We present 870 \micron ALMA observations of polarized dust emission toward the Class II protoplanetary disk IM Lup .
We find that the orientation of the polarized emission is along the minor axis of the disk , and that the value of the polarization fraction increases steadily toward the center of the disk , reaching a peak value of \sim 1.1 % .
All of these characteristics are consistent with models of self-scattering of submillimeter-wave emission from an optically thin inclined disk .
The distribution of the polarization position angles across the disk reveals that while the average orientation is along the minor axis , the polarization orientations show a significant spread in angles ; this can also be explained by models of pure scattering .
We compare the polarization with that of the Class I/II source HL Tau .
A comparison of cuts of the polarization fraction across the major and minor axes of both sources reveals that IM Lup has a substantially higher polarization fraction than HL Tau toward the center of the disk .
This enhanced polarization fraction could be due a number of factors , including higher optical depth in HL Tau , or scattering by larger dust grains in the more evolved IM Lup disk .
However , models yield similar maximum grain sizes for both HL Tau ( 72 \micron ) and IM Lup ( 61 \micron , this work ) .
This reveals continued tension between grain-size estimates from scattering models and from models of the dust emission spectrum , which find that the bulk of the ( unpolarized ) emission in disks is most likely due to millimeter ( or even centimeter ) sized grains .