基于Rydberg原子的超宽频带射频传感器

Rydberg原子具有极大的极化率和微波跃迁偶极矩,对外界电磁场非常敏感,可实现基于Rydberg原子的超宽频带射频电场的高分辨高灵敏测量.通过Rydberg原子的全光学无损的电磁感应透明探测手段,可以实现基于原子的快速免校准宽频带(0.01-1000 GHz)外电场的精密测量.对于频率大于1 GHz的微波场,由微波场耦合相邻Rydberg能级形成的Autler-Townes分裂进行测量;而对于频率小于1 GHz的长波射频场,由Rydberg能级的射频边带能级进行测量.这种方法是基于原子能级参数,可溯源到基本物理常量,不依赖于外界参考;且对电场无干扰,易于实现微型化和集成化,具有广泛的应用前景.本文主要综述了基于Rydberg原子的外电场测量的最新研究进展,重点介绍长波长射频场的测量,包括电场强度、频率以及极化方向的测量,详细介绍了其测量原理和探测灵敏度,并讨论了其应用前景及未来发展方向.     

Smoothing of Doppler spectra line shapes in electrophoretic analysis of biological cell suspensions

The study of cell and particle electrophoresis by laser Doppler techniques requires the application of an electric field to a conducting solution. Steady d.c. electric fields are usually not possible because of electrode surface reactions. Hence, it has been common practice to apply temporally varying electric fields, either pulses of alternating polarity or square-wave fields of constant amplitude. Oscillating particle motion leads to harmonic structure (multiples of the square-wave electric field frequency) in the scattered light spectra. This harmonic structure may be considerably smoothed by a suitable variation of the period of the applied square-wave field. The application of a pseudo-random period oscillating electric field yields line shapes that approach the line shapes to be expected from the application of a continuous d.c. electric field.     

Electron field emission from broad-area electrodes

In this review, measurements are surveyed for both dc and microwave electric fields. These show significant emission at applied fields typically 100 times smaller than those expected theoretically or measured experimentally for ideal microtip cathodes. Recent work is reviewed which studies on a microscopic scale this emission and the localized sites which produce it. Rather than forming sharp field enhancing projections, these sites often appear flat and are frequently associated with grain boundaries or insulating inclusions. Following an examination of various techniques for changing the emission of a given cathode, several theoretical models are considered which may explain some aspects of the observed emission characteristics.     

Electric field measurements and computational modeling at ultrahigh-field MRI

While magnetic resonance images essentially contain a map of the both circularly polarized components of the RF transverse magnetic fields (B(1) field), the thermal heat and electromagnetic power deposition is generated by the associated electric fields. Measurement of electric field distributions/intensities across a sample yields an indirect indication of possible cause of heating within the sample and potentially enables the detection of "hot spots," which can be present within inhomogeneous radiofrequency (RF) fields, such as the case with magnetic resonance imaging at high field strength. As a result, establishing a valid technique for direct measurements of the electric field and its correlation, obtained using computational electromagnetics, is essential in assessing (1) the safety of the RF coil designs and (2) the validity of the calculations. In this work, a probe was built and used to measure the transverse electric field (E(1) field) distributions within an empty 8 T (tuned to 340 MHz) RF head coil and within a saline water phantom loaded in the same coil. The measured E(1) field distributions were favorably compared to the distributions obtained utilizing a finite difference time domain in-house package.     

On the difference in the rise times of the two SES electric field components

The study of low frequency signal transmission in conductive media, reveals that the electric and magnetic fields follow diffusion type equations. In a previous paper (Varotsos et al.1)) experimental evidence was forwarded that for epicentral distances of the order of 100 km, the SES electric field variations precede those of the magnetic ones by a time of the order of 1 sec. In the present paper, we present evidence that this peculiarity still pertains (but to a smaller extent), when studying the differences in the components of the electric field. This cannot be probably observed in the scale of laboratory measurements, lying usually within the error bars of the current experimental facilities. A tentative theoretical justification, termed as τ-approximation, is presented which accounts for the measurements of electric field components. The present findings can provide a unique tool for the discrimination between remote and nearby sources by using data from electric measurements alone.     

Can an electric field induce an antiferroelectric phase out of a ferroelectric phase?

