Near-Field Microscope Manipulates Nanometer-Size Particles

Gary Boas

The use of optical fields to trap and manipulate particles is finding applications from atomic and nonlinear physics to biology. Most of the applications thus far, however, have involved particles no smaller than a micron in diameter. Now researchers at Institut Fresnel in Marseille, France, the National Institute of Standards and Technology in Gaithersburg, Md., and the Instituto de Ciencia de Materi-ales in Madrid, Spain, have theorized that an apertureless near-field microscope could be used to capture and manipulate particles as small as a few nanometers in diameter.

The scattering of two counterpropagating evanescent waves from the tungsten tip of an apertureless near-field microscope may be used to trap and manipulate nanoparticles. Switching between TE and TM polarization on the instrument may enable researchers to select, grab and release dielectric objects a few nanometers in diameter. Courtesy of Patrick C. Chaumet.

The method relies on the probe to scatter two counterpropagating evanescent waves, creating a localized, three-dimensional optical trap at the apex of the probe. When illumination of the sample at 500 nm with a 5-W laser is performed in TE polarization, the microscope could be used in its normal mode to locate a nanoparticle.

Once a nanoparticle has been selected, the researchers calculated, the microscope could be switched to TM polarization. The increase in the electrical field surrounding the tip then could be used to "lift" the nanoparticle and move it above the substrate. Switching back to TE polarization would deposit the particle back onto the surface in the desired location.

Researcher Patrick C. Chaumet of Institut Fresnel said there are several potential applications for the technique, including the assembly of nanostructures for experimental purposes. "With our configuration, quantum-dot nanocrystals -- CdSe/ZnS nanospheres a few nanometers in diameter -- could be placed in a specific geometry to study dot-dot correlations. Or, rather, one nanocrystal could be isolated to study single-dot structures."

The method would not require any special technology. He said that, although only a conventional apertureless microscope would be needed, certain conditions must be met if it is to be used successfully. For example, a very dry atmosphere would be required to avoid capillary force and an ultrahigh vacuum would be useful to avoid water absorption on the surface. Furthermore, switching the polarization must be as effortless as possible, and any mechanical vibrations of the microscope must be minimized, as these could have a negative effect on localization of nanoparticles.

There currently are no plans to commercialize the technique, primarily because the three authors of the study are working in theoretical laboratories. Whether it is developed with an eye toward commercialization, Chaumet said, will depend on the reaction of experimentalists to the idea.