Highly efficient, miniature transparent holograms can be fabricated from dielectric metasurfaces consisting of arrays of silicon nanopillars. That's the achievement of a collaboration between the Australian National University in Canberra, Nanjing University in China and Oak Ridge National Laboratory in the US (Optica 3, 1504–1505; 2016).

Metasurfaces, ultrathin patterned substrates composed of an array of resonant subwavelength-sized elements that alter the phase, amplitude and polarization of incoming light, have become a highly active area of research in recent years. While many designs feature metals and rely on plasmonic effects, several research groups are now designing all-dielectric versions that operate by resonant scattering for realizing a variety of flat, planar optical devices such as waveplates, Q-plates and lenses.

Now, Lei Wang and co-workers report that it is possible to design and fabricate grayscale “metaholograms” that exploit the Mie resonances from dielectric nanostructures and operate in the near-infrared with very high transmission and diffraction efficiency (as shown in the above image of the kangaroo as an example).

“Our metaholograms produce grayscale high-resolution images and transmit over 90% of light with a diffraction efficiency over 99% at a 1600 nm wavelength,” say the authors of the paper. “This is the highest efficiency of any metahologram demonstrated to date reproducing grayscale images over a broad spectral range.” In this context, diffraction efficiency is defined as the power in the holographic image with respect to the total power transmitted by the metahologram.

Credit: OSA

The team's metaholograms consist of a dense array of silicon nanopillars of identical height (865 nm) but varying radii (79–212 nm) arranged in a square lattice with 750 nm period and has a total length of 0.75 mm. When illuminated, the metaholograms produce images 5 mm in size at a distance of 10 mm.

The metaholograms are fabricated by depositing poly-silicon on a silica wafer by low-pressure chemical vapour deposition. Electron-beam lithography and refractive ion etching are then used to create the desired nanopillar pattern. Each nanopillar acts as a pixel for the hologram, with a size-dependent phase delay.

The metaholograms have a spectral bandwidth of operation of 375 nm and the team says that the design approach is both scalable to the visible spectral region and potentially compatible with high-index materials such as Ge, GaAs, TiO2 or diamond.