F. Persico

The role of the cloud of virtual photons in the shift of the ground state energy of a hydrogen atom

Abstract The division of the hamiltonian of a hydrogen atom into three parts: atomic, radiation and interaction, together with the use of non-relativistic second-order perturbation theory, is shown to yield a physical interpretation of the energy shift of the ground state which emphasizes the role of the field due to the cloud of virtual photons which surround the atom.

Canonical dressing of atoms by intense radiation fields

Abstract A canonical transformation is presented in order to diagonalize the radiation hamiltonian in the rotating wave approximation. Dressing of atomic operators in the single- and multi-atom cases is obtained explicitly and qualitatively discussed in connection with the problem of resonance fluorescence.

Virtual Field, Causal Photon Absorption and Photodetectors

The time-dependent electric-energy density surrounding a two-level atom fixed at r = 0 is studied, the atom being taken in its ground state at t = 0 and the field having initially only one photon in a delocalized mode. The atom-field coupling includes both rotating and counterrotating terms. The energy density in the rotating wave approximation is shown to behave noncausally, while in the presence of the complete coupling it is shown to be affected only within a sphere of radius r = ct centred on the atom. It is concluded that the counterrotating terms in the atom-field coupling are essential in order to ensure causality and cannot be neglected in any accurate treatment of photon absorption. Some consequences of this conclusion on the operation of photodetectors in one-photon absorption are discussed.

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.

Atoms dressed and partially dressed by the zero-point fluctuations of the electromagnetic field

An atom or a molecule is constituted by a set of bound electric charges with dynamics governed by the laws of quantum mechanics. These charges are sources of the quantized electromagnetic field which also binds them together, and thus the effects of their interaction with the field cannot be disregarded in principle. Consequently even overall neutral atoms, on which we focus our attention, are driven by a dynamics which is inextricably related to the dynamics of the quantized electromagnetic field. One of the most prominent aspects of the atom-held interaction is the existence of a cloud of virtual photons which dresses the atom even in the lowest possible energy state of the system. In this paper we review the static as well as the dynamic aspects of the theory of the virtual cloud around the neutral atoms. We begin by reviewing various forms of the atom-field coupling as well as various models of simplified atoms which will be used in the rest of the paper. The question then arises as how to characterize quantitatively the shape of the virtual cloud, and we show that the energy density of the electromagnetic field is a physical quantity suitable for this purpose. First a perturbative approach to calculating this shape is developed and applied to several physical models of a ground-state dressed atom. The next step is to consider virtual clouds which are out of equilibrium and examine their time development. This leads to the concept of half-dressed sources, which are discussed in a different physical context both in the absence and in the presence of real photons. In particular the role of the virtual cloud in ensuring causality of the field propagating in dressing and undressing processes is emphasized. Finally, the nature of the virtual cloud is further discussed in the light of theories concerning the dynamics of an atomic pair, the quantum theory of measurement and the effects of a driving electromagnetic field.

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.

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.

Dynamical Casimir-Polder energy between an excited- and a ground-state atom

We consider the Casimir-Polder interaction between two atoms, one in the ground state and the other in its excited state. The interaction is time dependent for this system, because of the dynamical self-dressing and the spontaneous decay of the excited atom. We calculate the dynamical Casimir-Polder potential between the two atoms using an effective Hamiltonian approach. The results obtained and their physical meaning are discussed and compared with previous results based on a time-independent approach, which uses a nonnormalizable dressed state for the excited atom.

Squeezing in a two-photon Dicke hamiltonian

Abstract The single-mode, two-level atom Dicke hamiltonian with two-photon atom-field coupling is treated exactly and it is shown to yield a certain degree of squeezing in the field variables. This result is briefly discussed in connection with the previously shown absence of squeezing in the two-photon laser model.