Grating couplers and methods of fabricating a grating coupler. The grating coupler may include a plurality of grating structures arranged on a substrate and a layer arranged over the grating structures. The grating structures are composed of a first material characterized by a first refractive index with a real part. The layer is composed of a second material characterized by a second refractive index with a real part. The real part of the second refractive index is greater than the real part of the first refractive index of the first material for electromagnetic radiation with a wavelength in a range of 1 micron to 9 microns.