Ultranarrow bandwidth nonlinear Faraday optical filter at rubidium D2 transition

We demonstrate a narrowband optical filter which operates on the 5(2)S(1/2) to 5(2)P(3/2) transition at 780 nm in rubidium vapor based on optical-pumping-induced dichroism combined with Faraday anomalous dispersion effects. Its peak transmission is 18.4(2)% at the 5(2)S(1/2), F = 2 to 5(2)P(3/2), F' = 1, 3 crossover transition, whereas at the 5(2)S(1/2), F = 2 to 5(2)P(3/2), F' = 2, 3 crossover transition the transmission is 18.6(2)%. Both transitions have a bandwidth (full-width at half-maximum of peak transmission) as narrow as 24.5(8) MHz, which is remarkably improved compared with the narrowest bandwidth as we know. This technique can also be applied to other alkali atoms.