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The technique, developed in the laboratory of New York University physics professor David Grier, is comprised of two components: making and recording the images of microscopic systems and then analyzing these images.

To generate and record images, the researchers created a holographic microscope based on a conventional light microscope. Instead of relying on an incandescent illuminator—which conventional microscopes employ—the holographic microscope uses a collimated laser beam that consists of a series of parallel light rays similar to a laser pointer.

When an object is placed into the path of the microscope’s beam, the object scatters some of the beam’s light into a complex diffraction pattern. The scattered light overlaps with the original beam to create an interference pattern reminiscent of overlapping ripples in a pool of water.

The microscope then magnifies the resulting pattern of light and dark and records it with a conventional digital video recorder. Each snapshot in the resulting video stream is a hologram of the original object. Unlike a conventional photograph, each holographic snapshot stores information about the three-dimensional structure and composition of the object that created the scattered light field.

The recorded holograms appear as a pattern of concentric light and dark rings. This resulting pattern contains a wealth of information about the material that originally scattered the light—where it was and what it was comprised of.

To analyze the images, researchers employed a quantitative theory explaining the pattern of light that objects scatter. The Lorenz-Mie theory maintains that the way light is scattered can reveal the size and composition of the object scattering it.

“We use that theory to analyze the hologram of each object in the snapshots of our video recording,” explains Grier, who is part of NYU’s Center for Soft Matter Research. “Fitting the theory to the hologram of a sphere reveals the three-dimensional position of the sphere’s center with remarkable resolution. It allows us to view particles a micrometer in size and with nanometric precision—that is, it captures their traits to within one billionth of a meter.”

“That’s a tremendous amount of information to obtain about a micrometer-scale object, particularly when you consider that you get all of that information in each snapshot,” Grier adds. “It exceeds other existing technology in terms of tracking particles and characterizing their make-up—and the holographic microscope can do both simultaneously.”

Because the analysis is computationally intensive, the researchers employ the number-crunching power of the graphical processing unit used in high-end computer video cards. Originally intended to provide high-resolution video performance for computer games, these cards possess capabilities ideal for the holographic microscope.

The team has already employed the technique for a range of applications, from research in fundamental statistical physics to analyzing the composition of fat droplets in milk. More broadly, the technique creates a more sophisticated method to aid in medical diagnostics and drug discovery.

Findings were reported in Optics Express.

NYU news: www.nyu.edu/public.affairs/