Rydberg-atom masers. I. A theoretical and experimental study of super-radiant systems in the millimeter-wave domain

Abstract

The operation of maser devices using highly excited Rydberg atoms as active medium is investigated. These systems, working in a transient regime reminiscent of super-radiance generate few-hundred-ns-long bursts of mm-wave radiation. They are characterized by extremely low inversion density thresholds ($N_0\sim {10}^4$ atoms typically) and very small peak power outputs (in the ${10}^{-11\mathrm{}}$- to ${10}^{-13\mathrm{}}$-watt range). This study is interesting in the context of new tests of super-radiance and electrodynamics theory. It also opens new possibilities in the important domain of mm-wave amplification and detection. In this paper, we present a simple semiclassical theory of transient Rydberg maser operation and we report the experimental observation of a large number of maser emission lines on Na atoms, spanning a wide frequency interval ($60<\mathrm{}\nu <950\mathrm{}$ GHz). The technique used to monitor the emission is the indirect field ionization procedure which reveals fast radiative transfers occurring between Rydberg levels. Another method involving the direct detection of the maser radiation by a heterodyne receiver is described in a subsequent paper, along with a report on the investigation of various effects related to the triggering of the transient maser emission by external radiation.