Extended Data Fig.3.

Brillouin shift gradient caused by the mismatch of refractive index. a, Brillouin image of a PDMS sphere immersed in 1% agarose gel. Arrow indicates the propagation of illumination beam. Black dished line indicates the location of interest for b. Scale bar is 10 μm. b, Shift profile across the PDMS sphere shows the gradient along illumination direction. c, Theoretical calculation model. ‘A’ represent an arbitrary point on the illumination axis, θ is the actual scattering angle collected by the spectrometer at 90° geometry, and α is the azimuthal angle of the sphere. d, The shift gradients of the calculation and the experiment agree with each other.