Phase-sensitive Sum Frequency Generation Spectroscopy Research Articles

Progress in phase-sensitive sum frequency generation spectroscopy.

Sum frequency generation (SFG) spectroscopy is a unique and powerful tool for investigating surfaces and interfaces at the molecular level. Phase-sensitive SFG (PS-SFG) is an upgraded technique that can overcome the inherent drawbacks of conventional SFG. Here we review several methods of PS-SFG developed and reported in 1990-2020. We introduce how and by which group each PS-SFG method was designed and built in terms of interferometer implementation for optical heterodyne detection, with one exception of a recent numerical method that does not rely on interferometry. We also discuss how PS-SFG solved some typical problems for aqueous interfaces that were once left open by conventional SFG. These problems and their solutions are good examples to demonstrate why PS-SFG is essential. In addition, we briefly note a few terminology issues related with PS-SFG to avoid confusion.

High-resolution phase-sensitive sum frequency generation spectroscopy by time-domain ptychography

We demonstrate that time-domain ptychography, when applied to a set of broadband vibrational sum frequency spectra, reconstructs amplitude and phase of the vibrational free induction decay from an interfacial sample with a resolution that is independent of up-converting pulse bandwidth and spectrometer resolution. These important improvements require no modifications to most standard homodyne setups, and the method is applicable to other coherent homodyne spectroscopies such as coherent anti-Stokes Raman spectroscopy and transient grating spectroscopy.

Quantitative determination of the nonlinear bulk and surface response from alpha-quartz using phase sensitive SFG spectroscopy

We investigate the nonlinear optical response of alpha-quartz with phase-sensitive vibrational sum frequency generation (SFG) spectroscopy. Phase and amplitude of the generated sum frequency signal are determined by a complex interplay between signal contributions from the surface and the bulk of the crystal with the experimental geometry, in particular, the quartz azimuth. By combining anisotropy measurements with spectral- and phase resolution in our SFG experiment, we study this interplay and show that we can quantitatively isolate and characterize the contributions from the surface and bulk of the crystal. The obtained optical constants in combination with the presented mathematical framework fully describe the nonlinear response and allow for a precise determination of the phase of the overall SFG signal of quartz for any experimental geometry. This knowledge is of particular relevance because quartz is commonly used as a reference in phase sensitive SFG experiments. In such studies, the surface contribution is typically neglected and a constant value for the phase of the sum frequency response is assumed. Our study shows that this assumption is not generally accurate by revealing the considerable impact that the surface contribution can have on the overall response of the crystal.

Measurements indicate alpha-quartz’s surface signal should not be neglected in spectroscopy calibration

Phase-sensitive sum frequency generation spectroscopy reveals that the contribution from the surface of alpha-quartz to the overall sum frequency response signal is more significant than previously assumed.

Determining the Orientation of Chemical Functional Groups on Metal Surfaces by a Combination of Homodyne and Heterodyne Nonlinear Vibrational Spectroscopy

We illustrate how phase-sensitive sum-frequency generation spectroscopy may be used to determine the polar orientation of organic species on metals, where there is a significant electronic contribution to the second-order signal. It turns out that traditional direct heterodyne schemes, as would be applied to the same molecules on dielectric substrates, are challenging to use here as a result of the large resonant/nonresonant amplitude ratio that diminishes the phase contrast observed in tuning through the vibrational mode. This is demonstrated in a variety of experimental surfaces that illustrate all limiting cases. We propose a scheme that can overcome this challenge and thereby determine the chemical functional group orientation through a combination of the homodyne spectrum and some phase information from a heterodyne approach.

Chemisorbed and Physisorbed Water at the TiO2/Water Interface.

The interfacial structure of water in contact with TiO2 is the key to understand the mechanism of photocatalytic water dissociation as well as photoinduced superhydrophilicity. We investigate the interfacial molecular structure of water at the surface of anatase TiO2, using phase-sensitive sum frequency generation spectroscopy together with spectra simulation using ab initio molecular dynamic trajectories. We identify two oppositely oriented, weakly and strongly hydrogen-bonded subensembles of O–H groups at the superhydrophilic UV irradiated TiO2 surface. The water molecules with weakly hydrogen-bonded O–H groups are chemisorbed, i.e. form hydroxyl groups, at the TiO2 surface with their hydrogen atoms pointing toward bulk water. The strongly hydrogen-bonded O–H groups interact with the oxygen atom of the chemisorbed water. Their hydrogen atoms point toward the TiO2. This strong interaction between physisorbed and chemisorbed water molecules causes superhydrophilicity.