Negative dispersion without absorption in atomic ensembles

Abstract

Dott. Alessandra Rocco Negative dispersion without absorption in atomic ensembles In this work the optical properties of a calcium atomic beam, strongly driven on the transition, are investigated, in order to obtain a coherently prepared medium with the right amount of negative dispersion at a point of zero absorption. From the theory it is well known that a strongly driven Two-Level system exhibits such properties. Such a medium could in particular be used to fill an optical resonator in order to obtain a broadband high finesse cavity (White Light Cavity). For the investigations a spectroscopic method had been used, consisting of a Mach-Zehnder interferometer with a phase modulated arm. In this way it is possible to perform a high sensitive simultaneous measurement of both phase shift and absorption profiles in a calcium atomic beam, for an increasing driving field, which frequency is stabilized on the transition 4s2 S0 4s4p P1 at 423nm. For the first time both absorption and phase shift were systematic investigated as function of the driving field intensity, while it was developed a new geometrical configuration for the atomic oven which permitted to reach a higher particle density in the interaction zone. It was then possible to calculate the values of derivative of phase shift and absorption at the transition frequency, where the dispersion results anomalous, having fitted the measured curves to the phase shift and absorption profiles for a strongly driven Two-Level Atom. An analysis of the evolution of the both the quantities as driving field function could be then performed. The theoretical analysis of the strongly driven two level system could then be experimentally verified, especially that the absorption decreases stronger than dispersion with an increasing driving field. It was in fact measured a decreasing of a factor 53 in the dispersion, between the not driven and driven atomic system while the corresponding absorption decreased of a factor 330, for a value of the driving field of D 2π54.4MHz. The possibility of inserting the medium inside an optical resonator to obtain a broadband high finesse cavity has also been discussed. The data analysis showed that even if the λ compensation was reachable in the achieved condition, the residual absorption was still too high to realize a White Light Cavity. The configuration for further development of the system which should allow this realization was also analyzed, together with a comparison with the performances of different atomic systems.