We consider four-photon resonant six-wave mixing and show that the efficiency of harmonic generation is limited due to destructive interference between different optical harmonics. Following the idea of ``induced transparency'' of Harris et al. [Phys. Rev. Lett. 64, 1107 (1990)], it is shown that in the presence of an auxiliary electromagnetic coupling between the four-photon excited level and a third level, the interference between various excitation channels can be made constructive. Depending upon the strength of the auxiliary wave, absorption and dispersion-free propagation of the generated wave lead to high conversion efficiency.
We calculate the electric-field-induced second-harmonic-generation (EFISH) susceptibility of a homogeneously broadened two-level system with permanent dipole moments. This susceptibility has contributions from the second-harmonic-generation (SHG) susceptibility of the same system without the presence of a dc field and the four-wave-mixing (FWM) component of the susceptibility ${\ensuremath{\gamma}}_{\mathit{e}}$(-2\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},0). The magnitude and phase of the various contributions to EFISH arising from SHG and from FWM depend on the frequency of the fundamental optical field, the transition dipole moment ${\mathrm{\ensuremath{\mu}}}_{\mathit{a}\mathit{b}}$, the permanent dipole moment of the ground-state level ${\mathrm{\ensuremath{\mu}}}_{\mathit{a}\mathit{a}}$, and the difference of the ground- and excited-state dipole moments \ensuremath{\Delta}\ensuremath{\mu}. We present non-rotating-wave-approximation results for both the quasi-steady-state in systems where the dephasing time
T2 is much shorter than the population decay time
T1, so the polarization is in a steady state but the population is not, and for the complete steady state. Extraction of the SHG susceptibility from EFISH measurements is considered.