Two phase-matching regimes in high-order harmonic generation.

High-order harmonic generation (HHG) provides scalable sources of coherent extreme ultraviolet radiation with pulse duration down to the attosecond time scale. Efficient HHG requires the constructive interplay between microscopic and macroscopic effects in the generation volume, which can be achieved over a large range of experimental parameters from the driving field properties to those of the generating medium. Here, we present a systematic study of the harmonic yield as a function of gas pressure and medium length. Two regimes for optimum yield are identified, supporting the predictions of a recently proposed analytical model. Our observations are independent on the focusing geometry and, to a large extent, on the pulse duration and laser intensity, providing a versatile approach to HHG optimization.

Α 10-gigawatt attosecond source for non-linear XUV optics and XUV-pump-XUV-probe studies.

The quantum mechanical motion of electrons and nuclei in systems spatially confined to the molecular dimensions occurs on the sub-femtosecond to the femtosecond timescales respectively. Consequently, the study of ultrafast electronic and, in specific cases, nuclear dynamics requires the availability of light pulses with attosecond (asec) duration and of sufficient intensity to induce two-photon processes, essential for probing the intrinsic system dynamics. The majority of atoms, molecules and solids absorb in the extreme-ultraviolet (XUV) spectral region, in which the synthesis of the required attosecond pulses is feasible. Therefore, the XUV spectral region optimally serves the study of such ultrafast phenomena. Here, we present a detailed review of the first 10-GW class XUV attosecond source based on laser driven high harmonic generation in rare gases. The pulse energy of this source largely exceeds other laser driven attosecond sources and is comparable to the pulse energy of femtosecond Free-Electron-Laser (FEL) XUV sources. The measured pulse duration in the attosecond pulse train is 650 ± 80 asec. The uniqueness of the combined high intensity and short pulse duration of the source is evidenced in non-linear XUV-optics experiments. It further advances the implementation of XUV-pump-XUV-probe experiments and enables the investigation of strong field effects in the XUV spectral region.

Quantum Optical Signatures in a Strong Laser Pulse after Interaction with Semiconductors.

Electrodynamical processes induced in complex systems like semiconductors by strong electromagnetic fields have traditionally been described using semiclassical approaches. Although these approaches allowed the investigation of ultrafast dynamics in solids culminating in multipetahertz electronics, they do not provide any access to the quantum-optical nature of the interaction, as they treat the driving field classically and unaffected by the interaction. Here, using a full quantum-optical approach, we demonstrate that the subcycle electronic response in a strongly driven semiconductor crystal is imprinted in the quantum state of the driving field resulting in nonclassical light states carrying the information of the interaction. This vital step towards strong-field ultrafast quantum electrodynamics unravels information inaccessible by conventional approaches and leads to the development of a new class of nonclassical light sources.

Attosecond pulse generation with an optimization loop in a light-field-synthesizer.

We developed an efficient, tailored optimization method for attopulse generation using a light-field-synthesizer [M. Hassan et al., Nature 530, 66 (2016)]. We adapted genetic optimization of single-cycle and sub-cycle waveforms to attosecond pulse generation and achieved significantly improved convergence to many target attosecond pulse shapes. Importantly, we show that the single-atom approach (based on strong field approximation) gives similar results to the more complex and numerically intensive 3D model of the attopulse generation process and that spectrally tunable attosecond pulses can be produced with a light-field synthesizer.

Attosecond pulse walk-off in high-order harmonic generation.

We study the influence of the generation conditions on the group delay of attosecond pulses in high-order harmonic generation in gases. The group delay relative to the fundamental field is found to decrease with increasing gas pressure in the generation cell, reflecting a temporal walk-off due to the dispersive properties of the nonlinear medium. This effect is well reproduced using an on-axis phase-matching model of high-order harmonic generation in an absorbing gas.

Reconstruction of attosecond pulse trains using an adiabatic phase expansion.

We propose a new method to reconstruct the electric field of attosecond pulse trains. The phase of the high-order harmonic emission electric field is Taylor expanded around the maximum of the laser pulse envelope in the time domain and around the central harmonic in the frequency domain. Experimental measurements allow us to determine the coefficients of this expansion and to characterize the radiation with attosecond accuracy over a femtosecond time scale. The method gives access to pulse-to-pulse variations along the train, including the timing, the chirp, and the attosecond carrier envelope phase.

Attosecond electron wave packet dynamics in strong laser fields.

We use a train of sub-200 attosecond extreme ultraviolet (XUV) pulses with energies just above the ionization threshold in argon to create a train of temporally localized electron wave packets. We study the energy transfer from a strong infrared (IR) laser field to the ionized electrons as a function of the delay between the XUV and IR fields. When the wave packets are born at the zero crossings of the IR field, a significant amount of energy (approximately 20 eV) is transferred from the field to the electrons. This results in dramatically enhanced above-threshold ionization in conditions where the IR field alone does not induce any significant ionization. Because both the energy and duration of the wave packets can be varied independently of the IR laser, they are valuable tools for studying and controlling strong-field processes.

Angular dispersion of femtosecond pulses in a Gaussian beam.

The angular dispersion of spatially Gaussian laser pulses, unlike for plane waves, changes with the distance between the disperser and the observer and between the beam waist and the disperser. The formula that is derived is experimentally verified by use of a high-precision measurement technique. Because the angular dispersion of a Gaussian beam is substantially different from that of a plane wave after the same disperser, the phenomenon may be of special interest for short-pulsed laser systems, for which alignment of the stretcher-compressor system for zero residual angular dispersion is essential.

Elevated hepatic glucocorticoid receptor expression during liver regeneration in rats.

In rats within the first week of partial hepatectomy reconstruction of the normal histological structure of the liver already starts. To approach the possible role of endogenous glucocorticoids in the process of regeneration we measured the changes in the expression of steroid glucocorticoid receptor gene after various regeneration intervals. After partial hepatectomy, between 0.5 168 hours from the surgery, the gene expression (mRNA) of glucocorticoid receptor was determined by reverse transcription followed by PCR and normalized to that of glycerolphoshate dehydrogenase. Two peaks of glucocorticoid receptor mRNA were detected first, between 3 and 6 hours (first peak) and a second between 24 and 36 hours. Immunoreactive glucocorticoid receptor was detected by immunohistochemistry using monoclonal anti-glucocorticoid receptor. Three days after the surgery immunohistochemical studies showed substantially more immunoreactive GcR protein in the regenerated liver than in the controls. These semiquantitative data provide evidence suggesting elevation of glucocorticoid receptor expression during regeneration of liver at mRNA and protein levels.