Optimized phase masks for absorption of ultra-broadband pulses by narrowband atomic ensembles

Abstract

By combining a genetic algorithm and spatial light modulator, we theoretically analyze how to improve two-photon cascaded absorption in atomic ensembles, inspecting the impact of various configurations and parameters in the optimized phase mask. At low atomic densities, we compare cases of sequential transitions with two photons coming from the same pulse or from two different pulses. For the former, we predict enhancement by a factor of 9.5, similar to what was previously reported in the literature [Phys. Rev. Lett. 86, 47 (2002)PRLTAO0031-900710.1103/PhysRevLett.86.47]. For the latter, on the other hand, we obtain an enhancement factor of 26 times. 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 enhancements by factors up to three for two-photon absorption after propagating through large optical depths. However, for the experimental conditions considered in a previous work by Carvalho et al. [Phys. Rev. A 101, 053426 (2020)PLRAAN1050-294710.1103/PhysRevA.101.053426], in terms of control power and optical depths, we show that this enhancement in two-photon absorption would still result in just a modest increase of the absorption of a weak probe pulse.