Superradiance macro dipole

Superradiance. We consider a system of several two-level atoms occupying a volume the dimension of which is small compared to A. In this case the total radiating dipole is given by eq. (36), multiplied by the difference of atom populations in the excited state and in die ground state. If the external resonant field is stopped at t-nHQa, we shall create a macro dipole given by ... 

So the first example of real superradiance was in fact the free induction decay and die decay of the photon echo observed in ruby by Kumit, Abella and Hartmann (1964). When a pulse from a ruby laser was sent onto a ruby crystal, the free induction decay and the echo decay observed were about 50 ns, when compared to the usual Cr " radiative decay time of 4 ms, showing clearly the radiation emission from the macro-dipole. 

Since the macro-dipole exists right after excitation, the superradiance shortened decay starts without delay, beginning with the (Ni -N2Y behavior. The only condition is that the dephasing rate is slower than the superradiant one so that the macro-dipole keeps on existing during the transition, that is... 

In the optical frequency domain, superradiance in the Dieke and Bonifacio sense (Dieke 1954, Bonifacio and Lugiato 1975), i.e. with strong radiation damping, seems to have been observed in solids up to now only in photon-echo experiments, as already mentioned. The difficulty lies in the preparation of the macro-dipole in a volume smaller than ). However the recent interest in restrained geometry, e.g. the so called 2D, ID... 

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