Abstract:
The study of collective light-matter interactions, where the
dynamics of an individual scatterer depend on the state of the entire
multi-scatterer system, has recently received much attention both in the area
of fundamental research and photonic technologies. Cold atomic vapors represent a particularly
exciting system for studying such effects because light-based manipulation of
the internal and center-of-mass states of the atoms can lead to reduced
instability thresholds and new phenomena. Previous investigations have required
single-mode cavities to realize strong light-mediated atom-atom interactions,
though, which limit the observable phenomena. Here we demonstrate steady-state,
mirrorless superradiance in a cold vapor pumped by a pair of weak optical fields.
Beyond a critical pumping strength, the vapor spontaneously transforms into a
spatially self-organized state: a density grating forms. Scattering of the pump
beams off this grating generates new optical fields that act back on the vapor
to synergistically enhance the atomic organization. This system has
applications in many-body physics with long-range interactions as well as
all-optical and quantum information processing.
I will discuss how non-equilibrium behavior of the gas is a crucial
component of our experiments and I will speculate on how our observations might
obscure or prevent the observation of recent proposals for quantum phase
transitions in this general type of system.