Determining nonlinear optical coefficients of metals by multiple angle of incidence heterodyne-detected sum-frequency generation spectroscopy

Abstract

We illustrate a technique by which heterodyne-detected sum-frequency generation spectroscopy is performed at multiple angles of incidence in order to decompose components of the second-order susceptibility tensor when all beams are polarized parallel to the plane of incidence. As an illustration we study the non-vibrationally resonant gold response. We benchmark our results by comparing with measurements obtained in a polarization scheme that isolates a single element of the susceptibility tensor. Our technique is particularly valuable in the case of metal substrates, where the surface selection rule often prevents spectra from being acquired in multiple beam polarizations.

Determining the Surface Potential of Charged Aqueous Interfaces Using Nonlinear Optical Methods

“…All of the established and proposed methods are based on a decomposition of χ total (2) ; it is therefore crucial to be able to extract this quantity from the measured signal of interest. In a typical SHG or SFG experiment, the detected signal is governed by the effective susceptibility that is in turn related to the susceptibility tensor value of interest through a combination of factors, most notably the local field corrections that relate the incident laser fields to the fields at the surface (appearing in eq ), , unit polarization vectors that transform s/p basis vectors to Cartesian coordinates, and other frequency- and angle-dependent factors. , In some cases, particularly off-resonance, the dispersion of these factors is small and then may or may not contribute to the success of the Φ 0 determination. However, in experiments that seek to modulate the coherence by changing the angle of incidence, it is of the utmost importance to consider the angle dependence of the magnitude and phase of these factors.…”

Phase-Sensitive Second-Harmonic Generation of Electrochemical Interfaces

“…This determination can be achieved by measuring the absolute phase with a standard nonlinear reference like alpha-quartz or using clever optical methods like multiple-angle heterodyne detection. 29 Given the importance of U pzc in our understanding of the electrode−electrolyte structure and the interfacial electron transfer process, 30−32 we envision this experiment as the next important step of the electrochemical SHG studies. Finally, we note that, as with the previous studies, 9−13 the SHG intensity minimum is close to U pzc in our experiment (Figure 3d).…”

“…Most critically, our work shows that if either α or θ is known, one can use eq to directly determine U pzc . This determination can be achieved by measuring the absolute phase with a standard nonlinear reference like alpha-quartz or using clever optical methods like multiple-angle heterodyne detection . Given the importance of U pzc in our understanding of the electrode–electrolyte structure and the interfacial electron transfer process, − we envision this experiment as the next important step of the electrochemical SHG studies.…”

Probing Molecular Interactions between Surface-Immobilized Antimicrobial Peptides and Lipopolysaccharides In Situ

“…One particular outstanding question is how the immobilization site of the same peptide on a surface affects the three-dimensional structure of the peptide that influences the AMP–LPS interactions. Sum frequency generation (SFG) vibrational spectroscopy is a powerful technique to probe surfaces and buried interfaces in situ with submonolayer sensitivity. ,− We have adopted SFG to study surface-immobilized peptides/water interfaces. − In this work, we studied molecular interactions in situ between SMAP29 peptide immobilized via each of the three sites, the N-terminus (NT), the C-terminus (CT), and a middle site (Y17c), and LPS samples extracted from three different bacterial strains [from E. coli O111:B4 (O111:B4), E.…”

“…SFG is a surface-sensitive vibrational spectroscopic technique involving a second-order nonlinear optical process. ,− It has been widely applied to examine molecular interactions at various interfaces including buried interfaces of proteins, peptides, polymers, and so forth. ,− ,− ,− ,− Details of SFG theory and instrumentation have been published elsewhere and will not be repeated here. ,− In the SFG setup, two input beams (a frequency-fixed 532 nm visible beam and a frequency-tunable infrared beam) are spatially and temporally overlapped on the sample to generate a signal beam with the sum frequency of t...…”