A superconducting diplexer for cosmic microwave background experiments

Cosmic microwave background (CMB) studies require observations in several frequency bands simultaneously. For example, the detection of clusters of remote galaxies can be performed by comparing CMB photon fluxes at different frequencies, using the spectral deformation of the Planck law, due to the Sunyaev–Zeldovich effect. We propose a single pixel design to receive and detect simultaneously two frequency bands of the (CMB), centered at 150 and 220 GHz. It is composed of a bow-tie antenna, sensitive to the polarization of the incident radiation, feeding a diplexer structure to separate the signal towards two kinetic inductance resonators associated respectively with the 130–170 and 200–270 GHz frequency bands. The diplexer makes use of 150 nm thick niobium-based superconducting transmission lines of the coplanar strip-lines (CPS) type, for filtering and separation purposes, deposited on a 30 μm thick quartz substrate. A new coplanar matching structure, taking into account the kinetic inductance of the superconducting films, is proposed and achieves a nearly perfect match over more than 25% of each frequency band, along with a signal separation ratio above 30 dB at both center frequencies. The size of a single pixel is 0.9 mm × 1.2 mm.