A photophoretic-trap volumetric display

Photophoretic optical trapping of cellulose particles and persistence of vision are used to produce real-space volumetric images that can be viewed from all angles, in geometries unachievable by holograms and light-field technologies. Three-dimensional moving images have long been the stuff of science fiction. In reality, the appearance of three dimensions is usually approximated by manipulating light, but such approaches have limitations such as narrow viewing angles and the need for special viewing headwear. Daniel Smalley and colleagues offer a solution in which a three-dimensional object can be represented in real space, viewed from any angle, and even coexist with—and wrap around—solid objects that occupy the same physical volume. A near-invisible light field traps and moves a small particle through a volume of space. As the particle moves, it is illuminated using red, green and blue laser light, producing an arbitrarily coloured point-source that maps out the surface of the object to be imaged. If the particle scanning is fast enough, persistence of vision, which results from the slower ''refresh rate'' of the human eye, gives the appearance of a solid three-dimensional surface. At even faster scanning rates, the imaged object can appear to move. Free-space volumetric displays, or displays that create luminous image points in space, are the technology that most closely resembles the three-dimensional displays of popular fiction1. Such displays are capable of producing images in ‘thin air’ that are visible from almost any direction and are not subject to clipping. Clipping restricts the utility of all three-dimensional displays that modulate light at a two-dimensional surface with an edge boundary; these include holographic displays, nanophotonic arrays, plasmonic displays, lenticular or lenslet displays and all technologies in which the light scattering surface and the image point are physically separate. Here we present a free-space volumetric display based on photophoretic optical trapping2 that produces full-colour graphics in free space with ten-micrometre image points using persistence of vision. This display works by first isolating a cellulose particle in a photophoretic trap created by spherical and astigmatic aberrations. The trap and particle are then scanned through a display volume while being illuminated with red, green and blue light. The result is a three-dimensional image in free space with a large colour gamut, fine detail and low apparent speckle. This platform, named the Optical Trap Display, is capable of producing image geometries that are currently unobtainable with holographic and light-field technologies, such as long-throw projections, tall sandtables and ‘wrap-around’ displays1.