| A series of nonlinear physical phenomena can emerge when ultraintense fem-tosecond laser pulses irradiate plasma targets,such as a number of relativistic scattering,intense electromagnetic radiations.Generation of coherent radiation by the interaction of femtosecond laser pulses with plasma medium remains an important topic all the time.In this paper,high-order harmonic generation(H-HG)by laser solid(plasma)target interaction is investigated by means of particle simulation method.Flat target scheme and grating target scheme are mainly in-volved.1.In the case where ultrashort intense laser pulse irradiates flat target surface,harmonic beam mainly emits along the reflection direction of target surface.It can be contributed to the reflection of incident light by the oscillating electron density interface driven by the ponderomotive force of incident pulse.This gives rise to the periodical phase modulation in reflected beam,producing high harmon-ic components of laser fundamental frequency.Under normal incidence,reflected harmonic spectrum just contains odd harmonic components while every odd and even harmonics can be included under oblique incidence.Due to the Gaussian distribution of transverse intensity distribution which leads to a denting target surface in the interaction area,the divergence of reflected harmonic beam may be increased.By irradiating the target surface with curved wave-front incident pulse can suppress the harmonic beam divergence caused by the denting surface.2.As a novel high harmonic generation scheme,grating targets irradiated by ul-trashort intense laser pulse can produce more intense harmonic radiation.Due to the effect of coherent superposition,harmonic intensities can be obviously en-hanced along specific directions.Under normal incidence,this phenomenon can be illustrated by the grating equation.PIC simulation results show that,intense har-monic emissions along target surface are produced,their intensities even obviously surpass the reflected harmonics from flat targets under the same interaction con-ditions.Analysis of surface electron density evolution and phase space behaviors tell us that intense near-surface harmonic emissions from grating target is linked to the coherent radiations by high density surface electron bunches formed near the grating protuberances and oscillate driven by the laser electric field.Under the effect of periodical grating structures,emissions from each bunches interfere with each other,giving rise to the specific intense near-surface harmonic distribu-tion.Different conditions(laser intensity,plasma density)and grating structure parameters can influence the formation and dynamics of surface electron bunches,which affect the harmonic generation process.3.Under oblique incidence,intense near-surface harmonics can still be produced,but the harmonic characteristics become more complex:harmonic components at different side of incident pulse are regular and their intensities are relatively high.While the harmonic emissions at the same side of incident pulse are weaker.Some“anomalous”harmonic components violating the expectations of grating equation also appeared.Near-surface harmonic components and overall intensities have strong dependence on the incident angle.In the cases investigated,near-surface harmonic intensities under 30~?incidence is the most intense.While when the incident angle becomes too large(60~?,for example),harmonic emissions along re-flection direction and near-surface directions are all obviously suppressed.Analysis of surface electron dynamics show that the asymmetric surface electron behaviors give rise to the asymmetric harmonic distribution features:along the near-surface direction at different side of incident light,highly dense electron bunches can be formed and accelerated along this direction.While along the other near-surface direction,surface electron(bunches)are severely diffused,emitting relatively weak harmonics along this direction. |
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