RELATIONSHIP BETWEEN 2ND-HARMONIC GENERATION AND ELECTRIC-FIELD-INDUCED 2-HARMONIC GENERATION

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 gamma-e(-2-omea;omega,omega,0). The magnitude and phase of the various contributions to EFISH arising from the SHG and from FWM depend on the frequency of the fundamental optical field, the transition dipole moment mu-ab, the permanent dipole moment of the ground-state level mu-aa, and the difference of the ground- and excited-state dipole moments DELTA-mu. We present non-rotating-wave-approximation results for both the quasi-steady-stage in systems where the dephasing time T2 is much shorter than the population decay time T1, so the polarization is in the steady state but the population is not, and for the complete steady state. Extraction of the SHG susceptibility from the EFISH measurements is considered.