Tunable Slow Light in Cesium Vapor

1. Tunable Slow Light in Cesium Vapor Aaron Schweinsberg, Ryan M. Camacho, Michael V. Pack, Robert W. Boyd, and John C. Howell The Institute of Optics, University of Rochester, Rochester, NY 14627 Frontiers in Optics Wednesday, October 11, 2006

2. Slow light • Goal: obtain large pulse delays for high-bandwidth pulses • The group velocity of a pulse is given by: • We can obtain exceptional pulse propagation speeds in spectral regions where refractive index changes rapidly with frequency (high dispersion). • We desire a region where dn/dw is large, but also constant over the bandwidth of the pulse.

3. Slow light in atomic vapors • Need dn/dw large, over a large bandwidth. • This condition can be met in the region between the absorption resonances of the ground-state hyperfine levels in an atomic vapor. • Working far from resonance, we find that pulse distortion is dominated by group velocity dispersion, rather than absorption.

4. Theory (cont.) (a) - absorption spectrum showing ~10 GHz ground state hyperfine splitting in cesium (can accommodate wide bandwidth pulses) (b) - associated index profile and group velocity

5. Experimental Setup • 852-nm diode laser is tuned between the hyperfine resonances. • Density of cesium atoms in the cell controlled by heater • Delay can also be tuned by application of resonant pump beams

6. Pulse delay through cesium vapor • Delay adjusted by changing cell temperature. Temperatures ranged from 90° C to 120° C. • 275 ps pulses delayed by up to 25 times the input pulse duration. • Useful delay limited by dispersive broadening.

7. Delay through cesium (740 ps input) • There is a trade-off between broadening and delay. • If we allow only minimal broadening, fractional delay can be greater for longer input pulses. • 740 ps pulses can be delayed up to 80 times their initial width! • Three 10-cm Cs cells were used in series. Temperatures ranged from 110° C to 160° C.

8. Measurements of broadening • Broadening data for delayed 740 ps pulses • Fractional broadening, defined as (T - T0) / T0, never exceeds 0.6. • Useful delay is likely to be limited by the reduction of the peak pulse height due to dispersive broadening.

9. Rapid tuning of the delay • Delay can be tuned by applying strong pump fields directly to the resonances. • Optical pumping reduces the effective number density of Cs atoms seen by the signal. • Used a 80 MHz AOM to turn two resonant 30 mW pump beams on and off • Delay of a pair of 275 ps pulses altered by 1 ns, equal in this case to the initial pulse separation. (one bit slot)

10. Measuring the switching speed • Pump turn-on time of 100 ns (as switched by AOM) • Reconfiguration of delay takes place over ~ 700 ns. • Higher pump powers could reduce reconfiguration time.

11. Summary • We can produce slow light in the high-dispersion spectral region between the ground-state hyperfine resonances of an atomic vapor. • Obtained delays much longer than the input pulses duration for high-bandwidth pulses. • 275 ps pulses - fractional delay of 25 • 740 ps pulses - fractional delay of 80 • Demonstrated rapidly tunable (700 ns) delays of 1 ns for sequential 275 ps pulses. • Support for this work has been provided by the slow light program of DARPA / DSO

12. Outline • Theory of slow light in cesium vapor • Large delay of high-bandwidth pulses • The effects of pulse broadening • Tuning the pulse delay • Conclusion