Abstract
Isolated attosecond pulses enables the studying and controlling of ultrafast electron processes in atoms and molecules. High-order harmonic generation (HHG) is the most promising way to generate such pulses, benefiting from the broad plateau structure of the typical HHG spectrum. We theoretically investigate high-order harmonic and attosecond pulse generation from helium atom in a three-color laser field, which is synthesized by 16 fs/1600 nm, 15 fs/1100 nm and 5 fs/800 nm pulse laser. Compared with harmonic spectrum generated by a two-color laser field synthesized by 16 fs/1600 nm and 15 fs/1100 nm, the harmonic spectrum generated from the synthesized three-color field exhibits high conversion efficiency and broadband supercontinuum characteristics. The continuous spectrum range covers from 230th to 690th harmonics, and the generation of 128 attosecond isolated short pulses with higher intensity is realized after Fourier transform. This result is attributable to the fact that the synthesized three-color electric field exhibits high-intensity and few-cycle mid-infrared femtosecond pulse laser characteristics, which can effectively control atomic ionization and recombination occurring within an effective optical period of the mid-infrared femtosecond pulse. This scheme solves the problems faced by the current femtosecond pulse laser technology, i.e. the few-cycle mid-infrared femtosecond pulse laser cannot have both carrier envelope phase stability and high power output.
Highlights
在本文中,我们提出利用三束不同波长的飞秒脉冲激光组合实现少周期且具 有稳定载波包络相位的高强度中红外飞秒脉冲激光光源,进而获得较高强度的孤 立阿秒脉冲相干辐射。这里,三束激光脉冲由 16 fs/1600 nm,15 fs/1100 nm 和 5.3 fs/800 nm 的钛宝石脉冲构成。双色场或三色场方案因可以扩展高次谐 波谱的超连续谱带宽较早就被提出用以获得理想的孤立阿秒脉冲产生。Zeng 等 人通过在基频脉冲上附加二次谐波脉冲获得了孤立的 65 as 短脉冲[20]。Lan 等
型成功解释了原子高次谐波谱的截止规则:Ecutoff = Ip+3.17Up,这里 Ip 和 Up =
图 2 中红色实线给出了三色场辐照氦原子得到的高次谐波发射谱。从图中可 以看出,谐波频谱的截止频率达到了 690 次,且从 230 次至截止位置谐波都呈现 规则分布且具有超连续特点。作为比较,图 2 中黑色实线给出了双色场辐照氦原 子得到的高次谐波发射谱。这里的双色场由峰值振幅为 0.1 a.u.的 16 fs/1600 nm 和峰值振幅为 0.06 a.u.的 15 fs/1100 nm 的两束中红外飞秒脉冲合成,与三 色场中的两束中红外飞秒脉冲激光参数一致。从图中可以明显看出,双色场的谐 波频谱的截止频率虽达到了 580 次,但具有规则结构的平台宽度明显变窄,特别 需要注意的是整个谐波平台效率低了将近 2 个数量级。可见,第三束钛宝石脉冲 激光的加入,不仅拓宽了平台区超连续谱的带宽,且大幅提高了谐波谱平台的效 率。这里需要指出的是,我们最初的设想是利用两束合适的中红外飞秒脉冲激光 组合达到获得少周期且功率较高的中红外飞秒脉冲激光的目的,基于这个原因选 择了上述双色场中的峰值振幅为 0.1 a.u.的 16 fs/1600 nm 和峰值振幅为 0.06 a.u.的 15 fs/1100 nm 的两束中红外飞秒脉冲。然而利用该双色场得到的谐波谱 效率偏低,基于此附加了第三束脉冲激光。下面将详细说明这样选取的双色场及 三色场在电场强度分布上的区别及对高次谐波发射谱的影响。
Summary
在本文中,我们提出利用三束不同波长的飞秒脉冲激光组合实现少周期且具 有稳定载波包络相位的高强度中红外飞秒脉冲激光光源,进而获得较高强度的孤 立阿秒脉冲相干辐射。这里,三束激光脉冲由 16 fs/1600 nm ,15 fs/1100 nm 和 5.3 fs/800 nm 的钛宝石脉冲构成。双色场或三色场方案因可以扩展高次谐 波谱的超连续谱带宽较早就被提出用以获得理想的孤立阿秒脉冲产生。Zeng 等 人通过在基频脉冲上附加二次谐波脉冲获得了孤立的 65 as 短脉冲[20]。Lan 等. 领域具有广泛应用。到目前为止,高次谐波光谱因具有独特的超连续平台结构是 实验室获得阿秒脉冲的唯一有效途径[1,2,3]。高次谐波是飞秒脉冲强激光辐照原子 或分子气体时产生的入射激光频率整数倍的相干辐射,其发射过程可由 Corkum 提出的半经典三步模型给出很好解释[4,5]:电子首先在入射驱动脉冲激光辐照下 型成功解释了原子高次谐波谱的截止规则:Ecutoff = Ip+3.17Up,这里 Ip 和 Up =
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