3D-Structured Polyethylene Windows for Low-Loss Transmission in Wideband Cryogenic Terahertz Systems
Authors
François Joint
Igor Lapkin
Pierre-Baptiste Vigneron
Emilie Hérault
Denis Meledin
Alexei Pavolotsky
Magnus Strandberg
Sven Erik Ferm
Mathias Fredrixon
Leif Helldner
Erik Sundin
Victor Belitsky
Vincent Desmaris
Abstract
We present the design, fabrication, and characterisation of a broadband vacuum window and infrared filter based on ultra-high molecular weight polyethylene (UHMWPE) for millimeter wave receivers operating across ALMA Band 6 and 7 (211-373 GHz). The window incorporates pyramidal anti-reflection (AR) structures, machined directly into the polyethylene using CNC machining, which provide impedance matching over a broad frequency range. The structured UHMWPE method was implemented in two distinct components: a vacuum window and a cryogenic infrared filter. This surface structuring approach provides mechanical robustness, cryogenic compatibility, and low insertion loss. We characterize the transmission properties using Terahertz Time-Domain Spectroscopy (THz-TDS), which demonstrates reflection below $5\%$ across the full band. Complementary heterodyne measurements confirm improved receiver noise performance. These results establish 3D structured UHMWPE as a promising platform for broadband cryogenic optics in high-sensitivity THz instrumentation.
