Introduction:

When molecules in the air absorb energy from a laser beam, they heat up. If the
laser beam is pulsed repetitively, the molecules alternately heat and cool. The cycle of heating and cooling produces an acoustic wave, which can be detected by a microphone and amplified electronically. The intensity of the sound wave is directly proportional to the concentration of molecules absorbing energy.
 

Research studies:

• Jaycor, an R&D company based in San Diego, is developing a device, based

On this well-known principle, for detecting narcotics, explosives, and hazardous  chemicals.