It has been widely accepted that electric fields favor the ferroelectric phase with parallel electric dipoles over the antiferroelectric phase. With detailed measurements in polycrystalline ceramics of Pb(0.99)Nb(0.02[(Zr(0.57)Sn(0.43)(1-y)Ti(y)](0.98)O(3), we demonstrate in this Letter that electric fields can induce an antiferroelectric phase out of a ferroelectric phase, i.e., trigger an apparently unlikely ferroelectric-to-antiferroelectric phase transition. We suggest that it is caused by the volume contraction from the converse piezoelectric effect at the coercive field with a reversed polarity.     

Anomalous plasma diffusion transverse to high magnetic fields in InSb

Investigations on the ambipolar diffusion of an electron-hole plasma transverse to a magnetic field have been carried out in InSb. A plasma layer, produced at the surface of the sample by a short laser pulse, was moved through the sample in crossed electric and magnetic fields by the Lorentz force. From the broadening of the plasma layer we found at 80K an enhanced diffusion coefficient which decreased proportional to 1/B for magnetic fields higher than 1T, constrary to the expected classical 1/B ² dependence. Furthermore, the diffusion coefficient was strongly dependent on the electric field. The ambipolar drift velocity, measured simultaneously showed a classical behaviour. Together with the enhanced diffusion we observed instabilites in the electric potential. The instability threshold decreased towards the cathode.     

Calculation of electric fields induced by body and head motion in high-field MRI

In modern magnetic resonance imaging (MRI), patients are exposed to strong, nonuniform static magnetic fields outside the central imaging region, in which the movement of the body may be able to induce electric currents in tissues which could be possibly harmful. This paper presents theoretical investigations into the spatial distribution of induced electric fields and currents in the patient when moving into the MRI scanner and also for head motion at various positions in the magnet. The numerical calculations are based on an efficient, quasi-static, finite-difference scheme and an anatomically realistic, full-body, male model. 3D field profiles from an actively shielded 4T magnet system are used and the body model projected through the field profile with a range of velocities. The simulation shows that it possible to induce electric fields/currents near the level of physiological significance under some circumstances and provides insight into the spatial characteristics of the induced fields. The results are extrapolated to very high field strengths and tabulated data shows the expected induced currents and fields with both movement velocity and field strength.     

Dipolar interactions in molecules aligned by strong AC electric fields

We observed magnetization transfer and spectroscopic splittings due to dipolar couplings in the solution NMR spectra of neat nitrobenzene aligned using AC electric fields. Weak dipolar splittings have been previously observed for nitrobenzene in a DC electric field (T. M. Plantenga, et al., Chem. Phys. 66, 1-9, 1982); the use of homogeneous pulsed AC fields has allowed us to establish stable experimental conditions, which were more tolerable to sample impurities and required no sample purification, and to carry out multidimensional experiments. A pulse sequence is discussed in which the electric field is present only for the indirect dimension: this sequence records the dipolar splittings for each proton in the indirect dimension; the direct dimension presents the isotropic chemical shift. Another pulse sequence is discussed that uses the applied electric field only in the mixing period to produce cross peaks between dipolar coupled pairs and correlate their isotropic chemical shifts. The order parameter describing molecular alignment was in good agreement with that previously determined from deuterium quadrupolar measurements of deuterated nitrobenzene in a similar range of electric fields: S(mol) approximately 0.025% for a field strength of 7.0 MV/m (rms). The dipolar splittings for ortho-meta, meta-para, and ortho-para protons were in qualitative agreement with the known geometry. Copyright 2000 Academic Press.     

Difference equation solutions for non-steady-state diffusion of charged particles in solids

The hopping transport of charged particles through a solid is described by means of difference equations based on the concept of classical thermal motion over discrete energy barriers. Homogeneous electric fields and concentration gradients are considered to be the driving forces for transport. Transient and steady-state currents are derived, and the concentration profiles are obtained for the mobile charged defect species. For the case of a slab geometry the discreteness of the potential barriers leads to a nonlinear dependence of current on voltage in the high electric field limit, with a more rapid increase of current with voltage than would be expected from an extrapolation of the low field linear dependence. The field-dependent relaxation of a non-steady-state defect concentration profile to the corresponding steady-state profile can be nearly exponential in the limit of large fields. Tracer distributions for the cases of semi-infinite and unbounded diffusion mediums are likewise affected appreciably in the high field limit. The velocity of the peak is increased over that obtained by linear extrapolation from the low field limit. It is concluded that the combination of high electric fields with the natural microscopic discreteness of a solid-state diffusion medium can result in readily observable nonlinear electric field effects which increase approximately exponentially with the atomic separation distances of the discrete barriers in hopping transport. Some of this nonlinear behavior can be retained in differential equations derived from the difference equations by means of Taylor series expansions of the carrier concentration with respect to position.     

