Abstract

Silicon immersion gratings will allow the Giant Magellan Telescope Near-IR Spectrograph (GMTNIRS) to achieve continuous coverage over the entire J, H, K, L and M photometric bands with resolution R~65,000 at J, H and K and R~80,000 at L and M. I describe the manufacturing process and metrology techniques for silicon immersion gratings and overview updates to the production process and new grating metrology techniques that I have developed in order to successfully manufacture silicon immersion gratings for GMTNIRS. Many of the changes to our manufacturing process that I have contributed to, were required because of our need to produce gratings on silicon substrates larger than those we have used in the past. Gratings for J, H and K will be blazed at R3, while the L and M gratings will be blazed at R4 to achieve the desired resolution. The higher blaze angle of the L and M gratings requires that we use 150mm diameter substrates rather than the 100mm substrates that our standard process was developed for. In order to accommodate the larger substrates my colleagues and I constructed a custom UV exposure system for contact printing of grating lines, and constructed fixtures for coating and etching of the larger substrates. Additionally, we implemented new process checks and metrology techniques to improve our overall grating yield for 100mm and 150mm gratings. These updates to our process have resulted in the successful production of a complete set of gratings for GMTNIRS.

In addition to my work in the area of manufacturing silicon immersion gratings, I have completed an observation based project using our existing instrument, the Immersion Grating Near-Infrared Spectrometer (IGRINS), which employs a silicon immersion grating as its primary dispersing element.  I measured the H and K band veiling for 142 young stellar objects in the Taurus-Auriga star forming region using IGRINS spectra and produced low-resolution spectra of the excess emission across the H and K bands. The low-resolution excess spectra allowed my collaborators and I to analyze the temperature of the excess and morphology of the low-resolution spectra. We compare the luminosity of the excess continuum emission in Class II and Class III young stellar objects and find that a large number of Class III sources display a significant amount of excess flux in the near-infrared. We conclude that the mid-infrared SED slope and therefore young stellar object classification is a poor predictor of the amount of remaining inner-disk (<1AU) material. We find that 6 members of our sample contain a prominent feature in their H-band excess spectra of unknown origin that is inconsistent with a single or multi-temperature blackbody.