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Optics and Spectroscopy

Generation of Isolated Attosecond Pulses with Large Electric Area in a Dense Resonant Medium

Rostislav M. Arkhipov, Mikhail V. Arkhipov, S. V. Fedorov, N. N. Rosanov

Published Mar 1, 2023• 1 citation
Publisher

Abstract

Abstract Obtaining unipolar half-cycle optical pulses of femto- and attosecond duration with a large electrical area is an urgent but difficult task. The reason for the emerging difficulties lies in the existence of the rule of conservation of the electrical area of the pulse, which does not allow converting a bipolar pulse into a unipolar one. In this work, it is shown that in a resonant medium a few-cycle pulse can be converted into two unipolar pulses separated in time by a distance that is an order of magnitude or more longer than the duration of the initial pulse. This allows in a number of problems to consider such pulses separately as unipolar. The estimation of the electric area value relative to its “atomic scale” is carried out.

DOI
10.1134/s0030400x23040045

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Abstract Obtaining unipolar half-cycle optical pulses of femto- and attosecond duration with a large electrical area is an urgent but difficult task. The reason for the emerging difficulties lies in the existence of the rule of conservation of the electrical area of the pulse, which does not allow converting a bipolar pulse into a unipolar one. In this work, it is shown that in a resonant medium a few-cycle pulse can be converted into two unipolar pulses separated in time by a distance that is an order of magnitude or more longer than the duration of the initial pulse. This allows in a number of problems to consider such pulses separately as unipolar. The estimation of the electric area value relative to its “atomic scale” is carried out.
Obtaining unipolar half-cycle optical pulses of femto- and attosecond duration with a large electrical area is an urgent but difficult task. The reason for the emerging difficulties lies in the existence of the rule of conservation of the electrical area of the pulse, which does not allow converting a bipolar pulse into a unipolar one. In this work, it is shown that in a resonant medium a few-cycle pulse can be converted into two unipolar pulses separated in time by a distance that is an order of magnitude or more longer than the duration of the initial pulse. This allows in a number of problems to consider such pulses separately as unipolar. The estimation of the electric area value relative to its "atomic scale" is carried out. Keywords: attosecond pulses, unipolar pulses, subcycle pulses, electrical area of pulses.
Obtaining unipolar half-cycle optical pulses of femto- and attosecond duration with a large electrical area is an urgent but difficult task. The reason for the emerging difficulties lies in the existence of the rule of conservation of the electrical area of ​​the pulse, which does not allow converting a bipolar pulse into a unipolar one. In this work, it is shown that in a resonant medium a low-cycle pulse can be converted into two unipolar pulses separated in time by a distance that is an order of magnitude or more longer than the duration of the initial pulse. This allows in a number of problems to consider such pulses separately as unipolar. The estimation of the electric area value relative to its "atomic scale" is carried out.
Here we numerically investigate the propagation of unipolar half-cycle attosecond pulses with a duration much shorter than the period of the proper oscillations T 0 of the medium and its relaxation times T 1 and T 2 in a two-level resonant absorbing and amplifying medium. We find that despite breaking down of the famous McCall and Hahn area theorem, some phenomena described by this theorem are still qualitatively preserved. In particular, it is shown that pulses with small initial electric pulse area become sign-alternating upon the propagation, similar to the traditional 0 π pulses; pulses with 4 π , 5 π , and 6 π area are unstable and split into lower-order pulses, exactly as in the long-pulse case. The electric pulse area conservation rule and limitations of unipolar pulse amplification in two-level gain media, which follows from this rule, are discussed.
It is shown that not only multicycle resonant pulses with zero electric area, but also nonresonant unipolar pulses can be used to efficiently excite quantum transitions in a medium. Arguments are presented in favor of the advantages of unipolar pulses for excitation of quantum systems in comparison with resonant radiation. A simple relationship has been found showing that in order to compare the effects of unipolar half-cycle and resonant multicycle pulses on atomic systems. It shows that it is necessary to compare the electric area of ??a unipolar pulse and the area of ??the envelope of a bipolar pulse, rather than their energies. Keywords: unipolar pulses, electric pulse area, pulse area, extremely short pulses, attosecond pulses.
Abstract Conventionally, attosecond pulses are obtained using generation of high-order optical harmonics upon excitation of various media by femtosecond laser pulses with the central frequency in the infrared range. Here, we consider theoretically an alternative possibility of obtaining an isolated extreme ultraviolet (XUV) attosecond pulse in a thin layer of helium atoms excited by a pair of half-cycle (quasi-unipolar) X-ray pulses. The approach is based on the free polarization decay of an atomic medium. An analogy of the processes under consideration with the phenomenon of superradiance is discussed.
Abstract Recently, the possibility of obtaining non-harmonic unipolar electromagnetic pulses with a specific electric field strength’s dependence on time (e.g. a rectangular pulse) has been actively discussed in optics. Unipolar pulses have a nonzero electric area and a wide spectrum: from zero frequency up to the visible region. This could open up wide application of such pulses, for example, for faster and more efficient control of the quantum systems’ properties with a high temporal resolution compared to conventional bipolar multicycle pulses. In this paper, based on an approximate solution of the time dependent Schrödinger equation and a numerical solution of the equation system for the density matrix, we show the possibility of creating electromagnetically induced population density gratings in an atomic medium using a pair of unipolar rectangular attosecond light pulses that do not overlap in the medium. The formation of such gratings can be explained on the basis of the concept of ‘interference’ of electric pulse areas, recently introduced into optics.