GRAVITY upgrade with high-performance grisms with factor >2 enhanced throughput

During the past years, the VLTI-instrument GRAVITY has made spectacular discoveries with phase-referenced interferometric imaging with milliarcsecond resolution and ten microarcsecond astrometry. Here, we report on the upgrade of the GRAVITY science spectrometer with two new grisms in October 2019, increasing the instrument throughput by a factor > 2. This improvement was made possible by using a high refractive index Germanium substrate, which reduces the grism and groove angles, and by successfully applying an antireflection coating to the ruled surface to overcome Fresnel losses. We present the design, manufacturing, and lab testing of the new grisms, as well as the results from the re-commissioning. We further give an outlook on observations of high-redshift active galactic nuclei and exoplanets with the new grisms.