Investigation Of High-Order Harmonic Generation Through The Interactions Of Ultrashort Ultraintense Laser Pulses With Solid Targets

High-order harmonics generated through the interactions of intense femtosecond laser pulses with overdense plasma surfaces are energetic coherent extreme ultraviolet(EUV)or soft X-ray sources.In the time domain,the radiation is confined in a very short temporal window in each optical cycle,leading to the generation of attosecond(as)or zeptosecond(zs)pulse trains.Even single isolated attosecond pulse could be obtained by various gating techniques.By shaping the plasma surface,the harmonics can be focused to reach Schwinger limit.Such promising source can be widely used in applications such as diagnosis of ultrafast dynamic processes,coherent diffractive imaging(CDI)with high resolution,probing magnetic materials,seeding the free-electron lasers(FELs)and studying the nonlinear quantum electrodynamics(QED).This thesis reports the investigation of harmonic generation from laser-target interac-tions in experiments and simulations.The generation processes are dominated by coherent wake emission(CWE)and relativistically oscillating mirror(ROM)mechanisms.We first carry out research on the effect of laser contrast on the harmonic generation process and prove that the harmonics can only be generated by high contrast laser pulse.Then,the influence of the plasma density scale length(L)on generation efficiency of the two mechanisms is investigated.The results prove that the scale length is a crucial physi-cal parameter in harmonic generation and there is an optimal scale length for efficient harmonic generation.At last,we explore the influence of the shaped plasma surface on harmonic divergence and its focusability.We demonstrate that the optically shaped plasma surface can not only control the harmonic divergence,but also precompensate the harmonic wavefront curvature induced by the denting effect.The thesis is organized as following:In the first chapter:the development of laser technologies and the basic absorption mechanisms of high-intensity laser interaction with overdense plasma are briefly intro-duced.Then the mechanism of high-order harmonic generated from inertial gas media is introduced.Although it is well studied,limited by the gas ionization threshold,the intensity of driving laser can not be beyond 10¹⁵W/cm².In contrast with the harmonic generation in atomic gases,the harmonics generated from solid-density plasma have no limitation on laser intensity,so it can generate much stronger EUV or soft X-rays.In the last,we focus on three mechanisms of harmonic generation from laser-plasma interaction and its relevant research results.In the second chapter:since the generation of relativistic harmonics requires a high contrast and ultraintense laser system,the 200 TW Ti:sapphire laser system at the Laboratory for Laser Plasmas of Shanghai Jiao Tong University and the plasma mirror system are introduced first.The laser contrast is improved by two orders of magnitude with the plasma mirror.In order to study the harmonic generation experimentally,we set up the solid target chamber,target system and alignment system.At last,we give a brief introduction of the two numerical simulation methods used in the study of relativistic harmonic radiation.In the third chapter:we first introduce the focusing mirror and the flat-field spec-trometer for harmonic diagnosis.By the laser system,target system and diagnostic system,the experimental investigation of laser contrast on harmonic generation is carried out.The experimental results show that the harmonics up to the 21st are observed at a small incidence angle of 15?when the plasma mirror system is adopted.The harmonic generation process is dominated by the CWE mechanism.No harmonics but only plasma continuum emission is obtained when the plasma mirror is not used.After that,we study the effect of the L on the harmonic generation efficiency of the CWE mechanism by introducing a prepulse before the main pulse.Experimental and simulation results indicate that with increasing L,the harmonic generation efficiency first increases very fast and then decreases slowly.At L=0.1?₀,the harmonic intensity is maximal.In the fourth chapter:when the incidence angle is increased from 15?to 40?,the ROM harmonics up to 47th are observed in experiments.Simulation results present that with increasing the incidence angle,the harmonic intensity first increases and then drops.The harmonic intensity reaches the maximum at 53?.The influence of plasma density gradients on the generation process from ROM mechanism is also studied in experiments and simulations.Results reveal that there are two optimal L for efficient ROM harmonic generation,one is shorter than 0.1?₀,and the other is longer than 0.1?₀.In order to quantitatively explain the experimental and simulation results,a quasi-1D analytical model is developed.By matching the potential energies provided by laser pulse and charge displacement in the plasma with an exponential distributed density profile,the two optimal L values can be quantitatively estimated at the given laser incidence angle and intensity.By increasing the incidence angle and laser intensity,the lower optimal L is lower and the larger optimal L gets larger.In the fifth chapter:we investigate the effect of plasma surface shape on harmonic divergence.Two kinds of prepulses are used to create different distributions of plasma surface.A prepulse with the same focal spot size with the main pulse is introduced by an ultrafast pulse shaper(Dazzler).The main laser interacts with a convex plasma surface.By inserting a reflection mirror in the optical path,we introduce a prepulse with larger focal spot size than the main pulse.The main laser interacts with a planar plasma surface.Under the condition with the laser intensity of 10¹⁹W/cm²,the divergence of harmonics from the convex plasma surface is found to be much larger than that from a planar plasma surface.When the laser intensity goes to much higher such as 10²¹W/cm²,according to our simulations,due to the optically shaped surface,the harmonics are radiated into a significantly smaller angle along with a much higher intensity.Our results suggest that a convex plasma surface can not only control the harmonic divergence,but also compensate the harmonic wavefront curvature.This could be critical to focus the relativistic high harmonics to achieve unprecedented intensity.