Lens-less bending and concentration of light by volume hologram

In this paper, a new approach is proposed to realize a lens-less and 90° light-bending by a volume-holographic element, consisting of 3D sub-wavelength index-gratings throughout the sample. In this approach, a top incident plane-wave is diffracted by 90° and guided into a planar volume hologram. Conversely, a side-incident light may be guided and diffracted out of the top sample surface. The diffraction-efficiency η of light was studied in real-time during light-exposure and also in the dark after exposure to observe “dark-enhancement”. It is shown that 90° light-bending at normal incidence can be achieved with a high efficiency, η~70%, and a small angular-tolerance, Δθ=0.02°. It is further shown, as a proof-of-concept, that a 5° cylindrical-wave top-incident beam may be used to improve angular tolerance to Δθ=5°, but with a reduced efficiency of η~0.80%.