Low-density polymer foams pre-ionized by a well-controlled nanosecond pulse are excellent plasma targets to trigger direct laser acceleration(DLA) of electrons by sub-picosecond relativistic laser pulses. In this work, the influence of the nanosecond pulse on the DLA process is investigated. The density profile of plasma generated after irradiating foam with a nanosecond pulse was simulated with a two-dimensional hydrodynamic code, which takes into account the high aspect ratio of interaction and the microstructure of polymer foams. The obtained plasma density profile was used as input to the three-dimensional particle-in-cell code to simulate energy, angular distributions and charge carried by the directional fraction of DLA electrons. The modelling shows good agreement with the experiment and in general a weak dependence of the electron spectra on the plasma profiles, which contain a density up-ramp and a region of near-critical electron density. This explains the high DLA stability in pre-ionized foams, which is important for applications.

The purpose of this study is to investigate the polarization conversion and amplification of electromagnetic terahertz (THz) wave incident normally upon graphene monolayer with direct electric current flowing at arbitrary angle to the elecric vector of incident wave. Methods. The expressions for the elements of the dynamic conductivity tensor of graphene were obtained in hydrodynamic approximation. The electromagnetic response is calculated by solving the Maxwell equations with standard boundary conditions for lateral components of the electric and magnetic fields. Results. It is shown that the dynamic conductivity of graphene depends on value and direction of the electron drift velocity even in the absence of the spatial dispersion. This results in the polarization conversion of electromagnetic radiation at THz frequencies. The real parts of elements of graphene dynamic conductivity tensor can become negative which leads to the amplification of THz oscillations. Conclusion. The polarization conversion and amplification of electromagnetic THz wave incident upon graphene with direct electric current is demonstrated. Polarization conversion efficiency can be as high as 97 percent.

We propose a tunable terahertz (THz) perfect absorber based on a metal groove with a graphene-loaded dielectric resonator, and theoretically study its basic properties. The proposed absorber allows switching between the regimes of perfect absorption at the Fabry-P & eacute;rot resonance excited near the cutoff frequency of the metal groove and almost total reflection away from the resonance by changing the Fermi energy in the graphene. For this purpose, we propose a 'bottom-up' approach, which is based on tuning the admittance of the input line (the metal groove in our case) instead of the structure admittance in order to reach the perfect admittance-matching condition. We demonstrate that this effect can be realized at arbitrarily selected frequencies in the entire THz range due to the dispersion of incoming waves in the metal groove, which ensures the large-scale tunability of its characteristic admittance. As a result, total absorption can be realized in the Fabry-P & eacute;rot resonance even in a simple graphene-loaded dielectric cavity for any admittance of the graphene layer, which is advantageous compared to the majority of existing THz absorbers with more complicated designs.

konstantin-m92@yandex.ru Abstract. Excitation of edge plasmon modes in a graphene rectangle by an incident electromagnetic wave is predicted. The problem is solved using a self -consistent electromagnetic approach based on the method of integral equations developed by the authors. It is found that the frequencies of edge plasmon resonances depend on both the width and length of graphene rectangle. For a graphene rectangle having the aspect ratio of 1 micron by 200 microns, the frequencies of edge plasmon resonances lie in terahertz frequency range

The polarization conversion of an electromagnetic wave normally incident on graphene with a direct electric current is investigated in the hydrodynamic approach. It is shown that the dynamic conductivity of graphene depends on the direction of electron drift even in the absence of spatial dispersion (i.e., for long electromagnetic waves). This changes the polarization of electromagnetic radiation at terahertz frequencies. The real part of the dynamic conductivity of graphene with electron drift can be negative, which leads to amplification of terahertz wave.

The excitation of terahertz plasmon modes in a graphene rectangle by normally incident linearly polarized electromagnetic wave has been theoretically studied. The complete electromagnetic approach based on formulation of the integral equations for sought-for electromagnetic quantities has been developed. The influence of edge-field effects on excitation of plasmon modes for different polarization of the incident wave and different shapes of graphene rectangle has been studied. The absorption cross-section spectra and the charge density distributions in graphene rectangle for different plasmon modes have been studied. It has been found that the edge-field effect, which results in spreading the plasmon field beyond the geometric boundaries of graphene rectangle, leads to considerable red shifts of the plasmon mode frequencies and modifies the plasmon mode dispersion.

We predict the plasmon instability associated with losses in hydrodynamic graphene with a drift-current bias. The instability appears even in a single-layer graphene and becomes stronger for greater losses in graphene in hydrodynamic regime but disappears in the lossless case. The unstable plasmon mode vanishes for zero drift velocity instead of transforming into any plasmon mode existing in graphene without carrier drift. The dissipative instability occurs for any finite drift velocity but the instability increment decreases down to zero for vanishing carrier drift velocity. The strongest instability develops for codirected carrier drift and plasmon propagation directions.

