Short, intense laser pulses can be used to access the transition regime between classical and quantum optical responses in dielectrics. In this regime, the relative roles of inter- and intraband light-driven electronic transitions remain uncertain. We applied attosecond transient absorption spectroscopy to investigate the interaction between polycrystalline diamond and a few-femtosecond infrared pulse with intensity below the critical intensity of optical breakdown. Ab initio time-dependent density functional theory calculations, in tandem with a two-band parabolic model, accounted for the experimental results in the framework of the dynamical Franz-Keldysh effect and identified infrared induction of intraband currents as the main physical mechanism responsible for the observations.
Transition metals with their densely confined and strongly coupled valence electrons are key constituents of many materials with unconventional properties 1 , such as high-Tc superconductors, Mott insulators and transition-metal dichalcogenides 2 . Strong electron interaction offers a fast and efficient lever to manipulate their properties with light, creating promising potential for next-generation electronics 3-6 . However, the underlying dynamics is a fast and intricate interplay of polarization and screening effects, which is poorly understood. It is hidden below the femtosecond timescale of electronic thermalization, which follows the light-induced excitation 7 . Here, we investigate the many-body electron dynamics in transition metals before thermalization sets in. We combine the sensitivity of intra-shell transitions to screening effects 8 with attosecond time resolution to uncover the interplay of photo-absorption and screening. First-principles time-dependent calculations allow us to assign our experimental observations to ultrafast electronic localization on d-orbitals. The latter modifies the whole electronic structure as well as the collective dynamic response of the system on a timescale much faster than the light-field cycle. Our results demonstrate a possibility for steering the electronic properties of solids prior to electron thermalization, suggesting that the ultimate speed of electronic phase transitions is limited only by the duration of the controlling laser pulse. Furthermore, external control of the local electronic density serves as a fine tool for testing state-of-the art models of electron-electron interactions. We anticipate our study to facilitate further investigations of electronic phase transitions, laser-metal interactions and photo-absorption in correlated electron systems on its natural timescale.A characteristic thermalization time of laser-excited hot electrons in solids is in the femtosecond regime and becomes faster with stronger electron interactions 7 . Therefore, attosecond time resolution is required to resolve coupled electron dynamics during the lasermatter interaction before electron thermalization has occurred. Attosecond transient absorption spectroscopy revealed electric-field-guided electron dynamics in simple dielectrics and semiconductors 9-14 . However, transient absorption studies of localized and strongly interacting electrons, such as on d-and f-orbitals of transition metals, have been limited to the fewfemtosecond regime 15 . Transition metal elements are the key constituents of many materials exhibiting remarkable properties, such as Mott insulators, high-Tc superconductors and transition metal dichalcogenides 2 . Understanding the coupled-electron dynamics in these systems is central for their applications in optoelectronics, energy-efficient electronics, magnetic-memory devices, spintronics and new solar cells 16 . Transition metal elements were studied with attosecond photoemission spectroscopy [17][18][19] . However, state-of-the-art theoretical
Magnetic phenomena are ubiquitous in our surroundings and indispensable for modern science and technology, but it is notoriously difficult to change the magnetic order of a material in a rapid way. However, if a thin nickel film is subjected to ultrashort laser pulses, it can lose its magnetic order almost completely within merely femtosecond times [1]. This phenomenon, in the meantime also observed in many other materials [2-7], has connected magnetism with femtosecond optics in an efficient, ultrafast and complex way, offering opportunities for rapid information processing [8-12] or ultrafast spintronics at frequencies approaching those of light [8,9,13]. Consequently, the physics of ultrafast demagnetization is central to modern material research [1-7,14-28], but a crucial question has remained elusive: If a material loses its magnetization within only femtoseconds, where is the missing angular momentum in such short time? Here we use ultrafast electron diffraction to reveal in nickel an almost instantaneous, long-lasting, non-equilibrium population of anisotropic highfrequency phonons that appear as quickly as the magnetic order is lost. The anisotropy plane is perpendicular to the direction of the initial magnetization and the atomic oscillation
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