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

In surface science, it is important to understand processes between the bulk and the surface of a material, such as the transport of charges or energy, or the coupling of states across boundaries. To study these interactions, researchers need to simultaneously probe the physical properties of both the bulk and surface, and parse out the signal from each.

Thämer et al. aimed to improve an existing laser-based technique that measures the difference in structural symmetry between bulk and surface, known as phase-sensitive sum frequency generation (SFG) spectroscopy. They used SFG spectroscopy to measure the nonlinear optical response of an alpha-quartz crystal – a common phase reference material for calibrating phase-sensitive SFG spectroscopy. Combining anisotropy measurements and phase resolution, the authors were able to quantitatively separate the surface and bulk’s contributions to the overall sum frequency response.

They found that the surface contribution can significantly affect the phase value of the sum frequency response of alpha-quartz. However, common practice ignores surface contribution by assuming a constant phase value of ±90 degrees for alpha-quartz, a value that only considers bulk contribution. The new results show the importance of including the surface contribution when measuring the phase value of reference materials to accurately calibrate SFG spectrometry, since even small phase errors can severely distort SFG spectra data.

Author Martin Thämer said that this technique could be applied beyond alpha-quartz to study couplings between bulk and surface states or to investigate transport phenomena between bulk and surface in other solid materials.

Source: “Quantitative determination of the nonlinear bulk and surface response from alpha-quartz using phase sensitive SFG spectroscopy,” by Martin Thämer, Tobias Garling, R. Kramer Campen, and Martin Wolf, The Journal of Chemical Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5109868 .