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Conference on Lasers and Electro-Optics

Quantum switching based on the dark resonance

Byoung Seung HAM

Published Aug 29, 2005• 0 citations
Publisher

Abstract

Summary form only given. In a three-level system, two resonant optical fields can excite spin coherence on the ground states via two-photon resonance. One of the two-superposition states created by the atom-field interactions is decoupled from the excited state. Thus, the population can be trapped in that state showing non-absorption resonance. The essential feature of this decoupled state is dark resonance. Therefore, the existence of the dark resonance is a basis of non-absorption resonance, coherent population trapping, and electromagnetically induced transparency. In a four-level system, however, the two-laser induced spin coherence can be broken when a third laser is applied. At the same time, new coherence between the third laser and one of the pre-existed lasers is created. This phenomenon is useful for optical switching owing to annihilation and creation of the dark resonance in the four-level system. It is noted that the switching time depends on the dark resonance build-up time, which is inversely proportional to the applied Rabi frequency. Numerical simulations are shown for potentially ultrahigh-speed optical switches. Each diffracted four-wave mixing signal is proportional to the spin coherence strength. For a preliminary experiment, a spectral hole-burning crystal is used. The intensity of the diffracted signals is reversed at line center. This coherence switching effect is based on the dark resonance interactions.

DOI
10.1109/iqec.2000.908124

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Summary form only given. In a three-level system, two resonant optical fields can excite spin coherence on the ground states via two-photon resonance. One of the two-superposition states created by the atom-field interactions is decoupled from the excited state. Thus, the population can be trapped in that state showing non-absorption resonance. The essential feature of this decoupled state is dark resonance. Therefore, the existence of the dark resonance is a basis of non-absorption resonance, coherent population trapping, and electromagnetically induced transparency. In a four-level system, however, the two-laser induced spin coherence can be broken when a third laser is applied. At the same time, new coherence between the third laser and one of the pre-existed lasers is created. This phenomenon is useful for optical switching owing to annihilation and creation of the dark resonance in the four-level system. It is noted that the switching time depends on the dark resonance build-up time, which is inversely proportional to the applied Rabi frequency. Numerical simulations are shown for potentially ultrahigh-speed optical switches. Each diffracted four-wave mixing signal is proportional to the spin coherence strength. For a preliminary experiment, a spectral hole-burning crystal is used. The intensity of the diffracted signals is reversed at line center. This coherence switching effect is based on the dark resonance interactions.
Summary form only given. The phenomenon of "dark resonances" or coherent population trapping is a well known concept in optics and laser spectroscopy. It can be considered as the basis for such effects as electromagnetically induced transparency and lasing without inversion. We report the observation of the novel spectral features which arise in systems where multiple quantum superposition states are coupled and interact coherently. These novel resonances correspond to the recently predicted "double-dark states". In the experiment, an optically thin ensemble of rubidium-37 atoms in zero magnetic field is excited in a /spl Lambda/ configuration with circularly polarized light from a pair of 789 nm diode lasers.
We experimentally observe a non-linear absorption resonance in one of the two probe beams of a double Λ system which is interacting with three optical beams. This probe beam counterpropagates to the other two beams, both of which are co-propagating. The absorption resonance in this probe beam arises due to interaction between the dark states of the double Λ created by the co-propagating beams. The double Λ configuration of levels is in the D2 hyperfine manifold of 87Rb atoms, and the interaction is studied at room temperature. The non-linear absorption resonance is shown to be a three-photon process arising whenever the two co-propagating beams satisfy the two-photon Raman condition, necessary for electromagnetically induced transparency. Through a dressed state analysis, we show that the absorption feature is a result of interference between the absorption pathways to the excited states, from the ground states of the double Λ. The non-linear nature of the probe beam absorption enables it to perform like a directional optical switch. This absorption switch operates conditioned on non-absorption of a single photon of the same frequency due to a coherent process, in a beam traveling in the opposite direction.
A new scheme is suggested to manipulate the probe transitions (and hence the optical properties of atomic vapors) via double-control destructive and constructive quantum interferences. The influence of phase coherence between the two control transitions on the probe transition is also studied. The most remarkable feature of the present scheme is that the optical properties (absorption, transparency and dispersion) of an atomic system can be manipulated using this double-control multi-pathway interferences (multiple routes to excitation). It is also shown that a four-level system will exhibit a two-level resonant absorption because the two control levels (driven by the two control fields) form a dark state (and hence a destructive quantum interference occurs between the two control transitions). However, the present four-level system will exhibit electromagnetically induced transparency to the probe field when the three lower levels (including the probe level and the two control levels) form a three-level dark state. The present scenario has potential applications in new devices (e.g. logic gates and sensitive optical switches) and new techniques (e.g. quantum coherent information storage).
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.
An optical switching method based on dark resonance is proposed for ultrafast/ultrawide fibre-optic communications. The proposed switching method is based on dark-resonance induced refractive index changes in a three-level system interacting with two-colour laser lights. Two different mechanisms in the proposed optical switching method are discussed for the use of electromagnetically induced transparency and two-photon coherence induction. They are also compared for suitable applications in fibre-optic communications. Finally, a hyper-THz optical modulator is proposed, limited by current technologies due to carrier lifetime or population relaxation decay time.