20 Apr 2004
Researchers use a mobile terawatt laser to profile and monitor the movement of biological aerosols in the air.
Detecting a cloud of biological warfare agent could become easier thanks to the development of a nonlinear lidar by a French-German consortium. What's more, the Teramobile team says that its technique should have a range of up to 4 km and a resolution of less than 0.5 m. (Applied Physics B 78 535)
The approach involves inducing two-photon fluorescence (2PEF) from natural fluorophores present in the aerosols. The fluorescence is then collected by a telescope and focused onto a spectrally-resolved detector where it is recorded as a function of distance and wavelength.
Teramobile is the world’s first mobile terawatt-class laser and emits femtosecond pulses at 800 nm courtesy of a Ti:sapphire system. In the aerosol tests, the researchers generated a plume of 1 micron-sized water droplets 45 m away from the Teramobile. Each water droplet contained a small amount of the riboflavin, a compound naturally found in bacteria that fluoresces at 540 nm. They then fired 80 fs, 5 TW pulses at the plume at a repetition rate of 10 Hz and collected the fluorescence.
“The detected spectrum clearly identifies the presence of riboflavin-containing particles and the lidar range resolution allows the precise spatial localization of the biological aerosol plume,” say the authors of the paper. “The plume is measured to be spread over some 10 m. The spatial resolution is 45 cm and is limited by the fluorescence lifetime of 3 ns for this transition.”
The team also simulated the performance of a laser that emits femtosecond terawatt pulses at 530 nm. Although such systems are not available today, they could be used to excite two-photon fluorescence from a fluorophore called tryptophan. The benefit is that the concentration of tryptophan is typically 104 times higher than riboflavin in bacteria.
Calculations show that the distance beyond which a two-photon system is more efficient than a one-photon fluorescence system depends on the ozone concentration. In particular, a two-photon system would prove invaluable in urban areas that have high ozone concentrations. While a one-photon system would be limited to a detection distance of several hundred meters, the team reckons a two-photon system could detect the presence of bacteria up to 4 km away.