The surface/interface species in perovskite oxides play essential roles in many novel emergent physical phenomena and chemical processes. With low eigen-energies in the terahertz region, such species at buried interfaces remain poorly understood due to the lack of feasible surface-specific spectroscopic probes to resolve the resonances. Here, we show that polarized phonons and two-dimensional electron gas at the interface can be characterized using surface-specific nonlinear optical spectroscopy in the terahertz range. This technique uses intra-pulse difference frequency mixing process, which is allowed only at the surface/interface of a centrosymmetric medium. Submonolayer sensitivity can be achieved using the state-of-the-art detection scheme for the terahertz emission from the surface/interface. Through symmetry analysis and proper polarization selection, background-free Drude-like nonlinear response from the two-dimensional electron gas emerging at the LaAlO3/SrTiO3 or Al2O3/SrTiO3 interface was successfully observed. The surface/interface potential, which is a key parameter for SrTiO3-based interface superconductivity and photocatalysis, can now be determined optically in a nonvacuum environment via quantitative analysis on the phonon spectrum that was polarized by the surface field in the interfacial region. The interfacial species with resonant frequencies in the THz region revealed by our method provide more insights into the understanding of physical properties of complex oxides.
2023, 3(1): 0042
A high-sensitivity DC magneto-optical Kerr effect (MOKE) apparatus is described in this Letter. Via detailed analysis on several dominating noise sources, we have proposed solutions that significantly lower the MOKE noise, and a sensitivity of is achieved with long-term stability. The sensitivity of the apparatus is tested by measuring a wedge-shaped Ni thin film on with Ni thickness varying from 0 to 3 nm. A noise floor of is demonstrated. The possibility of further improving sensitivity to via applying AC modulation is also discussed.high sensitivity direct current magneto-optical Kerr effect
Chinese Optics Letters
2022, 20(11): 111201