  We experimentally observe the dipole scattering of a nanoparticle using a
high numerical aperture (NA) imaging system. The optically levitated
nanoparticle provides an environment free of particle-substrate interaction. We
illuminate the silica nanoparticle in vacuum with a 532 nm laser beam
orthogonally to the propagation direction of the 1064 nm trapping laser beam
strongly focused by the same high NA objective used to collect the scattering,
which results in a dark background and high signal-noise ratio. The dipole
orientations of the nanoparticle induced by the linear polarization of the
incident laser are studied by measuring the scattering light distribution in
the image and the Fourier space (k-space) as we rotate the illuminating light
polarization. The polarization vortex (vector beam) is observed for the special
case, when the dipole orientation of the nanoparticle is aligned along the
optical axis of the microscope objective. Our work offers an important platform
for studying the scattering anisotropy with Kerker conditions.
