Abstract

Deformation potentials are important parameters for the description of static and dynamic properties of solids. There are considerable difficulties to determine these parameters, which describe the influence of an external stress on the electronic eigenstates, by classical methods of linear optical spectroscopy. For allowed transitions, like exciton or band to band transitions, one has to do piezo-reflectivity measurements. The main difficulties arise from the fact that one is dealing with polariton resonances, which cannot be isolated because of background contributions of oscillators at higher energies. For a detailed discussion of experimental techniques and the analysis of piezo-reflection measurements we refer to Cardona.^[1] We report a new method to determine deformation potentials with high accuracy. From the energy shift and splitting of paraexcitons in KI, which are measured by three-photon absorption, the hydrostatic, tetragonal and trigonal deformation potentials are determined. Due to the small linewidth of ΔE = 0.5 meV the splitting of the paraexciton can be resolved under static stress, the value of which can be determined with high accuracy. In Fig. 1 experimental data under uniaxial stress of about 640 bar in [110] direction are shown. In one-photon reflection spectroscopy one has to modulate the stress in order to resolve stress-induced effects of the rather broad spectra (ΔE ~ 100meV). In Fig. 2 we present the shift and the splitting of paraexcitons under uniaxial stress ([100], [110] and [111] direction). The different components are selectively excited with linearly polarized light. The detailed stress dependence for three orientations is shown in Fig. 2. The data are analyzed with the use of the Pikus-Bir Hamiltonian.^[1,2]

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