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Concept# Biexciton

Summary

In condensed matter physics, biexcitons are created from two free excitons.
In quantum information and computation, it is essential to construct coherent combinations of quantum states.
The basic quantum operations can be performed on a sequence of pairs of physically distinguishable quantum bits and, therefore, can be illustrated by a simple four-level system.
In an optically driven system where the and states can be directly excited, direct excitation of the upper level from the ground state is usually forbidden and the most efficient alternative is coherent nondegenerate two-photon excitation, using or as an intermediate state.
Three possibilities of observing biexcitons exist:
(a) excitation from the one-exciton band to the biexciton band (pump-probe experiments);
(b) two-photon absorption of light from the ground state to the biexciton state;
(c) luminescence from a biexciton state made up from two free excitons in a dense exciton system.
The biexciton is a quasi-particle formed from two excitons, and its energy is expressed as
where is the biexciton energy, is the exciton energy, and is the biexciton binding energy.
When a biexciton is annihilated, it disintegrates into a free exciton and a photon. The energy of the photon is smaller than that of the exciton by the biexciton binding energy,
so the biexciton luminescence peak appears on the low-energy side of the exciton peak.
The biexciton binding energy in semiconductor quantum dots has been the subject of extensive theoretical study. Because a biexciton is a composite of two electrons and two holes, we must solve a four-body problem under spatially restricted conditions. The biexciton binding energies for CuCl quantum dots, as measured by the site selective luminescence method, increased with decreasing quantum dot size. The data were well fitted by the function
where is biexciton binding energy, is the radius of the quantum dots, is the binding energy of bulk crystal, and and are fitting parameters.

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