George R. Welch

ULTRASLOW GROUP VELOCITY AND ENHANCED NONLINEAR OPTICAL EFFECTS IN A COHERENTLY DRIVEN HOT ATOMIC GAS

We report the observation of small group velocities of order 90 meters per second, and large group delays of greater than 0.26 ms, in an optically dense hot rubidium gas (≈ 360 K). Media of this kind yield strong nonlinear interactions between very weak optical fields, and very sharp spectral features. The result is in agreement with previous studies on nonlinear spectroscopy of dense coherent media.

Slow, Ultraslow, Stored, and Frozen Light

The article illustrates recent experiments in which light is slowed, frozen, reversed, and stored in hot atomic vapors via electromagnetically induced transparency.

Electromagnetically induced transparency controlled by a microwave field

We have experimentally studied the propagation of two optical fields in a dense rubidium (Rb) gas in the case when an additional microwave field is coupled to the hyperfine levels of Rb atoms. The Rb energy levels form a close-

Enhancement of Kerr nonlinearity by multiphoton coherence

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Nonlinear optics via double dark resonances

three-level system coupled to the optical fields and the microwave field. It has been found that the maximum transmission of the probe field depends on the relative phase between the optical and the microwave fields. We have observed both constructive and destructive interferences in electromagnetically induced transparency. A simple theoretical model and a numerical simulation have been developed to explain the observed experimental results.

Gain-swept superradiance applied to the stand-off detection of trace impurities in the atmosphere.

We propose a new method of resonant enhancement of optical Kerr nonlinearity that uses multilevel atomic coherence. The enhancement is accompanied by suppression of the other linear and nonlinear susceptibility terms of the medium. We show that the effect results in a modification of the nonlinear Faraday rotation of light propagating in an 87Rb vapor cell by changing the ellipticity of the light.

Radiation Trapping in Coherent Media

Double dark resonances originate from a coherent perturbation of a system displaying electromagnetically induced transparency. We experimentally show and theoretically confirm that this leads to the possibility of extremely sharp resonances prevailing even in the presence of considerable Doppler broadening. A gas of {sup 87}Rb atoms is subjected to a strong drive laser and a weak probe laser and a radio frequency field, where the magnetic coupling between the Zeeman levels leads to nonlinear generation of a comb of sidebands.

Large negative and positive delay of optical pulses in coherently prepared dense Rb vapor with buffer gas

We show that gain-swept superradiance can be used to detect low (parts per million) concentrations of various gases at distances on the order of kilometers, which is done by using pulse timing to create small regions of gain at positions that sweep toward a detector. The technique is far more sensitive than previous methods such as light detection and ranging or differential absorption light detection and ranging.

Vacuum squeezing in atomic media via self-rotation

We show that the effective decay rate of Zeeman coherence, generated in a (87)Rb vapor by linearly polarized laser light, increases significantly with the atomic density. We explain this phenomenon as the result of radiation trapping. Our study shows that radiation trapping must be taken into account to fully understand many electromagnetically induced transparency experiments with optically thick media.

Optical imaging beyond the diffraction limit via dark states

We experimentally study the group time delay for a light pulse propagating through hot {sup 87}Rb vapor in the presence of a strong coupling field in a {lambda} configuration. We demonstrate that the ultraslow pulse propagation is transformed into superluminal propagation as the one-photon detuning of the light increases due to the change in the transmission resonance line shape. Negative group velocity as low as -c/10{sup 6}=-80 m/s is recorded. We also find that the advance time in the regime of the superluminal propagation grows linearly with increasing laser field power.