ARTICLES https://doi.org/10.1038/s41566-020-00730-6 1 State Key Laboratory of High Field Laser Physics and CAS Center for Excellence in Ultra-intense Laser Science, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, P. R. China. 2 Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, P. R. China. 3 Department of Physics, Shanghai Normal University, Shanghai, China. 4 ShanghaiTech University, Shanghai, P. R. China. 5 These authors contributed equally: Chuliang Zhou, Yafeng Bai. ✉ e-mail: michaeljs_liu@siom.ac.cn; tianye@siom.ac.cn; ruxinli@mail.siom.ac.cn T he interaction of electrons and photons 1–3 is fundamental to microcosmic physics. The revelation of ultrafast electron dynamics driven by a light field has led to great progress in ultrafast electron diffraction and microscopy 4–6 , ultrafast streak cameras 7,8 and free-electron lasers 9,10 . These underlying dynamics are hidden below the femtosecond timescale, so the exploration and tracking of the charge dynamics involved in these applica- tions demands an ever greater temporal resolution to fully exploit their potential. Attosecond spectroscopy makes use of the oscillat- ing field as a probe to resolve dynamics, providing access to some transient dynamics between visible light and matter. Although atto- second spectroscopy or high-harmonic spectroscopy can provide insight into the collective electron motion in atoms or plasma with attosecond precision 3,11,12 , direct access to the optical field charac- terization of a free-electron pulse remains challenging because of difficulties in achieving phase matching between the optical field and the electron. It is also far from easy to generate electron pulses with less than femtosecond duration, which is critical for ultrafast electron microscopy and diffraction 4,13 . At present, pulsed electron guns based on planar photocathodes can produce electron bunches with a pulse duration of a few hundred femtoseconds, while bunch recompression-based systems can reach the 10–100-fs time domain 14–16 . However, both methods are severely hampered by the temporal duration of the driving laser and dispersive broadening within the bunch. Recent advances in compression techniques have promised the generation of isolated 30-fs electron pulses and attosecond pulse trains in ultrafast electron microscopy 17–19 . There are also rich attosecond dynamics in the laser–plasma interaction, which holds great promise as a candidate for attosecond electron generation. Both a comprehensive understanding of the ultrafast dynamics of attosecond electron pulses and their full exploitation require a time resolution that is as high as possible. During recent decades, streaking of photoelectrons with the optical field of an ultrafast laser has been used routinely to characterize the complete informa- tion of attosecond pulses. The energy and angular distributions of photoelectrons are dependent on the phase of the laser field at the instant of release 3,20,21 . This concept of an attosecond streak camera has been applied to the measurement and even the manipulation of free-electron pulses. Recent research on THz streaking of keV and MeV electrons 14,22,23 using the idea of field-induced time-dependent electron deflection has demonstrated exquisite ultrafast measure- ments of electron beams with sub-picosecond resolution. In this Article, direct probing of the attosecond dynamics of free-electron pulse trains is achieved with comparable resolution by a new embodiment of the basic concept of streak imaging, where the streaking, in this case, is controlled by a sub-relativistic infrared laser field (1 × 10 17 W cm −2 ) (Fig. 1a). We numerically investigate the generation of attosecond free-electron pulse trains and their subsequent different interaction mechanism with the laser field. The time structure of the attosecond electron pulses is character- ized using the laser streaking concept. As a proof of concept, the laser field-induced electron beam deflection allows us to probe this subcycle motion of charge with a streaking speed of ~60 μrad as −1 . Concept In general, when a femtosecond laser pulse of about relativistic intensity irradiates a flat solid target, the plasma ionized from the solid surface behaves as a high-flatness mirror—a plasma mirror— and a surface plasma wave can be excited. As shown in Fig. 1c, at each optical cycle, electrons are accelerated in the first half laser cycle at the plasma surface. Subsequently, when the electric field is reversed, some energetic electrons could escape the plasma wave when the laser electric field crosses zero, giving rise to a train of attosecond bunches as shown in Fig. 1b. It should be noted that such a narrow emission window exactly corresponds to a zero-crossing point of the reflected electric field where the field slope is the steepest in each laser cycle. In contrast to the general laser– plasma-based approaches, which are subject to broad emittance and a relatively noisy background, the strong correlation between the injection time of attosecond electron pulses and the phase of the laser electric field offers a glimpse of the fundamental physi- cal process from the surface in the time domain. These appealing Direct mapping of attosecond electron dynamics Chuliang Zhou  1,2,5 , Yafeng Bai  1,5 , Liwei Song 1 , Yushan Zeng 1,2 , Yi Xu 1 , Dongdong Zhang  1,2 , Xiaoming Lu 1 , Yuxin Leng 1,2 , Jiansheng Liu  3 ✉ , Ye Tian  1,2 ✉ , Ruxin Li  1,2,4 ✉ and Zhizhan Xu 1,2 The subcycle interaction of light and electrons has been one of the key frontiers in free-electron lasers, attosecond science and dynamical investigation of matter. Capturing the underlying subcycle dynamics of electrons with an optical field promises fas- cinating vistas with unprecedented temporal resolution. Yet the rigorous synchronization requirement has kept its realization out of reach. Here, by direct spatial observation of periodic electron bunch fringes, we demonstrate a laser streaking concept for revealing the dynamics of free electrons emitted from a plasma mirror under sub-relativistic laser intensity. Field-induced elec- tron beam deflection demonstrates subcycle charge dynamics with a streaking speed of ~60 μrad as −1 . This provides us with an attosecond-resolution metrology to obtain more direct evidence about the light-field-induced electron dynamics in the plasma mirror. Our results offer unprecedented characterization of attosecond dynamics and open the way to extensive experimental investigations of the interaction of attosecond electrons with intense lasers. NATURE PHOTONICS | www.nature.com/naturephotonics