  Young giant exoplanets emit infrared radiation that can be linearly polarized
up to several percent. This linear polarization can trace: 1) the presence of
atmospheric cloud and haze layers, 2) spatial structure, e.g. cloud bands and
rotational flattening, 3) the spin axis orientation and 4) particle sizes and
cloud top pressure. We introduce a novel high-contrast imaging scheme that
combines angular differential imaging (ADI) and accurate near-infrared
polarimetry to characterize self-luminous giant exoplanets. We implemented this
technique at VLT/SPHERE-IRDIS and developed the corresponding observing
strategies, the polarization calibration and the data-reduction approaches. By
combining ADI and polarimetry we can characterize planets that can be directly
imaged with a very high signal-to-noise ratio. We use the IRDIS pupil-tracking
mode and combine ADI and principal component analysis to reduce speckle noise.
We take advantage of IRDIS' dual-beam polarimetric mode to eliminate
differential effects that severely limit the polarimetric sensitivity
(flat-fielding errors, differential aberrations and seeing), and thus further
suppress speckle noise. To correct for instrumental polarization effects, we
apply a detailed Mueller matrix model that describes the telescope and
instrument and that has an absolute polarimetric accuracy $\leq0.1\%$. Using
this technique we have observed the planets of HR 8799 and the (sub-stellar)
companion PZ Tel B. Unfortunately, we do not detect a polarization signal in a
first analysis. We estimate preliminary $1\sigma$ upper limits on the degree of
linear polarization of $\sim1\%$ and $\sim0.1\%$ for the planets of HR 8799 and
PZ Tel B, respectively. The achieved sub-percent sensitivity and accuracy show
that our technique has great promise for characterizing exoplanets through
direct-imaging polarimetry.
