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arXiv:0807.2584 · Physical Review Letters

Refractive Index Enhancement with Vanishing Absorption in an Atomic Vapor

Nicholas Proite, Brett Unks, Jonathan Tyler Green, Deniz D. Yavuz

Published Sep 29, 2008• 70 citations
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Abstract

We report a proof-of-principle experiment where the refractive index of an atomic vapor is enhanced while maintaining vanishing absorption of the beam. The key idea is to drive alkali atoms in a vapor with appropriate control lasers and induce a gain resonance and an absorption resonance for a probe beam in a two-photon Raman configuration. The strength and the position of these two resonances can be manipulated by changing the parameters of the control lasers. By using the interference between these two resonances, we obtain an enhanced refractive index without an increase in the absorption.

DOI
10.1103/physrevlett.101.147401

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Physical Review A•Dec 15, 2011•26 citations
We report a proof-of-principle experiment where the refractive index of an atomic vapor is enhanced while maintaining vanishing absorption of the beam. The key idea is to drive alkali atoms in a vapor with appropriate control lasers and induce a gain resonance and an absorption resonance for a probe beam in a two-photon Raman configuration. The strength and the position of these two resonances can be manipulated by changing the parameters of the control lasers. By using the interference between these two resonances, we obtain an enhanced refractive index without an increase in the absorption.
It has recently been predicted and experimentally demonstrated that the refractive index of a vapor may be enhanced while maintaining vanishing absorption by using the interference of two Raman transitions, one absorptive and one amplifying in nature. In this paper, we present a detailed experimental study of this technique in a 1-mm-long rubidium (Rb) vapor cell with densities exceeding 1014{10}^{14}1014 cm${}^{\ensuremath{-}3}$. We study the optimization of the achieved refractive index as various experimental parameters are varied and discuss a number of limitations of the current experiments. We also present a detailed discussion of possible experimental improvements and future prospects of this technique.
A scheme is proposed to achieve substantial controllable phase modulation for a probe field propagating through a thermal atomic vapor in double-$\ensuremath{\Lambda}$ configuration. The phase modulation is based on manipulating the linear susceptibility of the probe field, paraxial diffraction is eliminated by exploiting the thermal motion of atoms, and residual absorption is compensated via an incoherent pump field. As a result of the homogeneous change in the refractive index, a strong controllable uniform phase modulation without paraxial diffraction is achieved essentially independent of the spatial profile or the intensity of the probe field. This phase shift can be controlled via the intensities of the control or the incoherent pump fields. A possible proof-of-principle experiment in alkali-metal atoms is discussed.
Enhancement and control of the index of refraction in a mixture of two three-level atomic species that form a pair of far-detuned $\ensuremath{\Lambda}$ schemes under two-photon resonance has been studied. We employ the density-matrix approach to properly take population relaxation into account and to describe the interaction of each $\ensuremath{\Lambda}$ system with the electromagnetic fields. Both $\ensuremath{\Lambda}$ systems are driven by a corresponding far-detuned coherent field at one atomic transition and are probed by the same weak field. In the dressed-state basis, it represents a superposition of effective two-level subsystems with the positions, widths, and amplitudes of the resonances controlled by the driving fields and allows for efficient control of the susceptibility of the total system; leading to refractive index (RI) enhancement with vanishing absorption in the absence of amplification. We analyze the experimental implementation of such a system in a cell of Rb atoms with a natural abundance of isotopes. An upper limit estimate of the RI enhancement is obtained.