  Development of novel radiation-absorbent materials and devices for millimetre
and submillimetre astronomy instruments is a research area of high interest,
and with substantial engineering challenges. Alongside low-profile structure
and ultra-wideband performance in a wide range of angles of incidence, advanced
absorbers in CMB instruments are aimed at reducing optical systematics, notably
instrument polarisation, far beyond previous specifications. This paper
presents an innovative design of flat thin flexible absorber operating in a
wide frequency range of 80-400 GHz. The structure comprises a combination of
sub-wavelength metal-mesh capacitive and inductive grids and dielectric layers,
making use of the magnetic mirror concept for large bandwidth. The overall
stack thickness is a quarter of the longest operating wavelength and is close
to the theoretical limit stipulated by Rozanov criterion. The test device is
designed to operate at 22.5deg. incidence. The iterative numerical-experimental
design procedure of the new absorber is discussed in detail, as well as the
practical challenges of its manufacture. A well-established mesh-filter
fabrication process has been successfully employed for prototype fabrication,
which ensures cryogenic operation of the hot-pressed quasi-optical devices. The
final prototype, extensively tested in quasi-optical testbeds using a
Fourier-transform spectrometer and a vector network analyser, demonstrated
performance closely matching the finite-element analysis simulations, viz.,
greater than 99% absorbance for both polarisations, with only 0.2% difference,
across the frequency band of 80-400 GHz. The angular stability for up to
+/-10deg. has been confirmed by simulations. To the best of the authors
knowledge, this is the first successful implementation of a low-profile
ultrawideband metamaterial absorber for this frequency range and operating
conditions.
