Phase-Sensitive Sum-Frequency Generation Measurements Using a Femtosecond Nonlinear Interferometer.

Ellen H G Backus,Patrick J Bisson,Mischa Bonn,Mary Jane Shultz,Shumei Sun

doi:10.1021/acs.jpcc.9b00861

Abstract

Phase-sensitive sum-frequency spectroscopy is a unique tool to interrogate the vibrational structure of interfaces. A precise understanding of the interfacial structure often relies on accurately determining the phase of χ(2), which has recently been demonstrated using a nonlinear interferometer in conjunction with a frequency-scanning picosecond laser system. Here, we implement nonlinear interferometry using a femtosecond laser system for broadband sum-frequency generation. The phase of the vibrational response from a self-assembled monolayer of octadecanethiol on gold is determined using the nonlinear femtosecond interferometer. The results are compared to those obtained using the more traditional heterodyne-detected phase measurements. Both methods give a similar phase spectrum and phase uncertainty. We also discuss the origin of the phase uncertainties and provide guidelines for further improvement.

Highlights

A precise understanding of the interfacial structure often relies on accurately determining the phase of χ(2), which has recently been demonstrated using a nonlinear interferometer in conjunction with a frequency-scanning picosecond laser system
Sum-frequency vibrational spectroscopy (SFVS) has emerged as one of the main tools for characterizing molecular structure and dynamics at interfaces because of its high surface sensitivity and molecular specificity.[1−7] in a conventional homodyne SFVS measurement, the SF signal is proportional to the square of the complex second-order nonlinear susceptibility
Since the seminal papers reporting the phase measurement methodology published by Shen et al.,[12,13] the technique of phase-sensitive sum-frequency vibrational spectroscopy (PSSFVS) has greatly developed in the past ten years,[12−21] and phase accuracy has been a prime concern

Summary

■ INTRODUCTION

Sum-frequency vibrational spectroscopy (SFVS) has emerged as one of the main tools for characterizing molecular structure and dynamics at interfaces because of its high surface sensitivity and molecular specificity.[1−7] in a conventional homodyne SFVS measurement, the SF signal is proportional to the square of the complex second-order nonlinear susceptibility. The sample signal interferes with the reference the phase standard to directly deduce the sample phase This nonlinear interferometer design has only been demonstrated using a picosecond laser system.[17,28] In the SFG community, an increasing number of experimental setups are based on a femtosecond laser system because a femtosecond laser system offers a broad IR spectrum, high peak intensity, and high repetition rate, which allows reducing the acquisition time. The signal and idler beams are sent to a difference broad IR wfrietqhueanbcyangdewniedrtahtoarro(ucnrydsta4l0:0AcgmG−aS1 2w),itphroadupcuilnsge energy around 5 μJ Another roughly 1 W of the laser output goes through an etalon to generate the narrowband (with just 15∼25 cm−1 line width), temporally lengthened, 800 nm wavelength pulse of ∼20 μJ used as the visible beam in the SFVS process.

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ REFERENCES