Guided, slow light in an ultracold medium has been demonstrated by
Mukund Vengalattore and Mara Prentiss at Harvard.
pulses in a sample of atoms had been accomplished before (see for
example PNU 521)
light pulses into a highly dispersive medium -- that is, a medium in
which the index of refraction varies greatly with frequency.
Previously, this dispersive quality had come about by tailoring the
internal states of the atoms in the medium. In the present Harvard
experiment, by contrast, the dispersive qualities come about by
tailoring the external qualities of the atoms, namely their motion
inside an elongated magnetic trap (see
Physics News Graphics).
In the lab setup, two pump laser
beams can be aimed at the atoms in the trap; depending on the
frequency and direction of the pump light, the atomic cloud (at a
temperature of about 10 micro-Kelvin) can be made more or less dispersive
in a process called recoil-induced resonance, or RIR. If now a
separate probe laser beam is sent along the atom trap central axis,
it can be slowed by varying degrees by adjusting the pump laser
beam. Furthermore, the probe beam can be amplified (the intensity
of the light can be increased by a factor of up to 50) or attenuated
depending on the degree of dispersiveness in the atoms. This
process can be used as a switch for light or as a waveguide.
According to Mukund (now working at UC Berkeley,
firstname.lastname@example.org), slowing light with the recoil-induced
resonance approach may be a great thing for nonlinear-optics
research. Normally, nonlinear effects come into play only when the
light intensities are quite high. But in the RIR approach,
nonlinear effects arise more from the strong interaction of the two
laser beams (pump and probe) and the fact that the slow light spends
more time in the nonlinear medium (the trap full of atoms).
these effects are enhanced when the atoms are very cold. Moreover,
because the slow light remains tightly focused over the length of
the waveguide region, intensity remains high; it might be possible
to study slowed single-photon light pulses, which could enhance the
chances of making an all-optical transistor. The light in this
setup has been slowed to speeds as low as 1500 m/sec but much slower
speeds are expected when the atoms are chilled further.
Vengalattore and Prentiss, Physical Review Letters, upcoming
MIT-Harvard Center for