We discuss the possibility of producing harmonics while the process of collective spontaneous emission (superradiance) takes place in a solid. Self correlations of dipole radiators induced by the emitted field lead to a reduction of the emission time and consequently to an increase of the intensity. Estimations of the superradiant energy flux show that, for crystalline densities of radiators, this flux may be comparable with the atomic unit. This assures the highly nonlinear behavior necessary to observe a pulse of high harmonics during superradiant pulse. We apply the Floquet–Bloch expansion to analyze the features of harmonic production in the framework of the two-level model.
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.
Using the formalism developed earlier, we treat spontaneous emission from a pair of identical two-level atoms
A1,
A2, whose separation
r21 can be comparable to the wavelength $\ensuremath{\lambda}$. We obtain expressions for time-dependent intensities and damping rates with the initial conditions (a) both atoms inverted, (b) prior excitation by a short $\frac{1}{2}\ensuremath{\pi}$ pulse, and (c) only
A1 inverted. The results in (a) are compared with those obtained for a model consisting of two initially excited harmonic oscillators
O1,
O2. The atoms exhibit superradiant behavior, whereas
O1,
O2 tend to trap radiation. In (a), the intensity pattern $\mathcal{R}(\ensuremath{\theta},t)$ develops lobes in different directions at different times, so that the spatial distribution of photons at time $t\ensuremath{\rightarrow}\ensuremath{\infty}$ is the same as in the independent-atom case ${r}_{21}\ensuremath{\gg}\ensuremath{\lambda}$. For the oscillators, the lobes of $\mathcal{R}(\ensuremath{\theta},t)$ do not change direction, but only become more pronounced as time increases. In (b) and (c), the lobes oscillate back and forth at frequency $2{\ensuremath{\Omega}}_{12}$ corresponding to the shifts $\ifmmode\pm\else\textpm\fi{}{\ensuremath{\Omega}}_{12}$ of the triplet and singlet states due to the ${A}_{1}\ensuremath{-}{A}_{2}$ interaction. The intensity can therefore have a sinusoidal component. Field correlation functions calculated for (a) and (c) show that
A2 and
A2 radiate simultaneously around the frequencies $\ensuremath{\epsilon}\ifmmode\pm\else\textpm\fi{}{\ensuremath{\Omega}}_{12}$, where $\ensuremath{\epsilon}$ is the single-atom resonant frequency. The spectrum is calculated for case (c), and shows the effects of coherent linewidth enhancement in addition to the frequency shifts.
We investigate the statistical aspects of the cooperative emission of radiation by a system of many atoms. By solving the equation of motion for a suitably defined quasiprobability distribution which represents the density operator of the atoms, we find explicit expressions for the expectation values of products of atomic observables. The system is found to exhibit large quantum fluctuations for the most highly excited initial states of the atoms. For all others it exhibits nearly classical behavior. Previous numerical evaluations of the radiated intensity and its variance are found to be in good agreement with the present results.
We investigate the statistical aspects of the cooperative emission of radiation by a system of many atoms. By solving the equation of motion for a suitably defined quasiprobability distribution which represents the density operator of the atoms, we find explicit expressions for the expectation values of products of atomic observables. The system is found to exhibit large quantum fluctuations for the most highly excited initial states of the atoms. For all others it exhibits nearly classical behavior. Previous numerical evaluations of the radiated intensity and its variance are found to be in good agreement with the present results.
The characteristics of the cooperative cascade emission is studied by taking into account of the nonlinearity due to depopulation. It is shown that the emitted radiation exhibits considerably different behaviors in three regimes of the atomic number density. In the lowest density regime the radiation is essentially a quasi-stationary amplified spontaneous emission (ASE), in the intermediate one it becomes superradiant, and in the highest one it shows a new cooperative behavior peculiar to the cascade system. By using the present theory the experimental results recently observed in Li vapour is explained. It is also shown that the present theory describes the two-level cooperative emission as a specific limiting case.