. at om - p h ] 1 4 N ov 2 00 8 Slow - light Faraday effect : an atomic probe with gigahertz bandwidth

The ability to control the speed and polarisation of light pulses will allow for faster data flow in optical networks of the future. Optical delay and switching have been achieved using slow-light techniques in various media, including atomic vapour. Most of these vapour schemes utilise resonant narrowband techniques for optical switching, but suffer the drawback of having a limited frequency range or high loss. In contrast, the Faraday effect in a Doppler-broadened slow-light medium allows polarisation switching over tens of GHz with high transmission. This large frequency range opens up the possibility of switching telecommunication bandwidth pulses and probing of dynamics on a nanosecond timescale. Here we demonstrate the slow-light Faraday effect for light detuned far from resonance. We show that the polarisation dependent group index can split a linearly polarised nanosecond pulse into left and right circularly polarised components. The group index also enhances the spectral sensitivity of the polarisation rotation, and large rotations of up to 15π rad are observed for continuous-wave light. Finally, we demonstrate dynamic broadband pulse switching, by rotating a linearly polarised nanosecond pulse from vertical to horizontal with no distortion and transmission close to unity. The phenomenon of reduced optical group velocity (slow light) is a topic of burgeoning interest [1]. In a slow-light medium, the group refractive index, ng, (the ratio of the speed of light in vacuo to the pulse velocity) is many orders of magnitude larger than the phase index, n. Hence an optical pulse propagates much more slowly than a monochromatic light beam. Large group indices of ∼ 10 are achievable in resonant optical processes, such as electromagnetically induced transparency (EIT), accompanied by a refractive index that is of the order of unity [2]. Such large group indices are the result of a rapid change