Transport measurements and diffusion of polaritons

Many papers have been devoted to the experimental determination of the exciton diffusion constant D. In most of the studies, D was determined by observing how the diffusion of excitons results in their capture by impurities (sensitized fluorescence) or in bimolecular quenching of excitons (reviews of these experiments may be found in (8),(21). The interpretation of such experiments requires that not only the diffusion of the excitons to the acceptor is taken into account, but also the character of the exciton interaction with the acceptor (i.e. with the impurity 

The decay rate of the transient grating signal versus 92 (9 is the angle between the pump pulses) for anthracene crystals at 10 and 20 K (23). The magnitude of the slope is proportional to the diffusion constant of the excitations in the crystal. With increasing temperature, the diffusion constant decreases. The average diffusion constant obtained from these data is about 10 times larger than the value expected for incoherent exciton motion (25). 

Indeed, at low temperature the polaritons are concentrated in the bottle neck region. Their diffusion constant along the surface of the film can be estimated as D vp p, where vp is the group velocity of the polaritons in this region and p is their mean free path. Since p = c/(wk), where k is the imaginary part of the refractive index ((2lj/c)k 104 — 105 cm-1), and vp a 105 cm/s, one obtains for the diffusion constant D 1 cm2/s, which agrees within an order of magnitude with the value obtained in the TG experiments. 

The discussion of the influence of bimolecular quenching and reabsorption of exciton fluorescence on the decay of the exciton gratings in organic crystals may be found in (26). The microscopic formulation of TG and four-wave-mixing experiments in molecular crystals in terms of polaritons may be found in (27). 

---