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Optics and Spectroscopy

Generation of an Attosecond Pulse in Helium Excited by Half-Cycle X-Ray Pulses

Rostislav M. Arkhipov, Mikhail V. Arkhipov, Ihar Babushkin, A. V. Pakhomov, Nikolay N. Rosanov

Published Apr 1, 2020• 7 citations
Publisher

Abstract

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.

DOI
10.1134/s0030400x20040025

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Optics and Spectroscopy•Mar 1, 2023•1 citation
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 A method of coherent controlling the polarization of a medium has been proposed recently for the generation of extremely short pulses. The method is based on the excitation of atomic polarization oscillations and their subsequent stop using a pair of ultrashort pulses. The so-called stopped pulse of the polarization of a medium, which arises in the interval between its excitation and deexcitation, can be a source of an extremely short radiation pulse. In this work, the above-indicated possibility of generation of an isolated attosecond ultraviolet pulse in a three-level resonant medium whose parameters correspond to a hydrogen atom excited by a pair of unipolar X-ray pulses is considered theoretically. In this case, the generation mechanism is of the antenna-type, that is, it is caused by the collective spontaneous emission of pre-phased atoms in the absence of a noticeable decay of their free polarization.
Our recent theoretical studies of generation of ultrashort pulses based on collective spontaneous emission from a thin resonant medium layer excited by a pair of high-frequency (compared to the transition frequency of the medium) pulses are reviewed. The source of generated radiation is a resonant polarization pulse in the medium (“stopped polarization pulse”), which is excited and switched off by a pair of light pump pulses. This method allows obtaining attosecond pulses in the optical and ultraviolet ranges, as well as femtosecond terahertz pulses. The effect of the superradiance in generation is discussed. The use of coherent and collective processes makes it possible to achieve a high efficiency of the generation of terahertz radiation. The proposed method is an alternative method of obtaining ultrashort pulses compared to the currently well-developed methods such as the high-harmonic generation method.
Using the numerical solution of the system of equations for the amplitudes of bound states, together with the wave equation, we theoretically consider stopped polarization pulse superradiance upon excitation by a pair of half-period attosecond pulses in a thin layer of a five-level resonant medium, the parameters of which are the same as in a hydrogen atom. It is shown that in the case of a multilevel medium, at certain parameters of the exciting field, the superradiance pulse near the medium is also a single-cycle pulse, the shape of which is determined by the first time derivative of the stopped polarization pulse, as in the case when approximate low-level and classical models were used to describe the response of the medium. Keywords: superradiance, attosecond pulses, stopped polarization pulse, unipolar pulses, subcycle pulses.
Here we demonstrate an unusual type of superradiance in an extended layer of a two-level medium excited by a pair of unipolar pulses. It is shown that, due to the interference of secondary waves emitted by the resonant medium, the response of such a layer in reflection represents a pair of half-cycle unipolar pulses of different polarity. They follow with a time delay proportional to the layer thickness. The source of such superradiance is a monopolar half-cycle pulse of the stopped polarization induced by the first excitation pulse and switched off by the second excitation pulse. The predicted effect can be used to control the shape of few- and subcycle pulses in resonant media.
Abstract Using the numerical solution of the system of equations for the amplitudes of bound states, together with the wave equation, we theoretically consider stopped polarization pulse superradiance upon excitation by a pair of half-period attosecond pulses in a thin layer of a five-level resonant medium, the parameters of which are the same as in a hydrogen atom. It is shown that in the case of a multilevel medium, at certain parameters of the exciting field, the superradiance pulse near the medium is also a single-cycle pulse, the shape of which is determined by the first time derivative of the stopped polarization pulse, as in the case when approximate low-level and classical models were used to describe the response of the medium.
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.