In this letter, we predict the effect of the total cross-polarization conversion of transverse-magnetic electromagnetic wave incident upon drift-current biased graphene into a reflected wave with transverse-electric polarization, which can be possible in the total reflection regime. It is shown that the degree of polarization conversion depends on the angle between the stationary drift-current direction and the plane of the wave incidence. Total polarization conversion is promoted by excitation of the hybrid electromagnetic mode (having a mixed polarization) in graphene. Remarkably, this effect is unrelated to the spatial dispersion of graphene conductivity. This is a unique feature of graphene, which does not exist in conventional two-dimensional electron systems with massive charge carriers. The hybrid electromagnetic mode in graphene can enter the self-excitation regime in the terahertz (THz) frequency range due to the negative real part of graphene conductivity induced by drift current. The studied effects can be useful for creating compact electrically and/or optically tunable polarization convertors and polarization-sensitive amplifiers of THz radiation.

We consider a graphene square located in the interface between two half-spaces with different dielectric constants. Using a self-consistent electromagnetic approach based on the integral equation method, we solve the problem of a linearly polarized terahertz electromagnetic wave scattering by the graphene square. The spectra of extinction, absorption, and scattering cross sections reveal the various plasmon modes excitation in graphene square. The properties of the plasmon modes different types are discussed.

We demonstrate experimentally that a one-dimensional array of silicon nanowires periodically placed on a nickel substrate enhances the transverse magneto-optical Kerr effect (TMOKE) compared to a nickel film. The enhancement mechanism is associated with the excitation of two types of resonances: multipole Mie resonances in each nanowire and surface lattice resonances (SLRs) emerging from the periodic arrangement of the nanowires. The maximal TMOKE values reached up to 1.9 and 2.6% due to the excitation of SLR and a magnetic dipole resonance, respectively. When the SLR is excited, the spectral width of the TMOKE enhancement is narrower compared to the case of the magnetic dipole resonance.

This work continues the study of two crystalline diamond powders with a size of 63–100 μm, rapidly synthesized at a high pressure of 5 GPa and a high temperature of 1650°C from a mixture of graphite and nickel powders with aluminum addition (9 wt %). Magnetization measurements performed with these samples in the temperature range 4.5–300 K and in a magnetic field of 0–100 kOe, revealed the presence of nickel in the form of superparamagnetic nanoparticles with a size of 7.5 nm. The addition of aluminum to the growth medium reduces not only the content of nitrogen impurities in diamond crystals, but also the nickel content (from 1760 to 270 ppm). XRD and glow discharge mass spectrometry confirm the decrease in the nickel content by an order of magnitude. The formation of nickel nanoparticles of 27–29 nm in size was also confirmed by XRD.

We study for the first time the interaction between the waveguide modes of graphene structure and freely propagating terahertz (THz) electromagnetic waves (this interaction takes place within the light cone). We revealed a new and rather unexpected physical phenomenon by showing that freely incident THz electromagnetic waves can resonate with the surface transverse electric (TE) modes of the graphene waveguide in virtue of these modes having their dispersions in the vicinity of the light cone. The dispersion and amplification of surface TE modes in a dielectric waveguide covered with two graphene layers biased by direct current (DC), as well the amplification and lasing of incident THz wave by excitation of TE mode resonances, are investigated. The DC flows perpendicular to the direction of the surface wave propagation and creates the capacitive complex conductivity of graphene at THz frequencies, which is necessary for the existence of surface TE modes in graphene. The real part of graphene conductivity can be negative at THz frequencies due to DC in graphene which leads to amplification and lasing of THz radiation. Such structure can be of great practical importance because an external THz wave can be amplified or generated in lasing process without using special coupling elements commonly needed for ensuring the interaction between external THz wave and surface waveguide modes. The use of a two-layer graphene structure makes it possible to reduce the charge-carrier drift velocity required for reaching the lasing threshold at those resonances, as compared to a structure with a single graphene layer.

The amplification of terahertz electromagnetic radiation in a structure with two layers of graphene through which a direct electric current flows is studied theoretically. The hydrodynamic conductivity of graphene is investigated. It is shown that the real part of the graphene conductivity can be negative in the terahertz-frequency range at the drift velocities of charge carriers in graphene that are lower than the phase velocity of an electromagnetic wave. For small wave vectors of a terahertz wave that is incident on a graphene structure, the spatial dispersion insignificantly contributes to the hydrodynamic conductivity of graphene. Due to this, the amplification efficiency does not depend on the direction of currents in each of the graphene layers. It is shown that graphene with a direct electric current can be used to create terahertz amplifiers operating at room temperature.

The dispersion, excitation, and amplification of electromagnetic transverse electric (TE) modes at terahertz (THz) frequencies in graphene in the hydrodynamic (HD) regime, with a direct electric current flowing perpendicular to the TE mode wavevector, were theoretically investigated. The expression for the nonlocal HD conductivity of graphene with a direct electric current flowing perpendicular to the TE mode wavevector was derived. The direct electric current in graphene leads to the capacitive nature of the graphene HD conductivity at THz frequencies, which makes TE modes exist in this frequency range. The excitation of TE modes in graphene by an incident THz wave was modeled for the attenuated total reflection geometry. A new physical mechanism of TE mode amplification in graphene effective for a low value of carrier drift velocity was predicted. THz lasing regimes with TE modes in graphene structure with direct electric current were found. The results of this work can be used to create miniature technologically feasible sources and amplifiers of THz radiation.

