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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.