'Super-Resolution' Microscope Possible for Nanostructures

Researchers have found a way to see synthetic nanostructures and molecules using a new type of super-resolution optical microscopy, representing a practical tool for biomedical and nanotechnology research.

"Super-resolution optical microscopy has opened a new window into the nanoscopic world," said Ji-Xin Cheng, an associate professor of biomedical engineering and chemistry at Purdue University. Conventional optical microscopes can resolve objects no smaller than about 300 nanometers, or billionths of a meter, a restriction known as the "diffraction limit," which is defined as half the width of the wavelength of light being used to view the specimen. However, researchers want to view molecules such as proteins and lipids, as well as synthetic nanostructures like nanotubes, which are a few nanometers in diameter.

"The diffraction limit represents the fundamental limit of optical imaging resolution," Cheng said. "Here, we demonstrate a new scheme for breaking the diffraction limit in optical imaging of non-fluorescent species. Because it is label-free, the signal is directly from the object so that we can learn more about the nanostructure."

The imaging system, called saturated transient absorption microscopy,or STAM,uses a trio of laser beams, including a doughnut-shaped laser beam that selectively illuminates some molecules but not others. Electrons in the atoms of illuminated molecules are kicked temporarily into a higher energy level and are said to be excited, while the others remain in their "ground state." Images are generated using a laser called a probe to compare the contrast between the excited and ground-state molecules.