The computer-generated holograms, called Fairy Lights (some are shaped like multicolored pixies), are quite small, occupying a maximum volume of 1 cm3, but could be scaled up using larger optical devices. By touching the mid-air light displays with a finger, a holographic heart breaks in half and returns to whole when the finger is removed, the word "Love" turns to "Hate" with a touch, and a floating box can be "checked" with a finger.
Through a series of lenses and mirrors, the researchers followed two methods of rendering their mid-air graphics made up of plasma voxels: through spatial light modulation, and by the scanning of a laser beam via a galvano mirror (a device commonly used in lasers light shows).
They likewise employed two ultrafast femtosecond lasers in their experiments. The first projected 1,000 pulses per second with 7 millijoules of energy per pulse, and the second with 200,000 pulses per second at 50 microjoules per pulse. The plasma generated by these femtosecond lasers is safer than nanosecond lasers, which burned human skin in earlier hologram experiments.
The spatiotemporal resolution of their 3D displays clocked in at 4,000 and 200,000 dots per second. And while laser-induced plasma was used in this instance, other rendering materials could also be used, such as fluorescence and microbubbles in liquid or solid materials.
Researcher Dr. Yoichi Ochiai said the technology could be used for three-dimensional communication, such as on construction sites or for medical instruction. Other uses might include a holographic computer keyboard projected onto a user's lap, or for video conferences that could add a "real" handshake. It could also be used for entertainment in a holodeck type scenario.
"If we can project an image in a three dimensional form, and if you can touch it, then you can make something where you'll think that there actually is something there," said Ochiai. "People's daily lives would change if we use a bigger laser in a bigger space where people can interact with it."
The team's paper can be found here.
The touchable holograms are demonstrated in the video below.