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SPhD

Propagation Velocity of an Intense Light Pulse in a Medium with Inverse Population

Nicolai G. Basov, Rafael V Ambartsumyan, V. S. Zuev, Peter G. Kryukov, Vladilen S Letokhov

Published May 1, 1966• 3 citations
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Propagation of light-pulses at a negative group-velocity

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The European Physical Journal D•Apr 1, 2003•111 citations
We present an analytical solution of the Maxwell-Schr\"odinger set of equations describing propagation of a two-component short electromagnetic pulse through a medium consisting of atoms with a @sV-type level scheme with an initially inverted population. This solution demonstrates solitonlike waves traveling at a velocity exceeding the vacuum speed of light c (or even a negative velocity) until they reach the forward (or, respectively, trailing) pulse front. The front propagation velocity recognized as information transmission velocity is explicitly found to be equal to c.
It is shown that the steady π -pulses of light in a lossy, resonant inverted medium can have an arbitrary duration greater than a critical value and are in general chirped. The non-chirped π -pulses of fixed duration (for a given population inversion) obtained by Arecchi and Bonifacio and by Lamb is a special case of our results when the pulse duration is set equal to the critical value. We find expressions for the pulse profile, propagation number, envelope velocity and atomic variables, and show that the expression for the area of the pulse in this case is different from that used commonly, and that the product of the envelope and mean phase velocities is equal to the square of the velocity of non-resonant light.
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.