Superradiant state

The behavior of HtS( in (4.4), as a function of the three parameters, may be sketched as follows. For large r0 (r0 d ), all domains are brought into coherence by the strong field of their neighbors (within A2), and we find the optical response of a perfect 2D lattice with fast surface emission for states K co/c (subradiant states). On the other hand, if the disorder width A dominates (A r0), then Rk may be treated as a perturbation of the localized states A >, resulting in a radiative rate for domain A... 

In (4.6), we have used the trace relation125 126 on the imaginary part of RK Klm RK = Nny0. Thus, domain A has a superradiant state with n times the radiation width of a single site, which is however much smaller than rK (RK = irK ir0), since a single domain is smaller than A2. Before discussing the intermediate case where r0 and An are comparable for the surface, we treat a simplified version where the molecular assembly is smaller than the wavelength A. 

Figure 4.5. Wave vectors around the center of the excitonic Brillouin zone for which coherent emission [solution of equations 4.10 and 4.25] is possible according to the disorder critical value Ac. We notice that r0 is the imaginary eigenvalue for K = 0 (emission normal to the lattice plane) and that K" and K1 indicate, respectively, components of K parallel and perpendicular to the transition dipole moment, assumed here to lie in the 2D lattice. The various curves for constant disorder parameter Ac determine areas around the Brillouin-zone center with (1) subradiant states (left of the curve) and (2) superradiant states (right of the curve). We indicate with hatching, for a large disorder (A,. r ), a region of grazing emission angles and superradiant states for a particular value of A.

Fig. 5.9 Time dependence of the population in Rb Rydberg states after the initial population of the 12s state showing the rapid superradiant cascades (from ref. 35).

Section III deals with the surface excitations of the anthracene crystal, confined in the first (SJ, second (S2) and third (S3) (001) lattice planes. The experimental observations are briefly summarized. A simple model shows how the fast radiative decay arises and how the underlying bulk reflection modulates this superradiant emission, as well as why gas condensation on the crystal surface strongly narrows this emission, thus accounting for the observed structures. An intrinsic process is proposed to explain the surface-to-bulk relaxation at low temperatures, observed in spite of the very weak surface-to-bulk coupling for k 0 states. 

This challenge has been met by a new technique based on the excitation of the surface I fluorescence, initiated in our laboratory, which allowed us to set up a powerful tool for photoselection of surface and subsurface states. This technique combines the superradiant 2D character of the surface emission, its very good spectral resolution at T < 80 K, and its strong sensitivity to gas coating. 

For An> T we have two states with the molecular decay rate rj2. For An< r we have two states with the same real energy (Rez1 0), but with different decay rates (superradiant y > r j2, subradiant y < r/2). We find a sudden qualitative change in behavior for the system for A = T the time decay passes from biexponential for An> T to a decrease with oscillating beats for A < T.153 This transition is not a special feature of the N = 2 case, but even survives in the continuous limit, as we shall see now. 

The standard measurement of different properties of quantum electromagnetic radiation is based on the photodetection, which is field destructive. Following our consideration of the possibility of the Aharonov-Bohm effect at optical frequencies [100], we propose here a new nondemolition method of polarization measurement in which the linearly polarized longitudinal mode of the field is detected without any perturbation of its quantum state (Section VI.D). The estimation of physical conditions shows that such a measurement can be done either for the photons propagating through the fiber, or for the superradiant photons in radioband frequencies. 

For example, a 450-A radius CdS cluster should have a l-ps radiative lifetime observable below 7 K. As the temperature increases, higher states are populated and the superradiant effect disappears. As a result, the radiative lifetime increases with increasing temperature in this size regime. 

The threshold condition for collective emission (superradiance) or maser action (gain in the medium exceeding the losses) is much lower for Rydberg atoms than it is for the same number density of atoms in low n states. To obtain an order of magnitude estimate for the point at which collective fects occur we estimate the amplitude of the electric dipole field E radiated by an atom at a distance corresponding to a neighbouring atom. That is, if we let L represent a linear dimension in the sample which contains N atoms,... 

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 ... 

Introduction. - Fundamental Physical Applications of Laser Spectroscopy. - Two and Three Level Atoms/High Resolution Spectroscopy. - Rydbeig States. - Multiphoton Dissociation, Multiphoton Excitation. - Nonlinear Processes, Laser Induced Collisions, Multiphoton Ionization. - Coherent Transients, Time Domain Spectroscopy, Optical Bistability, Superradiance. - Laser Spectroscopic Applications. - Laser Sources. - Postdeadline Papers. - Index of Contributors. 

---