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

Abstract

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 BiB 3 O 6 crystal-based optical parametric amplifier, which generates a strong broadband shortwave 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) × 10 12 s −1 cm 2. 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.

In Situ Spectroscopic Probing of Polarity and Molecular Configuration at Aerosol Particle Surfaces

“…The next step to improving the method is to develop a strong and broadband short-wave IR laser source with a high-repetition laser, as we demonstrated in a low-frequency 1 kHz laser system. 62 Gas molecules, ions, water, and adsorbates from particle, which define the boundary of gas−aerosol particles, largely determine the overall chemical and physical properties of aerosols. Thus, knowledge of binding and structure of gas molecules, ions, water, and organic adsorbates at the gas− aerosol particle surfaces is necessary to elucidate the significance of the surfaces in atmospheric processes.…”

“…Only a few techniques are available for investigating liquid surfaces, − not to mention the surfaces of liquid droplets. Second harmonic generation (SHG) and sum frequency generation (SFG) are two of the proven surface-sensitive methods to investigate liquid surfaces. ,− As a spectroscopic technique, SFG is anticipated to provide vibrational structures, ,− ,− as well as electronic characters, ,− both of which are essential to understanding chemical and physical properties of surfaces. Several groups demonstrated the applications of the SHG and SFG techniques in atmospheric science. ,− These pioneering works have shown cases of the powerful surface-specific techniques.…”

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

“…Despite its potential, there are limited examples of eSFG being utilized in the literature. − None of these existing studies have investigated the electronic structure of nanomaterials despite the importance of their surfaces. A recent SFG study probed the ligand vibrational modes on nanoparticles surfaces in the presence of a pulse resonant with the QD exciton transitions but was not able to extract any electronic structure information beyond the first exciton .…”

“…As such, at this time, we cannot extract quantitative information from the amplitudes of the peaks in isolation; however, given that sample oxidation does not change the QD orientations/number density and interferences between signals in the eSFG spectrum, we can compare relative changes in fresh and oxidized samples. Future experiments could explore the azimuthal angle dependence of the eSFG signal as others have done on macroscopic surfaces 70 to gain additional insight into the core and surface states.…”

Imaging Orientation of a Single Molecular Hierarchical Self-Assembled Sheet: The Combined Power of a Vibrational Sum Frequency Generation Microscopy and Neural Network

“…VSFG employs a broadband IR pulse (290 fs duration) centered at ~2850 cm -1 to excite CH vibrational modes of the SDS@2β-CD system, which is followed by a narrowband 1025nm near IR pulse to upcovert the vibrational coherence to a higher frequency macroscopic polarization emitted as the VSFG signal. As an even-order nonlinear optical process, only noncentrosymmetric systems produce VSFG responses, such as the air/liquid, air/solid and solid/liquid interfaces 29,31,[40][41][42][32][33][34][35][36][37][38][39] with a powerful demonstration of the techniques capabilities found in its application to the air/water interface. [43][44][45] Another class of systems that can be studied by VSFG that have not been paid much attention are materials without inversion centers.…”