Reactive ion plasma etched surface relief gratings for low/medium/high resolution spectroscopy in astronomy

Abstract

We report the fabrication of a binary-phase proof-of-concept astronomical diffraction grating embedded in a quartz substrate via reactive ion plasma etching. This grating operates at the first diffraction order within the 450 to 750 nm wavelength band. It features 1400-nm-deep, 188-nm-wide binary grooves at a 566-nm pitch, or 1767 lines/mm groove density, over a 25.4 × 25.4 mm2 area. A high depth-to-width ratio ( ∼ 8 ∶ 1 in this case) is one of the keys to near-theoretical diffraction efficiency being attained by the fabricated grating (94% at center wavelength and 70% at band edges) over a broad bandpass (>200 nm). This performance is also attributed to high-resolution micro-lithographic electron-beam patterning and anisotropic reactive ion etching process fabrication techniques. These types of binary gratings can potentially be high-throughput alternatives to Volume-Phase Holographic Gratings (VPHGs) for general spectroscopic applications. When scaled to appropriate sizes for astronomy, such gratings can serve as main or cross dispersion elements in low-, medium-, and high-resolution spectrographs not only in ground-based telescopes but also in those subject to challenging environmental conditions such as in space observatories.