Precise blaze angle measurements of lithographically fabricated Silicon immersion gratings and the design of simple prototype instruments for grating deployment
Creators: name: Lubar, Emily
name: 0000-0003-0790-7492
Publisher: name: The University of Texas at Austin
Year: 2022
Subjects: Gratings
Diffraction gratings
Silicon diffraction gratings
Grating blaze
Optical metrology
Optics
Characterization
Instrument design
Spectroscopy
Infrared
Stellar characterization

description: Silicon immersion gratings and grisms enable compact, high-throughput, near-infrared spectrographs. These instruments are used in ground-based efforts to characterize stellar and exoplanet atmospheres and in space-based observatories. Our grating fabrication technique uses x-ray crystallography to orient the crystal structure of silicon, followed by a specialized lithography and wet chemical etching to produce a blazed grating. The etching process takes advantage of the Silicon crystal structure and relative difference in etch rates between the [100] and [111] crystal planes. This allows us to produce parts that have phase uniformities of &lt;λ/4 at the operating wavelengths of each grating (J- through M-band). Previous measurements indicate that chemical etching may yield a final etched blaze that slightly differs from the orientation of the [111] plane. The presence of a discrepancy between expected blaze and actual blaze changes the optical performance of the grating and therefore jeopardizes instrument performance. Understanding the magnitude of the discrepancy is the first step toward controlling the process that produces it, so measuring the discrepancy resulting from fabrication is paramount. Knowing the fabricated blaze very precisely is especially critical for silicon gratings operating in immersion because discrepancies are inflated by Snell’s law at the interface between silicon and air. I overview the measurement method I developed and report on measurement results for the blaze of our in-house fabricated GMTNIRS silicon immersion gratings to ~0.05° precision. In addition to my work on blaze characterization, I have developed an instrument concept with science goals related to stellar characterization. Star spots introduce uncertainty in derived ages of young active stars. In recent years, stellar variability of exoplanet host stars has proven to be a bottleneck for progress in atmospheric characterization. JWST was successfully launched in December 2021 and will improve our ability to study exoplanetary atmospheres even in the presence of stellar variability, but is in high demand and not optimized for this science case alone. I propose an inexpensive, compact, and simple instrument concept to spectroscopically and photometrically observe and characterize star spots. This is a ground-based instrument concept with high resolving powers R~10,000 in the infrared H-band. Early instrument design is always motivated by science goals, and the process of converging on final instrument specifications is inherently iterative. Deen et al. 2017 outline a streamlined process of choosing the various specifications, significantly reducing the number of iterations required to converge on a preliminary set of parameters. I showcase this method in my design process.