Passive Fraunhofer-wavelength atomic filter at 422.7 nm.

We have demonstrated 25% internal photon conversion efficiency of a new passive atomic filter. The signal wavelength matches an intense Fraunhofer line at 422.7 nm, thereby offering enhanced sunlight rejection. A quasimolecular interaction promotes rapid energy transfer between the 4p (1)P(1) level and the 4p (3)P(J) level of neutral calcium for wavelength shifting of 422.7-nm light into 657.3-nm emission. We discuss augmentation of the photon conversion efficiency by radiation trapping.