  Polarized (sub)millimeter emission from dust grains in circumstellar disks
was initially thought to be due to grains aligned with the magnetic field.
However, higher resolution multi-wavelength observations along with improved
models found that this polarization is dominated by self-scattering at shorter
wavelengths (e.g., 870 $\mu$m) and by grains aligned with something other than
magnetic fields at longer wavelengths (e.g., 3 mm). Nevertheless, the
polarization signal is expected to depend on the underlying substructure, and
observations hitherto have been unable to resolve polarization in multiple
rings and gaps. HL Tau, a protoplanetary disk located 147.3 $\pm$ 0.5 pc away,
is the brightest Class I or Class II disk at millimeter/submillimeter
wavelengths. Here we show deep, high-resolution 870 $\mu$m polarization
observations of HL Tau, resolving polarization in both the rings and gaps. We
find that the gaps have polarization angles with a significant azimuthal
component and a higher polarization fraction than the rings. Our models show
that the disk polarization is due to both scattering and emission from aligned
effectively prolate grains. The intrinsic polarization of aligned dust grains
is likely over 10%, which is much higher than what was expected in low
resolution observations (~1%). Asymmetries and dust features are revealed in
the polarization observations that are not seen in non-polarimetric
observations.
