Electromagnetically induced waveguiding in double-Λ systems

Near the threshold for electromagnetically induced transparency (EIT) or coherent population trapping (CPT), two-photon-resonance-enhanced self-focusing of a Λ system can be exploited to induce spatial confinement in a second, diffracting Λ system. The diffracting Λ system is characterized by parameters below the EIT or CPT threshold, and the two Λ systems must be coupled to form a closed-loop double-Λ system. The waveguiding effect is shown to be strongly phase dependent, indicating that it derives from the phase-dependent effective third-order susceptibility rather than the phase-independent effective first-order susceptibility, as is the case in previously studied systems. We also show that when the second Λ system initially involves only a single laser beam, the loop is completed by the efficient generation of radiation at the four-wave-mixing frequency, within a propagation distance much shorter than the diffraction length. Both the applied and generated fields exhibit electromagnetically induced waveguiding.