Researchers at MIT's Lincoln Laboratory, a United States Department of Defense research facility, developed laser systems that can "transmit various tones, music and recorded speech at a conversational volume" to specific people without the recipient wearing any special equipment. Basically, the operator points a laser at someone from a distance and that individual hears the transmitted audio even though others in the area don't. Conspiracy theorists, start your engines. From the Optical Society of America:
"Our system can be used from some distance away to beam information directly to someone's ear," said research team leader Charles M. Wynn. "It is the first system that uses lasers that are fully safe for the eyes and skin to localize an audible signal to a particular person in any setting..."
The new approaches are based on the photoacoustic effect, which occurs when a material forms sound waves after absorbing light. In this case, the researchers used water vapor in the air to absorb light and create sound...
One unique aspect of this laser sweeping technique is that the signal can only be heard at a certain distance from the transmitter. This means that a message could be sent to an individual, rather than everyone who crosses the beam of light. It also opens the possibility of targeting a message to multiple individuals.