Direct three-dimensional measurement of refractive index via dual Brillouin scattering*

The spatial distribution of refractive index can help understanding several biomedical phenomena. However, current techniques to map refractive index rely on the measure of the phase delay induced by the sample. This approach requires knowing, measuring or assuming the geometrical path of the light, which is not trivial in most experimental conditions. 

To overcome these limitations, we designed a novel confocal microscopy technique based on two co-localized Brillouin scattering interactions probed inside a confocal voxel. The ratio between the two Brillouin shifts only depends on the local refractive index, which can therefore be determined in a direct manner, without assumptions on the size, symmetry or shape.

To test the consistency of our measurement, we compared the results of the dual Brillouin spectroscopy with a standard Abbe refractometer, obtaining strong agreement between the two methods (R²>0.99), reaching an accuracy of 10-3 RIU. To validate our 3D mapping capability we imaged a XY and a XZ section of a polymer drop, obtaining the correct index of refraction.