【摘要】 自啁啾脉冲放大（CPA）技术发明40年以来，激光峰值功率已突破拍瓦量级，但其进一步提升受限于固态光学元件的破坏阈值与有限尺寸。与固体材料不同，等离子体作为物质第四态，能承受更高能量密度的激光作用，为提升激光峰值功率提供了新可能。前期我们提出了一种等离子体光放大方案，利用两束同向传播的激光脉冲在等离子体中激发拉曼前向散射，在相位匹配条件下通过三波耦合机制，在极小时空尺度内实现对近红外种子光脉冲的高效放大。基于此方案，进一步探究了实际实验条件下该方案的放大效果，重点分析了非均匀等离子体密度分布、气体电离、电子温度等关键效应的影响。理论模型与大尺度二维数值模拟结果表明，该方案具备三大优势：一是可在百微米空间尺度、亚皮秒时间尺度内，将光斑尺寸为数百微米、初始光强极弱的种子脉冲光强放大4～5个数量级；二是在种子脉冲放大后，可通过自压缩过程，将脉宽压缩至准单周期；三是实验条件下的等离子体参数具有一定鲁棒性。该方案为近红外波段拍瓦量级极高功率激光脉冲的产生提供了全新路径，有望广泛应用于超快光学与高能量密度物理等领域。更多还原

【Abstract】 Objective Over the past 40 years since the invention of the chirped pulse amplification （CPA） technique,the peak power of lasers has reached the petawatt （PW） level,revolutionizing fields such as inertial confinement fusion,ultrafast science,and particle acceleration.However,further improvements in laser peak power are constrained by two critical limitations of solid-state optical components:their inherent damage thresholds (typically～10¹³ W/cm²) and limited physical sizes,which lead to bulky,high-cost amplification systems.Additionally,solid-state gain media （e.g.,laser crystals） restrict the tunability of laser wavelengths—most highpower femtosecond lasers are confined to 0.8μm or 1.06μm,lacking flexibility for amplifying pulses across diverse spectral bands.Plasma,as the fourth state of matter,offers a transformative alternative:it can withstand power densities up to～10¹⁷ W/cm² （four orders of magnitude higher than those of solids）,making it a promising medium for next-generation high-power laser amplification.Existing plasma-based amplification schemes,such as backward Raman amplification （BRA） and strong-coupling backward Brillouin amplification （sc-BBA）,rely on counter-propagating pump and seed pulses but suffer from harsh plasma conditioning requirements,limited amplification efficiency,and narrow wavelength tunability.Experimentally,BRA has not yet achieved output powers exceeding 0.1 terawatt （TW）.The present study aims to validate a novel plasma amplification scheme—forward Raman amplification（FRA）—which employs co-propagating laser pulses via the excitation of forward Raman scattering.The goal is to demonstrate efficient amplification of near-infrared （NIR） seed pulses within ultra-compact spatiotemporal scales,while evaluating its robustness against realistic experimental perturbations （e.g.,plasma inhomogeneity,gas ionization,and electron temperature fluctuations） and exploring its potential for generating PW-level,few-cycle NIR pulses.Methods The FRA scheme is based on the three-wave coupling mechanism in plasma,which requires phase-matching conditions:ω₀=ω₁+ω₂ and k₀=k₁+k₂,whereω₀/k₀,ω₁/k₁,andω₂/k₂ denote the frequency/wave number of the pump pulse,seed pulse,and electron plasma wave （EPW）,respectively.Under the low-temperature approximation,ω₂≈ω_pe （the plasma frequency）,defined as ■（where n_e is electron density,e is electron charge,?₀ is vacuum permittivity,and m_e is electron mass）.To validate the FRA scheme,both theoretical modeling and large-scale numerical simulations are conducted.The spatiotemporal evolution of the pump pulse,seed pulse,and EPW is described by a set of three-wave coupling equations.Analytical solutions are derived to obtain the linear gain coefficient and the nonlinear amplification scaling relation.Furthermore,we apply the open-source particle-in-cell （PIC）code EPOCH in one-dimensional （1D） and two-dimensional （2D） geometries to verify its feasibility.Results and Discussions 1D PIC simulations confirm the FRA scheme,in which over a plasma length of 200μm and a time scale of sub-picoseconds,the seed pulse intensity is amplified by～10⁵ orders of magnitude—from 1.0×10¹² W/cm² to 8.0×10¹⁶ W/cm²—with an energy transfer efficiency exceeding 25%（Fig.2）.After detaching from the pump pulse,the amplified seed pulse undergoes self-compression via self-phase modulation （SPM）,reducing its duration from 90 fs to 13 fs （near single-cycle）.No significant parasitic instabilities （e.g.,wave breaking or Landau damping） are observed during amplification.2D PIC simulations extend these results to large-scale spot sizes （200μm initial radius）.The seed pulse reaches a peak intensity of 5.0×10¹⁶ W/cm²,with a final spot radius of100μm （reduction attributed to inefficient amplification of low-intensity peripheral regions）（Fig.3）.Crucially,no transverse filamentation or modulation instabilities are detected,confirming the suitability of the scheme for high-power scaling.The 2D results also support the feasibility of a PW-level output:scaling to millimeter-scale spots （theoretical projection） will enable terawatt-topetawatt power levels.Additionally,multi-stage cascaded amplification is proposed:using a 1.0μm pump pulse to amplify a 1.8μm seed pulse,then reusing the 1.8μm pulse as a pump to amplify the 3.2μm seed pulse,extending wavelength tunability beyond the initialλ₀?2λ₀ range （whereλ₀ is pump wavelength）.Plasma density inhomogeneity will not compromise amplification in FRA.In the linear density gradient case,the seed pulse is still efficiently amplified near the resonance density,achieving a peak intensity of～5.0×10¹⁶ W/cm² （Fig.4）.Increasing temperature from 0 to 200 eV reduces the seed intensity slightly due to enhanced damping,but above 50 eV,amplification and compression performance stabilizes （Fig.5）.Co-propagating geometry suppresses the growth of temperature-induced instabilities,giving the scheme a wide temperature tolerance.Seed-pump time delays between-30 fs and 60 fs maintain effective amplification—within the range of standard laboratory timing control.Conclusions This study systematically validates the FRA scheme as a viable solution for next-generation high-power laser systems.Theoretical models and PIC simulations demonstrate that FRA can amplify NIR seed pulses by 4?5 orders of magnitude within hundreds ofμm （spatial） and sub-picoseconds （temporal）,while compressing pulses to near single-cycle durations （～13 fs）.Critically,FRA exhibits strong robustness to realistic experimental perturbations:plasma inhomogeneity,electron temperature variations （up to 200 eV）,and seed-pump timing delays do not significantly degrade its performance.Compared to solid-state amplifiers and existing plasma-based schemes,FRA offers three key advantages:1) ultra-compact spatiotemporal scales;2) broad wavelength tunability （extendable via cascading）;and 3) high efficiency and stability,supporting a PW-level output.These features make FRA a promising candidate for generating high-power,few-cycle NIR pulses,with potential applications in ultrafast optics（e.g.,high-harmonic generation,attosecond pulse production） and high-energy-density physics （e.g.,wakefield acceleration,photon deceleration）.更多还原