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Journal of Physics B Atomic Molecular and Optical Physics

Enhancing the index of refraction under convenient conditions

Carsten Szymanowski, Christoph H. Keitel

Published Dec 14, 1994• 33 citations
Publisher

Abstract

We investigate the possibility of increasing the achievable index of refraction and the slope of the dispersion while simultaneously minimizing absorption in a driven two-level system. The analysis is performed non-linearly in both the probing laser field and Doppler and collision broadening have been incorporated for the sake of a realistic treatment. We compare the achievable index of refraction without absorption quantitatively with known results of several favorable three- and four-level systems, find our system less sensitive to Doppler broadening in a collinear set-up of both fields and apply our considerations to the particular example of caesium.

DOI
10.1088/0953-4075/27/23/020

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We investigate the possibility of increasing the achievable index of refraction and the slope of the dispersion while simultaneously minimizing absorption in a driven two-level system. The analysis is performed non-linearly in both the probing laser field and Doppler and collision broadening have been incorporated for the sake of a realistic treatment. We compare the achievable index of refraction without absorption quantitatively with known results of several favorable three- and four-level systems, find our system less sensitive to Doppler broadening in a collinear set-up of both fields and apply our considerations to the particular example of caesium.
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Refractive index enhancement accompanied by vanishing absorption in a four-level atomic system interacting with two control and two probe fields in a regime of four-wave mixing (FWM) has been predicted and studied in the present work. We analyzed the maximal index enhancement in the four-level FWM system and gave index estimates for a real atomic gas of 40{}^{40}40K, taking into account its multilevel structure and collisional and Doppler broadenings at large atomic densities. We also discussed how vanishing absorption with no nearby amplification can be realized in a two species system, consisting of a four-level FWM and a two-level system, where the latter provides additional absorption for one of the probe fields. We numerically estimated the index change in a system composed of 40{}^{40}40K and 39{}^{39}39K gases.
. We investigate the possibility of enhancing the refractive properties in a nonabsorbing medium via two fundamentally different schemes. First there is the coherent preparation of three-level atoms where absorption is cancelled due to destructive interference while the refractivity is not hampered in the same way. There also is the possibility of cancelling absorption via a mixture of absorbing and emitting two-level atoms without the need of a coherent preparation. One drawback here, however, is high sensitivity to Doppler broadening, collisions and number fluctuations which makes this scheme practically infeasible. Introduction The various application of atomic coherence in laser physics and quantum optics has recently attracted considerable interest. It has been shown that atomic coherence can lead to absorption cancellation (Alzetta, et al., 1976; and Gray, et al., 1979) and quenching of spontaneous emission noise (Scully, 1985). More recently, the notion of noninversion lasing has received attention, and it was shown that atomic coherence leading to cancellation of absorption does not necessarily influence emission (Harris, 1989; Scully, et al., 1989; and Kocharovskaya and Khanin, 1988). There has been extensive research on many schemes that involve coherence between an upper or lower level laser doublet due to various means: microwave or Raman coherent coupling and spontaneous and incoherent pumping coupling just to name the most important.
The absorption and dispersion properties of an N-type four-level system with Doppler(broadening) effect were investigated.In this system we considered two coherent fields and a probe field.When the second coherent field is absent,this system is a single three-level configuration,and the absorption line is just one clear electromagnetically induced transparency(EIT) window.While the second coherent field exists,there are two EIT windows which become wide and deep with the(coherent) intensity increasing.The position width and absorbance of the transparency windows can be manipulated by adjusting the detuning of the driving field.In addition,we also found that the medium shows a large refractive index at each transparency window,and the group velocity of the probe laser can be reduced.
We investigate the influence of Doppler broadening on absorption-dispersion properties in a four-level atomic system that can evolve from a normal dispersion to an anomalous dispersion. Our results show that the absorption-dispersion properties become strongly dependent on the propagation directions of the applied fields if Doppler broadening is taken into account. Especially, the switchover in the sign of the dispersion is still achievable even in the presence of Doppler broadening if properly arranging the propagation directions of the applied fields, which is in contrast with the otherwise behaviours in some other configurations.