On the group velocity of whistling atmospherics

The dynamic spectrum of a whistling atmospheric is a signal of falling tone, and the group delay time of the signal as a function of frequency is formed as a result of propagation of a broadband pulse in a medium (magnetospheric plasma) with a quadratic dispersion law. In this paper, we show that for quadratic dispersion the group velocity is invariant under Galilean transformations. This means that, contrary to expectations, the group velocity is paradoxically independent of the velocity of the medium relative to the observer. A general invariance condition is found in the form of a differential equation. To explain the paradox, we introduce the concept of the dynamic spectrum of Green’s function of the path of propagation of electromagnetic waves from a pulse source (lightning discharge in the case of a whistling atmospheric) in a dispersive medium. We emphasize the importance of taking into account the motion of plasma in the experimental and theoretical study of electromagnetic wave phenomena in near-Earth space.

On the group velocity of whistling atmospherics

The dynamic spectrum of a whistling atmospheric is a signal of falling tone, and the group delay time of the signal as a function of frequency is formed as a result of propagation of a broadband pulse in a medium (magnetospheric plasma) with a quadratic dispersion law. In this paper, we show that for quadratic dispersion the group velocity is invariant under Galilean transformations. This means that, contrary to expectations, the group velocity is paradoxically independent of the velocity of the medium relative to the observer. A general invariance condition is found in the form of a differential equation. To explain the paradox, we introduce the concept of the dynamic spectrum of Green’s function of the path of propagation of electromagnetic waves from a pulse source (lightning discharge in the case of a whistling atmospheric) in a dispersive medium. We emphasize the importance of taking into account the motion of plasma in the experimental and theoretical study of electromagnetic wave phenomena in near-Earth space.

Tunable Terahertz Plasmon-Induced Transparency in Resonator-Coupled Dirac Semimetal Waveguides

The bulk Dirac semimetal (BDS) is an interesting material, similar to graphene, which can dynamically adjust its optical properties via a variation in its Fermi energy or electrical voltage. In this work, a BDS-based plasmonic device, which enables tunable terahertz plasmon-induced transparency, was proposed and designed. By using the finite element method, the surface plasmon polariton and plasmon-induced transparency of this device were systematically investigated. The results demonstrate that the plasmon-induced transparency of such device can be dynamically tuned by varying its Fermi energy. When the Fermi energy changes from 55 meV to 95 meV, the maximum group delay time of the device increases from 13.2 ps to 21 ps. In the case of a cascading device, the maximum group delay time can be further pushed up to 44.57 ps. The influence of the ambient refractive index on the optical properties of the proposed device was also considered and investigated.

Flexible Control of Two-Channel Transmission and Group Delay in an Optomechanical System with Double Quantum Dots Driven by External Field

With the presence of a driving field applied to double quantum dots and a control field applied on the cavity, the transmission performance and group delay effect of a probe field have been theoretically studied in a hybrid optomechanical system (HOMS). Due to the interaction between the mechanical mode and the double quantum dots system, double optomechanically induced transparency (OMIT) arises in the HOMS. With the assistance of a driving field, the system can be tuned to switch on any one of the two OMIT windows, switch on both of the two OMIT windows or switch off both of the two OMIT windows by dynamically adjusting control of the optical field and the driving field. Furthermore, the transmitted probe fields of the two OMIT windows can be tuned to be absorbed or amplified with proper parameters of the driving field and control field. Moreover, the transmission properties of the two OMIT windows are asymmetrical. One can obtain the maximum group delay time of the probe field by optimizing the amplitude and phase of the driving field. These results provide a new way for constructing optically controlled nanostructured photonic switch and storage devices.

Spin-dependent group delay time and Hartman effect in ferromagnetic bilayer graphene superlattice

We theoretically study the spin-dependent group delay time through ferromagnetic bilayer graphene superlattice in the absence and presence of the bandgap. It is found that the group delay time depends on the spin degree of freedom and exhibits an oscillatory behavior with respect to the Fermi energy and barrier width. Furthermore, in the absence of the bandgap, the superluminal or Hartman effect exists only for the normal angle of incidence. Moreover, when bandgap value is large enough [Formula: see text], the Hartman effect can be observed for all angles of incidence. These results are contrary to the observed behavior for monolayer graphene superlattice.

Variation of Earthquake Ground Motions with Focus on Site Amplification Factors: A Case Study

In this paper, an evaluation of the variation of earthquake ground motions with a focus on site amplification factors based on spectral analysis is presented. By using strong motion record obtained at six sites in Japan, probability distributions of site amplification factors were shown. The relations between standard deviations of site amplification factors and distances between the sites were studied. The variations of representative values of earthquake ground motions based on the variations of site amplification factors were discussed by using probabilistic seismic hazard analysis with focus on Fourier amplitude and group delay time. The distributions of peak ground accelerations and peak ground velocities were shown. It is suggested that design earthquake ground motions considering the average site amplification factors may lead the engineering design on the dangerous side.