S-polarized Laser Pulse Research Articles

Terahertz radiation in the interaction of a focused laser pulse with plasma

The theory of terahertz (THz) wave emission at the oblique incidence of a focused s-polarized laser pulse on the boundary of a rarefied plasma is developed. The angular, spectral and energy characteristics of the THz signal as a function of the focal spot size and the incidence angle of laser radiation, as well as the plasma density, are investigated. It is shown that the THz radiation energy increases with a decrease in the laser pulse focal spot and has the maximum value when the tightly focused laser pulse is incident at the angle of total reflection on the plasma boundary.

Post-solitons and electron vortices generated by femtosecond intense laser interacting with uniform near-critical-density plasmas

Generation of nonlinear structures, such as stimulated Raman side scattering waves, post-solitons and electron vortices, during ultra-short intense laser pulse transportation in near-critical-density (NCD) plasmas is studied by using multi-dimensional particle-in-cell (PIC) simulations. In two-dimensional geometries, both P- and S-polarized laser pulses are used to drive these nonlinear structures and to check the polarization effects on them. In the S-polarized case, the scattered waves can be captured by surrounding plasmas leading to the generation of post-solitons, while the main pulse excites convective electric currents leading to the formation of electron vortices through Kelvin–Helmholtz instability (KHI). In the P-polarized case, the scattered waves dissipate their energy by heating surrounding plasmas. Electron vortices are excited due to the hosing instability of the drive laser. These polarization dependent physical processes are reproduced in two different planes perpendicular to the laser propagation direction in three-dimensional simulation with linearly polarized laser driver. The current work provides inspiration for future experiments of laser–NCD plasma interactions.

Terahertz wave emission at an oblique incidence of the laser pulse on rarefied plasma

The terahertz wave packet emission theory is constructed when an s-polarized laser pulse is incident on a semi-infinite plasma whose density is much lower than the critical value. The angular, spectral and energy characteristics of a terahertz pulse are studied as a function of the plasma density, the angle of incidence and the duration of laser radiation. It is shown that the energy of the terahertz signal has the maximum value for grazing incidence of the laser pulse, when the effect of total reflection of the laser radiation from the plasma boundary takes place.

X-rays diagnostics of the hot electron energy distribution in the intense laser interaction with metal targets

A two-temperature hot electron energy distribution has been revealed by modeling of bremsstrahlung emission, measured by the radiation attenuation and half-shade methods, and Kα emission from a massive silver cylinder irradiated by a subpicosecond s-polarized laser pulse with a peak intensity of about 2 × 1019 W/cm2. To deduce parameters of the hot electron spectrum, we have developed semi-analytical models of generation and measurements of the x-rays. The models are based on analytical expressions and tabulated data on electron stopping power as well as cross-sections of generation and absorption of the x-rays. The Kα emission from thin silver foils deposited on low-Z substrates, both conducting and nonconducting, has been used to verify the developed models and obtained hot electron spectrum. The obtained temperatures of the colder and hotter electron components are in agreement with the values predicted by kinetic simulations of the cone-guided approach to fast ignition [Chrisman et al., Phys. Plasmas 15, 056309 (2008)]. The temperature of the low-energy component of the accelerated electron spectrum is well below the ponderomotive scaling and Beg's law. We have obtained relatively low conversion efficiency of laser energy into the energy of hot electrons propagating through the solid target of about 2%. It is demonstrated that the assumption about a single-temperature hot electron energy distribution with the slope temperature described by the ponderomotive scaling relationship, without detailed analysis of the hot electron spectrum, can lead to strong overestimation of the laser-to-electron energy-conversion efficiency, in particular, the conversion efficiency of laser energy into the high-temperature component of the hot electron distribution.

Absorption of the S-polarized short laser pulse in underdense plasma by considering the electrons ohmic heating and the laser pulse ponderomotive force effects

With the advent of ultrashort high intensity laser pulses, laser absorption during laser-plasma interactions has received significant attention during the past two decades since it is related to a variety of applications of high intensity lasers, including the hot electron production for fast ignition of fusion targets, bright X-ray and gamma-ray sources, ion acceleration, compact neutron sources, and generally the creation of high energy density matters. Although some absorption mechanisms appear for short laser pulses with nanosecond duration time, they usually appear for some nonlinear phenomena as electron ohmic heating and laser pulse ponderomotive force are not considered. In this paper, the absorption of the S-polarized laser pulse through an interaction with an underdense plasma in attendance of electron ohmic heating and ponderomotive nonlinearities is analyzed. It is shown that, due to the effect of the ponderomotive force in both isothermal and non-isothermal plasmas, the increasing laser radiation angle leads to the increasing electric field wavelength. Also, since the ponderomotive force modifies the electron density and temperature distribution, it is shown that the decreasing radiation angle leads to the steepening of the electron density profile and the narrowing of electron bunching. Furthermore, by increasing the radiation angle, the absorption coefficient is decreased strongly. It is found that due to the heat of the plasma at the expanse of the wave energy in the case of a non-thermal plasma, the absorption coefficient is increased intensively in comparison to a collisional plasma.

