  Direct optical detection and imaging of single nanoparticles on substrate in
wide field underpin vast applications across different research fields.
However, the speckles originating from the unavoidable random surface
undulations of the substrate ultimately limit the size of the decipherable
nanoparticles by the current optical techniques, including the ultrasensitive
interferometric scattering microscopy (iSCAT). Here we report a
defocus-integration iSCAT to suppress the speckle noise and to enhance the
detection and imaging of single nanoparticles on ultra-flat glass substrate and
silicon wafer. In particular, we discover distinct symmetry properties of the
scattering phase between the nanoparticle and the surface undulations that
cause the speckles. Consequently, we develop the defocus-integration technique
to suppress the speckles.We experimentally achieve an enhancement of the signal
to noise ratio by 6.9 dB for the nanoparticle detection. We demonstrate that
the technique is generally applicable for nanoparticles of various materials
and for both low and high refractive-index substrates.
