Dual Polarisation Interferometry

Dual Polarisation Interferometry

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Dual polarization interferometry (DPI) is an analytical technique that can probe molecular scale layers adsorbed to the surface of a waveguide
Waveguide (optics)
An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber and rectangular waveguides....

 by using the evanescent wave
Evanescent wave
An evanescent wave is a nearfield standing wave with an intensity that exhibits exponential decay with distance from the boundary at which the wave was formed. Evanescent waves are a general property of wave-equations, and can in principle occur in any context to which a wave-equation applies...

 of a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 beam confined to the waveguide. Typically used to measure the conformational change
Conformational change
A macromolecule is usually flexible and dynamic. It can change its shape in response to changes in its environment or other factors; each possible shape is called a conformation, and a transition between them is called a conformational change...

 in proteins or other biomolecules as they function (referred to as the conformation activity relationship
Conformation activity relationship
Conformation-activity relationship is the relationship between the biological activity and the conformation or conformational changes of a biomolecule...

).

DPI focuses laser light into two waveguides. One of these functions as the "sensing" waveguide having an exposed surface while the second one functions to maintain a reference beam. A two-dimensional interference pattern is formed in the far field by combining the light passing through the two waveguides. The DPI technique rotates the polarization of the laser to alternately excite two polarization modes of the waveguides. Measurement of the interferogram for both polarizations allows both the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 and the thickness of the adsorbed layer to be calculated. The polarization can be switched rapidly, allowing real-time measurements of chemical reactions taking place on a chip surface
Surface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...

 in a flow-through system. These measurements can be used to infer conformational information about the molecular interactions taking place as the molecule size (from the layer thickness) and the fold density (from the RI) change. DPI is typically used to characterise biochemical interactions by quantifying any conformational change
Conformational change
A macromolecule is usually flexible and dynamic. It can change its shape in response to changes in its environment or other factors; each possible shape is called a conformation, and a transition between them is called a conformational change...

at the same time as measuring reaction rates, affinities and thermodynamics.

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

A novel application for Dual Polarization Interferometry recently emerged 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. The latest versions of Dual Polarization Interferometers also have the capability to quantify the order and disruption in birefringent thin films. This has been used, for example, to study the formation of lipid bilayers and their interaction with membrane proteins.