Interesting article at physorg.com :

“Keio University and Burton Inc. noticed that, when laser beams are strongly focused, air plasma emission can only be induced near the focal point. (…) The emission time of the laser pulse light is approximately a nanosecond (10-9 sec). The device uses one pulse for each dot. The human eye will recognize the after-image effect of plasma emission from displays up to 100 dot/sec. By synchronizing these pulses and controlling them with software, the device can draw any 3D objects in air. ”