Federico Ferrari

Attosecond Electron Spectroscopy Using a Novel Interferometric Pump-Probe Technique

We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration.

Intense femtosecond extreme ultraviolet pulses by using a time-delay-compensated monochromator

Extreme-ultraviolet pulses, produced by high-order harmonic generation, have been spectrally selected by a time-delay-compensated grating monochromator. Temporal characterization of the harmonic pulses has been obtained using cross-correlation method: pulses as short as 8 fs, with high photon flux, have been measured at the output of the monochromator.

Time-delay compensated monochromator for the spectral selection of extreme-ultraviolet high-order laser harmonics.

The design and the characterization of a monochromator for the spectral selection of ultrashort high-order laser harmonics in the extreme ultraviolet are presented. The instrument adopts the double-grating configuration to preserve the length of the optical paths of different diffracted rays, without altering the extremely short duration of the pulse. The gratings are used in the off-plane mount to have high efficiency. The performances of the monochromator have been characterized in terms of spectral response, efficiency, photon flux, imaging properties, and temporal response. In particular, the temporal characterization of the harmonic pulses has been obtained using a cross-correlation method: Pulses as short as 8 fs have been measured at the output of the monochromators, confirming the effectiveness of the time-delay compensated configuration.

Temporal characterization of a time-compensated monochromator for high-efficiency selection of extreme-ultraviolet pulses generated by high-order harmonics

Ultrafast extreme-ultraviolet pulses are spectrally selected by a time-delay-compensated grating monochromator. The intrinsic very short duration of the pulses is obtained by exploiting the high-order harmonic generation process. The temporal characterization of the harmonic pulses is obtained using a cross-correlation method: pulses as short as 8 fs are measured at the output of the monochromator in the case of the 23rd harmonic. This value is in agreement with the expected duration of such pulses, indicating that the influence of the monochromator is negligible. The photon flux has been measured with a calibrated photodiode, pointing out the good efficiency of the monochromator, derived by the exploitation for the two gratings of the conical diffraction mounting.

Towards atomic unit pulse duration by polarization-controlled few-cycle pulses

We demonstrate the generation of broadband extreme ultraviolet (XUV) continua by high order harmonic generation in neon using carrier-envelope phase (CEP) stabilized, few-cycle pulses with a time-dependent polarization. The XUV spectra can support isolated attosecond pulses with a duration of about 50 as, opening the possibility of achieving time resolution close to the atomic unit of time. A theoretical model that calculates the spectral and temporal characteristics of the XUV radiation emitted by a single atom is used to interpret the CEP dependence of the harmonic structures.

Quantum path control in harmonic generation by temporal shaping of few-optical-cycle pulses in ionizing media

Temporal reshaping of the electric field of few-optical-cycle pulses in a low-density ionizing gas has been used to achieve control of the electron trajectories in the process of high-order harmonic generation. As a result of such a quantum path control mechanism, isolated or multiple attosecond pulses have been produced, depending on the carrier-envelope phase of the driving field. In particular, complete spectral tunability of the harmonic peaks over the whole spectral range has been demonstrated. Experimental results have been interpreted using a nonadiabatic three-dimensional propagation model and a nonadiabatic stationary phase model.

Chapter 8 – Principles and Applications of Attosecond Technology

Abstract Recent progress in attosecond technology has determined an increasing interest in the application of attosecond pulses in atomic, molecular, and solid state-physics. Novel techniques have been introduced for the generation of subfemtosecond pulses and for their applications. We report on recent advances in attosecond science, with particular emphasis on the generation and use of isolated attosecond pulses, produced by using the process of high-order harmonic generation in gases. Several techniques have been proposed and partly implemented for the confinement of the harmonic generation process to a single event: in this article, we analyze various temporal gating techniques. Various schemes for attosecond measurements have been proposed and experimentally demonstrated in the last decade; here, we review important applications of isolated attosecond pulses.

Efficient attosecond pulse generation by gating HHG with sub-cycle ionization dynamics

Nowdays attosecond technology has become an important branch of ultrafast science [1], in particular because of the possibility to take snapshots of the electronic motion in atoms and molecules. So far, the reported applications of isolated attosecond pulses have been limited by the low photon flux of the available sources. We have recently demonstrated a technique for the generation of isolated attosecond pulses with energy on target up to 2.1 nJ [2]. The key elements are th use of few-optical-cycle driving pulses with controlled electric field and peak intensity beyond the saturation intensity of the gas used for high-order harmonic generation (HHG), and efficient spatial filtering in the XUV beamline.

Sub-10-fs XUV Tunable Pulses at the Output of a Time-Delay-Compensated Monochromator

Extreme-ultraviolet pulses, produced by high-order-harmonic generation, have been spectrally selected by a time-delay-compensated monochromator. Temporal characterization has been obtained using cross-correlation method: pulses as short as 8 fs, with high photon flux, have been measured.

Attosecond excitation of electron wavepackets

We present experiments, supported by time-dependent Schrodinger simulations, on the dynamics of Helium bound states after an attosecond excitation in the presence of a strong infrared laser field.