Univ.
of Michigan physicists
used the electric fields generated by intersecting laser beams to trap and
manipulate thousands of microscopic plastic spheres, thereby creating 3D arrays
of optically induced crystals.
The technique could someday be used to analyze the structure
of materials of biological interest, including bacteria, viruses, and proteins,
said U-M physicist Georg Raithel.
Raithel is co-author of a research paper on the topic published
online in Physical Review E. The
other author is U-M research fellow Betty Slama-Eliau.
The standard method used to characterize biological molecules
like proteins involves crystallizing them, then analyzing their structure by
bombarding the crystals with x-rays, a technique called x-ray crystallography.
But the method cannot be used on many of the proteins of highest interest—such
as cell-membrane proteins—because there’s no way to crystallize those
molecules.
“So we came up with this idea that one could use,
instead of a conventional crystal, an optically induced crystal in order to get
the crystallization of a sample that could be suitable for structural
analysis,” said Raithel, professor of physics and associate chair of the
department.
To move toward that goal, Raithel and his colleagues are
developing the laser technique using microscopically small plastic spheres
instead of the molecules. Other researchers have created 3D optically induced
crystals, but Raithel said the crystals his team created are denser than those
previously achieved.
The process involves shining laser beams through two opposed
microscope lenses, one directly beneath the other. Two infrared laser beams are
directed through each lens, and they meet at a common focal point on a
microscope slide that holds thousands of plastic nanoparticles suspended in a
drop of water.
The intersecting laser beams create electric fields that vary
in strength in a regular pattern that forms a 3D grid called an optical
lattice. The nanoparticles get sucked into regions of high electric-field
strength, and thousands of them align to form optically induced crystals. The
crystals are spherical in shape and about 5 microns in diameter.
Imagine an egg crate containing hundreds of eggs. The
cardboard structure of the crate is the optical lattice, and each of the eggs
represents one of the nanoparticles. Stack several crates on top of each other
and you get a 3D crystal structure.
“The crate is the equivalent of the optical lattice that
the laser beams make,” Raithel said. “The structure of the crystal is
determined by the egg carton, not by the eggs.”
The optical crystals dissipate as soon as the laser is
switched off.
