| Scoping Out the Nanoworld
by David Pescovitz
Professor Xiang Zhang (ME '96) is the director of the National Science Foundation's Nano-scale Science and Engineering Center (NSEC) based at UC Berkeley.
UC Berkeley mechanical engineering professor Xiang Zhang is taking a close look at the nanoworld. Very close. He's building an optical microscope that's nearly ten times more powerful than today's best comparable systems. The superlens at the heart of the scope could someday be used to observe the tiniest machinations of living cells, fabricate much faster and smaller computer processors, and manufacture DVDs with thousands of movies on each disk. Already, it cast a new light on a centuries-old barrier faced by optical physicists and engineers.
A nanometer is one-billionth of a meter. State-of-the-art optical microscopes can only reveal details bigger than about 400 nanometers, the size of a very large virus. The superlens produces images with resolutions of approximately 60 nanometers.
"That's just the beginning," says Zhang, who built the lens with two graduate students and a research scientist in his group. "We're looking to move from 60 nanometers to 20 and possibly even 10."
At left (A) is an image of an array of nanowires 60 nanometers wide created with the silver superlens. The center distance between each nanowire is 120 nanometers. To the right (B) is an image of the same nanowires. In this image, created without the superlens, the individual nanowires are not distinct. The scale bar on both images is 1 micrometer. (Cheng Sun, UC Berkeley)
With a scope capable of such great resolution, biologists could zoom in on proteins moving along the microtubule fibers that form the skeleton of the cellular factory. Currently, that level of detail is captured using scanning electron microscopes and atomic force microscopes. While the electron microscopes produce valuable images, they work by scanning point by point just above the surface of the sample. As a result, they can take several minutes to generate a single image. Meanwhile, atomic force microscopes physically scan a sample much like a phonograph needle travels across a record. The problem is that as the sharp probe touches the sample, it can perturb it. On the other hand, the Berkeley researchers' superlens enables a single snapshot to be taken in a fraction of a second without any contact.
In April, Zhang and his colleagues published a paper outlining their technique in the journal Science. Lead author on the paper was Zhang's former PhD student Nicholas Fang-- now an assistant professor of mechanical engineering at the University of Illinois at Urbana-Champaign--graduate student Hyesong Lee, and research scientist Cheng Sun.
At top (A) is the higher resolution image of the word NANO created with a silver superlens. Below that (B) is an image created during a control experiment in which the superlens is replaced by spacer layer. The averaged line width is 89 nanometers in image A with the superlens, and 321 nanometer in image B without the superlens. The scale bar in both images is 2 micrometers. (Cheng Sun, UC Berkeley)
To prove their concept, the researchers focused a beam of ultraviolet light through the superlens, a silver film three thousand times thinner than a human hair. Employing a photolithography process like those used in microchip fabrication, the researchers then recorded crisp images of nanowires and the word "nano" spelled out on a layer of polymer. A similar approach could be used by the semiconductor industry to pack many more transistors on a chip than currently possible. The speed and power of a computer processor is determined in large part by the density of the transistors.
"The work we published demonstrated the physics principle of the lens," Zhang says. "Our next objective is to make an optical microscope that could be used in biomedical imaging and other applications."
The previous experiment was based on near-field microscopy, meaning the sample was placed very close to the light source and detector to achieve the high resolution. Now, the researchers are honing a superlens-equipped microscope where the distance between the sample and the detector (your eye in a standard microscope) is comparable to traditional optical microscopes. They expect to publish their results in the next several months.
Schematic drawing of nano-scale imaging using a silver superlens that achieves a resolution beyond the optical diffraction limit. The red line indicates the enhancement of "evanescent" waves as they pass through the superlens. (Cheng Sun, UC Berkeley)
The April publication made waves in the physics community not only because of the technology's promise but because it demonstrated that a well-known constraint in optical resolution, called the diffraction limit, could be beaten. When an object emits or reflects light, it produces two kinds of waves: propagation waves and evanescent waves. Conventional lenses such as those in eyeglasses capture the propagating waves of lights and focus them on a receptor, your eyeball for example. However, evanescent waves carry a great deal more detail about the object. The problem is that these waves decay very rapidly. For years, optical scientists and engineers have focused on capturing those waves to possibly produce a perfect image of an object. Two years ago, Zhang's group confirmed that a silver superlens could enhance optical evanescent waves. Now, they've shown how to build an optical microscope that does just that. The system doesn't capture all of the evanescent waves necessary for a perfect image, but it gets close.
"Between the diffraction limit and a perfect image, there is a huge land to look around in," Zhang says. "The superlens enables you to recover the lost treasures of the evanescent waves and build a sharper image."
"New superlens opens door to nanoscale optical imaging and high-density optoelectronic devices" by Sarah Yang (Media Relations, 21 April 2005)
Xiang Zhang's home page
Zhang Research Lab
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