ±1° but different attitude updating rate.     

Time-of-flight photoelectron spectroscopy of gases using synchrotron radiation

A gas-phase time-of-flight (TOF) photoelectron spectrometer has been developed for use with synchrotron radiation. The excellent time structure of the synchrotron radiation at the Stanford Positron Electron Accelerator Ring (SPEAR) has been used as the time base for the TOF measurements. The TOF analyzer employs two multichannel plates (MCPs) in tandem as a fast electron multiplier with a matched 50-Omega anode to form an electron detector with a timing resolution of 相似文献   

半导体表面场产生太赫兹电磁波的机理研究

介绍了两种由半导体表面场产生THz辐射的物理机制：一种是利用半导体表面的耗尽层电场,另一种是利用半导体表面的Dember电场加速载流子,形成瞬态光电流而辐射出电磁波.文中对这两种模型的辐射机理进行了深入的讨论,对利用表面场产生THz辐射的机理有了深刻的认识,为高效、便捷、低成本的THz辐射源的研究提供了基础.     

Two-dimensional measurement of edge impurity emissions using space-resolved extreme ultraviolet spectrometer in Large Helical Device

A space-resolved extreme ultraviolet (EUV) spectrometer working in wavelength range of 50-500 A? has been developed to measure two-dimensional distribution of impurity spectral lines emitted from edge plasma of Large Helical Device (LHD), in which the magnetic field is formed by stochastic magnetic field with three-dimensional structure called ergodic layer. The two-dimensional measurement of edge impurity line emissions is carried out by scanning horizontally the observation chord of the space-resolved EUV spectrometer during single LHD discharge. Images of CIV (312.4 A?) and HeII (303.78 A?) are presented as the first result. The results are compared with ones calculated from the edge chord length in the ergodic layer of LHD plasma.     

遥感图像双角度偏振大气校正仪

研制了一种双角度偏振大气校正仪航空样机用于卫星遥感图像数据的定量化。该仪器通过时间同步和空间覆盖的探测方式获取被校正图像对应的角度、光谱、偏振三个维度的大气信息,实现气溶胶和水汽的高精度参数的反演;将反演获取的大气参数作为输入条件,利用辐射传输模型进行遥感图像的高精度大气校正。仪器采用天底(0°)和前向(55°)两个方向观测,具有8个探测波段,覆盖可见到短波红外(0.49~2.25μm)波段,其中5个波段具备偏振探测能力;采用高精度一体化结构设计保证各偏振探测通道的视场重合精度,降低偏振探测目标不一致引起的偏振测量误差。实验室定标和测试结果表明,偏振波段的视场重合度优于95%,偏振测量精度优于1%(偏振度DoLP=0.3),满足仪器设计指标要求。     

Spin polarization of field-emitted electrons from half-metallic Co2MnSi thin films grown on a W(0 0 1) facet

We measured spin polarization of electrons field-emitted from half-metallic Co₂MnSi thin film grown on a W(0 0 1) facet via chromium buffer layer using Mott scattering. For spontaneously magnetized samples, values of polarization at room temperature were observed in a range from 10% to 46% and the polarization direction was 〈1 1 0〉 orientation of substrate tungsten, which agreed with an easy axis of magnetization of bulky Co₂MnSi. An enhancement of polarization was observed as a consequence of applying a magnetic field of 350 G perpendicular to the emitter axis after the annealing at 800 K. This result is considered to be caused by improvement in crystallinity of the evaporated film due to annealing.     

基于渡越-切伦科夫辐射原理太赫兹横向场频率特性的实验研究

基于渡越-切伦科夫辐射原理,单色飞秒激光脉冲聚焦到空气中形成等离子体进而辐射出径向偏振太赫兹波,径向偏振太赫兹波经过紧聚焦后在焦点处产生太赫兹波横向偏振分量。为了有效调控太赫兹横向分量的频率分布和振幅特性,首先,通过理论分析太赫兹横向场分量在外加电压下的分布规律,给出不同角度的外加电压对太赫兹横向场频率强度的影响。然后,采用对等离子体施加外部电场的方法,得到一个最佳的纵向电场角度产生高强度太赫兹横向偏振分量。对发展太赫兹波特性的基础研究以及太赫兹技术的应用具有重要的参考意义。     

Z(eff) measurement using extreme ultraviolet bremsstrahlung emission in LHD

Radial profile measurement of Z(eff) using visible bremsstrahlung (5300 A?) in the Large Helical Device (LHD) has often encountered difficulties because the intensity profile was largely deformed by the nonuniform visible bremsstrahlung emissions from the edge ergodic layer surrounding the core plasma. A space-resolved flat-field extreme ultraviolet (EUV) spectrometer has been newly adopted to measure the Z(eff) profile using the EUV bremsstrahlung continuum in the wavelength range of 70-75 A?. The EUV bremsstrahlung intensity profiles have been measured and checked for all the magnetic configurations with totally different magnetic field structures in the ergodic layer of LHD. It is found that the nonuniform bremsstrahlung emission from the thick ergodic layer can be entirely eliminated by use of the EUV emission with relatively high photon energy of 170 eV. As a result, the Z(eff) profile can be successfully measured for most of discharges regardless of magnetic field structures of the ergodic layer. The Z(eff) profiles measured in the EUV range are compared with those measured in the visible range at a magnetic configuration with the thinnest ergodic layer thickness. The result verifies that the use of the EUV bremsstrahlung continuum is an alternative way for the Z(eff) measurement in toroidal plasmas with nonuniform bremsstrahlung emissions at the edge. Typical results from the EUV bremsstrahlung measurement are presented showing a fairly flat Z(eff) profile with error bars of ±14%.     

Generation, transport, and detection of linear accelerator based femtosecond-terahertz pulses

The generation and detection of intense terahertz (THz) radiation has drawn a great attention recently. The dramatically enhanced energy and peak electric field of the coherent THz radiation can be generated by coherent superposition of radiated fields emitted by ultrafast electron bunches. The femtosecond (fs)-THz beamline construction at the Pohang Accelerator Laboratory (PAL) was completed in the end of 2009. The fs-THz beamline at PAL can supply ultrafast and intense fs-THz radiation from a 75 MeV linear accelerator. The radiation is expected to have frequency up to 3 THz (～100 cm(-1)) and the pulse width of <200 fs with pulse energy up to 10 μJ. This intense THz source has great potential for applications in nonlinear optical phenomena and fields such as material science, biomedical science, chemistry, and physics, etc.