一种基于旋转调谐的超材料

数值仿真研究了一种可调谐的双开口谐振环(DSRR)超材料.在平行入射的电磁波激励下,这种DSRR单元可以在不同的频段分别表现出磁谐振和电谐振.当外加电场E与DSRR的双开口平行时,DSRR受激励得到的磁谐振和电谐振强度最大.随着DSRR超材料沿外加磁场H方向顺时针旋转,其磁谐振和电谐振频率基本保持不变,但谐振强度均发生显著下降,同时对应透射相位的突变也逐渐降低.提出的超材料调谐方法只需要简单地旋转材料,而不需要改变原有超材料单元的结构或者增加额外的激励场,极大地简化了可调谐超材料的制备及应用,在电磁开关、相位调制等方面具有潜在的应用.同时,这种简单的方法有希望应用于更高频段的超材料调谐,可以有效地拓展太赫兹频段和光频段超材料的实际应用.     

Laser-spectroscopic investigations of the lithium resonance lines

High-resolution laser-atomic-beam spectroscopy allowed us to investigate the lithiumD lines in magnetic and electric fields and to determine hyperfine constants, isotope shifts, scalar and tensor polarizabilities and the absolute transition frequencies with high accuracy. The methods used are discussed and the results are summarized.     

Influence of the Stark effect on multiphoton ionization of atoms when the dynamic polarizability depends strongly on the laser frequency

The influence of the Stark effect on multiphoton ionization of Ba atoms under conditions when the dynamic polarizability depends strongly on the frequency of the laser radiation is investigated. It is found that for some electric field strengths ε of the laser radiation this effect gives rise to resonance peaks in the Ba⁺ ion yield as a function of the laser radiation frequency at frequencies corresponding to single-photon transitions between the excited states. These frequencies can differ substantially from the frequencies corresponding to the conventional multiphoton excitation of these states from the ground states of the atoms. Peaks in the ion yield as a function of ε behave differently from the conventional Stark effect—their position on the frequency scale does not depend strongly on ε. The conditions under which such an induced resonance structure appears are determined. Zh. éksp. Teor. Fiz. 113, 499–512 (February 1998)     

Collective electronic excitations in a semiconductor superlattice in the Landau and Wannier-Stark ladder regime

Using a mean-field approximation, we have developed a systematic treatment of collective electronic modes in a semiconductor superlattice (SL) in the presence of strong electric and magnetic fields parallel to the SL axis. The spectrum of collective modes with zero wavevector along the SL axis is shown to consist of a principle magnetoplasmon mode and an infinite set of Bernstein-like modes. For non-zero wavevector along the SL axis, in addition to the cyclotron modes, extra collective modes are found at the frequencies |Nω _c±Mω _s|, which we call cyclotron-Stark modes (ω _c and ω _s are respectively the cyclotron and Stark frequencies, N and M are integer numbers). The frequencies of the modes propagating in “oblique” direction with respect to the SL axis show oscillatory behavior as a function of electric field strength. All the modes considered have very weak spatial dispersion and they are not Landau damped. The specific predictions made for the dispersion relations of the collective excitations should be observable in resonant Raman scattering experiments. Received 29 August 2002 / Received in final form 25 February 2003 Published online 4 June 2003 RID="a" ID="a"e-mail: 612033@inbox.ru     

Nonlinear interaction of moving space-charge and photoconductivity gratings in Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> crystal

A nonlinear interaction of moving space-charge and photoconductivity gratings is experimentally investigated. In the presence of a dc electric field, a crystal is irradiated with an oscillating interference pattern with a spatial frequency K and an oscillation frequency ω. An ac electric field with a frequency \gM is also applied to the sample. At certain frequencies ω and \gM, the crystal exhibits two types of interacting oscillations: the space-charge grating moving with velocity |ω−Ω|/K and the photoconductivity grating moving with velocity −ω/K. The effect is studied using the method of the nonstationary photoelectromotive force in a photorefractive Bi₁₂SiO₂₀ crystal.     

Observation of a shift of the mobility threshold in the D − band of silicon in an electric field according to the photoconductivity spectra

We have discovered that the extrinsic photoconductivity spectrum of doped, uncompensated crystalline Si at liquid-helium temperatures is qualitatively different in electric fields E above a critical value E _c . Specifically, the red edge of the photoconductivity, associated with photoionization of a neutral impurity, is shifted strongly to lower frequencies. This result is explained by the appearance of a mobility threshold in the D ⁻-band (upper Hubbard band) and the shift of this threshold as E increases. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 2, 89–94 (25 January 1996)     

Pressure and temperature dependence of the electric field gradient (EFG) in CdS

Using the time differential perturbed angular correlation technique (TDPAC), the electric hyperfine interaction of¹¹¹Cd in the II-VI-semiconductor CdS was investigated. The results of the temperature and pressure dependence of the electric field gradient (EFG) are discussed. The binding energyE _b for¹¹¹In at a Cd lattice site and a Cd vacancy (¹¹¹In_Cd–V_Cd–pair) could be estimated to 340 meV±80 meV by means of an Arrhenius-Plot. The disappearance of the 79 MHz and 73 MHz frequencies under a pressure of about 30 kbar could be due to their vacancy character or to the beginning of the phase transition to rocksalt structure.     

