Biexcitonic states

Efficient MEG also requires strong coupling between exciton and biexciton states relative to YP Wc/ >> Yi- Fulfdling this condition satisfies the obvious requirement that the exciton thermalisation time should be longer than the coupling time between the optically created exciton and the asymmetric biexciton state (Nozik, 2002). 

The biexciton states described above have energies outside the free-exciton two-particle continuum and can decay only due to exciton-photon or exciton-phonon interactions. However, if the biexciton energy lies in the continuum, such a biexciton can decay spontaneously into states with two free (unbound) excitons. These quasibound states display themselves as resonant peaks in the two-particle continuum and can be observed in linear and nonlinear optical processes if these resonances are sufficiently narrow, e.g. if they are situated in the region of low density of two-particle states. 

For dipoles of the order of 5 Debye and nearest distance of molecules of the order of 5 A the energy M s, is of the order of 1000 cm-1 which can be large or of the order of the width of the exciton bandwidth. In such crystals we can expect the existence of biexciton states. The theory of biexcitons in layered organic anthracene type crystals has been developed by Spano et al. (23), (24). 

The first observation of biexciton states in a molecular structure has been performed by Chakrabarti et al. (25) for the ID 77-aggregate of an epitaxially grown trivalent-halogen-phthalocyanine with a unique packing arrangement of the monomers with nearest distance 4 A. The distinct photoinduced absorption to a continuum of two-exciton states and to a bound biexciton state was observed. The value of the exciton-exciton interaction for this structures was estimated to be equal to ss 0.3 eV. 

The AC Stark effect is relevant, not only in atomic spectroscopy, but also in solid state physics. The biexciton state (or excitonic molecule), where two Wannier excitons are bound by the exchange interaction between electrons, occurs in various semiconductors (see section 2.22). Various experiments on the AC Stark effect of excitons have been reported, but the clearest example to date is probably the observation of the Rabi splitting of the biexciton line in CuC reported by Shimano and Kuwata-Gonokami [477]. It is very interesting to consider how Bloch states in solids, which themselves are delocalised and periodic, are dressed or modified by the electromagnetic field, since their properties are rather different from those of purely atomic states, which are by definition completely localised. 

In high-quality ZnO epitaxial films grown by MBE on GaN templates, biexciton formation was also reported and radiative recombination of the biexciton state was observed at 3.35 eV for excitation densities lower than the threshold for exciton-exciton scattering-related SE [156]. The spectral shape of the biexciton PL, Jxx( ). is generally expressed by an inverse Boltzmarm distribution function. 

Fine Structure of the Lowest Exciton and Biexciton States in CdSe... 

The exdton and biexciton states, obtained from the diagonization of the... 

The transition dipole moment between an exciton and a biexciton state is calculated using ... 

After determining the allowed transitions between exciton and biexciton configurations, those between CSFs can be obtained. For example, the transition 64 bi2 is allowed because each exciton configuration in 4 can reach a biexciton configuration in b 2, thus p.4 12 = 1. On the other hand, the transition 64 bii is forbidden because it requires two steps in order to produce the biexciton configuration from each of the exciton ones therefore, /44 U = 0. Transitions between exciton and biexciton states, bx, are allowed when each e, in an allowed transition to a bj in Vbx- Figure 6.18 summarizes... 

The LE biexciton states are shown in Figure 6.22 right panel, black lines. By having CS configurations, there are an additional 75 biexciton states, making a total of 90 biexciton states. In this case, we also follow the conventional labeling of states, that is, Fx , where F is the total angular momentum and x = A,B,C,... for CS biexciton states cs is added between Fand x. 

Figure 6.23 Exciton-biexdton transitions in a type-II system - Allowed transitions between upper panel, LE exdton and biexciton states lower panel, CS exciton and biexdton states.

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