Transition radiation generated by a short laser pulse at a plasma-vacuum interface

A theory is presented of the generation of low-frequency transition radiation by a short laser pulse at a plasma-vacuum interface. The wave electromagnetic fields are excited by the vortex electric current that is generated at the plasma boundary by the ponderomotive force of the laser field. The spectral, angular, and energy parameters of the transition radiation, as well as the spatiotemporal structure of the emitted waves in vacuum and in plasma, are investigated. It is shown that the parameters of the transition radiation depend essentially on the ratio of the laser pulse duration to the plasma oscillation period. Under conditions typical of present-day laser-plasma experiments, the transition electromagnetic radiation is generated in the terahertz frequency range and its power can reach several megawatts.     

Generation of terahertz radiation from a low-density plasma slab irradiated by a laser pulse

The generation of terahertz electromagnetic radiation when a laser pulse propagates through a low-density plasma slab is considered. It is shown that terahertz waves are excited because of the growth of a weakly damped, antisymmetric leaking mode of the plasma slab. The spectral, angular, and energy parameters of the terahertz radiation are investigated, as well as the spatiotemporal structure of the emitted waves. It is demonstrated that terahertz electromagnetic wave fields are generated most efficiently when the pulse length is comparable to the slab thickness.     

Emission of terahertz waves in the interaction of a laser pulse with clusters

A theory of generation of terahertz radiation in the interaction of a femtosecond laser pulse with a spherical cluster is developed for the case in which the density of free electrons in the cluster plasma exceeds the critical value. The spectral, angular, and energy characteristics of the emitted terahertz radiation are investigated, as well as its spatiotemporal structure. It is shown that the directional pattern of radiation has a quadrupole structure and that the emission spectrum has a broad maximum at a frequency nearly equal to the reciprocal of the laser pulse duration. It is found that the total radiated energy depends strongly on the cluster size. Analysis of the spatiotemporal profile of the terahertz signal shows that it has a femtosecond duration and contains only two oscillation cycles.     

Emission of low-frequency electromagnetic waves by a short laser pulse propagating in a plasma with density fluctuations

It is shown that a short laser pulse propagating in a plasma with electron density fluctuations can emit electromagnetic waves with frequencies much lower than the laser carrier frequency. Emissions with frequencies close to the plasma frequency and the doubled plasma frequency in a nonisothermal plasma, as well as emission generated in a turbulent plasma, are examined. The effects in question are related to the transformation of the laser pulse wakefield into electromagnetic radiation by electron density fluctuations. The phenomenon under study opens new possibilities for diagnostics of both plasma fields excited by laser pulses and electron density fluctuations in a plasma.     

Dipole Mechanism Generation of Terahertz Waves under Laser-Cluster Interaction

The feasibility of dipole radiation of terahertz waves under the action of a femtosecond laser pulse on a cluster is demonstrated theoretically. It is shown that the dipole mechanism of terahertz radiation generation plays a decisive role in the interaction of a laser pulse with small-size clusters with a sufficiently high electron collision frequency. The dependences of the spectral, angular, energetic, and spatiotemporal characteristics of the terahertz signal on the density of free electrons in the cluster plasma under the conditions in which dipole radiation is dominant are investigated. It is shown that the energy of terahertz radiation is maximal under the resonance conditions, when the laser frequency coincides with the eigenfrequency of a spherical cluster.     

Generation of Terahertz Waves under Laser Action on Hot Dense Plasma

Generation of terahertz waves by hot dense plasma under the action of a femtosecond laser pulse in the regime of anomalous skin effect is considered. The spectral, angular, and energy characteristics of terahertz waves are studied as functions of the plasma and laser parameters. It is shown that, under the conditions of anomalous skin effect, which takes place in ultradense hot plasma, the total energy of the terahertz signal is independent of the electron density, proportional to the square of the electron temperature, and maximal at tight focusing of the laser pulse.     

Theoretical foundations of detection of terahertz radiation in laser-plasma interactions

A theory is developed enabling one to calculate the temporal profile and spectrum of a terahertz wave packet from the energy of the second harmonic of optical radiation generated during the nonlinear interaction between terahertz and circularly polarized laser pulses in the skin layer of an overdense plasma. It is shown that the spectral and temporal characteristics of the envelope of the second harmonic of optical radiation coincide with those of the terahertz pulse only at small durations of the detecting laser radiation. For long laser pulses, the temporal profile and spectrum of the second harmonic are mainly determined by the characteristics of optical radiation at the carrier frequency.     