The c-axis oriented AlN solidly mounted resonator operated in thickness shear mode using lateral electric field excitation

The AlN-based solidly mounted resonator operated in thickness shear mode using lateral field excitation is presented both in theory and experiment. The resonator configuration consists of the electrodes parallel to the surface, a highly c-oriented AlN film and an acoustic Bragg reflector. The theoretical analysis of the Christoffel equation predicts that the electric field in any direction normal to the c-axis can excite the shear mode wave along the thickness direction. The electric field characteristics are calculated by finite element modeling in order to design the electrode frame. The testing results of the finished devices show that the thickness shear mode wave can be excited by the lateral electric field in c-axis oriented AlN solidly mounted resonators. The experimental frequency corresponds well to the theoretical one. The resonators operated in thickness shear mode have resonant frequencies near 2 GHz with an average Q _s value of 323 and a K_eff²K_{\mathrm{eff}}^{2} value of 0.83%.     

Estimation of the action of lunisolar tides on the electric field in the atmospheric surface layer

The average amplitude of the vertical electric field strength in the atmospheric surface layer at the frequencies of lunisolar tides has been estimated. The special feature of the problem of estimation of the amplitude E_z at the frequencies of tides is related to the necessity of using a long time series of experimental data which is related to the frequency range of tidal variations and to the necessary frequency resolution. The signal amplitude at the frequencies of tides has been estimated by using the radio engineering method of quadrature correlation receiver. Experimental data on the electric field in the atmospheric surface layer over the range of lunisolar tides obtained at different stations of Hydrometeorological Service have been analyzed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 69–73, August, 2007.     

Optical absorption by quasiparticles in superconducting multilayers

The absorption of photons by quasiparticles in the Bloch bands of multilayers, formed by alternating normal (N) and superconducting (S) regions, is computed for the vector of the oscillating electric field being perpendicular to the N-S interfaces. The absorption spectrum extends down to frequencies below the gap of the S-layers, and its structure results from the combined effects of Andreev scattering and conventional scattering at the N-S interfaces. For not too different Fermi energies ?_FS and ?_FN in the S and N layers there is one prominent resonance peak. With increasing temperature low frequency absorption goes up. For ?_FS ? ?_FN, on the other hand, a sharp multipeak structure at low frequencies is found.     

Nichtlineare Wechselwirkung elektromagnetischer Wellen im Plasma mit konstantem Magnetfeld

The nonlinear interaction of waves in anisotropic plasmas is considered. Solving the hydrodynamics equations with a n-th order perturbation procedure, an equation for the nonlinear electric field is derived. The n-th order current produced by interaction of waves with lower order is represented by means of a nonlinear tensor of conductivity, which is calculated for n = 2. It is shown, that a miced plasma wave with combination frequencies is excited as the result of interaction of two transverse (ordinary) waves propagating perpendicular to the magnetic field.     

Quadrupole interaction of tantalum impurities in bismuth metal

The time differential perturbed angular correlation technique has been used in the measurement of the electric quadrupole interaction of Ta impuritiy in bismuth metal. The interaction frequencies at 293, 400 and 500 K have been observed to be 288±1.5, 266.9±3 and 244.5±4.3 MHz respectively. The electric field gradient at 293 K is 4.75±0.3 × 10¹⁷ V/cm² with the temperature coefficientB=2.2±0.2 × 10⁻⁵ (K)^−3/2.     

