We present few-femtosecond shadowgraphic snapshots taken during the non-linear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma wave period is induced by a lengthening of the first plasma period. Three dimensional particle in cell simulations support our observations.Laser-wakefield accelerators (LWFA) operating in the 'bubble'-regime [1] can generate quasimonoenergetic multigigaelectronvolt electron beams [2,3] with femtosecond duration [4,5] and micrometer dimensions [6,7]. These beams are produced by accelerating electrons in laser-driven plasma waves over centimeter distances. They have the potential to be compact alternatives to conventional accelerators [8]. In a LWFA, the short driving laser pulse displaces plasma electrons from the stationary background ions. The generated space charge fields cause the electrons to oscillate and form a plasma wave in the laser's wake. This wave follows the laser at almost c, the speed of light; for low amplitude it has a wavelength ofwhere n e is the electron density of the plasma. At high amplitude, electrons from the background can be injected into the wake and accelerated, producing monoenergetic electron pulses [9][10][11]. Significant progress has been made regarding achievable peak energy [3], beam stability [12] and the generation of bright X-ray pulses [13][14][15]. Until now, most of our knowledge about the dynamics of the self-injection process has been derived from detailed particle-in-cell (PIC) simulations. These simulations show that self-focusing [16] and pulse compression [17] play a vital role in increasing the laser pulse intensity prior to injection. Furthermore, simulations indicate that self-injection of electrons is associated with a dynamic lengthening of the first plasma wave's period (the 'bubble'). This lengthening can be driven by changes of the electric field structure inside the plasma wave caused by the injected electrons [18]. In contrast, the lengthening may also be due to an intensity amplification of the laser pulse caused by the non-linear evolution of the plasma wave [19,20] or due to a local increase in intensity caused by two colliding pulses [21]. In these latter scenarios, injection is a consequence of the lengthening of the bubble. However, experimental insight into these processes is extremely challenging due to the small spatial and temporal scales of a LWFA.The plasma wave, a variation in the electron density, has an associated refractive index profile which can be detected using longitudinal [22][23][24] or transverse probes [5]. Longitudinal probes cannot measure the rapid and dynamic evolution of the plasma wave that occurs in nonlinear wakefield accelerators and suffer from the strong refraction caused by the steep refractive ...

The velocity map of the above-threshold ionization electron spectrum at long laser wavelength exhibits a characteristic structure normal to the laser polarization, which has the appearance of a trident or a three-pronged fork. The forklike structure vanishes for few-cycle laser pulses. It is explained in terms of the classical-electrontrajectories model of strong-field ionization augmented so as to allow for rescattering. The analysis reveals its relation to the so-called low-energy structure, which was recently observed for very small transverse momenta. An atom exposed to an intense laser field provides one of the simplest physical realizations of a nonlinearly driven system. It has revealed various phenomena that have generated subfields of their own, such as the generation of high harmonics of the incident laser field, which in turn has brought us attosecond pulses [1]. In contrast to high-order harmonic generation, ionization of atoms can be investigated as a pure single-atom event; macroscopic effects have no significance. Especially, above-threshold ionization (ATI) has caught the interest of experimentalists and theorists alike, ever since its first observation 35 years ago [2,3]. ATI is characterized by the fact that the atom absorbs more photons than are necessary for ionization.In view of the simplicity of the system in question-in principle, as simple as hydrogen [4]-it is remarkable that novel features of ATI have continued to emerge. Recent examples include frustrated tunneling ionization (FTI) [5] and a carpetlike pattern in the ionization velocity map at about right angle to the laser polarization [6] and, for comparatively long laser wavelengths, the so-called low-energy structure (LES) [7] and spiderlike interference structures (SPIDER) that were interpreted as holograms [9][10][11]. The most recent such examples include a structure with an energy below the LES, the very-low-energy structure [12] and a strong enhancement at practically zero energy [13]. The latter two are assumed to be Coulomb-related effects, but there is no consensus as to their detailed origin. ATI is also the basis of various applications: For example, analysis of the velocity map at high energy allows one to extract the electron-ion scattering potential [14] while the details of the spider structure are sensitive to the atomic potential [11]. It also has been used to measure the carrier-envelope phase of few-cycle pulses [15,16].In this letter, strong-field ionization into states with low electron energy by a long-wavelength laser field is investigated experimentally and modeled theoretically. We compare the velocity map of the electron spectrum which is generated by a * max.moeller@uni-jena.de long laser pulse with the one generated by a few-cycle pulse. Besides retrieving the LES and the SPIDER, we observe a third-so far unadressed-fork-like structure which has a shape reminiscent of a trident or three-pronged fork (from here on, we will refer to it by the fork). The fork appears at close to right angle to the laser polar...

A precise, real-time, single-shot carrier–envelope phase (CEP) tagging technique for few-cycle pulses was developed and combined with cold-target recoil-ion momentum spectroscopy and velocity-map imaging to investigate and control CEP-dependent processes with attosecond resolution. The stability and precision of these new techniques have allowed for the study of intense, few-cycle, laser-matter dynamics with unprecedented detail. Moreover, the same stereo above-threshold ionization (ATI) measurement was expanded to multi-cycle pulses and allows for CEP locking and pulse-length determination. Here we review these techniques and their first applications to waveform characterization and control, non-sequential double ionization of argon, ATI of xenon and electron emission from SiO2 nanospheres.

High-order-harmonic-generation yield is remarkably sensitive to driving laser ellipticity, which is interesting from a fundamental point of view as well as for applications. The most well-known example is the generation of isolated attosecond pulses via polarization gating. We develop an intuitive semiclassical model that makes use of the recently measured initial transverse momentum of tunneling ionization. The model is able to predict the dependence of the high-order-harmonic yield on driving laser ellipticity and is in good agreement with experimental results and predictions from a numerically solved time-dependent Schrödinger equation.