In the recent years our collaboration acquired a lot of experience in the design and fabrication of plastic lenses (High Density Polyethylene and Polypropylene) currently used in instruments such as NIKA2, KISS and CONCERTO.

However, for the next generation instruments we need to develop higher Field-of-View and consequently larger lenses up to a diameter of the order of 300-400 mm.

For such large optical elements, plastic is no longer a suitable material because of its low refractive index. On the contrary, silicon is an excellent choice due to its low loss at millimeter and sub-millimeter wavelengths and to its high index of refraction leading to high-throughput and large field of view optical designs ideal for a large sky survey.

However the high index of refraction of the silicon lenses also means that a significant fraction of the incident light is reflected. This not only causes less light to reach the detectors, causing a decrease in the overall sensitivity, but it can also lead to other instrument systematics due to multiple reflections occurring in the instrument. The undesirable consequences of the high index means that the lenses must have Anti-Reflection (AR) coatings in order to deliver state-of-the-art measurements of the sky.

To solve this problem we develop meta-material AR coatings. Metamaterial AR coating consists of sub-wavelength features machined with a milling machine into an optical surface which act as effective dielectric layers. The shape and dimensions of the sub-wavelength features of the metamaterial coating can be tuned to result in sub-percent reflections across the identified bandwidth. This R&D work is possible through the use of a specific milling machining which is provided thanks to a LABEX ENIGMASS funding. The first prototypes of Silicon lenses are supposed to be produced in 2023.

Updated on 21 février 2022