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Applied Physics Letters

MEASURED PULSE VELOCITY GREATER THAN c IN A NEON ABSORPTION CELL

Frederick R. Faxvog, C. N. Y. Chow, T. Bieber, J. A. Carruthers

Published Sep 1, 1970• 50 citations
Publisher

Abstract

The pulse velocity in a Ne absorption cell placed inside a self-locked He–Ne (6328 Å) laser has been found to exceed c, the free-space velocity of light, by about 3 parts in 104. Conversely, the pulse velocity in a He–Ne gain cell was less than c by about the same fraction. These results are consistent with the classical group velocity dω/dk, where the frequencies are confined to the central region of the resonant line.

DOI
10.1063/1.1653360

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Oct 8, 2010•5 citations
The pulse velocity in a Ne absorption cell placed inside a self-locked He–Ne (6328 Å) laser has been found to exceed c, the free-space velocity of light, by about 3 parts in 104. Conversely, the pulse velocity in a He–Ne gain cell was less than c by about the same fraction. These results are consistent with the classical group velocity dω/dk, where the frequencies are confined to the central region of the resonant line.
Experiments are reported that measure the pulse velocity of a mode-locked 6328-Å He-Ne laser pulse in a resonant absorber as a function of the pulse intensity. At low intensities, pulse velocities greater than the velocity of light in the passive medium are observed. These results are compared with a simple theory based on the saturation of the dispersion associated with the absorption. At higher intensities where the pulse approaches a 2π pulse, pulse delays (that is pulse velocities less than the velocity of light in the passive medium) are observed.
For a pulse with carrier frequency detuned less than an atomic plasma frequency but outside a narrow Lorentzian gain line, the group velocity will be negative. Unlike propagation at the center of an absorption line, the energy velocity is approximately equal to the group velocity, and is also negative. We show that a classical Gaussian pulse with such a carrier frequency will propagate at the negative group velocity for many atomic plasma wavelengths, before dispersion deforms the pulse shape. The peak of the transmitted pulse leaves the gain medium before the peak of the incident pulse enters, i.e., the pulse is transmitted superluminally. For a sufficiently long medium the exit pulse is well resolved from a comparison pulse traveling through an equal distance of vacuum. There is no conflict with causality, as numerical simulations with a switched-on Gaussian demonstrate. We propose an experiment to observe this kind of propagation by sending a pulsed probe beam through a Xe gas cell pumped to achieve inversion.
It is shown that the prediction of Garrett and McCumber that a light pulse's velocity may exceed the speed of light in a resonantly absorbing medium is due to asymmetric absorption of energy from the light pulse. More energy is absorbed from the trailing half of the pulse than from the front half, causing the center of gravity of the pulse to move at a velocity greater than the phase velocity of light. It is also shown that in certain cases a pulse's maximum will propagate at the group velocity even when the pulse as a whole is distorted by dispersion.
Self-induced transparency in an absorbing neon discharge has been observed on the nonresonance transition $2{s}_{2}(J=1)\ensuremath{-}2{p}_{4}(J=2)$. Since linearly polarized light pulses excite two dipole moments differing by only 18%, the absorber is virtually nondegenerate and superior in simplicity to molecular absorbers. Light pulses, generated from the output of a cw single-frequency 1.15-\ensuremath{\mu}m He-Ne laser oscillator by a pulsed Ne laser amplifier, were transmitted through the absorber. Comparison of the oscilloscope traces of input and output pulses indicated the occurrence of pulse retardation, peak amplification, and pulse breakup. Complete pulse reshaping to the predicted hyperbolic-secant envelope has been observed. The periodicity of transmission versus pulse power, which provides direct evidence of optical nutation, is in agreement with the results of a pulse-area-pulse-energy model.
Comparison of absorption and fluorescence in a nano-cell containing Rb vapor with other Rb nano-cells with addition of neon gas is presented. It is shown that the effect of collapse and revival of Dicke-type narrowing occurs for Rb nanocells containing N2 as buffer gas under 6 and 20 Torr pressure for the thickness L = λ /2 and L = where λ is the resonant λ, laser wavelength 794 nm (D1 line). Particularly for 6 Torr the line-width of the transmission spectrum for the thickness L =λ/2 is 2 times narrower than that for L = λ. For an ordinary Rb cell with L = 0.1 - 10 cm with addition of buffer gas, the velocity selective optical pumping/saturation (VSOP) resonances in saturated absorption spectra are fully suppressed when the buffer gas pressure > 0.5 Torr. A spectacular difference is that for L = λ, VSOP resonances located at the atomic transitions are still observable even when Ne pressure is ≥ 6 Torr. Narrowband fluorescence spectra of a nano-cell with L = λ/2 can be used as a convenient tool for online buffer gas pressure monitoring for the conditions when ordinary pressure gauges are unusable. Comparison of electromagnetically induced transparency (EIT) effect in a nano-cell filled with pure (without a buffer gas) Rb with another nano-cell, where buffer gas nitrogen is added, is presented. The use of N2 gas inside Rb nano-cells strongly extends the range of coupling laser detunings in which it is still possible to form EIT resonance.