Light narrowing of magnetic resonance lines in dense, optically pumped alkali-metal vapor

A new and unusual phenomenon which we call light narrowing is reported and discussed in this paper. We discovered this effect in dense, spin-polarized cesium vapor optically pumped with a cw blue dye laser beam tuned to the second resonance D₁ line (4593 {}). We observe a significant narrowing of the radio-frequency power-broadened magnetic resonance lines (linewidths narrow by as much as a factor of 2.5) when the intensity of the circularly polarized incident dye laser beam is increased by either focusing the beam or by the removal of attenuating filters from the focused beam. The magnetic resonance linewidths in spin-polarized cesium vapor were measured over a wide range of cesium number densities (5×{}10¹² cm^-3 {≤}[Cs]{≤}1×{}10¹⁶ cm^-3). This corresponds to cesium spinexchange rates of 4.5×{}10³ to 9×{}10⁶ sec^-1. For low cesium number densities (5×{}10¹² [Cs]3×{}10¹⁴ cm^-3) the light-narrowing effect is large (a factor of 2.5) and independent of [Cs]. In the region of 3×{}10¹⁴ to 1×{}10¹⁵ cm^-3 in number densities, the light-narrowing effect decreases with increasing [Cs]. At high cesium number densities ([Cs]> 1×{}10¹⁵ cm^-3) this light-narrowing effect almost completely disappears. In the limit of low-radio-frequency power the magnetic resonance linewidths for focused and unfocused dye laser beam are nearly the same. Experimental observations on this new effect are presented in detail. In the latter part of this paper a self-contained theoretical treatment of the light-narrowing effect is developed. Using Bloch equations in the presence of optical pumping, spin relaxation (diffusion, electron randomization), rapid spin exchange, and radio-frequency magnetic field, expressions for magnetic resonance line shapes are derived. In general, we find good agreement between our experimental results and the theory.