  We propose a scheme for detecting and differentiating deeply subwavelength
particles based on their spatial features. Our approach combines scattering
from an ultrasonically modulated nanopatterend grating with heterodyne
techniques to enable far-field detection of high spatial frequency Fourier
components. Our system is sensitive to spatial features commensurate in size to
the patterning scale of the grating. We solve the scattering problem in Born
approximation and illustrate the dependence of the signal amplitude at
modulation frequency on grating period, which allows to differentiate between
model nanoparticles of size $\lambda/20$.
