Etched Silicon Gratings for NGST

The authors have developed the world's first etched silicon grisms at LLNL in September 1999. The high optical surface quality of the grisms allows diffraction-limited spectral resolution in the IR wavelengths where silicon has good transmission. They estimated that the scattering light level is less than 4% at 2.2 {micro}m. Silicon can significantly increase the dispersive power of spectroscopic instruments for NGST due to its very large refractive index (n = 3.4). For example, a silicon grism with 40 mm clear entrance aperture and a 46 wedge angle can provide R = 10,000--100,000 in {approximately} 1--10 {micro}m. The same grating working in the immersed reflection mode can provide {approximately} three times higher spectral resolution than in the transmission mode. To achieve a desired spectral resolution for NGST, the spectrograph size and weight can be significantly reduced if silicon gratings are used instead of conventional gratings.