Narrowband tunable filter based on velocity-selective optical pumping in   an atomic vapor
  We demonstrate a tunable, narrow-band filter based on optical-pumping-induced circular dichroism in rubidium vapor. The filter achieves a peak transmission of 14.6%, a linewidth of 80 MHz, and an out-of-band extinction >35 dB. The transmission peak can be tuned within the range of the Doppler linewidth of the D1 line of atomic rubidium at 795 nm. While other atomic filters work at frequencies far from absorption, the presented technique provides light resonant with atomic media, useful for atom-photon interaction experiments. The technique could readily be extended to other alkali atoms. 
Alessandro Cere, Valentina Parigi, Marta Abad, Florian Wolfgramm, Ana   Predojevic and Morgan W. Mitchell