Classical analog of electromagnetically induced transparency

Imagine a medium that strongly absorbs a light beam of a certain frequency. Add a second light source, with a frequency that would also be absorbed by the medium, and the medium becomes transparent to the first light beam. This curious phenomenon is called electromagnetically induced transparency ~EIT!. It usually takes place in vapors of threelevel atoms. The light sources are lasers that drive two different transitions with one common level. Most authors attribute the effect to quantum interference in the medium, involving two indistinguishable paths leading to the common state. In addition to the phenomenon of electromagnetically induced transparency, the dispersive properties of the medium are significantly modified as has been recently demonstrated by the impressive reduction of the group velocity of a light pulse to only 17 m/s and the ‘‘freezing’’ of a light pulse in an atomic medium. In this paper we develop a classical analog of electromagnetically induced transparency and discuss a simple experiment that can be carried out in an undergraduate physics laboratory. The stimulated resonance Raman effect has already been modeled classically in a system of three coupled pendula by Hemmer and Prentiss. Even though many aspects of electromagnetically induced transparency were already present in the stimulated Raman effect, as can be seen in Ref. 7, EIT had not been observed at that time, and the dispersive features were not considered. Our model involves only two oscillators with linear coupling. The experiment is performed with RLC circuits. The interest of such an experiment and the purpose of this paper is to enable undergraduate students to develop their physical intuition for the coherent phenomena that occur in atomic systems.