Dual-polarization interferometry

Dual-polarization interferometry (DPI) is an analytical technique that probes molecular layers adsorbed to the surface of a

Instrumentation

DPI

The technique is quantitative and real-time (10 Hz) with a dimensional resolution of 0.01 nm.[2]

Extensions of dual-polarization interferometry also exist, namely multiple pathlength dual-polarization interferometry (MPL-DPI)^[3][4][5] and absorption enhanced DPI. In MPL-DPI quantification of both layer thickness and refractive index (density) and therefore mass per unit area can be made for in situ and ex-situ coated films, where normal DPI can only calculate film properties if the interferogram is constantly monitored. Absorption enhanced DPI^[6] (AE-DPI) is used to separate the mass of different molecules on the surface, exploiting the absorption of one of the molecular species compared to the other species on the surface.

Applications

A novel application for dual-polarization interferometry emerged in 2008, where the intensity of light passing through the waveguide is extinguished in the presence of crystal growth. This has allowed the very earliest stages in protein crystal nucleation to be monitored.^[7] Later versions of dual-polarization interferometers also have the capability to quantify the order and disruption in birefringent thin films.^[8] This has been used, for example, to study the formation of lipid bilayers and their interaction with membrane proteins.^[9][10]

References