Identification of collective effects in recoil halos of ultracold strontium clouds

Abstract

Ultracold atomic clouds interacting with laser beams represent an ideal system for studying the light-matter interaction on a fundamental level, where the photonic recoil takes part in the dynamics. Surprisingly many seemingly obvious phenomena are yet not fully understood. An example is the question, brought up by Minkowski and Abraham a century ago, about the correct value for the momentum of light in a dielectricmedium. Although recent theoretical and experimental work seems to converge, a microscopic understanding of how the photon momentum is distributed over the atoms is still lacking. Another example, is a strange asymmetry experimentally observed in matter wave superradiance. According to theory, this phenomenon should not depend on the sign of the detuning of the incident laser beam. But experiments only observed it for red detuning.Our team of the Grupo de Optica at the Instituto de Física de São Carlos has, in collaboration with international researchers, developed a theory of cooperative scattering of light from atomic clouds with particular emphasis on radiation pressure effects. We were able to bridge the gap between scattering from localized particles and smooth density distributions, such as dielectric media. Within the framework of this theory, we recently predicted the occurrence of Mie resonances in atomic clouds. Currently,we investigate recoil halos produced in Bose-Einstein condensed clouds by the scatteringof single photons. Interestingly, the halos seem to contain direct measures of the distribution of photonic recoil inside the cloud, and they are asymmetric with respect to red or blue detuning.Beginning of 2011, we have started at the Instituto de Física de São Carlos the construction of an experiment aiming at preparing ultracold strontium clouds. In February 2012, we obtained our rst signal of a (still 100uK hot) strontium cloud confined in a (blue) magneto-optical trap. A second stage (red) magneto-optical trap and evaporative cooling will enable us to lower the temperature to about 100nK and toproduce single-photon recoil halos. One of our objectives is to study collective radiationpressure and its impact on the halos. We want to learn, how, instead of being localized to single atoms, the recoil is distributed over the cloud. We can already anticipate, that this will provide us with a microscopic insight in the Abraham-Minkowski controversy and with an explanation for the red-blue asymmetry.The experiments will be performed by Michael Holynski, who applies with this proposal for a post-doc position from FAPESP. He is a very motivated young PhD student currently at the University of Birmingham, UK. During his graduate studies, he has been trained in the high performance laboratories of Prof. Kai Bongs. Withhis post-doctoral stay at S~ao Carlos we hope to establish a stable cooperation between our team and his former research group. Both groups will, in this way, mutually profit from technological and scientific know-how available in the partner group. (AU)