Researchers at Brigham Young University in Utah made something they’re calling an Optical Trap Display (OTD). The device traps a tiny opaque particle in mid-air using an invisible laser beam, then moves the beam around a preset path in free space. At the same time, it illuminates the particle with red, green, or blue lights. When the particle moves fast enough, it creates a solid holographic image in the air. Move it even faster, and you can create the illusion of movement.
“We can think about this image like a 3D-printed object,” lead author Daniel Smalley, an assistant professor in electroholography at Brigham Young University, explained in a Nature video. “A single point was dragged sequentially through all these image points, and as it did, it scattered light. And the accumulated effect of all that scattering and moving was to create this 3D image in space that is visible from all angles.”

Scientifically, what Smalley and his team are creating are known as volumetric images, which differentiates them from 2D-hologram technologies. Other companies and scientists have made devices that create volumetric images, but the researchers say theirs is the first to generate free-floating images that can occupy the same space as other objects, as opposed to volumetric images that need to be contained inside a specially designed field. Other devices often require a much more elaborate set-up as well, while the OTD is relatively cheap, made with commercially available parts and low-cost lasers.
That said, the device does have its limitations. Namely, that the images produced right now are quite tiny: smaller than a fingernail. Making the images bigger will require the researchers learn how to manipulate more than one particle at a time. And it’s unlikely the device will be usable outdoors for the foreseeable future, since fast moving air particles can muck up the process. Video cameras also have a problem capturing the images the way our eyes or still cameras do—a video’s frame rate makes the image look like it’s flickering, while our eyes only see a solid image.

Source: Scientists Found a Way to Make Inexpensive, Solid-Looking 3D Holograms