Time‑, Energy‑, and Phase-Resolved Second-Harmonic Generation at Semiconductor Interfaces

Interfacial charge transfer is ubiquitous in many chemical and physical processes and can occur on ultrafast time scales of femtoseconds to picoseconds. Probing dynamics on such time scales necessitates the use of ultrafast laser spectroscopies, but signatures of interfacial charge transfer can be overwhelmed by the signal from bulk materials. This problem may be alleviated in second-harmonic generation, which can be specifically sensitive to interfacial charge transfer if other bulk and interfacial contributions to the measured second-harmonic signal can be resolved. We report the development of a femtosecond spectral interferometry technique for second-harmonic generation with time, energy, and phase resolution. Using the model systems of a passivated GaAs(100) surface and copper phthalocyanine/GaAs(100) interface, we demonstrate the application of this technique in unveiling the rich dynamics of band renormalization, charge carrier motion, and interfacial charge transfer, all induced by across-bandgap optical excitation of the semiconductor.