On the theory of Cherenkov emission from a short laser pulse in a magnetized plasma

Cherenkov emission from a short laser pulse propagating in an underdense plasma along a constant magnetic field is considered. The spectral, angular, energy, and spatiotemporal parameters of the emitted radiation are investigated. It is shown that the spectral content of the radiation and its directionality depend sensitively on the plasma and laser-pulse parameters. For instance, the most intense backward radiation at the upper hybrid frequency is generated by a tightly focused laser pulse.     

Influence of the electron density on the characteristics of terahertz waves generated under laser–cluster interaction

A theory of generation of terahertz radiation under laser–cluster interaction, developed earlier for an overdense cluster plasma [A. A. Frolov, Plasma Phys. Rep. 42. 637 (2016)], is generalized for the case of arbitrary electron density. The spectral composition of radiation is shown to substantially depend on the density of free electrons in the cluster. For an underdense cluster plasma, there is a sharp peak in the terahertz spectrum at the frequency of the quadrupole mode of a plasma sphere. As the electron density increases to supercritical values, this spectral line vanishes and a broad maximum at the frequency comparable with the reciprocal of the laser pulse duration appears in the spectrum. The dependence of the total energy of terahertz radiation on the density of free electrons is analyzed. The radiation yield is shown to increase significantly under resonance conditions, when the laser frequency is close to the eigenfrequency of the dipole or quadrupole mode of a plasma sphere.     

Excitation of surface waves at a plasma boundary by a short laser pulse

The excitation of surface waves by a laser pulse as it crosses a vacuum-plasma interface is considered. Surface waves are excited by a vortex electric current that is generated at the plasma boundary by the ponderomotive force of the pulse. The question is considered of how the duration and transverse dimensions of the pulse affect the spatiotemporal distribution and the spectral and energy parameters of the excited surface waves.     

Feasibility of stabilizing a vacuum-diode X-ray source with a laser-plasma cathode

Results are presented from experimental studies of discharge instabilities and the energy and temporal characteristics of a vacuum-diode X-ray source with a laser plasma cathode over a wide range of energies, intensities, and durations of the plasma-forming laser pulse. It is experimentally shown that the vacuum-discharge dynamics and radiation processes in different discharge stages substantially depend on the parameters of the laser radiation. The shortest recorded pulse duration (10 ns) of Ti K-line radiation (4.5 keV) with a total photon number of 10¹¹ is achieved when the laser plasma cathode is produced by a laser pulse with a duration of 27 ps and an intensity of 10¹³ W/cm². It is found that the contrast of characteristic emission against the bremsstrahlung background is maximum when discharge instabilities are suppressed and the accelerating voltage is three to four times higher than the threshold voltage for line excitation.     

Scattering of an ultrashort electromagnetic pulse in a plasma

An analytic approach is developed to describing how ultrashort electromagnetic pulses with a duration of one period or less at the carrier frequency are scattered in a plasma. Formulas are derived to calculate and analyze the angular and spectral probabilities of radiation scattering via two possible mechanisms-Compton and transition radiation channels-throughout the entire pulse. Numerical simulations were carried out for a Gaussian pulse. The effect of the phase of the carrier frequency relative to the pulse envelope on the scattering parameters is investigated.     

The biological effects of terahertz laser radiation as a fundamental premise for designing diagnostic and treatment methods

This review considers experimental research of the biological effects of terahertz radiation and their results, which provide a basis for medical application of laser terahertz radiation. Therapeutic and diagnostic uses of laser terahertz radiation are described with reference to preliminary basic investigations at the organismic, cell, and molecular levels. It is emphasized that preliminary basic studies of this kind are of importance as another avenue of research to design diagnostic and therapeutic techniques in addition to terahertz imaging and spectroscopy.

     

Nonlinear Thomson scattering of a relativistically strong tightly focused ultrashort laser pulse

The problem of nonlinear Thomson scattering of a relativistically strong linearly polarized ultrashort laser pulse tightly focused into a spot with a diameter of D _F ? λ (where λ is the laser wavelength) is solved. The energy, spectral, and angular distributions of radiation generated due to Thomson scattering from test electrons located in the focal region are found. The characteristics of scattered radiation are studied as functions of the tightness of laser focusing and the initial position of test particles relative to the center of the focal region for a given laser pulse energy. It is demonstrated that the ultratight focusing is not optimal for obtaining the brightest and hardest source of secondary electromagnetic radiation. The hardest and shortest radiation pulse is generated when the beam waist diameter is ?10λ.     

