Many optical biosensors detect molecules that either are attached to a surface, or are in close proximity to one. Therefore, understanding the optical properties of surfaces, and how these properties affect molecular interferometry, is a central topic of this book. Surfaces are planes of dielectric discontinuity that split the amplitude of waves into transmitted and reflected partial waves (Fig. 4.1). Surfaces impose electromagnetic boundary conditions that produce constructive or destructive interference of the incident and reflected waves. On the wrong type of surface (unity reflection with a π phase shift, as for a metal surface), a thin protein film can be entirely invisible to an incident plane wave (but not a surface wave), because the electric field strength at the molecular layer is canceled by destructive interference between the incident and reflected waves. In this situation, a light wave can pass right through the layer without ever polarizing it, and hence never sensing any change in the optical path length. This is perhaps the oldest and simplest form of optical cloaking .
KeywordsReflection Coefficient Partial Wave Optical Path Length Scattered Field Phase Quadrature
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