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Surface plasmonics with its unique confinement of light1,2is expected to be a cornerstone for future compact radiation sources and integrated photonics devices. The energy transfer between light and matter is a defining aspect that underlies recent studies on optical surface-wave-mediated spontaneous emissions3–5. However, coherent stimulated emission of free electrons, which is essential for free-electron light sources, and its dynamical amplification process remain to be disclosed in a clear, unambiguous and calibrated manner. Here we present the coherent amplification of terahertz surface plasmon polaritons via free-electron-stimulated emission: a femtosecond optical pulse creates an in-phase free-electron pulse with an initial terahertz surface wave, and their ensuing interactions intensify the terahertz surface wave coherently. The underlying dynamics of the amplification, including a twofold redshift in the radiation frequency over a one-millimetre interaction length, are resolved as electromagnetic-field-profile evolutions using an optical pump–probe method. By extending the approach to a properly phase-matched electron bunch, our theoretical analysis predicts a super-radiant surface-wave growth, which lays the ground for a stimulated surface-wave light source and may facilitate capable means for matter manipulation, especially in the terahertz band.

The effect of modifying microstructure and texture by electric current on mechanical properties of electrically assisted stationary shoulder friction stir welded Ti6Al4V alloy were investigated. The width of the ‘bowl-shaped’ stir zone gradually increased with the increasing current value and texture of the joints changed abruptly with an increasing current from 0 to 300 A. The macrozones consisting of similar orientated grains in the heat-affected zone have the highest deformation degree and the lowest hardness, while macrozones were not seen in the stir zone of both the friction-stir welded joints and the electrically assisted joints. Why the tensile strength and elongation reached the highest at a current of 300 A was explained in terms of microstructure and texture.

The effect of modifying microstructure and texture by electric current on mechanical properties of electrically assisted stationary shoulder friction stir welded Ti6Al4V alloy were investigated. The width of the ‘bowl-shaped’ stir zone gradually increased with the increasing current value and texture of the joints changed abruptly with an increasing current from 0 to 300?A. The macrozones consisting of similar orientated grains in the heat-affected zone have the highest deformation degree and the lowest hardness, while macrozones were not seen in the stir zone of both the friction-stir welded joints and the electrically assisted joints. Why the tensile strength and elongation reached the highest at a current of 300?A was explained in terms of microstructure and texture.

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 fascinating 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 electron 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.

The optical vortex has been widely studied owing to its specific characteristics such as the orbital angular momentum, hollow intensity distribution, and topological charge. We report the generation of electromagnetic solitons with angular momentum and the conversion of angular momentum via a circularly polarized (CP) laser and underdense plasma interactions on the basis of three-dimensional particle-in-cell simulations. We find that when a CP laser is incident into the underdense plasma, a longitudinal current will be induced off the laser axis, which is critical for the angular momentum conversion. This novel, to the best of our knowledge, regime will allow potential applications such as optical control and electron manipulation.

Microstructure, texture and mechanical properties of electrically assisted friction stir welding (EFSW) of Ti6Al4 V alloy were investigated. The base material (BM) and the heat-affected zone (HAZ) have a coarse equiaxed structure, and the fine equiaxed structure was observed in the stir zone (SZ). The boundary between the SZ and the HAZ changed abruptly from the FSW joint to the EFSW joint to make it smooth. The BM and the HAZ have a strong prismatic texture {100} and the texture of the SZ is randomly distributed. Hardness of the joints was explained by grain size, loss rate of elemental Al and dislocation density, as well as the advantages of the EFSW process in terms of improving mechanical properties as influenced by microstructure and texture.

Microstructure, texture and mechanical properties of electrically assisted friction stir welding (EFSW) of Ti6Al4?V alloy were investigated. The base material (BM) and the heat-affected zone (HAZ) have a coarse equiaxed structure, and the fine equiaxed structure was observed in the stir zone (SZ). The boundary between the SZ and the HAZ changed abruptly from the FSW joint to the EFSW joint to make it smooth. The BM and the HAZ have a strong prismatic texture {100} and the texture of the SZ is randomly distributed. Hardness of the joints was explained by grain size, loss rate of elemental Al and dislocation density, as well as the advantages of the EFSW process in terms of improving mechanical properties as influenced by microstructure and texture.

The electrically assisted friction stir welding (EFSW) of 5 mm thick AZ31B magnesium alloy with DC current ranged 0–200 A has been investigated. The effect of current on microstructure and mechanical properties of the EFSW joint was studied. EFSW increased the heat input by resistance heat, and minimized the weakly welded area at the root of the joints. With the increase of current, the stir zone (SZ) become gradually larger, and there is no obvious boundary line between the shoulder affected zone (SAZ) and the pin affected zone (PAZ), microstructure along the thickness direction of the SZ centre is much more uniform; the difference in grain size between the SZ and the thermo-mechanically affected zone (TMAZ) at the advancing side (AS) changed from abrupt to smooth. All the joints fractured in regions with the lowest hardness. The tensile strength of the joint increased with increasing current, the tensile strength of the EFSW joint with a current of 200 A is the highest, but the hardness of the weld is hardly affected by the current.

The capability of synchrotron radiation to produce ultrabright emission has attracted considerable interest over the last half a century. To date, magnetic undulators with a period of several centimetres are commonly used for wiggling relativistic electrons in a modulated field. Here, we propose a novel compact undulator with a period down to the submillimetre level based on a spontaneous electric field that is driven by a femtosecond laser. Both the guided energetic electrons and the gyrotron-like undulator are spontaneously produced by irradiating a thin metallic wire with an intense laser pulse. An intense radial electric field instantaneously created on the wire can guide the electrons' helical motion along the wire and induce periodic THz emission. We have demonstrated that this scheme can produce intense THz sources with a conversion efficiency of 1% that are frequency-tunable by adjusting the diameter of the wire. Amplified emission of THz radiation by more than tenfold has been observed.

We report on the experimental observation of the airflow motion induced by an 800 nm, 1 kHz femtosecond filament in a cloud chamber filled with air and helium. It is found that vortex pairs with opposite rotation directions always form both below and above the filaments. We do not observe that the vortices clearly formed above the filament in air just because of the formation of smaller particles with weaker Mie scattering. Simulations of the airflow motion in helium are conducted by using the laser filament as a heat source, and the simulated pattern of vortices and airflow velocity agree well with the experimental results.