Silicon-based vacuum window for millimeter- and submillimeter-wave astrophysics

We designed, fabricated, and characterized the properties of a silicon-based vacuum window suitable for millimeter-wave astrophysical applications. The window, which has a diameter of 124 mm, an optically usable diameter of 68 mm, and a thickness of about 4 mm, gives an average transmittance and reflectance of 99.6 ± 3.5% and 1.3 ± 1.4%, respectively, a fractional bandwidth of 67%, centered at 300 GHz. Absorptive loss is below the detection limit of our measurement. The anti-reflection coating is made with laser-ablated sub-wavelength structures (SWS), and the measured transmittance and reflectance values agree with the modeling based on the measured SWS shapes. The window has been integrated into DESHIMA 2.0, an astrophysics instrument that took year-long observations with the Atacama Submillimeter Telescope Experiment.