- Dual polarisation interferometry
Dual polarisation interferometry (DPI) is an analytical technique in chemistry that can probe layers adsorbed to the surface of a waveguide by using the
evanescent waveof a laserbeam confined to the waveguide. [cite journal|journal=J. Phys.: Condens. Matter | volume=16 | date=2004 | author= Neville J. Feeman, Louise L. Peel, Marcus J. Swann, Graham H. Cross, Andrew Reeves, Stuart Reeves, and Jian R. Lu| title=Real time, high resolution studies of protein adsorption and structure at the solid–liquid interface using dual polarisation interferometry | pages=S2493–S2496| doi=10.1088/0953-8984/16/26/023] DPI focuses laser light into two waveguides, one, the "sensing" waveguide, with an exposed surface and one to create a reference beam. A two-dimensional interference patternis formed in the far field by combining the light passing through the two waveguides. The DPI technique rotates the polarisationof the laser to alternately excite two polarisation modes of the waveguides. Measurement of the interferogram for both polarisations allows both the refractive indexand the thicknessof the adsorbed layer to be calculated. The polarisation can be switched rapidly, allowing "real time" measurements of chemistry taking place on a chip surfacein a flow-through system. These measurements can be used to infer structural information about the molecular interactions taking place and is typically used to characterise biochemical interactions.
A novel application for Dual Polarisation 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 Polarisation 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. [3,4]
Surface plasmon resonance
2. Attia Boudjemline, David T. Clarke, Neville J. Freeman, James M. Nicholson and Gareth R. Jones Early stages of protein crystallization as revealed by emerging optical waveguide technology J. Appl. Cryst. (2008). 41, 523–530. doi:10.1107/S0021889808005098
3. Tabrisur Rhaman Khan, Michelle Grandin, Alexander N. Morozov, Alireza Mashaghi,Marcus Textor, Erik Reimhult, Ilya Reviakine, Lipid redistribution in phosphatidylserine-containing vesicles adsorbing on titania in Biointerfaces 3(2), FA90 - FA95 (2008). ]
4. Alireza Mashaghi, Marcus Swann, Jonathan Popplewell, Marcus Textor and Erik Reimhult, Optical anisotropy of supported lipid structures probed by waveguide spectroscopy and its application to study of supported lipid bilayer formation kinetics Anal. Chem., 80 (10), 3666–3676, 2008. PMID: 18517221 Web Release Date: 19-Apr-2008; (Article) DOI: 10.1021/ac800027s Anal. Chem.; 2008; 80(13) pp 5276 - 5276; (Addition/Correction)]
* [http://www.farfield-group.com Farfield Group]
* [http://www.malvern.co.uk/Malvern/OnDemand.nsf/id/980232200 8 minute video about DPI]
Wikimedia Foundation. 2010.