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Optics Letters

Coherent versus incoherent modulation of a resonant level:?the effect of modulation noise on the propagation of light through quantum wells

D. S. Citrin

Published Apr 1, 1999• 0 citations
Publisher

Abstract

The propagation of a weak light beam through a quantum well in the vicinity of an excitonic resonance that is modulated harmonically in time at terahertz frequency is considered. It is found that the reflection probability of the light beam is considerably different in the cases of purely coherent versus Gaussian incoherent terahertz modulation, even in the limit of long coherence times.

DOI
10.1364/ol.24.000472

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The propagation of a weak light beam through a quantum well in the vicinity of an excitonic resonance that is modulated harmonically in time at terahertz frequency is considered. It is found that the reflection probability of the light beam is considerably different in the cases of purely coherent versus Gaussian incoherent terahertz modulation, even in the limit of long coherence times.
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The coherent propagation phenomenon of incoherent light through a resonant medium was experimentally demonstrated in the subpicosecond time region, using Na atoms and the light from an imperfectly mode-locked cw dye laser. The coherence spike with a 0.35-psec width in the cross-correlation profile between the propagated incoherent light and the initial light develops the same specific oscillatory structure as seen in the propagation of a weak coherent subpicosecond pulse. A theoretical analysis was also performed, and the experimental results are in considerably good agreement with the calculated ones.
Quantum-well excitons dressed by a terahertz field have multiple optical resonances. An optical beam tuned to any of the resonant frequencies excites the dressed-exciton state; therefore the optical beam experiences attenuation upon its transmission. For two coherent optical beams tuned to two different resonant frequencies, however, one can judiciously choose the amplitudes and phases in such a way that the beams do not produce any excitation, and thus they experience no attenuation, even though each of beams would be strongly attenuated in the absence of the other. This two-color scheme also suggests coherent control of dressed excitons by a pair of phase-locked pulses with different carrier frequencies and nonoverlapping spectra.
A theoretical analysis of the propagation of near resonant and linearly chirped light pulses through a resonant medium is presented under the restriction that the pulses are far enough off resonance or weak enough so that they do not significantly alter the atomic populations. If the pulses are of duration comparable to or less than the phase memory time T(2) of the atoms, it is found that the transient response of the medium gives rise to amplitude-and phase-modulating effects. Analytic solutions are presented to describe the propagation of an off-resonant pulse which is very short compared with T(2) and an off-resonant rectangular pulse. The evolution of a Gaussian pulse having a positive linear chirp is studied numerically.
Coherent Rayleigh scattering and incoherent luminescence are studied by femtosecond upconversion under resonant excitation as a function of exciton density in GaAs quantum wells. Comparing samples with different well widths allows to establish the relation between inhomogeneous broadening and the shape of resonant Rayleigh scattering. We find an excellent agreement between experiment and a classical model. In samples with moderate inhomogeneous broadening we provide evidence for the transition from coherent to incoherent emission. The comparison with excitonic dephasing times T2 obtained from four-wave-mixing experiments demonstrates that the incoherent exciton population rises within T2/2, at any exciton density when changed by more than an orders of magnitude. The experimental results confirm very clearly the concept predicting that the coherent polarization made up by localized excitons decays into an incoherent exciton population under conservation of the number of excited quanta.