Apparatus and method for detecting the presence or amount or rate of
binding of an analyte in a sample solution is disclosed. The apparatus
includes an optical assembly having first and second reflecting surfaces
separated by a distance "d" greater than 50 nm, where the first surface
is formed by a layer of analyte-binding molecules, and a light source for
directing a beam of light onto said first and second reflecting surface.
A detector in the apparatus operates to detect a change in the thickness
of the first reflecting layer resulting from binding of analyte to the
analyte-binding molecules, when the assembly is placed in the solution of
analyte, by detecting a shift in phase of light waves reflected from the
first and second surfaces.