Microwave generation by a superluminal source at limiting current densities

It is well known that high-power directed wideband electromagnetic radiation in the microwave range can be generated by a superluminal pulse of the electron emission current. The operation of a simple emitting element driven by a superluminal current pulse and consisting of an accelerating diode with a photocathode and a source of ionizing radiation that initiates electron emission from the cathode is considered. It is shown that the parameters of an elementary superluminal source obey scaling relations that are determined by the growth rate of the electron emission current from the photocathode and the parameters of the accelerating diode. The limiting anode current density and the limiting values of the characteristics of electromagnetic radiation achievable in such a system are determined. The effect of the finite dimensions of the accelerating system on the parameters of the emitter is investigated, and the spatiotemporal characteristics of the generated electromagnetic fields are obtained.     

Excitation of magnetic fields by a circularly polarized laser pulse in a plasma channel

The excitation of quasistatic magnetic fields by a circularly polarized laser pulse in a plasma channel is considered. It is shown that, to second order in the amplitude of the electric field of the laser pulse, circular rotation of the plane of polarization of the laser radiation in a radially nonuniform plasma gives rise to a nonlinear azimuthal current and leads to the excitation of the radial and axial components of the magnetic field. The dependence of the magnetic field distribution over the plasma channel on the spatial dimensions of the pulse and on the channel width is investigated for a moderate-power laser pulse. The structure of the magnetic fields excited by a relativistic laser pulse in a wide plasma channel is analyzed.     

High-power terahertz optically pumped NH3 laser for plasma diagnostics

The parameter of a terahertz (THz) laser intended for plasma diagnostics in electrodynamic accelerators and tokamaks with a strong magnetic field are discussed. Generation of THz radiation in an ammonia laser under the action of high-power pulsed optical pumping by the radiation of a 10P(32) CO₂ laser is simulated numerically. The main characteristics of the output radiation, such as its spectrum, peak intensity, time dependence, and total energy, are calculated.     

Terahertz spectroscopy of bacteriorhodopsin and rhodopsin: similarities and differences

We studied the low-frequency terahertz spectroscopy of two photoactive protein systems, rhodopsin and bacteriorhodopsin, as a means to characterize collective low-frequency motions in helical transmembrane proteins. From this work, we found that the nature of the vibrational motions activated by terahertz radiation is surprisingly similar between these two structurally similar proteins. Specifically, at the lowest frequencies probed, the cytoplasmic loop regions of the proteins are highly active; and at the higher terahertz frequencies studied, the extracellular loop regions of the protein systems become vibrationally activated. In the case of bacteriorhodopsin, the calculated terahertz spectra are compared with the experimental terahertz signature. This work illustrates the importance of terahertz spectroscopy to identify vibrational degrees of freedom which correlate to known conformational changes in these proteins.     

Process of commutation of a vacuum electric-discharge gap by laser plasma

The temporal parameters of a process of vacuum gap commutation under exposure to a nanosecond pulse of laser radiation incident on the cathode has been studied depending on the radiation energy. Based on the experiment data, it is suggested that a glow discharge is initially ignited in electrode erosion products under exposure to the laser pulse, which due to development of the ionization-overheating instability undergoes the contraction of current channel and transits to an arc discharge. With the radiation energy exceeding a threshold value, the radiation (incident on the cathode) accelerates directly the instability development and the glow discharge transition to the arc discharge due to the radiation absorption in the discharge plasma.     

Radiation from a pulsed dipole source in a moving magnetized plasma

The problem of radiation from a pulsed dipole source in a moving magnetized plasma described by a diagonal permittivity tensor is considered. An exact solution describing the spatiotemporal behavior of the excited electromagnetic field is obtained. The shape of an electromagnetic pulse that is generated by the source and propagates at different angles to both the direction of the external magnetic field and the direction of plasma motion is investigated. It is found that even nonrelativistic motion of the plasma medium can substantially influence the parameters of radiation from prescribed unsteady sources.