Origin of nonlinear optical characteristics of crystalline Ge-Sb-Te thin films for possible superresolution effects

An experiment was conducted to investigate the origin of the nonlinear optical characteristics of crystalline Ge₂Sb₂Te ₅ and Ge-doped SbTe thin films which have been demonstrated to have the capability of a superresolution readout. By means of the real-time measurement of the electrical resistance combined with concurrent optical reflectance/transmittance measurement during pulsed laser irradiation of varying power, it was found that the absorption coefficients of these films decrease with increasing laser power, accompanied most probably by an increase in the carrier concentration. This finding may be understood in light of the saturable absorption due to band filling by thermally assisted photoexcited carriers. This has been previously proposed as an important cause of the superresolution effect of PbTe, which belongs to the chalcogenide degenerate semiconductors such as the GeSbTe materials under study.