Spectral behaviour of the two-photon absorption coefficient in ZnO,CuCl and Bi4Ge3O12

Summary The spectral behaviour of the direct interband two-photon absorption coefficient α⁽²⁾ in ZnO, CuCl and Bi₄Ge₃O₁₂ has been investigated in a large excitation energy range. The experimental results have shown that the α⁽²⁾ spectral behaviour is well described by a parametric formula containing terms with different energy dependence. In particular, for 2ħω-E _g>≈400 meV, the first experimental evidence of 2ħω-E _g)^5/2 dependence has been obtained. As a consequence, each of the various models proposed to predict the α⁽²⁾ dispersion curve gives the correct energy dependence in a limited energy range, due to the poor approximation made in the evaluation of dipole matrix elements. To explain the spectral dependence of the two-photon absorption coefficient, consideration of all the intermediate states is required, with the energy dependence of the dipole matrix elements properly considered. It is shown that the greatest contributions to the oscillator strength come from transitions totally allowed at the critical points. Work partially supported by M.P.I.