凝聚态光物理学【英文版】.pptVIP

* * * 4 Excitons 4.1 The concept of excitons 4.2 Free excitons 4.3 Free excitons in external fields 4.4 Free excitons at high densities 4.5 Frenkel excitons 4.1 The concept of excitons Exciton: bound electron – hole pair Two basic types: Wannier – Matt excitons (free exciton): mainly exist in semiconductors, have a large radius, are delocalized states that can move freely throughout the crystal, the binding energy ~ 0.01 eV; Frenkel excitons (tight bound excitons): found in insulator and molecular crystals, bound to specific atoms or molecules and have to move by hopping from one atom to another, the binding energy ~ 0.1 -1 eV. The maximum energy of a thermally excited phonon ~ kBT = 0.025 eV (RT) Wannier – Matt excitons: stable at cryogenic temperature. Frenkel excitons: stable at room temperature. General properties: (补充) 4.2 Free excitons 4.2.1 Bing energy and radius The eigenfunction and eigenvalue eq: (Exciton Rydberg constant) The binding energy and radius of exciton: (Exciton Bore radius) Table 4.1 4.2.2 Exciton absorption Creating an electron-hole pair - the same k vector; Creating an exciton - the same group velocity. (high symmetry points) (high symmetry lines) At the zone centre: k=0 and zero gradient, strong excitons occur in the spectral region close to the fundamental band gap. The energy of exciton is: Strong optical absorption line at energies equal to En that appear in the spectra at energies just below the fundamental band gap Free excitons can only be observed in the absorption spectrum of very pure samples, because impurities release free elecrons and holes that can screen the Coulomb interaction in the exciton and thereby strongly reduce the binding froces. 4.2.3 Experimental data for free exciton in GaAs Excitonic ansorpti