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Optics Express

EIT-based all-optical switching and cross-phase modulation under the influence of four-wave mixing

Meng‐Jung Lee, Yi‐Hsin Chen, I-Chung Wang, Ite A. Yu

Published Apr 27, 2012• 21 citations
Publisher

Abstract

All-optical switching (AOS) or cross-phase modulation (XPM) based on the effect of electromagnetically induced transparency (EIT) makes one photon switched or phase-modulated by another possible. The existence of four-wave mixing (FWM) process greatly diminishes the switching or phase-modulation efficiency and hinders the single-photon operation. We proposed and experimentally demonstrated an idea that with an optimum detuning the EIT-based AOS can be completely intact even under the influence of FWM. The results of the work can be directly applied to the EIT-based XPM. Our work makes the AOS and XPM schemes more flexible and the single-photon operation possible in FWM-allowed systems.

DOI
10.1364/oe.20.011057

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Frontiers of Physics•Jul 21, 2014•5 citations
All-optical switching (AOS) or cross-phase modulation (XPM) based on the effect of electromagnetically induced transparency (EIT) makes one photon switched or phase-modulated by another possible. The existence of four-wave mixing (FWM) process greatly diminishes the switching or phase-modulation efficiency and hinders the single-photon operation. We proposed and experimentally demonstrated an idea that with an optimum detuning the EIT-based AOS can be completely intact even under the influence of FWM. The results of the work can be directly applied to the EIT-based XPM. Our work makes the AOS and XPM schemes more flexible and the single-photon operation possible in FWM-allowed systems.
In third order nonlinear mediums, the intensity of one optical field can modify the phase of another optical field. This is known as cross-phase modulation (XPM) and is important for Quantum Information Processing (QIP) as phase shifts of single-photons that exceed π rad can be used to implement two-qubit quantum logic gates. For a conventional XPM scheme, the Kerr nonlinearity responsible for XPM is too small for appreciable phase shifts to be achieved with low intensity optical fields. However, Electromagnetically Induced Transparency (EIT) can increase this nonlinearity by several orders of magnitude. This thesis discusses the experimental construction of a laser-atomic system capable of EIT modulated XPM. This includes construction of a rubidium magneto-optical trap (RbMOT), characterization of this RbMOT, demonstration of EIT in the RbMOT, and a system capable of detecting optical phase shifts produced by EIT. The talk will conclude with an outlook on further implementations to the experiment that will increase this nonlinearity such that phase shifts at the single-photon level are possible. | 76 pages
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.
We report on the direct measurement of low-light-level cross-phase modulation (XPM) based on electromagnetically induced transparency (EIT) in the pulsed regime. A phase shift of 0.02 rad of a probe pulse modulated by a signal pulse with a peak intensity of 3 $\mathrm{\ensuremath{\mu}}\mathrm{W}/{\mathrm{cm}}^{2}$ was observed, which is the lowest intensity ever achieved for the N-type EIT-based XPM system. The experimental data make a quantitative prediction of the single-photon-level XPM phase shift arising from two interacting pulses with unequal group velocities that is consistent with the theoretical predictions proposed by Harris and Hau [Phys. Rev. Lett. 82, 4611 (1999)]. Furthermore, a proof-of-principle experiment demonstrating an enhancement of the nonlinear optical Kerr effect by shining the signal pulse twice is presented.
The switch between the enhancement and suppression of the four-wave mixing (FWM) process in an electromagnetically induced transparency (EIT) window via quantum interference has been demonstrated experimentally. The enhancement and suppression of the FWM signal, as well as the EIT window, switch with a change in the probe detuning. Also, by controlling the powers of the dressing and probe fields, the enhancement and suppression of the FWM signal can be switched and an optimized enhancement can be obtained. Such a switch of the enhancement and suppression of the FWM signal intensity can find potential applications in optical switches, optical communication and quantum information processes.
We have found a novel symmetry for XPM(Cross Phase Modulation) systems .We prove that this symmetry is a necessary condition for a single multiphoton process to be visible in more than one field.We have found, two EIT(Electromagnetically Induced Transparency) systems that show cases of this symmetry.The above process has the possibility of leading to strong sources of temporally entangled photons of different frequency. For cases that do not show this symmetry,more than one species of multiphoton processes can take place and each one is visible in one field only. When there is a symmetry, we predict cross field noise correlations for the scattered fields.Cross field noise correlations have recently been seen in a different situation that can provide interesting variation to our study,namely the generated fields in FWM(Four Wave Mixing). We postulate or conjecture the existence of two types of asymmetric multiphoton processes.We show only one of these is possible. One of systems we use for this purpose generates a χ(9)\chi^{(9)}χ(9) absorption resonance that is observable on the same trace that shows linear features, in an experiment described in a previous work but the order of nonlinearity newly interpreted using this work.This high order nonlinearity should be very sensitive to cross field correlations.We show how it can be used to test our conjecture of asymmetric processes. We also come across an important χ(5)\chi^{(5)}χ(5) absorption,comparable to linear absorption, that is anomalous.The anomaly is that the Imaginary part of this XPM nonlinearity can cause squeezing.We give a qualitative argument which shows that we don't expect the Imaginary part of the XPM nonlinearities we usually come across, to cause squeezing.This promising source of squeezing, is qualitatively different from the other sources hitherto known.
The dressed four-wave mixing (FWM) in a four-level 85 Rb atomic system, experimentally demonstrated in this paper, is comprised by two coexisting processes. One is emission signal due to enhanced nonlinear via electromagnetically induced transparency (EIT). The other is the Bragg reflection of probe beam because of the created photonic band gap (PBG), which is affected by both linear and third-order nonlinear susceptibility. Moreover, we have demonstrated that different experimental parameters can significantly influence the measured signal with flexibly controlled PBG. These studies are found useful for understanding the fundamental mechanisms in generated FWM processing.