Scientists can probe the composition of clouds and atmospheric pollution with lasers using a process known as light detection and ranging (LiDAR). By measuring the amount of light reflected back by the atmosphere, it is possible to calculate the concentrations of fine drops of noxious chemicals such as nitrous oxide, sulfur dioxide and ozone. More detailed information regarding the size of the liquid droplets could lead to better understanding of the pollutants' movements, but such data is harder to come by. To that end, findings published in the July 15 issue of Physical Review Letters could prove helpful. Researchers report that extremely short laser pulses can generate an intense plasma within a miniscule water droplet, causing light to be reflected preferentially back toward the laser source. Liquid droplets of different sizes focus the laser pulse to varying degrees, producing distinctive wavelengths of emitted light that could provide important clues to aerosol size distribution.

Jean-Pierre Wolf of the University of Lyon 1 in France and his colleagues flashed femtosecond laser pulses on individual water droplets that were less than 70 microns in diameter. The team found that the light reflected by a microscopic sphere back toward the laser source was 35 times more intense than the light sent in any other direction. The researchers posit that this phenomenon arises because of a nanosized plasma that forms within the water drop and is hot enough to emit in the visible spectrum.

Though further tests are required to determine how the technique applies to situations that involve more than one liquid drop, the researchers suggest that ultrashort high-intensity laser pulses may enhance light-detection and ranging (LiDAR) signals. "The backward-enhanced plasma emission spectrum from water droplets or biological agents," they write, "could be attractive for remotely determining the composition of atmospheric aerosol."

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