The search for the B-mode of the cosmic microwave background (CMB) is one of the most
challenging measurements in cosmology. It requires an unprecedented level of sensitivity
and control of the instrumental systematics. For this reason, the optical system onboard
the next-generation CMB satellite LiteBIRD must meet several stringent constraints on
optical ghosting and, in general, straylight mitigation. The use of microwave absorbing
coatings on the 4K-cooled internal surfaces of the telescope enclosure, around the
aperture stop and the forebaffle, required to ensure stray light suppression, therefore
requires a selection of materials with suitable optical and thermo-mechanical
properties. For the absorbers of the LiteBIRD Medium and High Frequency Telescope
(MHFT), a baseline material recipe based on a mixture of epoxy resins and conductive
loading powders was selected. The design also features a 3D textured broadband
antireflective surface suitable for minimizing reflections in the 89-448 GHz
band. During the initial feasibility study, several prototype absorber plates were
fabricated and tested, yielding encouraging results in terms of specular and total
reflection compared to commercial absorber materials, most of which are unsuitable for
operation in the harsh space environment. Further work is now underway to mitigate the
differential thermal contraction relative to the aluminum telescope enclosure to ensure
survivability during cryogenic operation.