Population Inversion and Superluminality

R. Y. Chiao, Eric L. Bolda, Jason L. Bowie +3 more

Population inversion in an atomic or molecular medium leads to the possibility of superluminal propagation of off-resonance, finite-bandwidth electromagnetic wave packets, whose phase, group, energy, and 'signal' velocities, as defined by Sommerfield and Brillouin, all exceed the vacuum speed of light c. Einstein causality is not violated, since the front velocity is c. Here the authors propose an experiment to observe superluminal propagation of laser pulses detuned from a stimulated Raman transition in optically pumped rubidium vapour. The inversion of populations can also lead to a parelectric medium with negative DC electric susceptibility, thus implying the possibility of the levitation of an electrical charge in the vacuum above this medium. Stable electrostatic configurations of charges placed inside an evacuated cavity surrounded by this medium exist, in seeming violation of Earnshaw's theorem. An experiment is proposed to observe parelectricity in ammonia gas.

Philip R. Wallace

Matthew S. Bigelow, Nick N. Lepeshkin, Robert W. Boyd

We have observed both superluminal and ultraslow light propagation in an alexandrite crystal at room temperature. Group velocities as slow as 91 meters per second to as fast as -800 meters per second were measured and attributed to the influence of coherent population oscillations involving chromium ions in either mirror or inversion sites within the crystal lattice. Namely, ions in mirror sites are inversely saturable and cause superluminal light propagation, whereas ions in inversion sites experience conventional saturable absorption and produce slow light. This technique for producing large group indices is considerably easier than the existing methods to implement and is therefore suitable for diverse applications.

Steven Weinstein

Harshawardhan Wanare

We obtain the gain of the probe field at multiple frequencies in a closed three-level V-type system using a frequency-modulated pump field. There is no associated population inversion among the atomic states of the probe transition. We describe both the steady-state and transient dynamics of this system. Under suitable conditions, the system exhibits a large gain simultaneously at a series of frequencies far removed from resonance. Moreover, the system can be tailored to exhibit multiple frequency regimes where the probe experiences anomalous dispersion accompanied by negligible gain or absorption over a large bandwidth, a desirable feature for obtaining superluminal propagation of pulses with negligible distortion.

Galina A. Nemova