Intrinsic ambipolarity and rotation in stellarators

It is shown that collisional plasma transport is intrinsically ambipolar only in quasiaxisymmetric or quasihelically symmetric magnetic configurations. Only in such fields can the plasma rotate freely, and then only in the direction of quasisymmetry. In a non-quasi-symmetric magnetic field, the average radial electric field is determined by parallel viscosity, which in turn is usually governed by collisional processes. Locally, the radial electric field may be affected by turbulent Reynolds stress producing zonal flows, but on a radial average taken over several ion gyroradii, it is determined by parallel viscosity, at least if the turbulence is electrostatic and obeys the conventional gyrokinetic orderings. This differs from the situation in a tokamak, where there is no flow damping by parallel viscosity in the symmetry direction and the turbulent Reynolds stress may affect the global radial electric field.     

Propagation of terahertz waves in nonuniform plasma slab under "electromagnetic window"

The application of magnetic fields, electric fields, and the increase of the electromagnetic wave frequency are up-and-coming solutions for the blackout problem. Therefore, this study considers the influence of the external magnetic field on the electron flow and the effect of the external electric field on the electron density distribution, and uses the scattering matrix method (SMM) to perform theoretical calculations and analyze the transmission behavior of terahertz waves under different electron densities, magnetic field distributions, and collision frequencies. The results show that the external magnetic field can improve the transmission of terahertz waves at the low-frequency end. Magnetizing the plasma from the direction perpendicular to the incident path can optimize the right-hand polarized wave transmission. The external electric field can increase the transmittance to some extent, and the increase of the collision frequency can suppress the right-hand polarized wave cyclotron resonance caused by the external magnetic field. By adjusting these parameters, it is expected to alleviate the blackout phenomenon to a certain extent.     

High-resolution spectroscopy on trapped molecular ions in rotating electric fields: A new approach for measuring the electron electric dipole moment

High-resolution molecular spectroscopy is a sensitive probe for violations of fundamental symmetries. Symmetry violation searches often require, or are enhanced by, the application of an electric field to the system under investigation. This typically precludes the study of molecular ions due to their inherent acceleration under these conditions. Circumventing this problem would be of great benefit to the high-resolution molecular spectroscopy community since ions allow for simple trapping and long interrogation times, two desirable qualities for precision measurements. Our proposed solution is to apply an electric field that rotates at radio frequencies. We discuss considerations for experimental design as well as challenges in performing precision spectroscopic measurements in rapidly time-varying electric fields. Ongoing molecular spectroscopy work that could benefit from our approach is summarized. In particular, we detail how spectroscopy on a trapped diatomic molecular ion with a ground or metastable ³Δ₁ level could prove to be a sensitive probe for a permanent electron electric dipole moment (eEDM).     

Measurement of large parallel and perpendicular electric fields on electron spatial scales in the terrestrial bow shock

Large parallel (相似文献   

Direct measurement of the intersubband electro-optic coefficient in asymmetric multiple quantum wells

An extremely large electro-optic coefficient in asymmetric quantum well systems has been inferred from optical rectification measurements. To date, however, a direct measurement of the electro-optic coefficient in these materials has not been performed. We present a method to directly measure the electro-optic coefficient in an asymmetric quantum well systemconsisting of 30 Å GaAs wells, 65 Å Al(0.20)Ga(0.80)As step barriers, and 500 Å Al(0.40)Ga(0.60)As barriers. The devices consist of short waveguides with asymmetric quantum well cores. The waveguide endfaces are polished to form Fabry–Perot cavities. Electrodes are deposited running parallel to the waveguides to apply electric fields. The waveguides are analyzed using a Fourier transform infrared spectrometer. Interference fringes are observed in the reflected spectra at room temperature. The fringe spacing yields the refractive index of the material, while plots of the refractive index versus applied electric field yield the electro-optic coefficient. Absorption measurements through these waveguides showed the characteristic polarization dependent intersubband absorption. Because the electrodes appeared to be Schottky like, a large voltage was applied across the device and the voltage across the quantum wells was estimated from the device leakage current and the current-voltage characteristics of a similar device structure after scaling for length, area, and doping. There may be some uncertainty in this approximation. The maximum applied electric field is estimated to be 2.3 kV cm⁻¹. No anomalous effects were observed in the Fabry–Perot fringe pattern in the presence of this electric field, indicating the electro-optic coefficient in these waveguides is most likely less than 0.9 nm V⁻¹over a wide wavelength range. Clearly, however, measurements need to be performed at higher electric fields to accurately measure the electro-optic coefficient.     

