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Journal of Modern Optics

Kerr nonlinearity and optical multi-stability in a four-level Y-type atomic system

Mostafa Sahrai, Hamid Reza Hamedi, Meisam Memarzadeh

Published Jun 6, 2012• 23 citations
Publisher

Abstract

The nonlinear response to applied fields of a four-level Y-type atomic system is investigated. The effect of laser intensity and quantum interference induced by spontaneous emission on optical bistability, optical multi-stability and Kerr nonlinearity is then discussed. It is found that the threshold of the optical bistability can substantially be reduced by the quantum interference. So, an enhanced Kerr nonlinearity with reduced absorption can be achieved.

DOI
10.1080/09500340.2012.691561

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The nonlinear response to applied fields of a four-level Y-type atomic system is investigated. The effect of laser intensity and quantum interference induced by spontaneous emission on optical bistability, optical multi-stability and Kerr nonlinearity is then discussed. It is found that the threshold of the optical bistability can substantially be reduced by the quantum interference. So, an enhanced Kerr nonlinearity with reduced absorption can be achieved.
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We propose a new scheme for achieving giant enhancement of the self-Kerr nonlinearity in a four-level Y-type atom with two highest nearly degenerate lying levels. We have found that, owing to the quantum interference effect that originates from spontaneous emission from two closely lying levels, large Kerr nonlinearity can be achieved under appropriate conditions. In particular, in addition to the amplitudes, the phases of the applied fields also affect the Kerr nonlinearity. We attribute the enhancement of Kerr nonlinearity mainly to the presence of some extra atomic coherence terms induced by the spontaneously generated coherence. We present a physical understanding of our numerical results using analytical explanation.
A theoretical investigation is carried out into the effect of applied coherent fields on electromagnetically induced transparency(EIT) and Kerr nonlinearity in the four-level Y-type energy atom system.It is found that the Absorption curve of the atom system changes from one EIT window to a gain interval which has two transparency points when the intensity of coupling field and pumping field changes from equality to inequality.The position of transparency points,the magnitude of gain value together with the width of gain interval are all closely connected with the changed Rabi frequency of applied fields.It is also found that Kerr nonlinearity is enhanced but accompanied by linear Absorption when applied fields are all weak and their relative intensity reduces.When the intensity of applied fields is enlarged,Kerr nonlinearity is enhanced significantly and its peak moves to the position of transparency point with the enlargement of their relative intensity.That is to say,giant Kerr nonlinearity accompanied by vanishing linear Absorption is obtained.Finally,we find that the effect of the intensity variation of the pumping field on the enhancement of Kerr nonlinearity is more remarkable than that of the coupling field.
We explore the coherent control of nonlinear absorption of intense laser fields in four-level atomic systems. For instance, in a four-level ladder system, a coupling field creates electromagnetically induced transparency (EIT) with an Aulter-Townes doublet for the probe field while the control field is absent. A large absorption peak appears at resonance as the control field is switched on. We show how such a large absorption leads to optical switching. Further, this large absorption diminishes and a transparency window appears due to the saturation effects as the strength of the probe field is increased. We further demonstrate that the threshold of the optical bistability can be modified by suitable choices of the coupling and the control fields. In a four-level Y\mathsf{Y}Y-type configuration, the effect of the control field on saturable absorption (SA) and reverse saturable absorption (RSA) is highlighted in the context of nonlinear absorption of the probe field. We achieve RSA and SA in a simple atomic system just by applying a control field.