Pushing the Limits of Broadband and High-Frequency Metamaterial Silicon Antireflection Coatings

Abstract

Broadband refractive optics realized from high index materials provide compelling design solutions for the next generation of observatories for the Cosmic Microwave Background (CMB), and for sub-millimeter astronomy. In this paper, work is presented which extends the state of the art in silicon lenses with metamaterial antireflection (AR) coatings towards larger bandwidth and higher frequency operation. Examples presented include octave bandwidth coatings with less than 0.5% reflection, a prototype 4:1 bandwidth coating, and a coating optimized for 1.4 THz. For these coatings the detailed design, fabrication and testing processes are described as well as the inherent performance trade offs. Keywords Optics, Metamaterials IntroductionCurrent and planned ground-based CMB experiments [1-5] and sub-millimeter observatories [6, 7] use photon noise limited detectors. Improving the sensitivity of these instruments requires deploying a greater number of detectors. The two approaches being pursued are development of optical systems with large fields of view, and multichroic detectors. The former usually requires lenses fabricated from high index of refraction materials. The latter requires optical systems operating over broad bandwidths. Simultaneously meeting these design requirements necessitates the realization of high performance AR coatings.Silicon lenses with metamaterial AR coatings represent a rapidly maturing technology that can meet these needs. These coatings offer excellent optical performance (low reflection, low dielectric losses, and low scattering [8]), are robust to thermal cycling, and can be

The Simons Observatory Large Aperture Telescope Receiver

“…Inside each OT, three silicon lenses reimage the telescope focal plane onto a set of three hexagonal detector arrays (Dicker et al 2018). Silicon is selected as the lens material due to its low loss and the outstanding performance of developed metamaterial AR coatings (Datta et al 2013;Coughlin et al 2018;Golec et al 2020). The upper edge of each frequency channel is set by a set of low-pass edge (LPE) filters, which use a capacitive mesh design (Ade et al 2006) to set a range of cutoff frequencies (e.g., 12.5 cm −1 to 6.2 cm −1 for MF).…”

The Simons Observatory: modeling optical systematics in the Large Aperture Telescope

“…Using this approach, we launched randomly generated (time-reverse) rays from the focal plane and calculated the angular distribution of power emerging from the hexagonal cryostat window to determine spillover past the secondary mirror. For the three lenses, we assumed coatings representing the thickness, number of layers, and refractive index of the asmachined metamaterial [22,23]. For all filter surfaces, we assumed λ/4 coatings on all air transitions.…”

The Simons Observatory Large Aperture Telescope Receiver

“…Inside each OT, three silicon lenses re-image the telescope focal plane onto a set of three hexagonal detector arrays (Dicker et al 2018). Silicon is selected as the lens material due to its low loss and the outstanding performance of developed metamaterial AR coatings (Golec et al 2020;Coughlin et al 2018;Datta et al 2013). The upper edge of each frequency channel is set by a set of low-pass edge (LPE) filters, which use a capacitive mesh design (Ade et al 2006) to set a range of cut-off frequencies (e.g., 12.5 cm −1 to 6.2 cm −1 for MF), are incorporated to reduce optical loading on different temperature stages.…”