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Polarisation is a powerful remote-sensing tool to study the nature ofparticles scattering the starlight. It is widely used to characteriseinterplanetary dust particles in the Solar System and increasingly employed toinvestigate extrasolar dust in debris discs' systems. We aim to measure thescattering properties of the dust from the debris ring around HD 181327 atnear-infrared wavelengths. We obtained high-contrast polarimetric images of HD181327 in the H band with the SPHERE / IRDIS instrument on the Very LargeTelescope (ESO). We complemented them with archival data from HST / NICMOS inthe F110W filter reprocessed in the context of the Archival LegacyInvestigations of Circumstellar Environments (ALICE) project. We developed acombined forward-modelling framework to simultaneously retrieve the scatteringphase function in polarisation and intensity. We detected the debris discaround HD 181327 in polarised light and total intensity. We measured thescattering phase function and the degree of linear polarisation of the dust at1.6 micron in the birth ring. The maximum polarisation is 23.6% +/- 2.6% andoccurs between a scattering angle of 70 deg and 82 deg. We show that compactspherical particles made of a highly refractive and relatively absorbingmaterial in a differential power-law size distribution of exponent $-3.5$ cansimultaneously reproduce the polarimetric and total intensity scatteringproperties of the dust. This type of material cannot be obtained with a mixtureof silicates, amorphous carbon, water ice, and porosity, and requires a morerefracting component such as iron-bearing minerals. We reveal a strikinganalogy between the near-infrared polarisation of comets and that of HD 181327.The methodology developed here combining VLT/SPHERE and HST/NICMOS may beapplicable in the future to combine the polarimetric capabilities of SPHEREwith the sensitivity of JWST.

The polarisation properties of the HD 181327 debris ring. Evidence for sub-micron particles from scattered light observations