Plasma Polarization Puzzler

University of Illinois at Chicago researchers produced pairs of 80-fs, 800-nm pulses using a Ti:sapphire laser and a Michelson interferometer.

Scatterings imageFront row: Robert J. Gordon (left), Sima Singha and Tana Witt. Back row: Youbo Zhao (left) and Yaoming Liu.

Polarization can be a funny thing. Intuitively, one might expect light from a plasma to have random polarization. But sometimes plasmas are created in part by non-random processes that lead to some degree of polarization. For example, gamma ray bursts from a collapsing star are somewhat polarized, as is the output of a mercury lamp. Polarization can be a clue towards the processes that create the plasma.

Nevertheless, no one would have expected the extraordinary result that researchers in Robert J. Gordon’s group at the University of Illinois at Chicago found when they created plasmas by firing pairs of femtosecond near-infrared laser pulses at a silicon target: The ultraviolet emission was nearly 100 percent linearly polarized (Appl. Phys. Lett. 93, 161502).

More specifically, they produced pairs of 80-fs, 800-nm pulses using a Ti:sapphire laser and a Michelson interferometer. The pulses were focused on the target silicon crystal through a microscope objective. The laser was typically s-polarized with an angle of incidence of 30 degrees, although other configurations were studied. They collected the plasma light output at right angles to the laser beam, and measured the polarization using a Glan-Thompson polarizer mounted at the entrance slit of the spectrograph.

The researchers discovered that the degree of polarization depended on the timing relationship of the two pulses, on holding the focus of the laser at the surface, and on the wavelength. For the continuum emission, the polarization was greater at short wavelengths.

The degree of polarization in nature is quite small, and even in laboratory environments very strong polarization is rare. So what is going on in this experiment? “We speculate that it might be electron jets emitted from the surface,” says Gordon.

The group is planning to investigate the process, but the scientists have already found a potential use for the phenomenon. They have submitted a paper to Optics Letters on a method of extending laser-induced breakdown spectroscopy (LIBS), a method used for analyzing elements without having to prepare—or even get near—the sample. Not surprisingly, LIBS is used around blast furnaces, nuclear reactors, biohazard areas and similarly harsh environments. It is also part of NASA’s arsenal of unmanned instruments on the Mars landers.

In the upcoming article, the researchers describe how polarization-resolved laser-induced breakdown spectroscopy improves the sensitivity of LIBS. The polarization of the plasma emission suppresses the continuum—but only reduces the discrete atomic and ionic spectra slightly. They demonstrated the method on copper and carbon samples.

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Plasma Polarization Puzzler

University of Illinois at Chicago researchers produced pairs of 80-fs, 800-nm pulses using a Ti:sapphire laser and a Michelson interferometer.

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