Steady-state, cavityless, multimode superradiance in a cold vapor

Abstract

We demonstrate steady-state, mirrorless superradiance in a cold vapor pumped by weak optical fields. Beyond a critical pump intensity of 1 mW/cm 2 , the vapor spontaneously transforms into a spatially self-organized state: a density grating forms. Scattering of the pump beams off this grating generates a pair of new, intense optical fields that act back on the vapor to enhance the atomic organization. We map out experimentally the superradiant phase transition boundary and show that it is well-described by our theoretical model. The resulting superradiant emission is nearly coherent, persists for several seconds, displays strong temporal correlations between the various modes, and has a coherence time of several hundred µs. This system therefore has applications in fundamental studies of many-body physics with long-range interactions as well as all-optical and quantum information processing. 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 in the areas of fundamental research and photonic technologies [1,2]. One prominent example of collective behavior is superradiance [3], where light-induced couplings between initially incoherentlyprepared emitters cause the full ensemble to synchronize and radiate coherently [4]. While early studies of superradiance focused on collective scattering via the emitters' internal degrees of freedom, recent work demonstrates that formally identical behavior arises through the manipulation of the center-of-mass positions and momenta of cold atoms [5][6][7].In these studies, an initially uniformly-distributed gas of atoms pumped by external optical fields spontaneously undergoes a transition to a spatially-ordered state under certain circumstances [5][6][7][8]. This ordering arises from the momentum imparted to the atoms via optical scattering and can be understood as a form of atomic synchronization: instead of the atoms scattering light individually, the self-assembled density grating enables the entire ensemble to coherently scatter light as a single entity. The pump beams scatter off this grating and produce new optical fields that act back on the vapor to enhance the grating contrast. This emergent, dynamical organization can lead to reduced optical instability thresholds [9] and new phenomena [10] that are inaccessible using static, externally-imposed optical lattices [11].In order for superradiance to occur, the system must posses sufficient gain and feedback so that synchronization occurs more rapidly than dephasing. The main dephasing mechanisms are grating washout due to thermal atomic motion and the loss of photons from the interaction volume [5]. One can overcome the effects of thermal motion in free space by working at ultracold temperatures (T < 3 µK) and using optical fields detuned far from the atomic resonance in order to avoid recoilinduced heating [8,12]. Multi-mode superradiance has been observed in such systems [8], althou...

Second-order coherence of superradiance from a Bose-Einstein condensate

“…and, as a result, a complex nomenclature has evolved including the terms superradiance, superfluorescence, amplified spontaneous emission, mirrorless lasing, and random lasing [2, 4,[6][7][8][9], the distinctions among which we will not attempt to summarize here. The problem has recently seen renewed interest in the field of cold atoms [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. This is partly because cold atoms provide a reproducible, easily characterized ensemble in which Doppler broadening effects are small and relaxation is generally limited to spontaneous emission.…”

Enhancing light-atom interactions via atomic bunching

“…4 for b < 0.2 and T = 3/146. For a gas of rubidium atoms at 3 μK, achievable using Sisyphus cooling, for example [24,30], and | | = 7, χ (3) is more than two orders of magnitude larger than in the homogeneous case.…”

“…Our model explicitly connects the results of the zerotemperature models of the optomechanical physics community [14,15] with the finite-temperature models of the nonlinear optics community [28][29][30]. The results of our model also provide insight into the effects of high-order nonlinearities.…”

Cooperative Interactions in Lattices of Atomic Dipoles

“…1.3). These have been realised experimentally in a number of different systems, including quantum dots [2], nuclei [3], ions [4,5,6], Bose-Einstein condensates [7], cold atoms [8,9,10,11] and atoms at room-temperature [12]. Additional cooperative phenomena can include highly directional scattering [13], excitation localization [14,15,16], and modified optical transmission and scattering [17,18,19].…”