Dicke superradiance

This implies that the fluorescence intensity Ipi(t) at times t < T where all excited atoms oscillate still in phase, is N times larger than in the incoherent case (Dicke superradiance) [12.58]. 

This implies that the fluorescence intensity Ip- (t) at a time where all excited atoms oscillate in phase is N times larger than in the incoherent case (Dicke superradiance) [11.36]. This phenomenon of superradiance is used in the photon-echo technique for high-resolution spectroscopy to measure population and phase decay times, expressed by the "longitudinal" and "transversal" relaxation times T and T2. This technique is analogous to the spin-echo method in nuclear magnetic resonance [11.37]. Its basic principle may be understood in a simple model, transferred from NMR to the optical region. 

Often referred to as Dicke s superradiance,144 but with a very low excitation density. 

The lifetimes of molecular fluorescence emissions are determined by the competition between radiative and nonradiative processes. If the radiative channel is dominant, as in the anthracene molecule, the fluorescence quantum yield is about unity-and the lifetime lies in the nanosecond range. In molecular assemblies, however, due to the cooperative emission of interacting molecules, much shorter lifetimes—in the picosecond or even in the femtosecond range—can theoretically be expected an upper limit has been calculated for 2D excitons [see (3.15) and Fig. 3.7] and for /V-multilayer systems with 100 > N > 2.78 The nonradiative molecular process is local, so unless fluorescence is in resonance by fission (Section II.C.2), its contribution to the lifetime of the molecular-assembly emission remains constant it is usually overwhelmed by the radiative process.118121 The phenomenon of collective spontaneous emission is often related to Dicke s model of superradiance,144 with the difference that only a very small density of excitation is involved. Direct measurement of such short radiative lifetimes of collective emissions, in the picosecond range, have recently been reported for two very different 2D systems ... 

The subject of correlated or collective spontaneous emission by a system of a large number of atoms was first proposed by Dicke [1], who introduced the concept of superradiance that the influence on each atomic dipole of the electromagnetic field produced by the other atomic dipoles could, in certain circumstances, cause each atom to decay with an enhanced spontaneous emission rate. The shortening of the atomic lifetime resulting from the interaction between N atoms could involve an enhancement of the intensity of radiation up to N2. 

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