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Bulletin of the Russian Academy of Sciences Physics

Propagation of few-cycle pulses in homogeneous and nonhomogeneous media

Dmitry Yu. Zagursky, Irina G. Zakharova, Vyacheslav A. Trofimov

Published Dec 1, 2015• 3 citations
Publisher

Abstract

The interaction between a few-cycle pulse and an optically extended multi-level medium is analyzed via computer simulation. The cascade mechanism behind the excitation of the medium’s high energy levels is determined and the way in which it is affected by the pulse’s absolute phase is investigated. The effect structural disordering has on the absorption spectrum of a multi-level medium is also analyzed.

DOI
10.3103/s1062873815120254

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The interaction between a few-cycle pulse and an optically extended multi-level medium is analyzed via computer simulation. The cascade mechanism behind the excitation of the medium’s high energy levels is determined and the way in which it is affected by the pulse’s absolute phase is investigated. The effect structural disordering has on the absorption spectrum of a multi-level medium is also analyzed.
1D propagation of a THz Gaussian pulse, containing a few cycles, in multi-level media is studied using computer simulation. Electromagnetic field evolution is governed by Maxwell's equations whereas a medium response is described in the frame-work of the density matrix formalism. For the pulse with a sufficiently broad spectrum we demonstrate the appearance of a cascade mechanism for molecule energy level excitation. As a result, the spectrum of the THz pulse, transmitted through a medium layer, is broadened essentially in comparison with the incident pulse spectrum. We also take into account the incident pulse absolute phase influence on the pulse spectrum broadening.
The ultrabroad spectrum of few-cycle and especially subcycle pulses requires multiple energy levels in a medium to be properly considered in order to describe the pulse propagation effects. In this paper we go beyond the standard two-level or related few-level approximations and theoretically analyze the propagation of subcycle pulses in a nonequilibrium multilevel resonant medium. We specifically address the case of pulse durations, smaller than the periods of resonant transitions in a medium, where the appropriate analytical models can be provided, and assume accordingly a slow dissipation in the medium. We demonstrate a number of nonlinear phenomena, arising under such conditions, in particular the formation of stable subcycle dissipative solitons of various profiles in a nonequilibrium medium, including superluminal ones, as well as study possible scenarios of their collision.
The spectrum evolution of a few-cycle optical pulse in a resonant two-level atom medium is studied theoretically by using the full Maxwell-Bloch equations. On the propagating pulse, significantly much faster oscillation components separated with the main pulse appear due to strong self-phase modulation and pulse reshaping. In this case, ideal self-induced transparency cannot occur for a 2π pulse. The spectrum of the 4π pulse shows an evident oscillatory feature because of the continuum interference of the separate pulses. For larger pulse areas, continuum generation from near ultraviolet to infrared occurs.
Techniques for producing, measuring, and applying ever shorter electromagnetic pulses are being developed for incorporation in a variety of modern high-speed systems. In many cases these pulses are at most a few cycles in length, and so-called half-cycle electromagnetic pulses are also widely employed. The interaction of such pulses with two-level media is considered here in detail, and these media are basic to many of the absorbing and amplifying configurations of optics and laser studies. Significant delays and distortion of the resulting polarization and population pulses can occur, and nonlinear optical effects are also revealed. The limitations of the parity, rate-equation, and rotating-wave approximations for the characterization of such few-cycle interactions are also explored.
The spectrum evolution of a few-cycle optical pulse in a resonant two-level atom medium is studied theoretically by using the full Maxwell-Bloch equations. On the propagating pulse, significantly much faster oscillation components separated with the main pulse appear due to strong self-phase modulation and pulse reshaping. In this case, ideal self-induced transparency cannot occur for a 2 pi pulse. The spectrum of the 4 pi pulse shows an evident oscillatory feature because of the continuum interference of the separate pulses. For larger pulse areas, continuum generation from near ultraviolet to infrared occurs.