We present the first measurement of the vectorial response of strongly dressed helium atoms probed by an attosecond pulse train (APT) polarised either parallel or perpendicular to the dressing field polarisation. The transient absorption is probed as a function of delay between the APT and the linearly polarised 800 nm field of peak intensity 1.3 x 10(14) W cm(-2). The APT spans the photon energy range 16-42 eV, covering the first ionisation energy of helium (24.59 eV). With parallel polarised dressing and probing fields, we observe modulations with periods of one half and one quarter of the dressing field period. When the polarisation of the dressing field is altered from parallel to perpendicular with respect to the APT polarisation we observe a large suppression in the modulation depth of the above ionisation threshold absorption. In addition to this we present the intensity dependence of the harmonic modulation depth as a function of delay between the dressing and probe fields, with dressing field peak intensities ranging from 2 x 10(12) to 2 x 10(14) W cm(-2). We compare our experimental results with a full-dimensional solution of the single-atom time-dependent (TD) Schrodinger equation obtained using the recently developed abinitio TDB-spline ADC method and find good qualitative agreement for the above threshold harmonics.

Isolated attosecond pulses (IAP) generated by high-order harmonic generation are valuable tools that enable dynamics to be studied on the attosecond time scale. The applicability of these IAP would be widened drastically by increasing their energy. Here we analyze the potential of using multi-colour driving pulses for temporally gating the attosecond pulse generation process. We devise how this approach can enable the generation of IAP with the available high-energy kHz-repetition-rate Ytterbium-based laser amplifiers (delivering 180-fs, 1030-nm pulses). We show theoretically that this requires a three-colour field composed of the fundamental and its second harmonic as well as a lower-frequency auxiliary component. We present pulse characterization measurements of such auxiliary pulses generated directly by white-light seeded OPA with the required significantly shorter pulse duration than that of the fundamental. This, combined with our recent experimental results on three-colour waveform synthesis, proves that the theoretically considered multi-colour drivers for IAP generation can be realized with existing high-power laser technology. The high-energy driver pulses, combined with the strongly enhanced single-atom-level conversion efficiency we observe in our calculations, thus make multi-colour drivers prime candidates for the development of unprecedented high-energy IAP sources in the near future.

Quasifree field-driven electron trajectories are a key element of strong-field dynamics.Upon recollision with the parent ion, the energy transferred from the field to the electron may be released as attosecondduration extreme ultaviolet emission in the process of high-harmonic generation.The conventional sinusoidal driver fields set limitations on the maximum value of this energy transfer and the efficient return of the launched electron trajectories.It has been predicted that these limits can be significantly exceeded by an appropriately ramped-up cycle shape [L.E. Chipperfield et al., Phys.Rev. Lett.102, 063003 (2009)].Here, we present an experimental realization of similar cycle-shaped waveforms and demonstrate control of the high-harmonic generation process on the single-atom quantum level via attosecond steering of the electron trajectories.With our improved optical cycles, we boost the field ionization launching the electron trajectories, increase the subsequent field-to-electron energy transfer, and reduce the trajectory duration.We demonstrate, in realistic experimental conditions, 2 orders of magnitude enhancement of the generated extreme ultraviolet flux together with an increased spectral extension.This application, which is only one example of what can be achieved with cycle-shaped high-field light waves, has significant implications for attosecond spectroscopy and molecular self-probing.

We present transient absorption spectra of an extreme ultraviolet attosecond pulse train in helium dressed by an 800 nm laser field with intensity ranging from $2\times10^{12}$ W/cm$^2$ to $2\times10^{14}$ W/cm$^2$. The energy range probed spans 16-42 eV, straddling the first ionisation energy of helium (24.59 eV). By changing the relative polarisation of the dressing field with respect to the attosecond pulse train polarisation we observe a large change in the modulation of the absorption reflecting the vectorial response to the dressing field. With parallel polarized dressing and probing fields, we observe significant modulations with periods of one half and one quarter of the dressing field period. With perpendicularly polarized dressing and probing fields, the modulations of the harmonics above the ionisation threshold are significantly suppressed. A full-dimensionality solution of the single-atom time-dependent Schr\"odinger equation obtained using the recently developed ab-initio time-dependent B-spline ADC method reproduce some of our observations.

We investigate high-order-harmonic spectra from aligned diatomic molecules in intense driving fields whose components have orthogonal polarizations. We focus on how the driving-field ellipticity influences structural interference patterns in a macroscopic medium. In a previous publication [Phys. Rev. A 88, 023404 (2013)] we have shown that the nonvanishing ellipticity introduces an effective dynamic shift in the angle for which the two-center interference maxima and minima occur, with regard to the existing condition for linearly polarized fields. In this work we show through simulation that it is still possible to observe this shift in harmonic spectra that have undergone macroscopic propagation, and discuss the parameter range for doing so. These features are investigated for H-2 in a bichromatic field composed of two orthogonally polarized waves. The shift is visible both in the near- and in the far-field regime, so that, in principle, it can be observed in experiments.

