ZnSe immersion grating in the short NIR region

Abstract

ZnSe has a high refractive index (n~ 2.45) and low optical loss (< 0.1/cm) from 0.8 to 12 um. Therefore ZnSe immersion gratings can enable high-resolution spectroscopy over a wide wavelength range. We are developing ZnSe immersion gratings for a ground-based NIR high-resolution spectrograph WINERED. We previously produced a large prism-shaped ZnSe immersion grating with a grooved area 50 mm x 58 mm (Ikeda et al. 2010). However, we find two problems as NIR immersion grating: (i) serious chipping of the grooves, and (ii) inter-order ghosts in the diffraction pattern. We believed the chipping to be due to micro cracks just beneath surface present prior to diamond machining. Therefore we removed this damaged region, a few tens of microns thick, by etching the ZnSe grating blank with a mixture of HCl and HNO3. Ghosts appearing halfway between main diffraction orders originate from small differences in spacing between odd and even grooves. Apparently the blank shifts repeatably by about 120 nm in the direction orthogonal to the grooves depending on whether the translation stage holding the blank is moving right to left or left to right. Therefore we remachined the grating only cutting grooves with the stage moving from right to left. After re-cutting, we also deposit the Cu coating with an enhanced interface layer of SiO2 on the groove, which is developed in our previous study. We evaluated the optical performances of this immersion grating. It shows light scattering of 3.8 % at 1μm, no prominent ghosts, and a spectral resolution of 91,200 at 1 μm. However we measured an absolute diffraction efficiency of only 27.3% for TE and 25.9 % for TM waves at 1.55 μm. A non-immersed measurement of the diffraction efficiency of the facet blazed near 20o exceeded 60%, much closer to theoretical predictions. We plan to carry out more tests to resolve this discrepancy.