The researchers made their metasurface from an array of seemingly randomly oriented gold nanorods fabricated using electron beam lithography. Each nanorod supports a so-called plasmonic resonance and therefore functions like a small optical antenna.

Such antennas have long been used to transmit radio and television waves, but it is only recently that the concept has been extended to optical frequencies. All antennas work by oscillating charges along their structure, which means that the size of the antenna must fit to a resonant mode for the wavelength of the electromagnetic radiation it supports. To make an antenna work at optical frequencies, it must thus be scaled down to nanometre dimensions.

Each nanorod in the metasurface array collects incident circularly polarized light and re-emits this light with a certain delay – but polarized in the opposite direction. The delay from a rod depends only on the angle at which it is oriented, and, in this way, a particular phase profile for the light wavefront can be obtained. “To produce the hologram, the phase information contained in the light scattered from the 3D object being reproduced is first calculated by a computer and then the phase information at each local spot is accurately and continuously mapped onto the orientation of the nanorods at the metasurface,” explains team leader Shuang Zhang at Birmingham. “If this surface is then illuminated with circularly polarized light, a 3D image will form in the transmitted light path.”

Conventional holograms are usually made from relief gratings that are much larger than the wavelength of light, and a 3D image is reconstructed from the light waves diffracted by these gratings. Such structures are plagued by so-called multiple diffraction effects, twin images and a limited field of view (the angle range over which we can observe the reconstructed image),” explains Zhang. “In our work, we show that by using nanoscale gold rods to store information about a 3D object we can generate a phase hologram with the smallest pixel size ever and which does not suffer from the problems inherent in traditional 3D holography.”

“The good thing about our structures is that the phase profile of light falling on the nanorods is continuously encoded into their orientation, which means that each pixel contains much more information than is possible in conventional binary (black and white) holograms. And since the pixel size is smaller than the wavelength of light, there are no multiple diffraction effects either and the view field dramatically increases.”

In this work, the team focused on objects with diffuse surfaces so a phase hologram sufficed, but in the future they plan to make holograms in which they can separately control the phase and amplitude of the light wave emitted to generate high quality holograms of any type of 3D object.

More detailed high definition 3D images

“Our technique not only allows us to produce 3D holograms of an object but also opens the way to encoding arbitrary light wave phase and amplitude information into a ultrathin layer of nanostructures,” co-team leader Thomas Zentgraf at Paderborn told nanotechweb.org. “Manipulating the phase of a light wave on the micron scale is just not possible with traditional relief gratings because of their large size, but we show that we can generate certain wavefronts of light and even images on a much smaller scale. What is more, since the information density contained within a metasurface like ours is much higher, we can generate more detailed high definition 3D images.”

At the moment, the efficiency of the hologram (that is, the ratio of the amount of light used to form the holographic image compared with the amount of light falling on the object to be reconstructed) is only a few per cent. “We will now thus be looking to optimize the design of our nanostructures to further increase this value,” says Zentgraf. “Indeed, our preliminary numerical results show that efficiency can be upped to almost 90% with the right design.”

“Currently, we fabricate our metasurfaces using electron beam lithography, which is a very slow process and limits the area of the hologram to just half a millimetre across. We would therefore also like to develop large-area nanofabrication techniques to produce much bigger metasurface holograms that may even be observable with the naked eye.”

The current research is described in Nature Communications doi:10.1038/ncomms3808.

Further reading

Nanoantenna arrays go optical (Jan 2013)
Electron beams do the twist (Jan 2011)
Metalens doubles up (Nov 2012)
Metasurface couples SPPs and light (Apr 2013)