Branching of colloidal chains in capillary-confined nematics

We report on the observation of colloidal chain assembly and branching inside capillaries filled with a nematic liquid crystal. Because of the homeotropic anchoring of liquid crystalline molecules on the capillary and colloidal droplet surfaces, the assembly of droplets along the capillary axis is expected, producing a transformation of the nematic director field from an escape-radial to quasiradial configuration. However, the subsequent over time branching of the straight colloidal chains is counterintuitive. By numerical simulations, we demonstrate that chain branching can occur by overcoming an energy barrier and can at least dwell as a metastable configuration. Moreover, manipulation of colloidal chains by electric fields and their gradients demonstrates various regimes of chain behavior in electric fields.     

Long-term drifts of stray electric fields in a Paul trap

We investigate the evolution of quasi-static stray electric fields in a linear Paul trap over a period of several months. Depending on how these electric fields are initially induced, we observe very different timescales for the field drifts. Photo-induced electric fields decay on timescales of days. We interpret this as photo-electrically generated charges on insulating materials which decay via discharge currents. In contrast, stray fields due to the exposure of the ion trap to a beam of Ba atoms mainly exhibit slow dynamics on the order of months. We explain this observation as a consequence of a coating of the trap electrodes by the atomic beam. This may lead to contact potentials which can slowly drift over time due to atomic diffusion and chemical processes on the surface. In order not to perturb the field evolutions, we suppress the generation of additional charges and atomic coatings in the Paul trap during the measurements. For this, we shield the ion trap from ambient light and only allow the use of near-infrared lasers. Furthermore, we minimize the flux of atoms into the ion trap chamber. Long-term operation of our shielded trap led us to a regime of very low residual electric field drifts of less than 0.03 V/m per day.     

A method to measure electric field strengths in an argon glowdischarge plasma using laser spectroscopy

A method for electric field measurement based on laser spectroscopy of argon atoms has been developed and calibrated. Measurements were made using both laser optogalvanic spectroscopy and laser-induced fluorescence spectroscopy. Measurements using several different transitions in argon were carried out, and it was found that the 4s→7f and 4s→8f transitions were the most suitable for measurements in the sheath region of glow discharges. The lower limit for electric field measurements was estimated to be 500 V/cm. A minimal pressure of about 100 mtorr was required to detect optogalvanic signals and about 1 torr was required for the fluorescence signals to be detectable. These detection limits make this method applicable for measurement of sheath electric fields for a wide range of discharge conditions     

An exciton in a quantum well in the presence of crossed electric and magnetic fields

An analytical approach to the problem of the Wannier–Mott exciton in a semiconductor quantum well (QW) in the presence of external magnetic and electric fields is developed. The magnetic field is taken to lie in the heteroplanes while the electric field is directed perpendicular to the heteroplanes. Explicit dependencies of the energy levels and wave-functions of the exciton on the magnitudes of the fields for a wide range of the width of the QW are obtained. For the narrow QW, the results are valid for arbitrary electron and hole effective masses. In the case of intermediate and wide QWs, the adiabatic approximation implying the extreme difference of the electron and hole masses is used. In the intermediate QW, the states of the relative motion are the standard Coulomb states affected by the external fields while the states of the centre of mass are the size-quantized states in the QW. We focus particularly on the delocalized states caused by the external electric field and the motion of the excitons centre of mass in the magnetic field. These states are localized far away from the Coulomb centre. A strong influence of the boundaries of the wide QW on the delocalized exciton states is found to occur. Estimates of the expected values are made using typical parameters associated with GaAs QW.