Researchers have developed a new laser method that can detect electric charges and chemicals of interest with unprecedented sensitivity. The new approach could one day offer a way to scan large areas for radioactive material or hazardous chemicals for safety and security applications.
The new laser method, called mid-infrared picosecond laser-driven electron avalanche, detects extremely low charge densities — the number of electric charges in a certain volume — in the air or other gases. The researchers were able to measure electron densities in air produced by a radioactive source at levels below one part per quadrillion, equivalent to picking out one free electron from a million billion normal air molecules.
The researchers used the new method to calibrate lasers used to inspect irradiated air from 1 meter away. The approach could be applied to detecting other chemicals and species and could be scaled up for remote detection at distances of 10 meters and, eventually, 100 meters.
The new technique is based on a process known as electron avalanche in which a laser beam accelerates a single free electron in gas until it gains enough energy to knock a different electron off a molecule, resulting in a second free electron. This process repeats and develops into a collisional cascade, or avalanche, that grows exponentially until a bright observable spark appears in the laser focus.