Radiophysics and Quantum Electronics最新文献

We consider excitation of electromagnetic oscillations of a homogeneous dielectric ball by a radial dipole, oscillations forming a periodic sequence of radio pulses. It is shown that at high frequencies the field distribution is close to the field distribution of the eigenoscillations corresponding to these frequencies, whereas the neighboring harmonics also have an impact at low frequencies. It is also shown that an action at low frequencies characterized by a low Q-factor is needed for exciting short radio pulses in the resonator.

A study of open quantum systems and dynamical regimes that emerge in such systems is an actively developing field of the theoretical and experimental physics. Although the bifurcation analysis and the theory of dynamical chaos are very important branches of nonlinear dynamics, their use for describing the processes occurring in open dissipative quantum systems has been restricted until recently. In this work, we present a review of recent results on the generalization of the methods of the oscillation theory for such systems. We present quantum analogs of the classical bifurcations, which are observed in the structural changes of the asymptotic density matrix, namely, the pitchfork bifurcation, saddle-node bifurcation, transition to quantum chaos via a period-doubling cascade, and the Neimark–Sacker bifurcation. We also consider numerical characteristics of dissipative quantum chaos. The largest quantum Lyapunov exponent, which is based on analyzing the divergence rate of the initially close quantum trajectories, allows one to numerically study the structure of the regular and chaotic domains of various open quantum systems. Numerical characteristics of dissipative quantum chaos, which can be observed in a physical experiment, are also considered. It is shown that the qualitatively different statistics of the distribution of times between the successive emissions of individual photons by the system take place for the regular and chaotic regimes in an open quantum cavity.

We show that the strong spatial inhomogeneity of absorption (the change from the maximum to the minimum value was more than 50 times on a spatial scale of the order of 200–300 μm), which was observed in our earlier studies of ultrapure synthetic quartz glasses and a synthetic quartz crystal at a wavelength of 1070 nm, can be related to an inhomogeneous spatial distribution of the redox state of iron traces. Using experimental data on absorption of metal impurities in the studied glasses, we present a calculation of the iron ion concentration, which is of the order of a few ppb. This is several times better than the limit of detection of the mass spectrometry method used for certification of ultrapure quartz glasses. Based on the results of the mass spectrometry measurements of concentrations given by manufacturers and the values of the maximum (10⁻⁵ cm⁻¹) and minimum (10⁻⁷ cm⁻¹) absorption coefficient we have measured, we calculated the maximum possible concentrations of the Fe²⁺ and Cu²⁺ ions and OH groups in quartz glasses and hydrothermal synthetic crystalline quartz.

We present the results of theoretical and experimental studies of the surface-wave K_a-band resonators which are formed by sections of cylindrical waveguides with single-period wall corrugations. The electrodynamic characteristics of such resonance cavities are analyzed within the quasioptical approach and three-dimensional simulation by the finite-difference time-domain method. The results of the theoretical analysis agree quite well with the experimentally measured frequency dependences of the reflection coefficients, which demonstrate the existence of surface modes with different numbers of longitudinal field variations. The measured Q-factors and frequencies of the above-specified modes corresponded well to the calculated values. In order to perform “cold” electrodynamic tests, wideband waveguide converters of the TE mode of the standard rectangular waveguide to the TM mode of an oversized cylindrical waveguide, which have a sufficiently low level of ohmic loss at a conversion efficiency exceeding 95% in the 34–40 GHz frequency range, were manufactured by the the 3D printing method.

We present the first experimental results obtained with a setup created on the basis of the PEARL laser facility for studying the processes of generating terahertz radiation from laser wake fields which are formed during the propagation of a high-power femtosecond laser pulse in a rarefied plasma. In particular, the occurrence of terahertz generation in the case where the laser–plasma interaction region is located between a pair of dielectric prisms of total internal reflection is demonstrated. The dependence of the terahertz radiation energy on the energy of a femtosecond laser pulse and on the plasma density is studied.

We consider propagation and absorption of a narrow-band signal in a homogeneous dispersive medium. It is shown that all signals can be divided into two qualitatively different classes, namely, holonomic and piecewise-holonomic signals. When propagating in an absorbing medium, the holonomic signals are attenuated obeying an exponential law and, therefore, almost disappear at a sufficient long path lengths. The piecewise-holonomic signals are attenuated significantly slower (obeying a hyperbolic law) and, therefore, continue to exist even at substantially longer paths.

We study generation of Čerenkov radiation by an ultrashort laser pulse propagating in the magnetized plasma across an external magnetic field basing on the analysis of the dispersion relation and numerical simulation in the case where the cyclotron frequency exceeds the plasma frequency. It is shown that the radiation with a frequency in the range from the plasma frequency to the cyclotron one and with an angle between the wavevector and propagation direction below 45° can escape the plasma. The radiation power increases with the magnetic field before reaching the maximum at the cyclotron frequency being of the order of the inverse of the laser pulse duration and starts decreasing afterwards. It is also observed that, with an increase in the field, the pattern of the radiation escaping from the plasma narrows, the generated pulse is shortened, and the pulse spectrum widens.

We analyze the possibility of existence of electromagnetic-field pulses with a non-zero electric area (time integral of the electric field) within the framework of the Maxwell equations. It is demonstrated that in the absence of charges and currents in vacuum, the non-zero electric area of a pulse would lead to its infinite energy. A pulse with the non-zero area is shown to form already in the case of uniform and rectilinear motion of the charge. The conditions for formation of such pulses by a localized charge system are found, and an example of such a system is presented. The asymptotics of the electric area far from the system of charges is obtained.

We consider the problem of simultaneous detection of close and distant targets in an automotive millimeter-wave radar. Two approaches for solving the problem are proposed. The first approach is based on the subtraction of more powerful signals in the time domain and the subsequent detection of less powerful signals. The second approach is based on the most plausible estimate of the impulse response of the radar channel. The methods have different computational complexity. Numerical simulation of the proposed methods for processing of signals reflected from targets in terms of the false-alarm and missed-detection probabilities has been carried out. Simulations have shown that both close and distant targets can correctly be detected simultaneously with a low predetermined false-alarm probability.

We propose a method of multisignal adaptive spatial filtering, which generalizes the classical method of adaptive spatial filtering of a single signal with a known arrival direction. Additionally, the formulas for the modified Capon direction-finding method are found. Reliability of the relationships, which are obtained on the basis of the maximum-likelihood method, is confirmed by the limiting transitions and the results of numerical simulation for typical signal-reception conditions in radio-monitoring and passive radar systems.