Electrical tuning the radiative damping of "nonradiative" (weak) terahertz plasmon mode in a periodic dual grating-gate graphene structure is studied theoretically. Tuning the dc voltage value applied to one of the gate subgratings can change the radiative damping of the weak plasmon mode from zero to the dissipative damping of the plasmons in high-quality graphene allowing for maximum energy conversion from the incident wave to "nonradiative" plasmon mode. Radiative damping increases due to the increase of the dipole moment of this mode by virtue of its asymmetrical gating. Using the weak plasmon modes is important for effective excitation of the plasmon resonances in high-quality graphene as well as for realizing the concept of low-threshold terahertz lasers based on graphene plasmonic structures.Data availability: Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

We propose a concept of terahertz waveguide plasmon amplifier based on a metal groove with active graphene. It is shown that the power amplification factor of the longitudinal-section magnetic (LSM) waveguide plasmon (normalized to its wavelength) near the cut-off frequency of this mode can exceed the amplification factor of the transverse magnetic (TM) plasmon in a layered graphene structure by more than four orders of magnitude for the same frequency. This is caused by the increase of the LSM plasmon wavelength near the cut-off frequency, smaller energy velocity of the LSM mode, and greater energy release from graphene for the LSM plasmon due to stronger lateral confinement of the LSM waveguide plasmon as compared to the TM plasmon in a layered graphene structure. We show that the enhancement of the LSM plasmon amplification factor near the cut-off frequency is a stronger effect than that due to screening of graphene.

The effect of charge-carrier drift on the plasmon excitation modes (plasmons) in a Dirac electron liquid in graphene with a displaced Fermi level is considered. The dispersion relations for plasmons are obtained using the electromagnetic approach and hydrodynamic description of the electron liquid. Damped and amplified plasmon eigenmodes are studied numerically depending on the relationship of the magnitudes and directions of the direct electrical current and the plasmon phase velocity.

Terahertz (THz) waves have revealed a great potential for use in various fields and for a wide range of challenging applications. High-performance detectors are, however, vital for exploitation of THz technology. Graphene plasmonic THz detectors have proven to be promising optoelectronic devices, but improving their performance is still necessary. In this work, an asymmetric-dual-grating-gate graphene-terahertz-field-effect-transistor with a graphite back-gate was fabricated and characterized under illumination of 0.3 THz radiation in the temperature range from 4.5 K up to the room temperature. The device was fabricated as a sub-THz detector using a heterostructure of h-BN/Graphene/h-BN/Graphite to make a transistor with a double asymmetric-grating-top-gate and a continuous graphite back-gate. By biasing the metallic top-gates and the graphite back-gate, abrupt n + n (or p + p ) or np (or pn ) junctions with different potential barriers are formed along the graphene layer leading to enhancement of the THz rectified signal by about an order of magnitude. The plasmonic rectification for graphene containing np junctions is interpreted as due to the plasmonic electron-hole ratchet mechanism, whereas, for graphene with n + n junctions, rectification is attributed to the differential plasmonic drag effect. This work shows a new way of responsivity enhancement and paves the way towards new record performances of graphene THz nano-photodetectors.

Low-field ferromagnetic resonance in garnet ferrite epitaxial films of different composition with easy-plane anisotropy has been investigated. In ferromagnetic resonance spectra obtained by frequency sweeping, the modulation of the spectral line intensity by a set of narrow (about 100 kHz wide) lines equidistant in frequency has been observed. The lines are spaced in accordance with the resonance of the standing modes of transverse elastic vibrations along the thickness of the film–substrate structure. The modulation depth decreases with rising frequency, since the magnetoelastic coupling depends on the frequency of the elastic vibration mode. In some samples, the modulation depth drops monotonically, whereas in others it oscillates. Such behavior is explained by strong and weak spin pinning on the surface, respectively. The influence of the film–substrate interface and the hybridization of spin-wave and elastic vibration modes under magnetoacoustic resonance conditions have been discussed.

The paper deals with the formulation of a mathematical model for studying the end wall vibrations of a narrow annular channel filled with a pulsating viscous incompressible fluid. The narrow annular channel formed by two rigid coaxial cylinders. The case was considered when there was the end wall at the right end of the annular channel, and the pressure pulsation law was set at the opposite one. The right end face of the inner cylinder and the end wall form the end gap of the channel. The end wall possesses a flexible connection with the annular channel edge and can oscillate due to fluid pressure pulsation. The viscous fluid motion was studied as a creeping one. The mathematical model consisted of the dynamics equations for viscous fluid and the motion equation for the end wall as the spring-mass system. Initially, the problem for the annular gap of the channel was investigated and the pressure change law at the cross-section during the transition from the annular gap to the end one was determined. After that, the problem for the end gap of the channel was considered and the oscillations law of the end wall for the case of the steady-state vibration was determined. The hydroelasticity problem solution made it possible to determine the hydroelastic response of the end wall. The study results showed that the damping properties of the oscillatory system under consideration were determined by the fluid physical properties, as well as the sizes of the annular and end gaps.