Three-dimensional films and TVs may seem cutting-edge, but existing technologies all require optical tricks to create the illusion of depth (in some cases, very old tricks). The only truly 3D display technology we have, holography, has primarily been limited to displaying static images. That situation has slowly begun to change, but the existing technology is complicated and expensive, and it suffers from a slow refresh rate.

Now, some researchers have come up with a completely different method of creating the light pattern necessary to build a holographic image. The functional units in their device can be manufactured for pennies: the researchers suspect they could build a large holographic display for as little as $500, one that could potentially be driven by a commodity PC sporting a suite of high-end graphics cards.

The key to building a hologram is the ability of photons to interfere with each other, creating patterns where some regions have constructive interference and become bright, while others experience destructive interference and go dark. A carefully crafted diffractive can bend and redirect light so that this interference pattern recreates patterns of light that look as if they just reflected off the surface of a three-dimensional object. Most importantly, this 3D appearance is retained even as the viewer's perspective shifts around the surface.

We have very mature technology that allows us to create a static surface that consistently displays a single image. But to get that image to move or to replace it with a different image entirely involves wholesale modification to the hardware that is creating the diffraction pattern. Even assuming that you can calculate what the new configuration of the hardware must be (something that's not especially easy), you would then have to reconfigure the hardware and rescan light over it. The existing examples of hardware that can do this have some serious issues.

Most of them involve liquid crystal, micro-mechanical hardware that physically alters its configuration. The authors of the new paper provide a laundry list of this technology's limitations: "relatively low bandwidth, high cost, low diffraction angle, poor scalability, and the presence of quantization noise, unwanted diffractive orders, and zero-order light." (The latter factors create visual artifacts in the display image.)

The device the researchers have created instead involves an array of devices called anisotropic leaky-mode couplers. These devices act as waveguides for light while allowing the light travelling through them to be manipulated. When exposed to radio-frequency radiation, the hardware will form acoustic waves that alter the light travelling through them. This allows each coupler to rapidly alter the timing and direction of the light it emits in response to changes in the radio waves. By placing a number of them in close proximity, it's possible to get the light they emit to interfere (creating a hologram) and then change this hologram simply by altering the radio frequencies.

Other good features of the couplers are that they can be made to emit light with a single polarization, allowing a simple filter to cut out any imaging artifacts. And they can work with red, green, and blue light simultaneously, allowing a true-color hologram. This can also work with just about any light source.

You don't need many of these couplers to build a significant device; the authors estimate that about 500 of them would be all that you'd need to build a single horizontal line of a one-meter wide display. In the paper, they showed a device with 40 channels, and they're already testing one with 1,250 channels. The devices also easy and cheap to make. Their 40-channel hardware cost $50 to make at MIT's custom fabrication facility, but the author's estimate that an equivalent could be made at a commercial fabrication facility for somewhere in the area of $3.

As further progress has been made in getting graphics cards to generate holographic information and the radio frequency control signals are compatible with analog video displays, the authors think that a holographic display could probably be driven by a commodity PC with a bank of high-end graphics cards. The graphics cards would end up being the biggest expense in the hardware, so the actual array of couplers would only cost about $500 to make. Any reasonably bright color LED could provide the light source in this case. The end result would be a full-color hologram at standard video resolution with a refresh rate of about 30 fps.

Nature, 2013. DOI: 10.1038/nature12217 (About DOIs).