26 Apr 05. The Engineer Online announced that 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 nanometres. In comparison, current optical microscopes can only make out details down to one-tenth the diameter of a red blood cell, or about 400 nanometres.
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.
“The field of optics is involved in much of today’s technology, including imaging and photolithography, which is used to make semiconductors and integrated circuits,” said Xiang Zhang, UC Berkeley associate professor of mechanical engineering and principal investigator of the study. “Our work has a far reaching impact on the development of detailed biomedical imaging, higher density electronic circuitry and ever-faster fibre optic communications.”
Nicholas Fang, one of Zhang’s former Ph.D. students and lead author of the paper, said a nearer term application would be the development of medical imaging devices that could reveal never-before-seen details with optical microscopy.
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 travelling 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 nanometres. 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.
“Optical microscopes can capture an entire frame with a single snapshot in a fraction of a second,” said Fang, who is now an assistant professor of mechanical engineering at the University of Illinois at Urbana-Champaign. “That opens up nanoscale imaging to living materials, which can help biologists better understand cell structure and function in real time, and ultimately help in the development of new drugs to treat human diseases.”
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 2000, British physicist John Pendry theorised that a material capable of a negative index of refraction could capture and “refocus” evanescent waves into a perfect image. Pendry’s proposed “perfect lens” theory came more than 30 years after Russian physicist Victor Veselago first conceived of a negative refraction material that could reverse k