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Abstract
We propose a novel scheme to realize an optical waveguide induced by an active Raman gain (ARG) process in a four-level N-type atomic system. Because of the nature of the ARG, there are two distinct features related to the waveguide: i) It is not absorptive, on the contrary, weak gain is presented; ii) It can be improved by the Doppler effect in the sense that the dispersion is enhanced while the gain is further reduced. This is in sharp contrast to the previously considered schemes where usually the optical induced waveguide is passive and is severely attenuated by the Doppler effect. We then study the paraxial light propagation in the waveguide which shows that the propagation dynamics is lossless and diffractionless.
Highlights
Optical diffraction, originating from the wave nature of light, is inevitable during beam propagation
Apart from the physical mechanism based on structured control beam, another interesting approach is to explore atomic motions and collision to cancel paraxial diffraction[24,25,26,27]. In most of these proposals, the strong control and weak probe beams are coupled to the atomic transitions in the near-resonance regime
This may be subject to two drawbacks: (i) In the near-resonance regime, the probe beam propagates without diffraction owing to the induced waveguide but often accompanied with strong absorption, leading to a severely attenuated output. (ii) The atomic response will be usually weakened by the Doppler effect, making it hard and impractical to work at room temperature
Summary
We plot the real and imaginary parts of the linear susceptibility for different n respectively It would be worthwhile to discuss the underlying physics resulting in better waveguide structure as n increases We understood this as follows: in the dressed-state picture, the effect of the Raman field would be to introduce a space-dependent Stark shift equal to Ωr2(x)/∆r for the probe transition. Better waveguide structures can be obtained by increasing the temperature, and the probe field would experience diffractionless and gain-free propagation
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