Short-pulse lasers allow the most accurate measurement of fast phenomena such as the dynamics of atoms and even electrons. Unfortunately, pulses in the visible region of the spectrum cannot be shorter than the period of one optical cycle: a couple of femtoseconds (one femtosecond is a millionth of a billionth of a second). However, this limit can be overcome when the laser field is very strong, so that its interaction with matter becomes extremely nonlinear, enabling one to zoom in and initiate processes that evolve during a fraction of an optical cycle [1]. This breakthrough, known as high-harmonic generation or HHG, lets researchers produce electromagnetic pulses with durations of attoseconds (thousandths of a femtosecond) and wavelengths in the extreme-ultraviolet or x-ray regime. In Physical Review X, a collaboration lead by Andrius Baltuška of the Technical University of Vienna, Austria, demonstrates an important advance in HHG, showing that carefully chosen combinations of three synchronized optical beams can produce higher frequencies and photon flux [2]. Controlling the generation of these pulses is essential for the observation and manipulation of extremely fast phenomena, such as multielectron processes.

We realize a multicolor, multi-cycle combination of commonly CEP-locked three waves from a single femtosecond OPA driven by a CEP -stable 7-mJ kHz Yb laser system and report HHG driving with individual and combined colors.

The strong-field process of high-harmonic generation is the foundation for generating isolated attosecond pulses 1 , which are the fastest controllable events ever induced. This coherent extreme-ultraviolet radiation has become an indispensable tool for resolving ultrafast motion in atoms and molecules 2 , 3 . Despite numerous spectacular developments in the new field of attoscience 2 , 3 , 4 , the low data-acquisition rates imposed by low-repetition-rate (maximum of 3 kHz) laser systems 5 hamper the advancement of these sophisticated experiments. Consequently, the availability of high-repetition-rate sources will overcome a major obstacle in this young field. Here, we present the first megahertz-level source of extreme-ultraviolet continua with evidence of isolated attosecond pulses using a fibre laser-pumped optical parametric amplifier 6 for high-harmonic generation at 0.6 MHz. This 200-fold increase in repetition rate will enable and promote a vast variety of new applications, such as attosecond-resolution coincidence and photoelectron spectroscopy 7 , or even video-rate acquisition for spatially resolved pump–probe measurements.

Quasi-free field driven electron trajectories are a key element of strong-field dynamics. Upon recollision with the parent ion, the energy transferred from the field to the electron may be released as attosecond duration XUV emission1,2 in the process of high harmonic generation (HHG). The conventional sinusoidal driver fields set limitations on the maximum value of this energy transfer, and it has been predicted that this limit can be significantly exceeded by an appropriately ramped-up cycle-shape3.Here, we present an experimental realization of such cycle-shaped waveforms and demonstrate control of the HHG process on the single-atom quantum level via attosecond steering of the electron trajectories. With our optimized optical cycles, we boost the field-ionization launching the electron trajectories, increase the subsequent field-to-electron energy transfer, and reduce the trajectory duration, to obtain greatly enhanced HHG efficiency as well as spectral extension compared to sinusoidal drivers. This application, which is only one example of what can be achieved with cycle-shaped high-field light-waves, has far-reaching implications for attosecond spectroscopy and molecular self-probing.

High-order harmonic spectra are composed of a coherent sum of half-cycle emissions, the cut-off energy of which depend sensitively on different sub-cycle portions of the driving laser field. By selecting the correct focal geometry the half-cycle cut-off emissions can be preferentially selected over the lower energy plateau emissions through phase matching, such that they form macroscopic half-cycle cut-off features in the far-field spectrum. The energy of these macroscopic half-cycle cut-offs can then be used to retrieve the waveform of the driving laser field. The processes through which these macroscopic half-cycle cut-offs are formed and their applications, both for measuring the laser waveform and the generation of wavelength tunable isolated attosecond pulses, are reviewed in detail.A wavelet transform of the simulated on-axis harmonic field generated by an atomic gas jet driven by an intense few-cycle laser pulse. The focal geometry has been selected to only phase match the half-cycle cut-offs. The energies of these half-cycle cut-offs form a "fingerprint" of the laser field, which can be used to determine important properties of the field waveform. (C) 2010 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In the article of L. E. Chipperfield et al. [Phys. Rev. Lett. 2009, 102, 063001], the ideal waveform was presented that maximised the recollision energy of a strong-field driven electron. In this paper we expound the derivation of this perfect wave, as well as a more general waveform for optimising electron trajectories for which the recollision occurs some distance from the place of origin. The ideal waveform acts as a guide for the synthesises of more practical waveforms that maximise the electron recollision energy, and so extend the high-order harmonic cut-off to higher frequencies.

