Emilio Fiordilino

Generation of isolated attosecond pulses using unipolar and laser fields

A new scheme to generate isolated attosecond pulses is presented that involves the use of a laser field and of a unipolar field. The laser field has a pulse of intensity I = 1.5×1014 W cm−2 and wavelength λ = 820 nm. The unipolar pulse is an asymmetric pulse consisting of a sharp peak, lasting approximately half a laser period, i.e. nearly 1.4 fs, followed by a long and shallow tail. We show that on combining these two fields, it is possible to generate isolated attosecond pulses as short as 1/10 of a laser period, i.e. approximately 270 as. Moreover, it is argued that this scheme is robust either against small variations of the laser envelope, or against small changes in the delay between the laser pump and the unipolar pulse.

A paradigm of fullerene

We study the dynamics of an electron constrained over the surface of a rigid sphere, with geometrical parameters similar to those of the C60 fullerene, embedded in a low intensity linearly polarized laser field. The model is shown to emit odd harmonics of the laser even at very low field intensity. For more intense laser fields, the spectrum presents odd harmonics and hyper-Raman lines shaped in a broad plateau. The spectrum of the model is compared to that theoretically obtained by other authors for more realistic models of C60. It is concluded that the model can be used as a paradigm for mesoscopic molecules in the fullerene family, particularly in practical applications where it is convenient to have a simple model of fullerene molecules. It is also concluded that the model is an example lending support to the concept of universality introduced by other authors some time ago.

Wavelet temporal profile of high-order harmonics emitted by a two-level atom in the presence of a laser pulse

The acceleration of the electron in a two-level atom driven by a short laser pulse of intermediate strength is calculated and therefore analysed through two wavelet transforms in order to obtain the temporal evolution of the spectrum of the emitted radiation. We show (i) the full spectrum at different instants and (ii) the time profile of some representative harmonics. The growth of the harmonics as well as the appearance and erratic behaviour of the hyper-Raman can be followed with the laser intensity.

Laser pulse shape effects in harmonic generation from a two-level atom

Abstract The harmonic spectrum emitted by a two-level atom driven by a resonant laser field is calculated. The spectrum is seen to depend on the shape of the pulse. In the case of an asymmetric pulse profile, harmonic emission is present also at a relatively low intensity field.

A three-colour scheme to generate isolated attosecond pulses

We propose a new scheme to produce isolated attosecond pulses, involving the use of three laser pulses: a fundamental laser field of intensity I = 3.5 × 1014 W cm−2 and of wavelength λ = 820 nm, and two properly chosen weak lasers with wavelengths 1.5λ and 0.5λ. The three lasers have a Gaussian envelope of 36 fs full width at half maximum. The resulting total field is an asymmetric electric field with an isolated peak. We show that a model atom, interacting with the above-defined total field, generates an isolated attosecond pulse as short as 1/10 of a laser period, i.e. approximately 270 as.

Direct theoretical evidence of nuclear motion in H2+ by means of high harmonic generation

The numerical solution of the time-dependent Schrodinger equation for vibrating hydrogen molecular ions in many-cycle laser pulses shows that high-order harmonic generation is sensitive to laser-induced molecular vibrations. In particular, the odd harmonic lines in the emitted spectra are surrounded by additional regular peaks whose spacing is given by the vibrational frequency of the nuclei motion. Analytical theory relates these satellite peaks to the molecular vibrations in terms of an approximated effective potentials. These results are not affected by the dimensionality of the system.

Pulse shape control of the spectrum emitted by a two level atom

We find an approximated analytic expression of the acceleration spectrum of a quasi-degenerate two level atom in an external laser field with a non-constant pulse shape function. The spectrum contains odd harmonics and side bands of even harmonics. We show that the relative amplitude of these lines can be controlled by using different rise times of the laser pulse and different switching on functions. In particular, we find that by choosing the laser pulse parameters appropriately the emission of odd harmonics is forbidden and the spectrum contains only even harmonics.

Harmonic generation from nanorings driven by a two-color laser field

We study the high harmonic generation and the polarization of the harmonics emitted by a nanoring driven by two laser fields of angular frequency ?1 and ?2, with ?2?=?2?1, and ?1 resonant between the ground and the first excited state. We show that by varying parameters, such as laser intensity, photon energy and the delay between the two laser pulses, we can control the number of harmonics and the polarization of the radiation. In particular we show that with this choice of two-laser photon frequency rate we obtain more harmonics with respect to other configurations. We also calculated the average absorbed energy and the average angular momentum acquired by the electron as a function of the intensity of the two lasers obtaining spikes of angular momentum.

Wavelet analysis of short harmonics generated in presence of a two colour laser field

Abstract The Morlet wavelet spectrum of the radiation emitted by a two level atom in presence of two laser pulses with very close frequency is obtained. The wavelet spectrum gives information on the time evolution of the full spectrum and of a particular line. The beating condition stimulates the atom to emit pulses of harmonics with duration of the order of a few optical cycles of the pumping radiation. Pulse trains of 3 optical cycles (FWHM) are observed.

Temporal evolution of the spectrum emitted by a two-level atom in the presence of a laser field

Abstract The temporal evolution of the spectrum emitted by a two level atom irradiated by a laser pulse is obtained by means of the Gabor transform of the acceleration of the electron. This transform is useful in revealing fine structures of the spectrum which are otherwise invisible.