Microdroplet makes mighty microscope

March 23/30, 2005

There is a point beyond which it is not possible to magnify using lenses simply because a lightwave cannot be focused tighter than about half its wavelength -- the diffraction limit.

Electron microscopes, which bounce electron beams off objects to create images, and scanning probe microscopes, which trace objects with mechanical probes, sidestep the problem by avoiding lightwaves. Near-field microscopes also beat the diffraction limit because they are positioned closer to the subject in the wavelength of light used.

Researchers from the University of Maryland have found a way to reach nanometer-scale resolution using ordinary far-field optics. Far-field optics are generally less expensive and more convenient then electron, scanning probe and near-field microscopes.

The method could be used for bioimaging and optical nanolithography, and promises to make it easier to make movies of extremely small objects, according to the researchers.

The researchers' microscope consists of an ordinary optical microscope, a gold film, a microscopic glycerine droplet on the gold surface, and a laser.

The laser produces electromagnetic waves on the gold that are much smaller than the laser's wavelength. These surface waves are reflected by the inside surface of the droplet, much like radio waves are reflected onto a receiver by a dish antenna. An enlarged image of any nanoscale object in the droplet's focal point is reflected onto the gold film, and this image is picked up by the optical microscope.

The researchers' prototype uses 515-nanometer wavelength light to resolve objects smaller than 60 nanometers. Sixty nanometers is about 20 times smaller than an E. coli bacterium. A nanometer is one millionth of a millimeter.

The technology could be ready for practical use in two or three years, according to the researchers. The work appeared in the February 11, 2005 issue of Physical Review Letters. -TECHNOLOGY RESEARCH NEWS

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