Maximizing the grating efficiency is a key goal for the first light
instrument IRIS (Infrared Imaging Spectrograph) currently being designed to
sample the diffraction limit of the TMT (Thirty Meter Telescope). Volume Phase
Holographic (VPH) gratings have been shown to offer extremely high efficiencies
that approach 100% for high line frequencies (i.e., 600 to 6000l/mm), which has
been applicable for astronomical optical spectrographs. However, VPH gratings
have been less exploited in the near-infrared, particularly for gratings that
have lower line frequencies. Given their potential to offer high throughputs
and low scattered light, VPH gratings are being explored for IRIS as a
potential dispersing element in the spectrograph. Our team has procured
near-infrared gratings from two separate vendors. We have two gratings with the
specifications needed for IRIS current design: 1.51-1.82{\mu}m (H-band) to
produce a spectral resolution of 4000 and 1.19- 1.37 {\mu}m (J-band) to produce
a spectral resolution of 8000. The center wavelengths for each grating are
1.629{\mu}m and 1.27{\mu}m, and the groove densities are 177l/mm and 440l/mm
for H-band R=4000 and J-band R=8000, respectively. We directly measure the
efficiencies in the lab and find that the peak efficiencies of these two types
of gratings are quite good with a peak efficiency of ~88% at the Bragg angle in
both TM and TE modes at H-band, and 90.23% in TM mode, 79.91% in TE mode at
J-band for the best vendor. We determine the drop in efficiency off the Bragg
angle, with a 20-23% decrease in efficiency at H-band when 2.5 degree deviation
from the Bragg angle, and 25%-28% decrease at J-band when 5{\deg} deviation
from the Bragg angle.Comment: Proceedings of the SPIE, 9147-33
