Phonons at functional oxide interfaces: An in situ sum-frequency spectroscopic perspective

Functional oxides host emergent interfacial phenomena from superconductivity to catalysis, intimately tied to lattice phonons that mediate many-body interactions. Sum-frequency generation (SFG) spectroscopy, a second-order nonlinear optical technique with intrinsic surface specificity, enables in situ and operando probing of lattice vibrations at surfaces and interfaces. In this review, we introduce the basic theory of SFG and survey its application in investigating functional oxide surfaces and interfaces — tracking oxygen vacancies on anatase TiO₂, unveiling electron–phonon coupling modulation in LaAlO₃/SrTiO₃ heterostructures, and detecting polaronic signatures in Nb-doped SrTiO₃ — thereby offering microscopic insights into the physics of oxide interfaces.