Optical Measurement of the Mass Density of Biological Samples

Mass density is a vital property for the improved biophysical understanding of and within biological samples. It is increasingly attracting active investigations, but still lacks reliable, noncontact techniques to accurately characterize it in biological systems. Contrary to popular belief, refractive index information alone is insufficient to determine a sample's mass density, as we demonstrate here theoretically and experimentally. Instead, we measured the nonlinear gain of stimulated Brillouin scattering to provide additional information for the mass density estimation. This all-optical method reduces the estimation error 10-fold, offering a more accurate and universal technique for mass density measurements.