Photoinduced phase transition in strongly correlated TTTA crystals probed with two-photon luminescence

Two-photon processes are used for studying photoinduced phase transition (PIPT) from diamagnetic low-temperature phase to paramagnetic high-temperature phase in strongly correlated organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystals. We have measured the Raman and luminescence spectra in the low-temperature phase after two-photon excitation with various excitation photon densities. After correction of re-absorption of the luminescence, the shape of the two-photon luminescence spectrum is in good agreement with that of the one-photon luminescence spectrum, which strongly suggests that the initial states of the two-photon luminescence are the same as those of the one-photon luminescence and are self-trapped excitons (STEs). As increasing the excitation photon density, PIPT to the high-temperature phase takes place with a threshold behavior as a sudden decrease of the two-photon luminescence intensity. This result suggests that STEs are the precursors of the PIPT and the two-photon luminescence can be a useful probe of whole PIPT dynamics from the microscopic lattice distortion to the macroscopic phase transition.