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The theory of optical effects in hydrogenated graphene (graphane) in the terahertz and infrared range is developed, including the analysis of complex conductivity, reflection coefficient for graphane on a substrate and dispersion of surface plasmon-polaritons. The calculations are based on quite simple analytical approximation of graphane band structure in the vicinity of Γ-point and on the modified model of quantum coherence relaxation. Comparison of the obtained theoretical results with corresponding experimental data can be used both for the determination of graphane characteristics (Fermi level, relaxation rate etc) and for the investigation of potential applications of this material in the design of new optical elements.

In this talk we summarize our latest theoretical and experimental results on the asymmetry of the electronic distribution function induced by femtosecond laser pulses and its manifestation in the sub-terahertz electromagnetic response. Non-reciprocal generation of THz pulses in monocrystalline bismuth and a new mechanism of magnetic field generation inside metals are discussed. We show that the processes of electron momentum relaxation are critical for electrodynamical properties of the discussed media at sub-picosecond timescale.

Spontaneous optical emission of graphene irradiated by intense single-cycle terahertz pulses was investigated experimentally and explained theoretically. We found that emitted photons are polarized predominantly perpendicular to the electric field of the terahertz pulse, which proves that the terahertz field not only heats the electrons, but also creates a strongly nonequilibrium momentum distribution. Comparison of the measured optical spectrum and polarization anisotropy with the results of numerical modeling allowed us to estimate a momentum isotropization time for electrons in graphene to be ~25 fs and roughly reconstruct the distribution function evolution in k-space.

We show theoretically that anisotropy of the electronic distribution function inside the laser-irradiated metal leads to the formation of edge currents at the timescale of distribution isotropization. When the electronic pressure in the skin layer is anisotropic, the pressure gradient appears to be non-potential force producing a low-frequency magnetic field. In the case of femtosecond laser pumping, the estimated internal magnetic field reaches magnitude up to 1 T even in the non-damaging interaction regime. We demonstrate that this field is localized inside the metal, while just a minor part of its energy is radiated into free space as a sub-terahertz signal.

We report on the observation of terahertz (THz) signal during the femtosecond laser ablation of metals. Strong correlation between the regimes of ablation and THz generation was found for different metals (copper, zinc, nickel and others). According to previous results, the interconnection of these two effects is caused by their thermal nature.

We report systematic theoretical studies of the inverse Faraday effect in materials with massless Dirac fermions, both in two dimensions such as graphene and surface states in topological insulators, and in three dimensions such as Dirac and Weyl semimetals. Both semiclassical and quantum theories are presented, with dissipation and finite size effects included. We find that the magnitude of the effect can be much stronger in Dirac materials as compared to conventional semiconductors. Analytic expressions for the optically induced magnetization in the low temperature limit are obtained. Strong inverse Faraday effect in Dirac materials can be used for the optical control of magnetization, all-optical modulation, and optical isolation., 28 pages, 5 figures

The talk is devoted to the nonlinear effects in metallic structures (like gratings and nanoparticle arrays) irradiated by intense femtosecond laser pulses. Possible mechanisms of delayed THz response and specific non-quadratic nonlinear regimes of conversion are analyzed. In contrast to previous models, only low-frequency currents inside the metal are considered without involving electron emission and acceleration. Special attention is paid to the role of plasmonic resonances at optical frequency in the enhancement of low-frequency nonlinear response.

We consider the possibility of generation of broadband terahertz (THz) radiation upon reflection of a p-polarised femtosecond laser pulse from the surface of a semi-metal. The hydrodynamic model of an instantaneous quadratic response of metals is generalised, and analytical results and numerical simulation data are presented. It is shown that transition from highly conductive metals to semi-metals is accompanied by a significant increase in the efficiency of the THz signal generation due to the reduction of the effective charge carrier mass and attenuation of the shielding of optical and THz fields.

A new method for the experimental determination of the characteristics of the electron scattering in a metal on the basis of optical-to-terahertz conversion has been proposed. It has been shown that the measurement of the characteristics of a low-frequency signal generated at the reflection of a femtosecond laser pulse from the surface of the metal makes it possible to determine the temperature dependence of the electron collision frequency and to reconstruct the dynamics of heating and heat transfer. The existing experimental data on the optical-to-terahertz conversion on the surface of the metal have been analyzed in terms of diagnostics.

