  We study scattered light properties of protoplanetary discs at near-infrared
wavelengths for various dust size and structure by performing radiative
transfer simulations. We show that different dust structures might be probed by
measuring disk polarisation fraction as long as the dust radius is larger than
the wavelength. When the radius is larger than the wavelength, disc scattered
light will be highly polarised for highly porous dust aggregates, whereas more
compact dust structure tends to show low polarisation fraction. Next, roles of
monomer radius and fractal dimension for scattered light colours are studied.
We find that, outside the Rayleigh regime, as fractal dimension or monomer
radius increases, colours of the effective albedo at near-infrared wavelengths
vary from blue to red. Our results imply that discs showing grey or slightly
blue colours and high polarisation fraction in near-infrared wavelengths might
be explained by the presence of large porous aggregates containing sub-microns
sized monomers.
