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arXiv:1202.3353

Directed Spontaneous Emission from $N$ -atom Extended Ensemble

Harry J. Lipkin

Published Feb 15, 2012• 0 citations

Abstract

Coherence and interference play crucial roles in emission and absorption of photons to and from large systems with many atoms. Confusion has arisen because nuclear X-ray physicists and atomic quantum-optics physicists do not understand one another's individual descriptions of related phenomena. Basic physics same for all wave lengths from optical to nuclear gamma ray photons. But different languages are used to describe this physics in different domains. Crucial parameters vary over many orders of magnitude and what is intuitive or counterintuitive varies widely. Differences in parameters arising from differences between coherent emission effects in different domains produce very different results. Unified general treatment of the entire photon spectrum makes basic physics intelligible to all. In the optical region the mean distance between the scattering atoms is much longer than the photon wave length, Dicke superradiant scattering is isotropic and multiple scattering, Fano couplings are important and the lifetimes of intermediate stats are sufficiently short to be negligible. In X-ray scattering the mean distance between atoms is comparable to the photon wave length, Dicke superradiance is concentrated in a forward peak, multiple scattering and Fano effects are negligible, lifetimes are measurably long, speedup shortening the lifetime is important and most of the radiation is not elastically scattered but lost to absorption. Explicit calculations for a one-dimensional array shows the great difference between the case where the photon wave length is much shorter or comparable to the distance between nearest neighbors. A full investigation of the angular distribution and speedup of the intensity for two and three-dimensional scatterers give very different results for the two cases.

DOI: 10.48550/arxiv.1202.3353

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It is well known that the optical properties of multi-level atomic and molecular system can be controlled and manipulated efficiently using quantum coherence and interference, which has led to many new effects in quantum optics for e.g. lasing action without population inversion, ultraslow light, high resolution nonlinear spectroscopy etc. Recent experimental and theoretical studies have also provided support for the hypothesis that biological systems uses quantum coherence. Nearly perfect excitation energy transfer in photosynthesis is an excellent example of this.\n\nIn this dissertation we studied quantum coherence and interference effects in the transient and the continuous-wave regimes. This study led to (i) the first experimental demonstration of carrier-envelope phase effects on bound-bound atomic excitation in multi-cycle regime (~15 cycles), (ii) a unique possibility for standoff detection of trace gases using their rotational and vibrational spectroscopic signals and from herein called Coherent Raman Umklappscattering, (iii) several possibilities for frequency up-conversion and generation of short-wavelength radiation using quantum coherence (iv) the measurement of spontaneous emission noise intensity in Yoked-superfluorescence scheme.\n\nApplications of the obtained results are development of XUV (X-Ray) lasers, con- trolled superfluorescent (superradiant) emission, carrier-envelope phase effects, coherent Raman scattering in the backward direction, enhancement of efficiency for generating radiation in XUV and X-Ray regime using quantum coherence with and without population inversion and to extend XUV and X-Ray lasing to ~4.023 nm in Helium-like carbon.

Coherent spontaneous emission from a system of

N

two-level atoms interacting with a quantized radiation field is treated for the case in which Dicke's "cooperation number"

r

is macroscopically large. It is shown how to modify the quasiclassical approach to this problem to incorporate quantum effects that are lost in the self-consistent-field approximation. The statistics of the emitted radiation are found to vary markedly with the initial state of the system of atoms. The photon statistics tend to that typical of blackbody radiation when the initial state of the atomic system is that which would result from incoherent pumping ($m\ensuremath{\sim}r$). When, on the other hand, the atoms are initially in a superradiant state ($m\ensuremath{\ll}r$), the emitted radiation may be represented approximately by a coherent state.

The present state of theoretical and experimental research on collective spontaneous emission (superradiance) in a system of radiators (atoms, molecules, or nuclei) is reviewed. The distinction between superradiance and the amplification of spontaneous emission is discussed. There is also a discussion of conditions under which the effect can be observed and of various theoretical methods for describing superradiance: the quantum single-mode and multimode models and the semiclassical approach. Theoretical papers on superradiance in systems with dimensions smaller than the wavelength of the radiation and also in extended systems are reviewed. It is shown that superradiance may occur in weakly amplifying media. A situation in which the superradiance is oscillatory is described. The possible use of superradiance to generate coherent emission in the x-ray and ? ranges is discussed. The superradiance accompanying Raman scattering of light in atomic and molecular media is studied. The theoretical results are compared with experimental observations of superradiance in the optical range. In an appendix, the nature of the phase transition in a system of radiators interacting through an electromagnetic field is discussed.