We consider theoretically magnetic dipole mechanism of the IR-light absorption at intersubband transitions in wide-gap quantum wells (QW). In contrast to the electric dipole resonance, discussed mechanism manifests in the interaction with the s-polarized component of electromagnetic radiation. We demonstrated that a) the frequencies of magnetic and electric dipole intersubband transitions in 2D layer are equal in the framework of one-particle approximation; b) collective plasma effects should lead to a significant frequency shift between the magnetic and electric dipole resonances in typical QW. That is why independent measurements of the magnetic and electric dipole resonant frequencies could allow both the extracting the QW parameters and the experimental verifying of theoretical models of collective fermion interaction. Taking into account relative weakness of the magnetic dipole absorption, we propose a waveguide scheme for its experimental observation., Comment: 14 pages, 4 figures

Optical frequency comb (OFC) generators based on whispering gallery mode (WGM) microresonators have a massive potential to ensure spectral and energy efficiency in wavelength-division multiplexing (WDM) telecommunication systems. The use of silica microspheres for telecommunication applications has hardly been studied but could be promising. We propose, investigate, and optimize numerically a simple design of a silica microsphere-based OFC generator in the C-band with a free spectral range of 200 GHz and simulate its implementation to provide 4-channel 200 GHz spaced WDM data transmission system. We calculate microsphere characteristics such as WGM eigenfrequencies, dispersion, nonlinear Kerr coefficient with allowance for thermo-optical effects, and simulate OFC generation in the regime of a stable dissipative Kerr soliton. We show that by employing generated OFC lines as optical carriers for WDM data transmission, it is possible to ensure error-free data transmission with a bit error rate (BER) of 4.5 &times, 10&minus, 30, providing a total of 40 Gbit/s of transmission speed on four channels.

We propose a microscopic theory of terahertz (THz) radiation generation on metal gratings under the action of femtosecond laser pulses. In contrast to previous models, only low-frequency currents inside the metal are considered without involving electron emission. The presented model is based on plasmon-enhanced thermal effects and explains the resonant character of optical-to-THz conversion giving an adequate estimation for the full signal energy. Numerical modeling reproduces specific experimental features like delayed character of THz response and low conversion efficiency when the grating depth is too large., Comment: 8 pages, 4 figures

Ultrafast heating of metal nanoparticles by the laser pulse found to be the source of coherent terahertz (THz) radiation due to the heat redistribution processes. Numerical modeling showed that after the femtosecond laser pulse action the time-dependent gradient of the electronic temperature induces low-frequency particle polarization with the characteristic timescale of about fractions of picosecond.

We demonstrate theoretically that ultrafast heating of metal nanoparticles by the laser pulse should lead to the generation of coherent terahertz (THz) radiation during the heat redistribution process. It is shown that after the femtosecond laser pulse action, the time-dependent gradient of the electronic temperature induces low-frequency particle polarization with the characteristic timescale of about fractions of a picosecond. In the case of the directed metallic pattern, the THz pulse waveform can be controlled by changing the geometry of the particle. The proposed THz generation mechanism can be the basis for interpretation of recent experiments with metallic nanoparticles and nanostructures.

We present the results of experimental and theoretical studies of generation of terahertz radiation during reflection of a femtosecond-long laser pulse from a metal surface. It is shown that a greater part of the experimental results (narrow directivity of the terahertz waves and dependence of its energy on the polarization and incidence angle of the laser radiation) can be interpreted within the model of Cerenkov radiation. Some new features of generation of terahertz radiation are considered, which appear during an optical breakdown of the medium near the metal surface. Structural changes in the terahertz signal, which take place in the process of formation of the near-surface plasma, are studied.

We observe enhanced second-harmonic generation in monolayer graphene in the presence of an ultra-strong terahertz field pulse with a peak amplitude of 250 kV/cm. This is a strongly nonperturbative regime of light-matter interaction in which particles get accelerated to energies exceeding the initial Fermi energy of 0.2 eV over a timescale of a few femtoseconds. The second-harmonic current is generated as electrons drift through the region of momenta corresponding to interband transition resonance at an optical frequency. The resulting strongly asymmetric distortion of carrier distribution in momentum space gives rise to an enhanced electric-dipole nonlinear response at the second harmonic. We develop an approximate analytic theory of this effect which accurately predicts observed intensity and polarization of the second-harmonic signal.

We present systematic theoretical studies of both bulk and surface electromagnetic eigenmodes, or polaritons, in Weyl semimetals in the minimal model of two bands with two separated Weyl nodes. We derive the tensors of bulk and surface conductivity taking into account all possible combinations of the optical transitions involving bulk and surface electron states. We show how information about electronic structure of Weyl semimetals, such as the position and separation of Weyl nodes, Fermi energy, and Fermi arc surface states, can be unambiguously extracted from measurements of the dispersion, transmission, reflection, and polarization of electromagnetic waves.

