Optimized Phase Masks for Absorption of Ultra-Broadband Pulses by Narrowband Atomic Ensembles

By combining genetic algorithm and a spatial light modulator we demonstrate theoretically how to improve a two-photon cascade absorption in atomic ensembles using optimized phase masks. At low atomic densities, we compare the cases of sequential transitions with the two photons coming from the same pulse and from two different pulses. We achieved an enhancement by a factor of 9.4 ± 0.2, about 30% higher than previously reported. For the later, we obtained 20 ± 3 for the enhancement factor. This absorption of two photons by different pulses is of particular interest for the storage of ultra-broadband single photons by atomic ensembles, in which case the second photon would come from a control pulse. We investigate this process as a function of the atomic density, demonstrating still enhancements by factors on the order of 6 for the two-photon absorption after propagating through large optical depths, on the order of hundreds.