Speckle based sensing of chemicals by an acoustic excitation in aqueous solutions

The optical activity of glucose in aqueous solutions offers a very high specificity in detecting the presence of glucose. In this presentation we will present several concepts for non-invasive detection of glucose, being realized in-vitro as well as in-vivo. In all cases the sensing concept is based upon analysis of time changing spatial statistics of back scattered speckle patterns when being analyzed by properly defocused optics. We will focus on an experimental approach in which we try to employ contactless measurement of acoustic excitations in solutions containing various chemical while using analysis of those time changing speckle pattern. Solutions containing glucose should response differently than those where glucose is absent. To perform this measurement, we excited acoustic waves in a solution and measured the changes in the speckle pattern. The basic concept is that while the solution is acoustically excited the acoustic waves modulate the density of the fluid under examination. This modulation will have two effects on the speckle pattern, the first is a spatial and time-varying modulation of the effective refractive index, and the second is a spatial and time-varying modulation of the optical rotation which is induced by the presence of glucose. Both of these effects should change the speckle pattern, which if recorded with an exposure time which is longer than the acoustic period, will be seen as a smearing of the pattern. By analyzing properties such as speckle size, contrast and/or correlation between images, it is possible to extract a signal which is proportional to the amplitude of the acoustic excitation.