Development of a Fine Grating on ZnS for a Wideband Spectral Disperser in Characterizing Exoplanets using Space-borne Telescopes

TL;DR

This paper reports the fabrication and testing of a high-quality fine grating on ZnS for wideband infrared spectral dispersion, aimed at characterizing exoplanet atmospheres with space telescopes.

Contribution

It introduces a precise fabrication process for a ZnS grating with high diffraction efficiency suitable for infrared spectroscopy in space applications.

Findings

Diffraction efficiencies closely match theoretical calculations (0.9%-2.4% difference).

Surface roughness of the fabricated grating is 2.6 nm rms.

The grating design is suitable for infrared spectral dispersers in space telescopes.

Abstract

We present the fabrication and optical testing of a fine grating on a ZnS substrate to be used as a wideband infrared spectral disperser and for which the primary application is measurement of the composition of the atmospheres of transiting exoplanets using space-borne infrared astronomical telescopes. A grating with a blaze angle of 2.1 deg. and pitch of 166.667 midron was constructed on a roughly flat 10 mm x 10 mm substrate with a maximum thickness of 1 mm. To obtain high accuracy, the sample was fabricated on a ZnS monocrystal using a high performance processing machine at Canon Inc. The surface roughness measured with a microscope interferometer was 2.6 nm rms. The shape of the fabricated grating edges was examined with a scanning electron microscope. The diffraction efficiency was evaluated by optical experiments at {\lambda} = 633 nm, 980 nm, and 1550 nm, and compared with the efficiencies calculated using a Fourier Modal Method. The results showed that the differences between the diffraction efficiencies obtained from experiment and by calculation were between just 0.9 % and 2.4 %. We concluded that the quality of the fabricated ZnS grating was sufficiently high to provide excellent diffraction efficiency for use in the infrared wavelength region. We also present the design of a spectral disperser in CdTe for future more advanced performance.