Controlled Fabrication of Micro/Nano-Structures on Germanium Using Ultrashort Laser Pulses under Ambient Conditions

A technique for ordered fabrication of periodic freestanding micro/nanostructures on the crystalline germanium (Ge) <100> surfaces with 1064 nm wavelength ultrashort laser pulses under ambient conditions is presented. The laser radiation fluence used for obtaining the structures is close to the melting threshold (0.1 J/cm2) of Ge. The dimensions of structures range from hundreds of nanometres to a few microns. The orientation of the periodic surface structures depends on laser beam polarization direction. Arrays of structures are formed in rows parallel to the sample movement direction for samples machined with s-polarized laser pulses, but formed in the direction perpendicular to the movement for p-polarized pulses. The structures are fabricated under variable temperatures on sample surface owing to the changed interference between incident and reflected laser beams. A micro-Raman analysis of the processed surfaces shows a minor change in the spectral intensity as compared to the unprocessed surface and the material retains its crystallinity after laser irradiation.

Time-resolved measurements on reflectivity of an ultrafast laser-induced plasma mirror

Using a linearly chirped laser pulse to irradiate antireflection coated targets, the time-varying reflectivity of a plasma mirror (PM) has been measured at various laser intensities from 1012 to 1017 W/cm2. The onset of plasma generation as well as the formation process of a PM with the highest reflectivity has been observed. The rise time of the PM’s reflectivity reaching up to the maximum varies from 300–500 fs at lower laser intensities but goes up to 900 fs at higher intensity of >1016 W/cm2. This long rise time can be attributed to a slowly rising shoulder of the laser pulse, which will trigger the generation of preplasma well in advance of the laser peak. The detailed measurements on both time-integrated and time-resolved reflectivity of a PM, which is induced by p- and s-polarized laser pulses, respectively, indicate that an s-polarized pulse is favorable to obtain the maximal reflectivity and the best contrast improvement as well. This difference can be attributed to the weaker absorption and a smaller plasma scale length generated for s-polarization.

Electron radiography using hot electron jets from sub-millijoule femtosecond laser pulses

Electron jets produced in the intermediate intensity range of 1015 to 1017 W/cm2 from submillijoule 120 fs Ti:Sapphire laser pulses focused to spots of a few microns in diameter have been characterized. The experimental results show strong emission of hot electrons with energies from 80 keV to above 250 keV from microplasmas created with both p- and s-polarized 250 μJ laser pulses. The electron jets with energies above 250 keV are observed to be highly directional. The electron jets are observed in the plane of polarization of the laser electric field for both p- and s-polarized laser pulses. The hot electrons emitted from these femtosecond laser plasmas have also been used for radiographic imaging. It is expected that the short initial duration of these electron pulses would make them useful for time resolved applications.

High-energy electrons produced in subpicosecond laser-plasma interactions from subrelativistic laser intensities to relativistic intensities.

The characteristics of the forward hot electrons produced by subpicosecond laser-plasma interactions are studied for different laser polarizations at laser intensities from subrelativistic to relativistic. The peak of the hot electron beam produced by p-polarized laser beam shifts to the laser propagation direction from the target normal direction as the laser intensity reaches the relativistic. For s-polarized laser pulse, hot electrons are mainly directed to the laser axis direction. The temperature and the maximum energy of hot electrons are much higher than that expected by the empirical scaling law. The energy spectra of the hot electrons evolve to be a single-temperature structure at relativistic laser intensities from the two-temperature structure at subrelativistic intensities. For relativistic laser intensities, the forward hot electrons are less dependent on the laser polarization under the laser conditions. The existing of a preplasma formed by the laser amplified spontaneous emission pedestal plays an important role in the interaction. One-dimensional particle-in-cell simulations reproduce the most characteristics observed in the experiment.

Effects of Polarization on Super-hot Electron Generation inFemtosecond Laser-Plasma Interaction

The effects of laser polarization on super-hot electron (>100 keV) generationhave been studied in the interaction of femtosecond laser light (800 nm,150 fs, 6×1015W·cm-2) with a pre-formed plasma from aslab Cu target. For p-polarized laser pulses, high-energy γ-rays ofthe energy ~400 keV were detected. The electron temperatures deducedfrom the γ-ray spectra were 66 and 52 keV, respectively, in normal andreflective directions of the solid target, and hot electrons were emittedout of the plasma mainly in the normal direction. In contrast, there werenearly no γ-rays >100 keV found for s-polarized laser pulses. The hotelectron temperature was 26 keV and the emission of hot electrons wasparallel to the laser field. The superposition of resonant field withelectrostatic field excited by escaping electrons may contribute to thehigh-energy γ-ray or super-hot electron (>100 keV) generation.

Prepulse effects on the interaction of intense femtosecond laser pulses with high-Z solids

Kalpha emission of high-Z solid targets irradiated by an intense, short (<100 fs) laser pulse in the 10 keV region is shown to be sensitive to the electron energy cutoff, which is strongly dependent on the density gradient of the plasma corona formed by a long prepulse. The absorption rate of short laser pulses, the hot electron distribution, and x-ray emission from a Cu slab target are studied via a hybrid model, which combines the hydrodynamics, collisional particle-in-cell, and Monte Carlo simulation techniques, and via a direct spectroscopic measurement. An absorption mechanism originating from the interaction of the laser pulse with plasma waves is found to increase the absorption rate by over 30% even for a very short, s-polarized laser pulse. Calculated and measured x-ray spectra are in good agreement, confirming the electron energy cutoff.