In the interest of completeness, the specifics of the fabrication process employed are detailed here. First, a silicon wafer of the desired orientation is coated with two process layers: a 30 nm layer of silicon nitride () deposited via low-pressure chemical vapor deposition and a 95-nm-thick layer of NXR-1025 nanoimprint resist deposited via spin coater (Fig. 2, step #1). These layers are deposited over the thin native silicon dioxide layer present on the substrate. Next, a grating premaster with the desired groove distribution is obtained and prepared for use (step #2). The premaster is a grating that has the desired groove density and radial convergence, is identical in size to the final flight gratings, and will serve as a mold for the nanoimprint process. However, the premaster has a laminar (i.e., square wave) groove profile and lacks the figure quality required for flight gratings. Prior to use, the premaster is coated with a mold release agent to aid in separation of the mold from the substrate after imprinting. The premaster is then aligned to a foundry-provided wafer flat indicating the direction and the grating pattern imprinted into the resist using a Nanonex NX-1006 nanoimprint tool (step #3). Any nanoimprint resist remaining in the groove troughs is then etched with a reactive ion etch in plasma performed at 10 mTorr and 40 W RF, and the layer etched in an plasma at 100 mTorr and 150 W RF (step #4). A rinse step (step #5) in acetone removes any remaining nanoimprint resist, leaving a silicon nitride hardmask matching the grating mold pattern in negative. A dip in buffered hydrofluoric acid (HF) (step #6) removes the native layer of silicon dioxide, exposing bare silicon between strips of the nitride hardmask. The sample is then transferred to a chemical bath for an anisotropic KOH wet etch (step #7) to sculpt the triangular shape of the groove facets. After terminating the KOH etch with a brief soak in deionized water, the silicon nitride mask is removed by a soak in HF (step #8).