Second-harmonic phase determination by real-time in situ interferometry

Second Harmonic Generation (SHG) has emerged as a highly sensitive probe of protein conformation. SHG can also be used to determine the tilt angle of an SHG-active moiety bound to a surface-adsorbed protein through polarization-dependent measurements. However, due to the coherent nature of SHG, interference occurs between the SHG produced by the SHG-active moieties and background sources at a solid-liquid interface, obscuring the signal of interest. In order to separate the protein-specific signal from the background signal, the phase difference between these two different sources of SHG must be determined. Although the phase difference can be obtained through a conventional interferometric approach involving a phase-modulated SHG source external to the sample, it can be sensitive to drift and other instabilities. We present here a simple, convenient, and crucially, model-independent method to determine the phase difference for any system in which the intensity of SHG-active moieties can be varied. We demonstrate the approach with time-resolved measurements of an SHG-active labeled protein binding to a supported lipid bilayer surface using a total internal reflection (TIR) geometry. This approach requires no additional optics beyond what is required to measure SHG and is highly stable since the interferometry occurs in situ , within the sample over a nanometer length scale, rather than external to it. To validate our measurements and the general approach, we constructed a dual-beam, external SHG interferometer in a TIR geometry. We also validated our approach by applying the in situ method to previously published measurements of the phase difference, obtaining the same values without recourse to a specific adsorption model. We present a interferometer-free, model-independent method of phase determination between an SHG-active moiety and coherent background sources.