Double-Flare Angle Bowtie Slot Antenna for Multichroic CMB Polarization Detectors

Lens antenna-coupled detectors have emerged as a prominent technology for millimeter-wave astronomy over recent decades. The future ground-based cosmic microwave background (CMB) observations that target B-modes require the receiver to operate across multiple frequency bands while maintaining effective polarization selectivity. In this context, we have reconfigured the conventional broadband bowtie slot antenna into a dual-flare angle design, exclusively targeting the CMB emission wavelength at two bands centered at 150 GHz and 220 GHz. This antenna design facilitates partial independent tuning of resonant frequencies, leading to improved impedance-matching bandwidth,that achieves a return loss of ( \(S_{11} < -10\) $S_{11}<-10$ dB) spanning over an octave from 100 to 300 GHz. Simultaneously, it maintains effective linear polarization sensitivity, with cross-polarization remaining below −15 dB at both sub-bands when coupled with a dielectric lens. The integration of on-chip band-pass filters enables the effective separation of antenna signals to microwave kinetic inductance detectors. This results in a compact, polarization-selective, multichroic pixel solution that perfectly aligns with the demands of CMB B-mode observation.