Development and prospect on driving laser for attosecond pulse

Abstract

<p indent="0mm">Attosecond light source is a new type of light source that was born at the beginning of the 21st century, which has a short pulse, broad spectrum, high temporal and spatial coherence and wide tunability, thus being widely employed in various research fieds. From ultrafast motion of electrons in atoms to charge transfer in biological macromolecules, attosecond pulses is currently the only tool that can track and capture these ultrafast dynamics. Attosecond pulse enables us to investigate ultrafast dynamics of micro-world both in nanometer and attosecond scales. However, the mechanism of attosecond pulse generation is completely different from general ultrafast lasers. Instead, attosecond pulse is generated by a highly nonlinear interaction between strong ultrafast femtosecond laser and matter, which is called high-order harmonic generation (HHG). The mechanism of HHG can be understood by a classical three step model. First, an electron is ionized from an atom by a strong laser electric field through tunnel ionization. Then the free electron is accelerated by the laser field and gains energy. Finally, the electron recombines with the parent ion when the laser field changes its sign with emission of a photon, whose energy equals its kinetic energy gained in the laser field plus ionization potential of the atom. Apparently, HHG is strongly affected by the laser waveform. Several key parameters of driving laser, such as wavelength, intensity, and carrier envelope phase strongly affect the process of HHG. Thus, the characteristics of attosecond pulses are determined by the driving laser. The rapid development of attosecond pulse technology strongly depends on the development of driving laser technology. In the beginning, chirped pulse amplification (CPA) technology greatly promoted the development of attosecond light sources. The femtosecond CPA systems based on Ti:sapphire crystal has been the main driving laser to generate attosecond light pulses. The driving laser wavelength is in the near infrared region near <sc>800 nm,</sc> which generally has a pulse duration of multiple optical cycles. By employing post-compression technology to shorten pulse durations of CPA systems to few-cycle, isolated attosecond pulses in the extreme ultraviolet (XUV) can be generated, which is called the first generation of attosecond light source. Recently, optical parametric amplification (OPA) systems have been widely used as driving laser due to its flexible wavelength tunability. Using OPA, longer driving laser wavelength up to the midinfrared (MIR) can be obtained, which pushes the attosecond pulses to the soft X-ray region, and has been called the second generation of attosecond light sources. Due to broad spectrum and higher photon energy, attosecond pulses have a shorter duration in the soft X-ray region compared with XUV region, given that the temporal chirp is properly compensated. In 2017, reseachers generated soft X-ray isolated attosecond pulses, which was driven by mid-infrared pulses centered at <sc>1.8 µm.</sc> These attosecond pulses, whose duration reaches 53 as and spectrum surpasses carbon K-edge, provide a tool for studying the ultrafast dynamics of diamond, graphene and other carbon materials. In this paper, we start with the principle of attosecond pulses generation based on HHG. Then, we introduce different technologies and their development for obtaining driving laser pulses for HHG. Finally, we introduce prospect on development of driving laser pulses for attosecond pulses generation.