Investigation of insulator-metal transition in Ti4O7 using terahertz probe pulse

The ultrafast photo-induced insulator-metal transition in a Ti4O7 single crystal is investigated with different excitation densities using an optical-pump terahertz (THz)-probe technique. The oxide shows a two-step phase transition at 130 K from a low temperature (LT) phase to an intermediate phase and to a high temperature metallic phase at 150 K. The photo-irradiation in the LT phase induces an ultrafast decrease in the transmittance of the THz probe within 3.6 ps, corresponding to the generation of a metallic domain. This is followed by an additional slow decrease when the excitation is above the threshold level. The nonlinear dependence of the amplitude and lifetime on the excitation density indicates that the metallic domains grow spontaneously and are stabilized depending on their resultant amount.