Guided Slow Light

Guided, slow light in an ultracold medium has been demonstrated by Mukund Vengalattore and Mara Prentiss at Harvard.

Slowing light pulses in a sample of atoms had been accomplished before (see for example PNU 521) by sending 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, mukundv@calmail.berkeley.edu), 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).

All of 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 article;
MIT-Harvard Center for Ultracold Atoms