G. Ferrante

Monte Carlo Analysis of the Efficiency of Tera-Hertz Harmonic Generation in Semiconductor Nitrides

The high-order harmonics generation in bulk InN, subjected to a high-frequency alternating electric field of strong-amplitude is investigated theoretically using Monte Carlo simulations of carrier interacting with a multi-mode electric field. This approach allows on the one hand to account in a natural way for the effect of the reabsorption of the generated radiation by the nonlinear medium itself, and on the other hand to estimate the constraints, which must be satisfied by the external experimental setup to achieve the maximum conversion efficiency. It is demonstrated, that InN is a promising material for such a conversion up to the THz frequency range. Under optimum conditions, the power conversion efficiency is found to reach several percents.

Harmonic generation and wave mixing in a plasma in the presence of two linearly polarized laser fields

A theoretical treatment of the high-order harmonic generation by electrons undergoing collisions with ions in a plasma in the presence of two single-mode linearly polarized fields is presented. Except for polarization, the two radiation fields may have arbitrary parameters. The treatment is based on an approximate solution of the Fokker–Planck equation for the plasma electron velocity distribution function. A set of representative numerical calculations is carried out. It is found that the generation of odd harmonics of each field is followed by the generation as well of satellites at mixed frequencies. For a given odd harmonic of one field, the satellites are shifted from it by multiple numbers of twice the frequency of the other field. When there are no coincident frequencies among the generated ones, the intensity of the odd harmonics of one field is found to decrease owing to the action of the other field. When, instead, the satellite frequencies of low-order harmonics coincide with the frequencies of high-order harmonics, a considerable enhancement of the latter is predicted. It is shown also that the harmonic intensity and the polarization direction depend significantly on the angle between the polarization directions of the two mixing fields.

Inverse bremsstrahlung in a plasma with electron temperature anisotropy

Inverse bremsstrahlung absorption of electromagnetic radiation in plasma with anisotropic two-temperature bi-Maxwellian electron distribution function over velocities is investigated. In the case of a weak field, absorption is more effective if the radiation field is polarized in the plane in which the plasma electrons have the smaller of the two temperatures. In the case when the distribution function is highly anisotropic, absorption changes strongly when the field polarization changes its direction with respect to the temperature anisotropy axis. In the intermediate domain, when the field is strong in directions not very close to that of the larger temperature, both absorption efficiency and degree of its anisotropy decrease. The conditions when the absorbed power practically does not depend on the field are established. Finally, in the case of a strong field, absorption decreases further while the degree of anisotropy is a weakly changing logarithmic function of effective electron temperatures.

Electron scattering in strong laser fields. Theoretical models versus recent experiments

Using recent measurements of multiphoton free-free transitions in e-Ar scattering in the presence of a CO2 laser as reference data, an analysis is made of the role different properties of realistic lasers may play in affecting the cross sections of multiphoton transitions. Single-mode and N-mode fields are considered with and without given statistics, emphasis being placed on few-mode cases. Spatial and temporal inhomogeneities, accounting respectively for the focusing and pulsed regimes, are also taken into account. While a complete and detailed agreement is probably out of the question at the present due to incomplete knowledge of all the experimental conditions, the basic and most important features are reproduced with satisfactory agreement. To achieve this agreement a decisive role is played by spatial and temporal inhomogeneities irrespective of the particular laser model in which they are considered.

A simple model of high harmonic generation in a plasma

We apply the average electron model to describe high harmonic generation in a collisional plasma in a laser field. The model is based on the coupled kinetic equations for electron velocity and electron temperature. The harmonic intensities are obtained from the Fourier coefficients of the nonlinear part of the electron velocity. Numerical calculations are reported for both strongly and weakly ionized plasmas in the nonstationary regime. We show the role of collisions frequencies as well as of quiver and thermal velocities in the harmonic generation efficiency.

Even harmonics generation of high frequency radiation in current-carrying plasmas

Generation of high frequency radiation harmonics in a current-carrying plasma is studied. The physical mechanism responsible for harmonics generation is provided by electron-ion collisions. The current in the plasma is sustained by a constant electric field. It is shown that the electron distribution function anisotropy due to the static field yields generation of even harmonics. As a result, the radiation spectrum emitted by the current-carrying plasma contains both even and odd harmonics, the latter being attributed to currentless plasma. For a broad range of plasma and high frequency radiation parameters, a detailed analysis of the even harmonics properties is reported.

Monte Carlo simulation of harmonic generation in InP

Harmonics spectra in an n-type InP bulk semiconductor showing up to the 13th harmonic are reported. The external field is linearly polarized with the frequency v = 100 GHz. Calculations are based on a Monte Carlo simulation for the electron motion in the conducting band and on an electrodynamics equation for the harmonics generation. The effect of a significant reduction of efficiency in the generation of particular harmonics is found, and is traced back to a sort of alternating field Gunn effect. A short analysis of the different physical mechanisms giving rise to harmonics generation is presented.

Comments on charged particle scattering in the presence of a quantising magnetic field

Transition probabilities and scattering cross sections are derived for charged particle potential scattering in the presence of a quantising constant magnetic field. They are derived within the S-matrix and the Greens function approaches and found to differ significantly from the ones defined for spherical geometries. Three types of scattering potential are considered: screened Coulombic, pure Coulombic and exponential. For all of them the first-order matrix elements and accordingly, the transition probabilities and the cross sections are exactly calculated and given in analytic closed form. The optical theorem is derived, with the result that the total scattering cross section is found to be expressed through the real part of the elastic scattering amplitude. The transition probabilities and the scattering cross sections are found to have singularities and accordingly to exhibit giant growth (cyclotron resonances) at values of the incident particle energy (along the z axis) exactly matching the energy differences between the initial and the final state Landau levels.

Radiation absorption and reflection by a plasma with cold and hot electrons

Reflection and collisionless absorption of a test wave by a plasma, with a sharp boundary and containing a small amount of hot electrons, besides the bulk of more cold electrons, are investigated. It is established that, in the high-frequency skin-effect regime, the conditions are possible, when the absorption coefficient is basically determined by the hot electrons. On the contrary, in the transition to the anomalous skin-effect, the absorption by the cold electrons becomes dominant. In both the cases of high-frequency and anomalous skin-effect it is found that the reflected wave phase shift is determined by the cold electron bulk.

Fast plasma heating in the high-frequency skin-effect regime

Dense plasma heating by an ultrashort laser pulse is investigated in the regime of the high-frequency skin effect. The electron temperature evolution on the plasma surface and the properties of the classical heat transfer into the plasma interior are studied. Three possible plasma heating mechanisms are considered. At relatively not large temperatures the plasma heating, due to radiation absorption, is controlled by the electron–ion collisions in the skin layer. The others two pathways are related to high temperatures and take place in the regime of collisionless radiation absorption when the electrons are reflected by the plasma surface either specularly or diffusely. It is shown that, compared with the specular reflection, the diffuse one yields a larger increase of the electron temperature on the plasma surface and a faster propagation of the heat flux in the plasma interior, even when the fraction of diffusely reflected electrons is not high.