Piezoelectric and ferroelectric properties in the two-dimensional (2D) limit are highly desired for nanoelectronic, electromechanical, and optoelectronic applications. Here we report the first experimental evidence of out-of-plane piezoelectricity and ferroelectricity in van der Waals layered α-InSe nanoflakes. The noncentrosymmetric R3m symmetry of the α-InSe samples is confirmed by scanning transmission electron microscopy, second-harmonic generation, and Raman spectroscopy measurements. Domains with opposite polarizations are visualized by piezo-response force microscopy. Single-point poling experiments suggest that the polarization is potentially switchable for α-InSe nanoflakes with thicknesses down to ∼10 nm. The piezotronic effect is demonstrated in two-terminal devices, where the Schottky barrier can be modulated by the strain-induced piezopotential. Our work on polar α-InSe, one of the model 2D piezoelectrics and ferroelectrics with simple crystal structures, shows its great potential in electronic and photonic applications.

We report direct measurement of hot-electron temperatures and relaxation dynamics for peak electron temperatures between 3400 and 11000 K utilizing two-pulse-correlation femtosecond (fs) thermionic emission. The fast relaxation times ( &1.5 ps) are described by extending RT characterizations of the thermal conductivity, electron-phonon coupling, and electronic specific heat to these high electron ternperatures.Hot-electron relaxation dynamics In the femtosecond laser intensity regime utilized here, the e1ectmn emission is dominated by the thermionic process which is strongly space charge suppressed.

T abletop plasma accelerators can now produce GeV-range electron beams 1-5 and femtosecond X-ray pulses 6 , providing compact radiation sources for medicine, nuclear engineering, materials science and high-energy physics 7 . In these accelerators, electrons surf on electric fields exceeding 100 GeV m −1 , which is more than 1,000 times stronger than achievable in conventional accelerators. These fields are generated within plasma structures (such as Langmuir waves 8 or electron density 'bubbles' 9 ) propagating near light speed behind laser 2-4 or charged-particle 5 driving pulses. Here, we demonstrate single-shot visualization of laser-wakefield accelerator structures for the first time. Our 'snapshots' capture the evolution of multiple wake periods, detect structure variations as laser-plasma parameters change, and resolve wavefront curvature; features never previously observed.These previously invisible features underlie wave breaking, electron injection and focusing within the wake, the key determinants of charge, energy, energy spread and collimation of the accelerated beam. Because of their microscopic size and luminal velocity, these critical structures previously eluded direct singleshot observation, inhibiting progress in producing high-quality beams and in correlating beam properties with wake structure. Here, in contrast, we reconstruct wake morphology in real-time, enabling rapid feedback and optimization.Recent advances in laser-wakefield accelerators dramatically illustrated the link between beam quality and plasma structure 1-4 . Earlier laser-wakefield accelerators produced electron beams with large divergence and energy spread, but by introducing a plasma channel guide 1,2 or by carefully adjusting the laser-plasma conditions to produce an electron density cavity behind the driving pulse 3,4 , collimated, nearly mono-energetic beams from 80 MeV to 1 GeV were demonstrated. Nevertheless, the plasma structures themselves remained invisible. Previous direct measurements of laser wakes with spatial resolution better than a plasma wavelength 10-13 (l p ) used frequency-domain interferometry 14 , in which a focused femtosecond probe pulse measured local electron density n e (ζ) at only a single time delay ζ behind the driving pulse within the co-propagating wake for each laser shot. Wake structure was then accumulated painstakingly by probing a different ζ on each subsequent shot. However, multi-shot techniques average over

Electron self-injection into an evolving plasma bubble: Quasi-monoenergetic laser-plasma acceleration in the blowout regime" (2011 An electron density bubble driven in a rarefied uniform plasma by a slowly evolving laser pulse goes through periods of adiabatically slow expansions and contractions. Bubble expansion causes robust self-injection of initially quiescent plasma electrons, whereas stabilization and contraction terminate self-injection thus limiting injected charge; concomitant phase space rotation reduces the bunch energy spread. In regimes relevant to experiments with hundred terawatt-to petawatt-class lasers, bubble dynamics and, hence, the self-injection process are governed primarily by the driver evolution. Collective transverse fields of the trapped electron bunch reduce the accelerating gradient and slow down phase space rotation. Bubble expansion followed by stabilization and contraction suppresses the low-energy background and creates a collimated quasi-monoenergetic electron bunch long before dephasing. Nonlinear evolution of the laser pulse (spot size oscillations, self-compression, and front steepening) can also cause continuous self-injection, resulting in a large dark current, degrading the electron beam quality.