Laser Auditory Effect
Laser communication using the photoacoustic effect
https://www.ll.mit.edu/news/laser-can-deliver-messages-directly-your-ear-across-room
Researchers from the MIT Lincoln laboratory created a laser communication system based on the the photoacoustic effect which consists of the generation of pressure waves or sound waves in a material following light absorption. They determined that water vapor, which is found in all environments, absorbs light at the infrared wavelength of 1907 nm and as a result generates mechanical/pressure waves which are perceived as sound when they reach a person's ear. They used a laser of the above wavelength as carrier on which they modulated sound information or messages. In one technique, the message is heard by all persons crossing the beam and in another technique the message is delivered at a specific distance from the transmitter and is heard only by the person found at the specified location.
Laser communication using the laser-induced plasma effect
Initial efforts of generic sound (and light) generation
Transcript of video (slightly modified): We have whole propagated weakly-focused laser pulses, one of which is a femtosecond 30 mJoule igniter pulse which creates a very dilute plasma. The plasma is subsequently densified by copropagating a nanosecond pulse with about 3 Joules of pulsed energy.
At this point only a nanosecond pulse is propagating. The femtosecond pulse is blocked. As you can see, although the nanosecond pulse has huge energy, it doesn't have any visible effect and does not produce any amount of plasma except for occasional sparks due to the dust in the lab.
You can see that if I insert different objects in the beam like this piece of paper, then immediately on the surface there is dielectric breakdown (electrical breakdown). This is all left-over dust triggering the breakdown.
Depending on the type of materials I put in the beam, dark or light and so forth, the amount of noise that the plasma produces is different, the amount of plasma is different.
This is a blackened foil. Of course immediately the amount of plasma and noise increases.
Now let me turn on the femtosecond pulse. The femtosecond pulse will trigger the plasma formation in the gas and the nanosecond pulse will enhance the plasma, and create another plasma in the air.
Now the igniter and heater pulses propagate together and the level of sound is (...) maybe at about a loud voice level. The plasma is created in the form of a string which expands maybe to a distance of 10 cm.
If I block the igniter femtosecond pulse, the effect disappears even though the Joule pulse is still propagating.
Now I am again opening again the igniter pulse and I see the stable formation of plasma in the air.
Version imprimable