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Time-reversal symmetry for elastic wave propagation breaks down in a resonant mass-in-mass lattice whose inner-stiffness is weakly modulated in space and in time in a wave-like fashion. Specifically, one-way wave transmission, conversion and amplification as well as unidirectional wave blocking are demonstrated analytically through an asymptotic analysis based on coupled mode theory and numerically thanks to a series of simulations in harmonic and transient regimes. High-amplitude modulations are then explored in the homogenization limit where a non-standard effective mass operator is recovered and shown to take negative values over unusually large frequency bands. These modulated metamaterials, which exhibit either non-reciprocal behaviours or non-standard effective mass operators, offer promise for applications in the field of elastic wave control in general and in one-way conversion/amplification in particular.

The ionosphere can be artificially modified by employing ground-based high-power high-frequency electromagnetic waves to irradiate the ionosphere. This modification is achieved through the nonlinear interaction between the electromagnetic waves and the ionospheric plasma, leading to changes in the physical properties and structure of the ionosphere. The degree of artificial modification of the ionosphere is closely related to the heating energy density of high-frequency pump waves. Due to the high density of neutral constituents in the lower ionosphere and the high frequency of electron-neutral collisions, the energy of heating pump waves will be absorbed and attenuated during the penetration of the low ionosphere, seriously affecting the heating effect. This paper proposes a method to reduce the absorption of ionospheric heating pump waves by releasing electron attachment chemicals into low ionosphere to form a large-scale electron density hole. A model for mitigating pump waves absorption based on SF 6 release is established, and the absorption at different frequencies is quantitatively calculated. The propagation characteristics of high-frequency signals in ionospheric holes are studied using a three-dimensional ray tracing method, and the results demonstrate that the chemical release method not only reduces the absorption attenuation of heating pump waves but also forms spherical electron density holes, which exhibit a focusing effect on the heating beam and enhance the heating effect. The results are of great significance for understanding the nonlinear interaction between electromagnetic wave and ionospheric plasma and improving the ionospheric heating efficiency.

Ducts (field-aligned plasma density enhancements) provide a link into the magnetosphere and can guide whistler waves. Inside ducts, wave-particle interactions occur efficiently; therefore, their presence contributes to the removal of energetic particles from the magnetosphere. We present experimental results concerning the characteristics, behavior, and excitation thresholds of ducts induced by extraordinary (X-mode) polarized high-power HF radio waves emitted towards the magnetic zenith (MZ) into the upper ionosphere at EISCAT (European Incoherent SCATter). The features and behavior of ducts were diagnosed by the EISCAT incoherent scatter radar (ISR) at Tromsø and the CUTLASS (SuperDARN) Finland radar at Hankasalmi. The state of the ionosphere was monitored by the Dynasonde in Tromsø. It was found that the electron density Ne enhancements inside ducts were of 50–80% above the background Ne values and their transverse size (normal to the magnetic flux tube) corresponded to about 3–4° in the north–south direction. They were generated during magnetically quiet periods and extended from ~300 to 320 km up to the upper altitude limit of the EISCAT radar measurements (600–700 km), when heater frequencies were both below and above the critical frequency of the F2 layer (fH ≤ foF2 and fH > foF2), regardless of whether HF-induced plasma and ion lines were generated or not. Comparing the O-/X-mode effects from the EISCAT radar observations, it was shown that the creation of the strong Ne ducts is a typical characteristic of the X-mode pulses. As a rule, electron density enhancements were not observed during O-mode pulses. A plausible mechanism for the creation of X-mode artificial ducts is discussed.

A one-dimensional nonlinear problem of parametric generation of low-frequency radiation is considered in the case of a pulsed high-frequency initial excitation capable of forming shock fronts in the wave profile. A numerical algorithm for solving the Burgers equation in the time domain using a Godunov-type shock-capturing scheme is developed. Examples of the propagation of model frequency-modulated signals with different envelope shapes at different ratios of nonlinear and dissipative effects, which limit the interaction length of pump waves, are considered. Examples of the evolution of waveforms and spectra in a process of self-demodulation of a high-frequency pump signal are given, with self-demodulation manifesting at shorter distances in strongly nonlinear propagation regimes due to additional attenuation of wave energy at shock fronts. It is shown that the efficiency of low-frequency generation increases in shockwave propagation regimes.

A technique for studying parametric instabilities in magnetic nanostructures is worked out using the developed computational algorithm for determining the bifurcation points of the nonlinear Maxwell operator. The instability of the process of parametric excitation of magnetostatic and spin waves (oscillations) in 2D gratings of magnetic nanoparticles (MNPs) is studied numerically as a function of the bifurcation parameters (the pump wave amplitude and frequency). The threshold values of the parametric instability of magnetostatic and spin oscillations in 2D grating of MPs (of micro- and nanosizes) are calculated depending on the distance between MNPs (the grating period).

The generation of a picosecond pulse train is demonstrated taking advantage of the cross-gain occurring in a dispersion-oscillating fibre. The resulting frequency-converted signal is detuned by more than 20 nm from the pump and can be temporally compressed by a factor of 2 compared with the input sinusoidal pump wave.

We develop a reference-free automated method for measuring the frequency-response of an acoustic receiver based on the nonlinear interaction of two collinearly propagating pump waves. We use the identical dependence of the amplitudes of waves of difference and sum frequencies on the product of the amplitudes of the pump waves, varying with the tuning of the difference frequency.

This research is to facilitate the current understanding of long wave dynamics at coasts and during on-land propagation; experimental and numerical approaches are compared against existing analytical expressions for the long wave run-up. Leading depression sinusoidal waves are chosen to model these dynamics. The experimental study was conducted using a new pump-driven wave generator and the numerical experiments were carried out with a one-dimensional discontinuous Galerkin non-linear shallow water model. The numerical model is able to accurately reproduce the run-up elevation and velocities predicted by the theoretical expressions. Depending on the surf similarity of the generated waves and due to imperfections of the experimental wave generation, riding waves are observed in the experimental results. These artifacts can also be confirmed in the numerical study when the data from the physical experiments is assimilated. Qualitatively, scale effects associated with the experimental setting are discussed. Finally, shoreline velocities, run-up and run-down are determined and shown to largely agree with analytical predictions.

The first experimental demonstration of a continuous-wave dual-pump fibre optical parametric amplifier operating near 1 µm is reported. A flat gain band of 23 dB spanning over 16 nm (4.5 THz) with <2 dB variation is reported. This fibre optical parametric amplifier is achieved with a dispersion stabilised microstructured optical fibre having a high figure of merit, i.e. high nonlinearity and low loss.

A rigid open-ended pipe is submerged in the ocean below the troughs of the surface waves and held fixed in the vertical position, the lower end being at or below the depth of wave influence. When surface gravity waves propagate past the pipe, water flows up as long as waves are present. The steady upward vertical velocity in the center of the pipe is calculated to be proportional to the square of both the average wave steepness and the pipe’s radius. An application is to bring nutrient rich waters up into the sunlit surface layers of the open oceans.