Tunable Slow Light in Cesium Vapor

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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

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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.

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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.

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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

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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

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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.

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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.

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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.

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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)

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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.

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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

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Outline

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