Optical-Pump and Terahertz-Probe Spectroscopic Evidence of Polaron Quasiparticles in La_0.3Sr_0.7TiO₃ Thin Films: A Push toward High-Frequency Oxide Optoelectronics

Strongly correlated materials, due to competing energies, give rise to exciting ground states, such as metal-to-insulator transitions (MIT). Understanding MIT in strongly correlated materials is of immense interest, owing to their potential applications in future electronics. Generally, the physics near MIT is driven by electron quasiparticles resulting from electron–electron interactions. However, we show the profound effects of polaron quasiparticles resulting from the 3d electron clouds coupled to phonons on the properties near MIT. The nature of these quasiparticles, resulting from strong electron–phonon coupling in our La0.3Sr0.7TiO3 (LSTO3) thin films, is investigated through a low-energy carrier dynamics study using time-domain terahertz (THz) spectroscopy, which shows that the electrons move in the heavy 3d band influenced by the optical phonons. The low-temperature results agree with polaron hopping. The nonequilibrium behavior and the role of photoinduced polarons are further explored using our ultrafast carrier dynamics studies utilizing optical pump terahertz probe spectroscopy. Our findings demonstrate the dimensionally dependent electron–phonon interaction in the system, and the conduction mechanism occurs through polaron hopping. The increased effective mass of dressed electrons in our system is directly related to the large thermoelectric properties seen in the 3d transition systems because of the strong electron–phonon coupling. In this case, we offer a tuning parameter for the polaron energy with a direct practical consequence.