Spectral line narrowing due to velocity selective optical pumping on the D2 line hyperfine transitions in spatially restricted Cs vapor

We report on the investigation of narrow sub-Doppler-width resonances observed at the D2 line of Cs atoms (Fg = 4 → Fe = 3, 4, 5 hyperfine transitions), confined in thin optical cells. These cells have a thickness of L = 6λ (λ = 852 nm) and L = 700 μm, and are irradiated by two narrow-band laser radiations (one with fixed and the other with scanned frequency). We show that for zero frequency detuning of the fixed-frequency laser from the Fg = 4 → Fe = 5 hyperfine transition, its absorption spectrum in the thin cell consists of resonances, centered at the respective hyperfine transitions. The spectral widths of these resonances range from 2 to 20 MHz, and they are characterized by a good contrast and extremely small Doppler background. This spectrum is highly sensitive to the frequency offset of the fixedfrequency laser. The analysis leads to the conclusion that the spatial restriction of Cs vapor suppresses the interaction of the light with atoms with significant velocity component along the direction of the laser beam propagation.