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• Cuprates. • Polarons. • HTc superconductivity. • Photoinduced IR spectroscopy. • Raman spectroscopy. The discovery in 1986 of superconductivity at high critical temperatures attracted the attention of many researchers not only for its potential applications but also because it represented a theoretical challenge to understanding the mechanism that goes beyond the BCS model. Over 35 years of research, both theoretical and experimental, have not been sufficient to reach a comprehensive description of mechanism governing high Tc superconductivity. We summarize here the experimental signature of polarons and of the anharmonic behavior of the strongly polarizable O−2 ion, two of the possible ingredients of superconductivity in HT c materials, evidenced by Raman and by photoinduced infrared spectroscopy. [ABSTRACT FROM AUTHOR]