A volumetric bubble display using lasers allows viewers to see 3D images without 3D glasses or headsets. Researcher Kota Kumagai of Utsunomiya University in Japan said the new work is currently a proof of concept, but one day might allow full-color updatable volumetric displays. "Creating a full-color updatable volumetric display is challenging because many three-dimensional pixels, or voxels, with different colors have to be formed to make volumetric graphics," said Kumagai. "In our display, the microbubble voxels are three-dimensionally generated in a liquid using focused femtosecond laser pulses. The bubble graphics can be colored by changing the color of the illumination light." This mermaid graphic was created from laser-generated bubbles suspended in a liquid "screen" and illuminated with a halogen lamp. Courtesy of Kota Kumagai, Utsunomiya University. These types of displays could be used for art or museum exhibits, where viewers can walk all the way around the display. They are also being explored for helping doctors visualize a patient's anatomy prior to surgery or to let the military study terrain and buildings prior to a mission. The bubbles are created by a phenomenon known as multiphoton absorption, which occurs when multiple photons from a femtosecond laser are absorbed at the point where the light is focused. Multiphoton absorption allowed the researchers to create microbubbles at very precise locations by moving the focus of the laser light to various parts of a liquid-filled cuvette that acted as a "screen." Using a thick liquid prevents the bubbles, once formed, from immediately rising to the top of the liquid. The bubble graphics are viewable when light from an external light source, such as a halogen lamp or high-power LED, is scattered. The researchers produced monochromatic images in white, red, blue and green by switching the color of the illuminating LED. Rather than creating each bubble one by one, the researchers used a computer-generated hologram to form 3D patterns of laser light that let them control the number and shapes of the microbubble voxels. This approach also increased the amount of light scattered from the microbubbles, making the images brighter. Microbubble formation depends on the irradiation energy of the laser and the contrast can be modified by changing the number of laser pulses used to irradiate the liquid. "Our bubble graphics have a wide viewing angle and can be refreshed and colored," said Kumagai. "Although our first volumetric graphics are on the scale of millimeters, we achieved the first step toward an updatable full-color volumetric display." The researchers are now developing a system that would use a stream inside the liquid to burst the bubbles, allowing the image to be changed or cleared. They are also working on methods that could allow the formation of larger graphics, which requires overcoming spherical aberrations caused by the refractive index mismatch between the liquid screen, the glass holding the liquid and air. The research has been published in the journal Optica (doi.org/10.1364/OPTICA.4.000298).