Superradiance

SuperPro software, 26 1040 Superradiant lasers, 14 690 Supersensitization, 9 510 Supersoft copolymers, 26 538, 540 Super spring copper alloys, 7 723t Superstructure, creating and optimizing, 20 726-728... 

SUPERIMPOSABILITY (or Superposability) SUPERRADIANCE SUPERSATURATION BIOMINERALIZATION Suprafadal,... 

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. 

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 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 superradiant states) and no emission for 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. 

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. 

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.

K 0/c (Fig. 3.7). Then, coherent emission is always possible for sufficiently grazing emission angles, in directions normal to the transition dipole (see Fig. 4.5). For a normal or nearly normal emission (K = 0), the transition to the coherent emission regime takes place for A = gcr0. If our model of static disorder may be extrapolated to dynamical disorder induced by thermal fluctuations, this transition should be observable as a function of temperature. A similar buildup of superradiance below a critical temperature in the condensed phase has been recently reported by Florian et al.154... 

Laser A source of ultraviolet, visible, or infrared radiation which produces light amplification by stimulated emission of radiation from which the acronym is derived. The hght emitted is coherent except for superradiance emission. 

Nitrogen laser A source of pulsed semi-coherent superradiance mainly around 337 nm. The lasing species is molecular nitrogen. 

Superradiance Spontaneous emission amplified by a single pass through a population inverted medium. It is distinguished from trae laser action by its lack of coherence. The term superradiance is frequently used in laser technology. 

Following the second pulse the systems proceed with its free evolution marked by the grey arrow in panel (d). However, this free evolution now causes rephasing of the dephased molecular dipoles After time t, which is equal to the time elapsed between the two light pulses, the system will be completely rephased as indicated by the final (ftill-line) vector in panel (d). This analysis then predicts that at that point in time the superradiance emission by the molecular system will resume. In other words, following a second light-pulse at time t after the first pulse, the system will respond with an echo at time It. An experimental example is shown in Fig. 18.14. 

H. J. Brouwer, V. V. Krasnikov, A. Hilberer, G. Hadziioannou, Blue superradiance from neat semiconducting alternating copolymer films, Adv. Mater. 1996, 8, 935. 

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