We report observations and analysis of high harmonic generation driven by a superposition of fields at 1290 nm and 780 nm. These fields are not commensurate in frequency and the superposition leads to an increase in the yield of the mid-plateau harmonics of more than two orders of magnitude compared to using the 1290 nm field alone. Significant extension of the cut-off photon energy is seen even by adding only a small amount of the 780 nm field. These observations are explained by calculations performed in the strong field approximation. Most importantly we find that enhancement is found to arise as a consequence of both increased ionization in the sum-field and modification of the electron trajectories leading to an earlier return time. The enhanced yield even when using modest intensity fields of 5 x 10(13) Wcm(-2) is extended to the 80 eV range and is a promising route to provide a greater photon number for applications in XUV imaging and time-resolved experiments at a high repetition rate.

Control over the polarization of an attosecond pulse train (APT) is demonstrated theoretically using orthogonally polarized two-color fields. The carrier envelope phase of the two pulses is used as a control parameter to generate both an APT with linear polarization in two nearly perpendicular planes or a train of elliptically polarized pulses of alternating helicity. By using few-cycle driving laser fields an isolated attosecond pulse with elliptical polarization is shown to be generated after selecting the cut-off region of the harmonic spectrum. The control mechanism is explained in terms of classical trajectories.

We present the perfect waveform which, during a strong field interaction, generates the maximum possible electron recollision energy for any given oscillation period, over 3 times as high as that for a pure sinusoidal wave. This ideal waveform has the form of a linear ramp with a dc offset. A genetic algorithm was employed to find an optimized practically achievable waveform composed of a longer wavelength field, to provide the offset, in addition to higher frequency components. This second waveform is found to be capable of generating electron recollision energies as high as those for the perfect waveform while retaining the high recollision amplitudes of a pure sinusoidal wave. Calculations of high harmonic generation demonstrate this enhancement, by increasing the cutoff energy by a factor of 2.5 while maintaining the harmonic yield, providing an enhanced tool for attosecond science.

In this paper, we present an extension of the cut-off frequency in high-order harmonic generation using two delayed few-cycles laser pulses, identical in all parameters except the carrier envelope phase (CEP), which is shifted by pi in the second pulse. We use two pulses of the same colour at fixed CEP difference and control the delay between the two pulses which enables us to shape the fundamental field with a high accuracy under one optical cycle. These quantities correspond to macroscopic parameters that can be manipulated experimentally.

It is well known that strong field processes, such as high order harmonic generation (HHG), depend upon the sub-optical cycle dynamics of the strongly driven electrons. We will discuss some of our recent work where the sub-optical cycle dynamics of the driven electrons are examined using both theoretical and experimental approaches. Techniques for measuring the sub-optical cycle electron dynamics from the HHG spectrum are identified and control methods to optimise HHG, which may soon be used in practice, are treated theoretically.

One of the major causes of injury and loss of life in structures subjected to blast loading is the glass windows. Common window glass is annealed glass. When annealed glass is subjected to impulsive loads such as the one generated by a blast, it breaks into relatively large shards with sharp cutting edges. This is responsible for for most of the injuries incurred in explosions. In the bombing of the Alfred P. Murrah Federal Building in Oklahoma City in 1995, more than 75% of the injuries were due to flying shards of glass. The current approach to testing blast-resistant glazing materials such as polycarbonates and laminated glass is to carry out tests in either a shock tube or an open-air arena. Limited access to such facilities is the main obstacle hampering the development of standardised tests for glazing under blast loading conditions. This paper will present a simple and economical physical simulation technique for testing glass panels under impact and blast loading conditions. A technique that can generate pressure shocks simulating blasts of different magnitude and duration is based on a high-capacity drop hammer machine to produce impulsive loads which are distributed over the glass panel using the airtight chamber with a fluid medium. Experimental and numerical simulation results for the annealed glass panels with and without fragment retention films will demonstrate the effectiveness of the system to deliver a blast impulse with a given characteristic.

We show how to optimize the process of high-order harmonic generation (HHG) by gating the interaction using the field gradient of the driving pulse. Since maximized field gradients are efficiently generated by self-steepening processes, we first present a generalized theory of optical carrier-wave self-steepened (CSS) pulses. This goes beyond existing treatments, which only consider third-order nonlinearity, and has the advantage of describing pulses whose wave forms have a range of symmetry properties. Although a fertile field for theoretical work, CSS pulses are difficult to realize experimentally because of the deleterious effect of dispersion. We therefore consider synthesizing CSS-like profiles using a suitably phased subset of the harmonics present in a true CSS wave form. Using standard theoretical models of HHG, we show that the presence of gradient-maximized regions on the wave forms can raise the spectral cutoff and so yield shorter attosecond pulses. We study how the quality of the attosecond bursts created by spectral filtering depends on the number of harmonics included in the driving pulse.