Passively Stabilized Phase-Resolved Collinear SFG Spectroscopy Using a Displaced Sagnac Interferometer

Sum-frequency generation (SFG) vibrational spectroscopy is a powerful technique to study interfaces at the molecular level. Phase-resolved SFG (PR-SFG) spectroscopy provides direct information on interfacial molecules' orientation. However, its implementation is technologically demanding: it requires the generation of a local oscillator wave and control of its time delay with sub-fs accuracy. Commonly used noncollinear PRSFG provides this control naturally but requires very accurate sample height control. Collinear PR-SFG spectroscopy is less demanding regarding sample positioning, but tuning the local oscillator time delay with this beam geometry is challenging. Here, we develop a collinear PR-SFG setup using a displaced Sagnac interferometer. This scheme allows full, independent control of the time delay and intensity of the local oscillator and provides long-time phase stabilization (better than 5 degrees over 12 h) for the measured signal. This approach substantially reduces the complexity of an experimental setup and combines the advantages of collinear and noncollinear PR-SFG techniques.

Automated summary

SFG vibrational spectroscopy is a powerful technique to study interfaces. However, implementing this technique is challenging. In this study, a collinear PR-SFG setup using a displaced Sagnac interferometer is developed, which allows independent control of time delay and intensity and provides long-time phase stabilization for the measured signal.