Reflective diffraction gratings employing efficiency enhancement or etch barrier layers

A diffraction grating comprising:

• (a) a substrate comprising a dielectric or semiconductor substrate material substantially transparent over a range of operational wavelengths with a substrate refractive index nsub, and having a first surface facing an optical medium with a medium refractive index nmed that is less than nsub;
• (b) a dielectric or semiconductor layer formed on the first surface of the substrate, substantially transparent over the operational wavelength range, and characterized by a layer refractive index nL and a layer thickness t, wherein nL differs from both nsub and nmed;
• (c) a set of diffractive elements formed on the layer on the first surface of the substrate, wherein (i) the diffractive elements comprise a set of protruding ridges of a dielectric or semiconductor ridge material, (ii) the ridge material is substantially transparent over the operational wavelength range and has a ridge refractive index nR that differs from nmed, (iii) the ridges are characterized by a ridge spacing Λ, a ridge width d, and a ridge height h, and (iv) the ridges are separated by intervening trenches substantially filled with the optical medium,
• wherein:
• (d) λ/2nsub<Λ<λ/(nsub+nmed) over the operational wavelength range;
• and
• (e) nsub, nmed, nL, nR, Λ, d, h, and t result in wavelength-dependent, first-order diffraction efficiency of the grating greater than a prescribed level over the operational wavelength range for both s- and p-polarized optical signals incident on the diffractive elements from within the substrate at an incidence angle θin that exceeds a critical angle θc=sin−1(nmed/nsub).