Applications of nanofabrication for x-ray astronomical reflection gratings

The physics governing astrophysical objects is encoded in observed spectra.One of the most pressing objectives within the astrophysics community is to complete a modern day census the Universe's normal matter, or baryons (e.g. protons and neutrons). Measurements of the cosmic microwave background (CMB) constrain the number of baryons that should exist to a high degree of accuracy; however, 30% - 50% of these baryons are yet to be observed in the present day -- leading to the colloquial title "the missing baryon problem." It is believed these missing baryons should compose the diffuse gas in intergalactic space, well outside bright galaxies and clusters. Cosmological simulations indicate some of this gas, often referred to as the Warm-Hot Intergalactic Medium (WHIM), exists in a hot phase (T > 10^6K). At this temperature, the WHIM imprints key absorption features on background spectra from active galactic nuclei (AGN) in the soft X-ray bandpass of 12 - 50 A. Currently operating observatories like Chandra and XMM-Newton lack the sensitivity to make statistically significant observations of the hot component of the WHIM. Thus, high resolution X-ray spectrometers are required to complete a census of baryons in the present day. The next generation of X-ray observatories such as Arcus will require diffraction gratings that are efficient and capable of separating closely-spaced absorption features. We require these gratings to be fully customizable based on a mission's science objectives; the blaze angle, groove spacing, and groove pattern are all important considerations which require specialized fabrication techniques, including: electron beam lithography, nanoimprinting, and potassium hydroxide (KOH) etching. Other mission designs utilizing exotic grating geometries will be enabled by an emerging fabrication technique known as thermally-activated selective topography equilibration (TASTE). This essay offers an overview of critical advances in X-ray reflection grating manufacture over the last few decades and is intended to serve as a reference for future instrumentalists on the current state-of-the-art.