V. Tosa

Phase-matching effects in the generation of high-energy photons by mid-infrared few-cycle laser pulses

We report on our experimental and theoretical investigations on the generation of high-order harmonics driven by 1500 nm few-cycle laser pulses in xenon. In contrast to the common belief, we found experimental evidence suggesting that harmonic generation driven by mid-infrared laser pulses can be realized with high efficiency; in particular, an enhancement of very high harmonic orders can be achieved under suitable conditions of the laser–medium interaction. The experimental results were simulated by a 3D non-adiabatic model. The theoretical outcomes confirm the experimental findings and provide a physical explanation for the counter-intuitive results. In particular, a time-dependent phase-matching analysis threw light on the generation mechanisms at a timescale of half optical cycle of the fundamental pulse.

Isolated Attosecond Pulse Generation by Two-Mid-IR Laser Fields

The availability of tunable mid-infrared laser systems capable to deliver high intensity few cycle pulses offers the perspective of generating ultrashort radiation in the extreme UV range through the high-order harmonic generation (HHG) of the fundamental. In particular, such systems make possible the superposition of multiple colors of incommensurable frequencies that gives rise to time gating of HHG, thereby realizing the necessary condition leading to isolated attosecond pulse (IAP) generation. In this paper, the HHG resulting from the superposition of two incommensurable frequencies is analyzed with the aim to find out the conditions that the relative intensity and frequency of the two fields have to fulfill in order to generate isolated attosecond pulses. It is observed that the two fields can be superimposed in parallel and perpendicular polarization, both giving rise to IAP under appropriate conditions.

Influence of generalized focusing of few-cycle Gaussian pulses in attosecond pulse generation

In contrast to the case of quasi-monochromatic waves, a focused optical pulse in the few-cycle limit may exhibit two independent curved wavefronts, associated with phase and group retardations, respectively. Focusing optical elements will generally affect these two wavefronts differently, thus leading to very different behavior of the pulse near focus. As limiting cases, we consider an ideal diffractive lens introducing only phase retardations and a perfect non-dispersive refractive lens (or a curved mirror) introducing equal phase and group retardations. We study the resulting diffraction effects on the pulse, finding both strong deformations of the pulse shape and shifts in the spectrum. We then show how important these effects can be in highly nonlinear optics, by studying their role in attosecond pulse generation. In particular, the focusing effects are found to affect substantially the generation of isolated attosecond pulses in gases from few-cycle fundamental optical fields.

Non-collinear high-order harmonic generation by three interfering laser beams

High order harmonic generation (HHG) has shown its impact on several applications in Attosecond Science and Atomic and Molecular Physics. Owing to the complexity of the experimental setup for the generation and characterization of harmonics, as well as to the large computational costs of numerical modelling, HHG is generally performed and modelled in collinear geometry. Recently, several experiments have been performed exploiting non-collinear geometry, such as HHG in a grating of excited molecules created by crossing beams. In such studies, harmonics were observed at propagation directions different from those of the driving pulses; moreover the scattered harmonics were angularly dispersed.In this work we report on a new regime of HHG driven by multiple beams, where the harmonics are generated by three synchronized, intense laser pulses organized in a non-planar geometry. Although the configuration we explore is well within the strong-field regime, the scattered harmonics we observe are not angularly dispersed.

Single attosecond pulse generation by two laser fields

In this paper, the high order harmonic generation resulting from the superposition of two incommensurable frequencies is analyzed. The aim is to find out the conditions that the relative intensity and frequency of the two fields have to fulfill in order to generate isolated attosecond pulses. It is observed that the two fields can be superimposed in both parallel and perpendicular polarization, both giving rise to a single attosecond pulse under appropriate conditions. The two geometrical configurations lead to similar output, and we anticipate the possibility to generate attosecond pulses shorter than 280 as and with energy centered around 80–100 eV.