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The Onset of Matter-Wave
Amplification in a Superradiant Bose-Einstein Condensate
We have developed a consistent physical picture of
the surprising observation that short laser pulses cause
superradiance that results in atoms gaining momentum
against the direction of propagation of the pulse. This
picture elucidates the nature of bosonic stimulation
in the four-wave mixing of light and atoms and the interplay
of optical and atomic stimulation. The results were
published in Science [1].
Here is a short summary of the main findings. Light
at low intensity is scattered into all directions –
this is the well-known case of Rayleigh scattering.
Stronger laser pulses can give rise to collective “superradiant”
light scattering. In the case of elongated Bose-Einstein
condensates, the light is predominantly emitted along
the long axis of the condensate in the so-called endfire
modes [2]. The recoiling atoms appear as distinct peaks
in momentum space corresponding to the momentum transfer
of an absorbed pump photon and a superradiantly scattered
photon.
Short and intense laser pulses were found to generate
patterns of recoiling atoms that were strikingly different
from those seen in previous experiments [1]. Superradiantly
emitted photons were reabsorbed and re-emitted into
the pump beam, leading to atoms, which were recoiling
out of the condensate with a velocity component antiparallel
to the pump beam. Since this process is non-resonant,
it occurs only for short pulse durations. These experiments
show that the previous description of superradiance
as atomic stimulation was incomplete and that optical
stimulation plays a crucial role. The reabsorption of
superradiant photons implies that matter wave amplification
in superradiant Bose-Einstein condensates is suppressed
at early times.
Superradiant
scattering of a laser beam (arrow) from a Bose-Einstein
condensate in the short-pulse (A) and long-pulse (B)
limit. Absorption images of the atomic density distribution
were taken after 30 ms of ballistic expansion. In case
A, the detuning was -420 MHz and the pulse duration
was 6 ms. In case B, the detuning was -4400 MHz and
the pulse duration was 800 ms. The field of view of
both images is 2.0 mm by 2.0 mm.
- D. Schneble, Y. Torii, M. Boyd, E.W. Streed, D.E.
Pritchard, and W. Ketterle, Science 300, 475 (2003).
Link
- S. Inouye, A.P. Chikkatur, D.M. Stamper-Kurn, J.
Stenger, D.E. Pritchard, and W. Ketterle, Science
285, 571 (1999).
Link
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