Incoherent emission

Section IV is devoted to excitons in a disordered lattice. In the first subsection, restricted to the 2D radiant exciton, we study how the coherent emission is hampered by such disorder as thermal fluctuation, static disorder, or surface annihilation by surface-molecule photodimerization. A sharp transition is shown to take place between coherent emission at low temperature (or weak extended disorder) and incoherent emission of small excitonic coherence domains at high temperature (strong extended disorder). Whereas a mean-field theory correctly deals with the long-range forces involved in emission, these approximations are reviewed and tested on a simple model case the nondipolar triplet naphthalene exciton. The very strong disorder then makes the inclusion of aggregates in the theory compulsory. From all this study, our conclusion is that an effective-medium theory needs an effective interaction as well as an effective potential, as shown by the comparison of our theoretical results with exact numerical calculations, with very satisfactory agreement at all concentrations. Lastly, the 3D case of a dipolar exciton with disorder is discussed qualitatively. 

The analysis of (4.12) shows that in the general case as well, we have a threshold Ac = gcr (with gc 1) above which no coherent emission can build up only an incoherent emission of isolated domains is then possible. Below the threshold (An < Ac), the coherent state has y linear in A — Ac [the function... 

As in the model of Section IV.A.e, eigenstates with small imaginary parts (Va < 7c) exist. These states lead to a much slower regime of incoherent emission, with the rate corresponding to a single domain (yA — ny0). Numerical calculations are probably the best way to study the regime of emission in the intermediate-coupling case (T0 A ).141... 

All these phenomena can occur simultaneously within the same material, as illustrated by the spectral response of an oriented polymer doped with DCM dye (4-dicyanomethylene-2-methyl-6-p-dimethylamino-styryl-4H-pyran) under 1.06 iJ,m laser irradiation (Figure 1.1). The two sharp signals at 532 and 354 nm are coherent emission induced by SHG and THG, whereas the broad band is incoherent emission of two-photon excited fluorescence (TPEF). 

We may say that for the unobserved system the emission is a cooperative effect of the N parts, whereas for the observed system we have an ordinary spontaneous emission from the N pieces. More to this, for the unobserved case the N emitters are stimulated by the same vacuum, imparting phase correlations between them. On the observed system the pieces are influenced by different and statistically independent vacuum fields leading to mutually incoherent emissions. 

To this point, we have been concerned with phosphors consisting of many small particles, each of which lies at a different orientation to all of the others. This results in incoherent emission. But, if we now grow a single crystal containing the activator, we would find that, upon excitation of the ciystal phosphor, the emitted light is now polarized. The plane of polarization has a specific relation to the crystallographic axes of the... 

The coherent emission intensity of an ensemble of N molecules is therefore N times stronger than the incoherent emission. This result is due to the N(N— 1) cross-terms in the expansion as first shown by Dicke. A closer look at this enhanced spontaneous emission shows that the coherent emission is also highly directional, in fact in a sample of macroscopic size the constructive interference effects only occur in the direction of the exciting laser beam. 

Fig. 12. Ratio R3 of coherent/incoherent emission measured for AX = 0.7 A and a solid angle of 0.2 mrad. Experimental points are compared with theoretical curve a. Data in b and c are of f3, the modulation rate of spontaneous emission measured respectively with and without the laser. Abscissa is stored current. E = 166 MeV X] = 1.06 nm, X3 = 0.355 nm.

Rather different circumstances are encountered when considering THz remote sensing of extraterrestrial sources. The major source of THz opacity in the Earth s atmosphere is water vapour, and from either high, dry mountain sites or from space there are windows in which the background becomes very small. Incoherent instruments which detect the faint emission from astronomical sources can therefore be considerably more sensitive than their laboratory... 

We still need to consider the coherence properties of astronomical sources. The vast majority of sources in the optical spectral regime are thermal radiators. Here, the emission processes are uncorrelated at the atomic level, and the source can be assumed incoherent, i. e., J12 = A /tt T(ri) (r2 — ri), where ()(r) denotes the Dirac distribution. In short, the general source can be decomposed into a set of incoherent point sources, each of which produces a fringe pattern in the Young s interferometer, weighted by its intensity, and shifted to a position according to its position in the sky. Since the sources are incoherent. 

Keywords coherent detection, incoherent source, thermal emission, Shottky noise, photon... 

Figure 9.20 Light emission (a) normal spontaneous emission produces incoherent light, and (h) stimulated emission produces coherent fight.

Light is emitted from the bulk material at random times and in all directions, such that the photons emitted are out of phase with each other in both time and space. Light produced by spontaneous emission is therefore called incoherent light. 

The computation of far-field radiation from a collection of incoherently radiating dipoles is in general quite a complicated problem. To calculate the angular dependence of the far-field intensity, the volume distribution of excited states must first be obtained, which, as we have seen, depends on the volume distribution of the absorbers and the electromagnetic field which stimulates them. The fields in turn depend on the frequency and linewidth of the exciting light source. Then the emission problem for the excited-state distribution (both spatial and frequency) must be solved including reorientation and depolarization effects. 

Selected entries from Methods in Enzymology [vol, page(s)] Additive properties of polarization, 246, 286 angle-resolved, assessment of peroxidation effects on membranes, 233, 274-275, 281-283, 287-288 binding isotherm construction, 246, 287-288 effect of inner filter effects, 246, 288 incoherent systems, 246, 263-264 orientational averaging, 246, 265, 269-270 Perrin equation, 246, 284-285 polarization of emission, 246, 284 rotational diffusion, 246, 9, 260 time-resolved, assessment of peroxidation effects on membranes, 233, 274, 283-285, 285-287. 

A new development in powerful EE STEM/TEM by Boyes et al (2001), called 2-2-2 200 kV held emission STEM/TEM, is now becoming available which combines atomic-resolution imaging with atomic level chemical and crystallographic analyses with 2 A (0.2 nm) resoluhon in each of the TEM, STEM and chemical analysis modes (hence 2-2-2), providing new oppormnihes in catalysis. In the 2-2-2 EESTEM, the incoherent HAADE-STEM provides... 

Stimulated (laser) excimer emission can be generated in pulsed high-pressure glow discharges. Dielectric barrier (silent) discharges or micro-wave discharges can be used to produce quasi-stationary or continuous incoherent excimer radiation. 

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