Polarization-interference effects in the bremsstrahlung of quasiclassical electrons on ions with a core

The polarization dependence of the stimulated bremsstrahlung and inverse bremsstrahlung (SBIB) of quasiclassical electrons on highly charged ions with a core is calculated in the approximation of a specified Coulomb current. Emission frequencies close to an eigenfrequency of the ion core are considered. The contributions of the static and polarization channels are taken into account in the amplitude of the process. When the nondipole nature of the interaction between the incident particle and a resonant transition in the ion core is taken into account, interference between these channels causes the spectral-amplitude characteristics of the process to assume a specific dependence on the angle α between the electric field intensity vector of the electromagnetic wave and the initial velocity vector of the incident particle. This dependence, which persists after integration of the cross section over the scattering angle of the incident particle, causes interference effects, viz., asymmetry of the line shape and dips in the dependence of the SBIB cross section on electric field intensity, to appear for α=π/2 and significantly reduces them for α=0. Zh. éksp. Teor. Fiz. 115, 1619–1629 (May 1999)     

High-frequency properties of KNiPO<Subscript>4</Subscript> in alternating magnetic and electric fields

The dependences of the antiferromagnetic resonance frequencies on the constant magnetic field H and constant electric field E are calculated for a KNiPO₄ crystal with spontaneous electric polarization and antiferromagnetic order. It is demonstrated that the KNiPO₄ crystal is characterized by an exchange-enhanced effect of the electric field E on the antiferromagnetic resonance frequencies. This effect is not revealed in the magnetoelectric materials studied earlier. It is established that oscillations of both magnetization and electric polarization exhibit resonance response at antiferromagnetic resonance frequencies. The expressions for these responses in alternating magnetic and electric fields are presented.     

Magneto-electric response functions for simple atomic systems

We consider a simple atomic two-body bound state system that is overall charge neutral and placed in a static electric and magnetic field, and calculate the magneto-electric response function as a function of frequency. This is done from first principles using a two-particle Hamiltonian for both an harmonic oscillator and Coulomb binding potential. In the high frequency limit, the response function falls off as 1/ω ² whilst at low frequencies it tends to a constant value.     

Response of Granular La1??xCaxMnO3 Film to 10?GHz and 35?GHz Frequency Electromagnetic Radiation

The nonbolometric response of La_1 − xCa_xMnO₃ film to 10 GHz and 35 GHz frequency electromagnetic radiation is investigated in the case when, in addition to the strong electric field of the wave, the film is subjected to a stationary electric bias field. Dependences of responses on the radiation power P at temperature T = 80 K are presented. In the low power region, a linear dependence of the response on P is observed at both frequencies whereas for high powers the dependence behaves as ~P ^1/2. The obtained results are explained taking into account that the nonbolometric response originates from the intergranular junctions that operate in the reverse current regime. There two effects take place: (i) at low powers the detection resistance decreases with increasing power P, and (ii) at higher powers in addition to that the film resistance decreases as P ^1/2 due to the avalanche of charge carriers in the electric field of the electromagnetic wave.     

GHz magnetic response of split ring resonators

Measurements of engineered subwavelength microstructures can be designed to have positive or negative and μ at desired frequencies. We present transmission measurements of a metamaterial consisting of split ring resonators (SRR). Results for different polarizations and propagation directions are presented. The transmission shows a dip even for propagations perpendicular to the SRR plane, provided that the incident electric field is parallel to the sides of the split ring resonators (SRRs) which contain the cuts. The experimental results agree well with the theoretical calculations.     

Theory of the electroacoustic echo in single crystals of order-disorder ferroelectrics

The mechanism governing the shaping of the echo response in a monodomain order-disorder ferroelectric crystal to excitation by two pulses of a microwave electric field at the frequencies ω and 2ω [(ω, 2ω) echo] and at the frequency ω [(ω, ω) echo] is described in the framework of the pseudospin formalism (S=1/2). The parameters of the echo response are calculated, and the results obtained are in good qualitative agreement with experiment. Fiz. Tverd. Tela (St. Petersburg) 39, 697–703 (April 1997)     

同时实现介电常数和磁导率为负的H型结构单元左手材料

基于用一种结构同时实现材料介电常数和磁导率为负的思想，提出了H型结构单元左手材料模型.采用矩形波导法测试表明，H型结构单元在微波频率范围出现左手透射峰，并且可以由参数t对左手特性区域进行调控.同时利用相位法、棱镜折射法和散射参量法从实验和理论证明了在左手透射峰区域材料的折射率为负值，介电常数和磁导率亦同时为负.相对由金属开口谐振环与金属线两种结构组合实现的左手材料，H型结构集磁谐振与电谐振于一体，结构简单、制备方便，对微波器件左手材料表现出更多的优越性.     

Influence of chemical composition on confinement and interface character of optical phonons in GaAs/Ga1−xAlxAs superlattices

The influence of alloying on optical vibrations in GaAs/Ga_1−xAl_xAs superlattices is theoretically studied. To calculate frequencies and effective atomic displacements two different approaches are applied. The random-element-isodisplacement model is generalized for layered structures under inclusion of the macroscopic electric field. Parallel to it vibrational properties are studied by means of numerical simulations of alloying and randomness. Two effects, confinement versus folding and interface character, are discussed in particular.