Lens allows optical microscopy down to 60 nanometers
Results: A team from the University of California, Berkeley, has devised a silver “superlens” that could increase the resolution of light microscopy by about a factor of six. The lens doesn’t diffract light like conventional glass lenses. Instead, it uses evanescent waves, which are produced when light hits a lens at such an angle that it bounces off instead of passing through. Evanescent waves emerge on the other side of the lens and add optical information to normal “propagating” light waves, but they decay very quickly over short distances. By capturing and amplifying these weak waves, the researchers obtained images with 60-nanometer resolution.
Why it Matters: High-resolution imaging methods such as electron microscopy can’t image living tissue. Light microscopy can. Its resolution, however, is limited by the wavelength of the light used. And 400 nanometers is the shortest wavelength that doesn’t damage tissue. Evanescent waves allow researchers to get around this limitation. The technique could eventually allow researchers to watch, in real time, biological processes such as protein interactions in samples of living tissue – events that can now be studied only indirectly.
Previous research has used evanescent waves to construct images in piecemeal fashion. The Berkeley team, led by Xiang Zhang, has shown that it’s possible to take a clear and complete picture in one shot.
Methods: The researchers made a lens out of a 35-nanometer-thick film of silver. They chose a light source whose frequency matched the resonant frequency of the lens’s surface electrons. The light shone through the word “NANO,” inscribed in letters with a 40-nanometer line width on a piece of chromium through ion beam lithography. When the light hit the lens, the silver electrons resonated with the evanescent waves, boosting their energy. The superlens directed the waves onto light-sensitive material to capture the image.
Next Step: The superlens didn’t spread out the evanescent waves enough that the human eye could see the image directly; it had to be observed with an atomic force microscope. Future research will curve the lens so that it can further spread the waves and pass them into, say, a fiber-optic cable. Superlenses might then be integrated into light microscopes. – By Stu Hutson
Source: Fang, N., et al. 2005. Sub-diffraction-limited optical imaging with a silver superlens. Science 308:534-7.