All sorts of 3D-imaging technologies tend to get lumped under the label "hologram." But there's actually a variety of distinct technologies that can create the appearance of depth. Now, we can add another to the list: the photophoretic-trap volumetric display. The device uses one set of optical hardware to control the motion of a tiny sphere and a second set to illuminate the sphere as it travels. Provided the sphere can be kept moving fast enough, the result is a true-color image that has real depth since it's built from light reflected from different locations.

The downside is that a single sphere can't cover all that much ground in the amount of time our brain needs to construct an image. As a result, photophoretic-trap volumetric display is currently limited to either small images or showing only part of an image at a time.

The recent work, from a team at Brigham Young University, is a variation on volumetric displays. These involve projecting a changing image onto a moving reflective surface. If the change in the image is properly matched to the changing location of where it's projected, the result will be the appearance of depth, since the light you see will actually be reflected at different locations. On the plus side, this doesn't require the viewer to wear any hardware, and multiple people can view the image at the same time, each seeing it from the appropriate perspective.

On the downside, this means that any images are going to be limited to the area where you put moving hardware. And attempts to interact with the image will likely end when your finger is whacked by a moving plate.

The new work involves getting rid of these moving reflective surfaces and replacing them with what's essentially just a single moving dot. The dot in this case is a small polymer bead about a micrometer across. Given this size, the bead is largely invisible. The display system makes the bead visible by shining lasers at it, allowing viewers to see the reflected light. With a laser each for red, green, and blue, the dot can glow with a full range of colors.

Of course, left on their own, beads will tend to sit peacefully on whatever surface they find themselves. This would obviously not make for a compelling display, even with the lasers. So instead, the BYU team figured out how to do a sort of controlled levitation. This involves using even more lasers.

While lasers can produce force as objects absorb and emit photons, these forces are relatively weak, making it difficult to move a bead around with enough speed and precision. So the team relied on what's called a photophoretic trap, in which the lasers create rapid heating of the bead and its environment. This can allow the bead to hover and can drive its motion. In the BYU case, the system used a low-visibility laser near the blue edge of the visible spectrum.

Overall, the researchers were able to move the bead at speeds up to 1.8 meters a second, though it mostly moved a bit slower than that when it was navigating curved paths. That's a decent speed if all you're drawing is a line one pixel wide, but it was a limitation for more complex images. Even small objects with simple geometry were limited to 13 frames a second. The more complex images the team created relied on long exposures—if you were there, you'd only see a subset of the image at any given time.

Despite that limit, this system has a few advantages. For one, it doesn't suffer from viewing angle issues (or clipping), in which part of the image is cut off because the image source isn't visible from a viewer's perspective (even though the image itself should be). If you're in the room and there's nothing between you and the image, you'll see it, all without special equipment. With an appropriately located laser system, it's possible to get the bead to move around physical objects, allowing a mixed-media display. It's easy to imagine using this to highlight specific details or features of an object.

But the most exciting prospect might be the fact that there's no reason to limit this to one bead. A handful of beads could easily trace out a far larger volume quickly, greatly expanding the size of the images that could be generated. Red, green, and blue fluorescent beads could be mixed, providing a better color representation.

Handing off beads between optical traps might even be possible. This would allow a single line to be traced across a very large area. The limit here seems to be how many trapping lasers you can fit into a given area before they start interfering with each other.

Nature, 2017. DOI: 10.1038/nature25176 (About DOIs).