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

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.