The effects of velocity-selective optical pumping (VSOP) on Doppler-free resonance signals have been investigated in the Na $D_1$ and $D_2$ hyperfine lines. Three Doppler-free spectroscopic techniques, saturated absorption spectroscopy, polarization spectroscopy, and polarization modulation spectroscopy were employed using a frequency-stabilized, single-mode cw R6G dye laser as their two counterpropagating pump and probe beams. The experiments were carried out for various polarization configurations of both beams in two different intensity regimes of the probe beam, the "Weak-probe" and "strong-probe". These experimental approaches and conditions allow us to investigate the various interesting effects of VSOP.
In the weak probe regime, a theoretical analysis was extended to include the VSOP and saturation effects of the pump beam on the basis of the steady-state density matrix or stationary rate-equation in a four level model of two ground and two excited states. The theoretical results were in good agreement with the experimental ones.
In the strong probe regime, an additional narrow structure (possibly narrower than the natural linewidth) superimposed on the Doppler-free, power-broadened resonance was observed in the saturated absorption and polarization spectroscopy of the Na $D_1$ line. This structure was recorded only in the $F_g=2-F_e=1$ hyperfine resonance and $F_g=2-F_e$=1,2 crossover resonance under the present experimental conditions. The experimental observations include some new features in the line shape which are not fully understood by an existing VSOP theory, non-stationary rate-equation theory. The coherent population trapping due to VSOP is qualitatively discussed as a mechanism responsible for the effect. Possible applications of the subnatural dip are considered.
