Breadboard model assembly and characterization of a sapphire achromatic half-wave plate for LiteBIRD low-frequency telescope

We present the development of a breadboard model achromatic half-wave plate (AHWP) for the <i>LiteBIRD</i> Low-Frequency Telescope (LFT). <i>LiteBIRD</i> is a JAXA-led strategic L-class satellite mission to probe the cosmic microwave background polarization. The breadboard model (BBM) polarization modulator unit (PMU) of the LFT uses an AHWP, which achieves an observational frequency coverage 34-161 GHz using a five-layer sapphire stack with a diameter of 330 mm based on the Pancharatnam recipe. The sub-wavelength structures on both end surfaces mitigate the reflection over broadband frequencies. The designed single sapphire plate thickness is 4.95 mm, with a corresponding half-wave shift center frequency of 97.5 GHz. We use hydro-catalysis bonding to glue the sapphire surfaces and assemble the five-layer AHWP. The AHWP is successfully assembled and measured. The transmittance and polarization properties are consistent with the theoretical prediction that neglects the effect of the bonding interfaces. In this work, we present the AHWP design, the assembly process, and the polarimetric characterization. We also discuss the path-forward for this BBM AHWP including the cryogenic and vibrational tests, and the development plan for the flight-size engineering model.