4.2. QUANTUM CUTTING
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siderably more difficult than for direct bandgap materials, in view of important contributions of nonradiative recombination channels. Since these processes are usually very fast, they complicate interpretation of induced absorption dynamics in terms of carrier multiplicity, as carrier trapping and Auger processes are difficult to separate in absorption transients. Microscopic mechanism of MEG The microscopic origin of MEG is under debate and several possibilities have been put forward. These include: 1. Impact ionization by a hot carrier created as a result of photon absorption [64, 65] 2. Coherent superposition of single and multiexciton states [59] due to the strong Coulomb interaction of carriers confined in NCs, which should take place when the energy relaxation rate of a single electron-hole pair is lower than both the two-exciton state thermalization rate and the rate of Coulomb coupling between single and two-exciton states. 3. Multiexciton formation via a virtual state [77, 78]. This process can be described by second order perturbation theory and here two possible scenarios, with comparable rates, have been proposed. The first one proceeds via a virtual single exciton state. In this case, the direct optical transition from vacuum to a single exciton state is followed by the transition into the final two-exciton state due to Coulomb interaction [77]. In the second channel, the first step is the transition initiated by the Coulomb interaction from vacuum to a bi-exciton state, and the second step is optical intraband transition [78]. 4. More recent theoretical modeling [79] suggests that a prominent role could be played by defects in the bandgap of NC. While this idea is of general character and not restricted to a particular material, it could be especially important for the case of Si NCs embedded in an oxygen-rich environment of SiO2 , for which oxygen-induced defects are known to appear in the bandgap for sufficiently small NC sizes. In particular, a relation could be sought with the well known SiO2 -related 420 nm (âˆź 3 eV) defect band [80]. The processes (2) and (3) can be responsible for MEG without any delay, i.e., in the moment of the photon absorption, and can be effective for production of multiple excitons in a single NC. However, extra excitons occurring in the same NC are recombining on a picosecond time scale [54]. The impact ionization process (1) and defect mediated process (4) start with some delay after the absorption and proceed via a real state when the hot electron-hole pair is created by the