Abstract

It is well known that the coherent, population trapping (CPT) leads to electro-magnetically induced transparency [1], while at the same time large coherence on a dipole-forbidden (Raman) transition (off-diagonal element of density matrix) may be created [2,3]. This coherence may be considered as strong atomic oscillator. A third field, interacting with the oscillator, creates a nonlinear polarization, which can produce radiation at sum or difference frequency. The nonlinear polarization in this case may be larger than that in traditional resonant nonlinear frequency conversion processes in gases. Thus, using CPT one may combine the minimum of absorption with maximum of nonlinear polarization responsible for frequency conversion, and. at the same time, with improvements in phase-matching and increasing of a medium density. Here, in contrast to [3], we investigate the processes of resonance four-wave mixing ω₄ = ω₁ − ω₂ + ω₃ of cw laser radiations under both single- and two-photon resonances in conditions of CPT. We assume that the fundamental fields at frequencies ω_1,2 are strong and their frequencies are resonant to corresponding transitions while the others are weak and arc off-resonant. For numerical analysis and illustrations the characteristics of transitions 6S – 6P − 5D − 6¹P⁰ – 6S of Ba atom are used.

© 1998 IEEE