中国激光, 2021, 48(5): 0501001, 网络出版: 2021-05-01

阿秒光学进展及发展趋势

Progresses and Trends in Attosecond Optics
作者单位

1中国科学院物理研究所北京凝聚态物理国家研究中心, 北京 100190

2松山湖材料实验室, 广东 东莞 523808

3中国科学院大学, 北京 100049

4北京交通大学理学院微纳材料及应用研究所, 北京 100044

5西安电子科技大学物理与光电工程学院, 陕西 西安 710071

摘要
自从2001年人们首次实现单个独立的阿秒(1 as=10 -18 s)脉冲以来,阿秒脉冲作为超快光学最前沿的内容,在近20年的时间内得到了长足的发展,为人们在电子运动的自然时间尺度中观测量子世界的基本动力学过程提供了崭新的研究手段,并开启了阿秒科学这一全新的研究领域,覆盖了原子、分子、凝聚态物理、化学、生物等诸多学科的不同研究需求。随着飞秒激光驱动器技术的不断发展,目前阿秒脉冲不仅脉冲宽度突破了50 as,而且也进一步朝着更高单脉冲能量(高通量)、更短波长(高光子能量)、更高重复频率的方向发展。本文将结合高次谐波相位匹配及高能量飞秒超强激光、双色及多色相干合成飞秒激光、中红外飞秒激光、高重复频率飞秒激光等驱动技术,综述介绍阿秒脉冲在上述各方面的新进展,并展望了未来进一步的发展趋势。
Abstract

Significance Attosecond (1 as=10 -18 s) light pulses provide new approach to the basic mechanics in the quantum world in its natural time scale. A novel research area called attosecond science was opened up since the first observation of attosecond pulses in 2001. Owing to the advances of ultrafast laser techniques and the in-depth understanding of the attosecond pulse generation mechanism, a world record of 43 as light pulse has been demonstrated in 2017, which is shortest pulse ever obtained by human beings. Nowadays table-top attosecond sources based on high harmonic generation (HHG) have been routinely achieved by many groups worldwide. It is widely applied in the measurements of various ultrafast phenomena like photoionization time delay in atoms, molecules, and solids, electron correlation effects such as Fano resonance, Auger decay, and inner shell ionization, charge migration and dissociation in molecules, and manipulation of dielectrics. Attosecond pulses has achieved impressive progress in different fields such as atomic and molecular physics, condensed matter physics, chemistry, and biology in the past two decades.

Progress The limited photon flux of the attosecond pulses due to low conversion efficiency and phase mismatch of HHG process prevents the potential applications in multi-photon ionization, single shot coherent diffraction imaging, and attosecond pump-probe. HHG driven by TW or even PW high power laser is the straightforward way to generate intense attosecond pulses. Loose focusing geometry is proposed to overcome the over-ionized plasma that will destroy the phase matching process. Attosecond pulse with μJ pulse energy and 10 14W/cm 2power density is obtained using loose focusing geometry and adaptive optics. It serves as an alternative to free electron laser with shorter pulse duration and better stability to investigate ultrafast nonlinear phenomena.

Various gating technique is utilized to isolate singleattosecond burst from an attosecond pulse train. Few-cycle driving laser with stabilized carrier envelope phase (CEP) is typically required for isolated attosecond pulse (IAP) generation. Such driving laser with high pulse energy is still challenging even nowadays. The coherent synthesizer consisting of two-color or multi-color laser fields might produce “perfect” waveform to optimize the HHG conversion efficiency as well as relaxing the pulse duration limitation required for IAP gating. Sub-cycle light transients from waveform synthesizer which is ideal for IAP generation has also been demonstrated.

The so-called “water window” wavelength ranging from 2.3 nm to 4.4 nm between the K-edge of carbon and oxygen elements is very important in chemistry and biology. HHG in water window wavelength or even higher photon energy can be obtained using long wavelength driving laser combined with high gas density waveguide and transient phase matching to compensate the unfavorable scaling of HHG efficiency with driving laser wavelength. The world record of light pulse (43 as) is reported using mid-inferred driven HHG in 2017.

High repetition rate attosecond pulses are required to fulfill coincidence counting or to avoid space charge effect in precise photoelectron spectroscopy. According to the HHG scaling principle, tight focusing, and high pressure are needed to generate high harmonics using low pulse energy laser. The high repetition rate, high average power driving laser, and frequency up conversion technique make it an ideal source for high flux HHG.

HHG from solid phase material follows different mechanism with that from gas phase. The intraband HHG is due to the nonlinear radiation of the Bloch oscillation in the conduction band while the interband HHG is resulted from the transition between electron-hole pairs in different bands. It is not only a potential method to generate high efficiency harmonics, but also an important approach to the band structure and electron interaction of the material.

Conclusions and Prospects The frontier of attosecond science has been paved by the advances in the laser technique. 10 μJ attosecond pulse is obtained by loose focusing geometry of the intense driving laser and phase matching optimization. The mid-inferred driving laser enables the high photon energy HHG up to 1.6 keV and sub-50-as short attosecond pulse. The high repetition rate laser source allows >100 kHz attosecond pulse with photon flux as high as 1015 s-1 which is ideal for coincidence measurements. Last but not least, the recent progress of HHG in solid state material provides new approaches to both attosecond pulse generation and all optical measurement of laser-matter interaction. All these novel attosecond sources towards the true attosecond-pump-attosecond-probe measurements will give new insight into the microscopic mechanics in their natural time scale.

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