MIT boffins have used low-power lasers to beam audio directly into a subject's ear.
The technique relies on the interaction between light and water in the air, a phenomenon known naturally enough as the "optoacoustic effect". In their experiments, the researchers found they could use this to transmit an audio signal which, while "silent" between transmitter and receiver, is detected at 60 decibels at a target 2.5 metres from the transmitter.
Water is among a class of materials that can emit sound after they absorb light, and according to a paper in Optics Letters, a correctly tuned laser can be modulated to deliver human-audible sounds this way.
In their experiments, MIT's Ryan Sullenberger, Sumanth Kaushik, and Charles Wynn concentrated on a 1.9-micron wavelength because that matches an atmospheric water vapour absorption line, and therefore maximises sound pressure without using a dangerously powerful laser.
In the Optical Society's announcement, Charles Wynn said the technique "can work even in relatively dry conditions because there is almost always a little water in the air, especially around people. We found that we don't need a lot of water if we use a laser wavelength that is very strongly absorbed by water. This was key because the stronger absorption leads to more sound."
Another technique to amplify the audible signal was to sweep the signal at the speed of sound using a fast steering mirror. The sweeping technique came from dynamic photoacoustic spectroscopy (DPAS), developed as a chemical detection technology.
The setup modulates audio onto the laser as changes to the length of the laser sweeps to encode different audio frequencies onto the light.
If you like imagining yourself a spy, this sweeping technique has another interesting effect: "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."
The group also tested a static photoacoustic setup. Sullenberger said: "The traditional photoacoustics method provides sound with higher fidelity, whereas the laser sweeping provides sound with louder audio." ®