Abstract

When a high-density (~10¹⁵-cm⁻³) sodium vapor is illuminated by an intense pulsed dye laser detuned to the blue of the D₂ line, the forward radiation contains a coherent emission to the blue of the D₁ line. Such off-resonant emission (the blue peak at ω_B) near the D₁ line was observed both in potassium¹ and sodium^2–4 in different experimental conditions. Electronic Raman and FWM processes (Fig. 1) are excluded, since the blue peak frequency changes very little with laser detuning. Collision-induced processes are not involved as no property of this emission depends on the buffer gas pressure or kind. Laser ASE is rejected since the exciting radiation was spectrally filtered or produced by Raman shifting of the ruby laser.¹ Self-phase modulation as a primary for the blue peak emission has major difficulties as it produces a laser detuning-dependent emission frequency and does not allow for a narrow bandwidth (1 cm⁻¹) of the emission. The model of FWM by a three-level system, proposed to explain the blue peak,³ is inconsistent with a Manley-Rowe relation due to the experimental absence of the obligatory counterpart to the ω_B photon.

© 1987 Optical Society of America