New look for optical microscopy

Physicists in Switzerland and Germany have made a new type of optical microscope that can produce images without capturing light from the sample. The new device relies on measuring changes in the properties of a gold nanoparticle placed next to the sample. The nanoantenna could have application in sensing devices (Phys Rev Lett 95 200801).

Scientists have known for many years that the electronic structure of an atom can be modified by placing it close to a boundary. Now, Vahid Sandoghdar and colleagues at the Swiss Federal Institute of Technology (ETH) in Zurich, the Zuse Institute in Berlin, and the University of Potsdam, also in Germany, have exploited this phenomenon to perform high-resolution microscopy.

The new technique is very different from other forms of optical microscopy because it does not involve the detection of photons from the object being imaged. Instead, it relies on measuring how the intrinsic properties of the gold nanoantenna such as its resonance frequency and line width change when it is placed close to a sample.

Sandoghdar and co-workers began by mounting a single gold nanoparticle on the end of a glass fibre tip. Next, they focused white light from a Xenon lamp onto the end of the tip, which excited a resonance frequency in the nanoantenna. Finally, they measured the resonance wavelength and line width of the antenna while scanning it across the surface of a sample. By plotting these quantities for different positions of the tip with respect to the sample, they were able to obtain an image.

Although Sandoghdar and co-workers used photons to read the spectrum of the nanoantenna, they say that electric current could easily be used to excite a resonance frequency in the gold instead. Moreover, the method is capable of sub-wavelength resolution because imaging takes place very close to the sample in its 'near field'.