Massless Dirac electrons in graphene and on the surface of 3D topological insulators such as Bi2Se3 demonstrate strong nonlinear optical response and support tightly confined surface plasmon modes. Although both systems constitute an isotropic medium for low-energy in-plane electron excitations, their second-order nonlinear susceptibility becomes non-zero when its spatial dispersion is taken into account. In this case the anisotropy is induced by in-plane wave vectors of obliquely incident or in-plane propagating electromagnetic waves. In this work we develop a rigorous quantum theory of the second-order nonlinear response and apply it to the parametric amplification of mid-infrared and Thz radiation. We show that a strong near-infrared or mid-infrared laser beam obliquely incident on graphene can experience a parametric instability with respect to decay into lower-frequency (idler) photons and THz surface plasmons. The parametric gain leads to efficient generation of THz plasmons. Furthermore, the parametric decay process gives rise to quantum entanglement of idler photon and surface plasmon states. This enables diagnostics and control of surface plasmons by detecting idler photons. A similar parametric process can be implemented in topological insulator thin films.

The features of terahertz radiation generated upon oblique incidence of a femtosecond laser pulse on a metal surface are investigated theoretically. We propose a Cherenkov-type generation mechanism associated with excitation of low-frequency surface currents by a p-polarized optical field. The nonlinear surface currents and corresponding electrodynamic characteristics of low-frequency radiation are calculated analytically and by numerical simulation using the hydrodynamic model. The features of Cherenkov generation of terahertz radiation are analyzed, and techniques are proposed for increasing efficiency.

We report on the first experimental observation of graphene optical emission induced by the intense THz pulse. P-doped CVD graphene with the initial Fermi energy of about 200 meV was used, optical photons was detected in the wavelength range of 340-600 nm. Emission started when THz field amplitude exceeded 100 kV/cm. For THz fields from 200 to 300 kV/cm the temperature of optical radiation was constant, while the number of emitted photons increased several dozen times. This fact clearly indicates multiplication of electron-hole pairs induced by an external field itself and not due to electron heating. The experimental data are in a good agreement with the theory of Landau-Zener interband transitions. It is shown theoretically that Landau-Zener transitions are possible even in the case of heavily doped graphene because the strong THz field removes quasiparticles from the region of interband transitions during several femtoseconds, which cancels the Pauli blocking effect.

The paper is devoted to the very recent experimental and theoretical results on the laser-induced THz generation from metals and semimetals. Despite the relatively low efficiency, study of the optical-to-THz conversion on metal surface are still relevant, mostly because of possible applications in structural diagnostics and due to the fundamental interest in transformation mechanisms. Here we consider a thermal model of THz response and present some new experimental results on the anisotropic THz generation from the surface of mono- and polycrystalline bismuth samples.

We show that a strong infrared laser beam obliquely incident on graphene can experience a parametric instability with respect to decay into lower-frequency (idler) photons and THz surface plasmons. The instability is due to a strong in-plane second-order nonlinear response of graphene which originates from its spatial dispersion. The parametric decay leads to efficient generation of THz plasmons and gives rise to quantum entanglement of idler photons and surface plasmon states.

Laser induced terahertz (THz) waves generation from metals is a result of thermal nonlinear effects in the electron gas near the surface. We discuss the generation mechanism and the possibility of electron scattering investigation with a help of nondestructive optical-to-THz conversion on the surface. It is shown that measurement of THz response can be used to determine electron scattering frequency for electron gas temperatures up to 1–2 eV.

We describe a simple experiment giving the opportunity to estimate the time of collision of a freely falling ping-pong ball and a table. The method is based on the ball deformation measurement and uses just elementary geometry and kinematics.

We report on the first experimental observation of terahertz (THz) wave generation from bismuth mono- and polycrystalline samples irradiated by femtosecond laser pulses. Dependencies of the THz signal on the crystal orientation, optical pulse energy, incidence angle, and polarization are presented and discussed together with features of the sample surfaces. The optical-to-THz conversion efficiency was up to two orders of magnitude higher than for metal at a moderate fluence of ∼1 mJ/cm2. We also found nonlinear effects not previously observed using other metal and semiconductor materials: (a) asymmetry of THz response with respect to a half-turn of a sample around its normal, (b) THz polarization control by orientation of the sample surface, and (c) strong enhancement of optical-to-THz conversion after structuring of a sample with optically large-scale parallel strips pattern.

We propose a simple experiment on the air density measurement which does not require any special equipment: just an A4 sheet of paper, a stopwatch and a ruler. The discussed method uses the most basic air resistance model.