TIR and Evanescent Waves
LFIRE™ uses the advantages of performing ellipsometric measurements in total internal reflection mode. In this configuration, also known as the Kretschmann configuration, interrogating light enters the underside of a microarray or wellplate and is reflected from the solid-liquid interface between the substrate and the sample solution.
Light will reflect from this interface provided the index of refraction of the substrate is lower than the index of the material on the reflection side, and the angle of incidence of the beam is equal to or larger than the critical angle. For angles less than the critical angle, a portion of the light will be transmitted, and some reflected.
When light is totally internally reflected at the glass/liquid interface, a small portion of the beam 'leaks' into the liquid roughly 100-200 nm getting exponentially weaker further from the interface. This localized light wave is called the "evanescent wave". When matter (i.e.- proteins and DNA) diffuses into the evanescent wave region of the liquid sample, the polarization state of the light is changed and can be measured accurately. The evancescent field is thus, a sensing region for local changes in mass and these changes are evident in the polarization components of the reflected beam. Analysis of this polarization change is a highly sensitive and reproducible method for determining thin layer optical characteristics and can be directly correlated to excess surface mass density on the glass substrate.
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