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

TL;DR

This work presents a silicon-based vacuum window for millimeter/submillimeter astrophysics designed to be mechanically robust and highly transparent. A laser-ablated sub-wavelength-structured anti-reflection coating (SWS-ARC) is implemented on both faces to achieve broadband anti-reflection, with RCWA modeling confirming the measured performance. The window demonstrates a fractional bandwidth of $67\%$ with an average transmittance of $99\%$ and reflectance of $1\%$ across $200$-$400$ GHz, while absorptive loss remains below detection. Fielded in DESHIMA v2.0 aboard the ASTE telescope, it operated for about one year without leaks, marking the first field deployment of a broadband silicon vacuum window with laser-ablated SWS for millimeter-wave astrophysics, and highlighting potential extensions to higher frequencies and larger diameters.

Abstract

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, optically active diameter of 68 mm, and thickness of about 4 mm, gives an average transmittance and reflectance of 99% and 1%, respectively, a fractional bandwidth of 67%. 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 modeling based on the measured SWS shapes. The window has been integrated into DESHIMA v2.0, an astrophysics instrument that took year-long observations with the Atacama Submillimeter Telescope Experiment.

Figures (9)

• Figure 1: The design shape of three elements in the periodic SWS (left) and the predicted transmittance of this ARC (right) with the DESHIMA pass-band highlighted (cyan). The vertical scale of the inset, between 0.9 and 1, highlights the $\sim$1% difference between the spectrum without loss (blue), which gives an in-band average transmittance of 99.5%, and the spectrum with loss (red), which gives an average of 98.8%.
• Figure 2: Cross section of the vacuum window assembly. An aluminum ring (A, grey) clamps the 4.1 mm thick outer rim of the silicon window (B, blue) into a housing with an o-ring (D, red and C, green, respectively). The window's center area, with a diameter of 68 mm, has SWS-ARC. For that area, only the solid substrate of 2.8 mm thickness is shown.
• Figure 3: A small section of the fabricated SWS-ARC (left), a photograph of the window (middle), and a sketch of the parameters used to characterize the SWS (right). In the middle panel, the black circle is the 68 mm diameter optically active area with SWS and the silver ring around it is bare silicon. The numbers denote areas in which SWS shape measurements were conducted. In the right panel, ${\rm w}$ and ${\rm p}$ parameters are length measurements, and ${\rm d}$ are height measurements, see Table \ref{['tab:summaryfabshapes']}.
• Figure 4: Sketch of the experimental setup used for the reflectance and transmittance measurements. For transmittance: A is the sample and B is a flat mirror, and the measurement is normalized against a measurement without a sample. For reflectance: Nothing is placed in location A and B is the sample, and the measurement is normalized against a measurement when B is a mirror.
• Figure 5: Reflectance and transmittance spectra of the flat sample (data points, upper panels, left and right, respectively), the best fit model (line), and the residuals (lower panels). Values for the best fit $n$ and $\tan \delta$ are given in Table \ref{['tab:bestfit']}.