Development of ultrafast broadband electronic sum frequency generation for charge dynamics at surfaces and interfaces

Understandings of population and relaxation of charges at surfaces and interfaces are essential to improve charge collection efficiency for energy conversion, catalysis, and photosynthesis. Existing time-resolved surface and interface tools are limited to either under ultrahigh vacuum or in a narrow wavelength region with the loss of spectral information. There lacks an efficient time-resolved surface/interface-specific electronic spectroscopy under ambient conditions for the ultrafast surface/interface dynamics. Here we developed a novel technique for surface/interface-specific broadband electronic sum frequency generation (ESFG). The broadband ESFG was based on a stable two-stage BiB₃O₆ crystal-based optical parametric amplifier, which generates a strong broadband short-wave infrared (SWIR) from 1200 nm to 2400 nm. A resultant surface spectrum covers almost all visible light from 480 nm to 760 nm, combined a broadband electronic second harmonic generation (ESHG) with the ESFG from the SWIR laser source. We further developed the steady-state and transient broadband ESFG and ESHG techniques to investigate the structure and dynamics of charges at oxidized p-type GaAs (100) semiconductor surfaces, as an example. Both the steady-state and transient experiments have shown that two surface states exist inside the bandgap of the GaAs. The kinetic processes at the GaAs surface include both the population and recombination of the surface states after photoexcitation, in addition to the build-up of the space photo-voltage (SPV). The build-up SPV occurs with a rate of 0.56 ± 0.07 ps^-1, while the population rate of the surface states exhibits a two-body behavior with a rate constant of (0.012 ± 0.002) × 10¹² s^-1 cm². The photo-generated electron-hole pairs near the surface recombine with a rate of 0.002 ± 0.0002 ps^-1 for the oxidized p-type GaAs (100). All the methodologies developed here are readily applied to any optically accessible interfaces and surfaces, in particular buried interfaces under ambient conditions.