The operation of a narrow-linewidth optical filter based on the 4P1/2 → 8S1/2 excited-state transition in potassium vapor is reported. The 4P1/2 state is excited by a circularly polarized, 769.9-nm, 10-ns pulse from a dye laser. A linearly polarized, time-sequenced, and spatially overlapped probe pulse at 532.33 nm completes the transition to the 8S1/2 state. The peak filter transmission is ~40% with a bandwidth of less than 4 GHz. Corroborative experimental results suggest that the rotation of probe-pulse polarization by an induced circular birefringence is the dominant mechanism behind the filter operation.
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