April 22, 2005, Berkeley, CA--A group of scientists at the University of California, Berkeley, is giving new relevance to the term "sharper image" by creating a superlens that can overcome a limitation in physics that has historically constrained the resolution of optical images.
Using a thin film of silver as the lens and ultraviolet (UV) light, the researchers recorded the images of an array of nanowires and the word "NANO" onto an organic polymer at a resolution of about 60 nanometers. In comparison, current optical microscopes can only make out details down to one-tenth the diameter of a red blood cell, or about 400 nanometers.
The breakthrough, reported in the April 22 issue of the journal Science, opens the door to dramatic technological advances in nanoengineering that could eventually lead to DVDs that store the entire contents of the Library of Congress, and computer processors that can quickly search through such a huge volume of data, the researchers said.
With current optical microscopes, scientists can only make out relatively large structures within a cell, such as its nucleus and mitochondria. With a superlens, optical microscopes could one day reveal the movements of individual proteins traveling along the microtubules that make up a cell's skeleton, the researchers said. Scanning electron and atomic force microscopes are now used to capture detail down to a few nanometers. However, such microscopes create images by scanning objects point by point, which means they are typically limited to non-living samples, and image capture times can take up to several minutes.
The study is the latest entry in a hotly debated topic among physicists and engineers surrounding the creation of a lens that can break the so-called diffraction limit of optics through negative refraction.
Conventional lenses, whether manmade or natural, create images by capturing the propagating light waves all objects emit and then bending them. The angle of the bend is determined by the index of refraction and has always been positive. Yet objects also emit "evanescent" waves that carry a great deal of detail but are far more elusive. Such evanescent waves decay exponentially and thus never make it to the image plane, an optics threshold known as the diffraction limit. Breaking this diffraction limit and capturing evanescent waves are critical to the creation of a 100-percent perfect representation of an object, considered the Holy Grail in optics.
In 2003, Zhang's group was the first to confirm that optical evanescent waves are enhanced as they pass through a silver superlens in carefully designed conditions. But it wasn't until this latest experiment by Zhang's group that optical imaging with a superlens was demonstrated. Zhang and his research team used UV light at a 365-nanometer wavelength in the new experiments, so the image created actually has more detail than is possible with beams in the microwave range.