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arXiv:1602.07596 · Physical Review A

Optical switching and bistability in four-level atomic systems

Pardeep Kumar, Shubhrangshu Dasgupta

Published Aug 30, 2016• 34 citations
arXivHTMLPDF

Abstract

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 $\mathsf{Y}$-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.

DOI
10.1103/physreva.94.023851

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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.
We consider the behavior of optical bistability (OB) and multistability (OM) in a four-level atomic system involving a Rydberg state illuminated by a probe field as well as control and switching laser beams of larger intensity. When the switching field is absent, no OB arises because of the effect of Rydberg electromagnetically induced transparency. However, by application of the switching field, the hysteresis cycle appears to give rise to optical bistability, thanks to Rydberg electromagnetically induced absorption. It is further demonstrated that one can efficiently modify the OB threshold via suitable choices of system-controlling parameters. Interestingly, it is observed that this model can produce an optical switching between OB and OM with potential applications in logic-gate devices for optical communication.
Abstract We present a theoretical study on optical bistability (OB), optical multistability (OM) and tunable all-optical switching in a four-level cascade type atomic system. The density matrix formalism is used to find out the response of the probe laser field. We explore OB and OM under unidirectional optical ring cavity configuration. A comparison between the effects of electromagnetically induced transparency and electromagnetically induced absorption windows on OB is presented. The threshold of OB and the hysteresis width can be tuned effectively by adjusting the applied coherent fields. A transition from OB to OM and vice-versa can be easily achieved by tuning the field parameters and the atomic cooperation parameter. Dynamic control of the probe laser propagating through the medium has been investigated. We also demonstrate conversion of a continuous wave input probe field into output switched pulse. The ON/OFF time of the switched probe pulse can also be controlled by tuning the field parameters.
We present a novel approach to achieving intrinsic optical bistability and multistability through the interplay between saturable and reverse saturable absorption. Our findings reveal that the stable points on the optical bistability curve align with the switching points of intensity-dependent nonlinear absorption. By analyzing the optical properties of a four-level cascade quantum system, we demonstrate that these switching behaviors can be triggered by applying laser fields to induce nonlinear absorption. Furthermore, we propose a comprehensive model to describe the intensity-dependent nonlinear absorption within this quantum system. Finally, it is shown that the optical bistability switches to the multistability when the detuning of applied fields exceeds the multi-photon resonance condition.
Generation and control of the reverse saturable absorption (RSA) and optical limiting (OL) are investigated in a four-level Y-type quantum system. It is demonstrated that the applied laser fields induce the RSA and it can be coherently controlled by either intensity or frequency of the applied laser fields. The effect of the static magnetic field on the induced RSA is studied and we obtain that it has a constructive role in determining the intensity range in which the OL is established in the system. In addition, we find that the transmission of the suggested optical limiter can be decreased either by increasing the length of the medium or by getting the atomic system denser. Finally, the Z-scan technique is presented to confirm our theoretical results. The proposed scheme can be used in designing the coherent optical limiters with controllable threshold and intensity range of the OL.
We have studied nonlinear quantum optical effects in a doubly driven four-level atomic system in a homogeneously broadened regime. On application of two strong fields the system can be made to display double electromagnetically induced transparency (EIT) because of constructive quantum interference giving rise to nonlinear absorption at three-photon resonance condition. This fact may be used to obtain other phenomena like the effect of photon switching in a pulsed regime and large cross phase modulation (XPM) effect. We also show that the behaviour of spatial modulation of transparency of the weak probe beam in a three-level system interacted with a strong standing wave pump field can be dramatically changed in our four-level ladder type system. Here, strong coupling field acting on the uppermost transition is represented by a standing wave whereas strong pump and weak probe fields acting successively on the lower transitions are represented by travelling waves. The effect of double EIT on nonlinear light generation based on the four-wave-mixing (FWM) technique is also investigated. We have examined that the four wave mixing efficiency can be effectively controlled by varying the strength of the pump field.
The optical properties of a four-level double-V-type closed-loop system is studied in the multiphoton resonance condition. It is shown that the phase-dependent electromagnetically induced transparency (EIT) window is established and by changing the relative phase of the applied fields, EIT switches to electromagnetically induced absorption. Moreover, the optical bistability (OB) behavior of the system is studied, and it is demonstrated that the cross-Kerr-effect-induced OB depends on the relative phase of the applied fields, and it can be controlled by either intensity or relative phase of laser fields. By applying a microwave field to drive the hyperfine transition between two lower states, the optical properties change and OB switches to optical multistability. This can be used for detection of longer wavelengths than visible light.