Rydberg Atoms and Radiation in a Resonant Cavity Ii. Experiments

We present various experiments involving Rydberg atoms interacting with resonant microwave electromagnetic fields inside a cavity, allowing to check different features of the theory presented in the previous paper. We first describe experiments about superradiant emission by Rydberg states in the cavity, also called transient Rydberg atoms maser; the evolution of the system is detected either on the populations by selective field ionization technique or on the microwave field itself by heterodyne detection. Very low thresholds, down to 100 atoms, for the maser emission have been observed. We then describe various aspects of collective absorption of radiation by very excited atoms including the observation of Rabi nutation in a small applied resonant field, the study of the quantum efficiency of this set-up considered as a microwave photon counter, and finally the unexpected characteristics of the blackbody radiation absorption in the cavity. In this second paper, we present various experiments on Rydberg states interacting with resonant millimetric electromagnetic waves inside a cavity. These experiments show various interesting physical effects calculated in the previous paper and provide a test for the theoretical models. In a first paragraph, we will describe the experimental techniques which are common to all the experiments that will be presented afterwards. We will devote the second paragraph to an experimental study of the different characteristics of the collective Rydberg atom transient emission in the cavity (the "Rydberg atom maser"). In the last paragraph, we will describe several experiments involving the absorption of radiation by very excited states in the cavity and showing the collective behaviour of the Rydberg atoms in this absorption process. 1. Experimental techniques 1.1. P£ep_ar.ation_of_the_p_hy_sicaX_sy_stem In order to prepare the physical system which has been theoretically studied in the previous paper, i.e. a set of 2-level Rydberg atoms in resonant interaction with an electromagnetic mode inside a cavity, we use the experimental set-up which Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982221