Attosecond light source in material science investigation

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Since the dawn of the 21st century, attosecond (1 as =10–18 s) science has been explored and is now fast developing. It brought us the unprecedented time-resolved spectroscopy of quasiparticle dynamics in materials. To date, femtosecond (1 fs =10–15 s) time-resolved pump-probe technology is widely used to study the ultrafast quasiparticle dynamics of quantum materials, such as strong correlated quantum materials, topological materials, superconductors, and semiconductors. In a typical pump-probe experiment, the laser source will be divided into two beams, namely the pump and the probe beams. The pump beam excites the quasiparticles in target materials, and the time-delayed probe beam scans the ultrafast dynamics of the excited state carriers and the coherent lattice vibrations. By employing this method, we are able to observe the lifetimes due to various interactions, such as the electron-electron scattering, electron-phonon coupling, phonon-phonon scattering, etc. By manipulating the various laser pulse characteristic parameters, such as the photon energy, polarization, pulse duration and laser fluence, femtosecond ultrafast spectroscopy excels in investigating the physical properties of materials, which are largely determined by the outer electrons. To investigate the excited states of various materials, femtosecond pump-probe spectroscopy is often conducted under extreme conditions, such as low temperature, strong magnetic field, and high pressure. As a comparison, attosecond ultrafast spectroscopy can not only get access to exploring even faster ultrafast processes, but also probe the inner shell electrons of various types of materials. Attosecond pulses are usually prepared by high-order harmonic generation (HHG). The most common way of generating HHG is in an atomic gas. The excited unbound electrons are accelerated to the nuclei, and the electron-core collisions lead to multiple ionizations. The photons are emitted as trains of ultra-shot pulses. Recently, HHG has also been realized in liquids and solids. The separation of isolated attosecond pulses from trains of attosecond harmonics can be achieved by manipulating the polarization gating, the ultrashort few-cycle driving lasers, or both. Both attosecond pulse trains and isolated attosecond pulses can be applied to the investigation of quantum materials. With the aid of attosecond time-resolved ultrafast pump-probe spectroscopy, more and more fundamental physical properties and relevant applications will be revealed, which will greatly lead the development of condensed matter physics, material sciences, and their applications. Conversely, the attosecond science and technology can also benefit from the deeper understanding of the physical properties of solid state materials. In this article, we first introduce two types of attosecond time-resolved pump-probe experimental techniques, attosecond transient absorption spectrum (ATAS) and attosecond time-resolved angle-resolved photoemission spectroscopy (atto-ARPES). We review typical experiments carried out using such facilities in dielectrics, semiconductors, layered materials, transition metals, etc., and make an outlook for the potential applications of attosecond laser spectroscopy in this field of material science. Furthermore, as the attosecond pulses are often generated from the HHG of mid-infrared laser beams, the mid-infrared ultrafast spectroscopy itself can also be applied to material investigations. Moreover, we review the applications of mid-infrared ultrafast spectroscopy in condensed matter physics, with intriguing laser-induced novel phenomena.