基于固体等离子体的阿秒高次谐波产生理论与实验进展

Theoretical and experimental progress of high-order harmonics and attosecond pulse generation based on solid overdense plasma

自从2001年首次产生并测量了阿秒(attosecond,1 as=10^(-18)s)脉冲之后,高次谐波和阿秒脉冲在原子分子物理、材料科学等领域得到了广泛的应用.但是,由于气体高次谐波方法产生的阿秒脉冲效率较低,阿秒脉冲能量受限,限制了阿秒时间动力学研究的探测方式(目前主要是IR(infrared)+XUV(extreme ultraviolet)泵浦/探测)及其在许多领域的应用.如何获得高亮度、大能量的阿秒脉冲一直是该领域的追求.高强度的相对论飞秒激光脉冲与固体密度等离子体相互作用,在高亮度、大能量高次谐波和阿秒脉冲产生上具有独特的优势,甚至可能获得远高于泵浦激光场强的谐波电场强度.本文对基于固体等离子体的阿秒高次谐波产生的物理机制和目前的实验研究进展作简要介绍.

Since the laser was invented,harmonic generation and wavelength conversion have been important research topics in nonlinear optics regime.In 1987,Mc Pherson A et al.used sub-picosecond Kr F laser(248 nm wavelength)to interact with noble gas to obtain high-order harmonics for the first time.In 2001,the first single attosecond(10–18 s)pulse with a pulse width of 650 as was measured experimentally by Krausz et al.by using the broadband high-order harmonic generation.Since then,high-order harmonics and attosecond pulses have been widely used in atomic and molecular physics,materials science and other fields.However,due to the low efficiency of attosecond pulses generated by the gas media,the energy of attosecond pulses is limited,which limits the detection methods of attosecond time dynamics research(currently mainly IR+XUV pumping/detection)and its applications in many fields.How to obtain high-brightness,high-energy attosecond pulses has always been the pursuit in this field since the generation of the first attosecond pulse.With the development of femtosecond laser technology,the output peak power of the femtosecond laser system has been rapidly increased in the past two decades.At present,the laser system can output the pulse with a peak power of up to 10 PW(1 PW=1015 W).Its intensity can be much higher than 1018 W/cm2 and its interaction with the overdense plasma can efficiently generate highorder harmonics at the level of hundreds of e V or even ke V,which brings new opportunities for the development and application of high-order harmonics and attosecond pulses.Generally speaking,the interaction of high peak power femtosecond laser pulses with solid-density plasmas has unique advantages in high-luminance,high-energy high-order harmonics and attosecond pulse generation.In the ELI-ALPS plan,solid plasma high-order harmonics and attosecond pulse generation are very important parts,especially to promote their high brightness and their high photon energy.In terms of the attosecond pulse energy,the goal of their first phase is that the attosecond pulse energy reaches theμJ level,which will be advanced to the m J level in the next phase,and the photon energy of the attosecond pulse will reach a few ke V.This review gives a brief introduction to its physical mechanisms and their current experimental research progress.Three mechanisms(coherent wake emission(CWE),relativistic oscillating mirror(ROM),coherent synchrotron emission(CSE))will be discussed.They are working under different laser intensity.CWE can emit high efficiency harmonics at the laser intensity below 10^(18 )W/cm^(2).But when the peak laser intensity is very high,the research of the CSE mechanism shows the generation of high-luminance high-order harmonics under super-relativistic light intensity,which can be used to produce extremely high field-strength pulse of attoseconds,even tens of zeptoseconds.If the laser parameters and the plasma condition can be well controlled,the laser pulse may even generate higher harmonic radiation with field strength much higher than the pump laser field strength,which may be a possible way to study the 39 th question("What is the most powerful laser researchers can build?")among the 125 most challenging scientific questions published by Science on its125 th anniversary.