Optical thin-film structures exhibiting high reflectivity and a nearly constant negative group-delay dispersion over frequency ranges as broad as 80 THz are presented. This attractive combination makes these coatings well suited for intracavity dispersion control in broadband femtosecond solid-state lasers. We address design issues and the principle of operation of these novel devices.The relevance of intracavity dispersion control in passively mode-locked ultrashort-pulse laser was recognized soon after the appearance of the first systems operating in the femtosecond domain.' Negative dispersion that is due to wavelengthdependent refraction in a pair of Brewster-angled prisms combined with positive material dispersion proved to be an efficient and convenient means of controlling the net group-delay dispersion (GDD) inside the laser cavity. 2 In solid-state lasers femtosecond pulse generation invariably relies on a net negative intracavity GDD because of an ultrafast self-phase modulation caused by the optical Kerr effect in the laser medium. Hence prism pairs have become standard components in these systems. The interplay between negative GDD and Kerr-induced self-phase modulation, often referred to as solitonlike shaping, appears to be the dominant pulse-forming mechanism that determines the steady-state pulse duration in femtosecond solidstate lasers. 3 In practical prism-pair-controlled broadband laser systems a major limitation to ultrashort pulse generation originates from the variation of the intracavity GDD with wavelength. The principal source of this high-order dispersion was found to be the prism pair. 3 ' 4 In this Letter we report the novel development of chirped multilayer mirror coatings that can exhibit essentially constant negative GDD over a frequency range as broad as 80 THz. Careful design permits higher-order contributions to the mirror phase dispersion to be kept at low values or to be chosen such that high-order phase errors introduced by other cavity components (e.g., the gain medium) are canceled. Replacing the prism pair with these novel devices offers the potential of generating pulses that are shorter than previously achievable directly from the laser. In addition, this simplifies the cavity design and may permit the construction of more compact and reliable femtosecond sources.The first thorough investigations of the frequencydependent phase retardation (phase dispersion) of multilayer dielectric coatings date back to the early 1960's.5, The emergence of femtosecond lasers in the 1980's has led to a revival of interest in this field. 7 -' 3 Whereas standard quarter-wave dielectric mirrors were shown to introduce negligible dispersion at the center of their reflectivity bands, 7 ' 8 various specific high-reflectivity coatings (Gires-Tournois interferometers, double-stack mirrors, etc.) with adjustable GDD (through angle tuning) were devised and used for the precise control of intracavity dispersion in femtosecond dye lasers.' 4 " 5 However, the GDD introduced by these mirr6r coatings ...

A feasibility study of controlling the carrier phase in ultrashort light-wave packets emitted by a sub-10-fs laser is reported. An experimental apparatus capable of exploring the phase sensitivity of nonlinear-optical interactions is presented.

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 ...

Laser-induced ablation has been extended down to a pulse duration of 20 fs generated by a Ti sapphire laser system at a wavelength of 780 nm. Barium aluminum borosilicate glass with an extremely high glass transformation temperature (∼600 °C) served as target material. The most significant observation was a substantial decrease of the ablation threshold fluence at pulse durations below 100 fs. All results indicate a dominant role of multiphoton absorption in addition to collisional ionization in this time domain.