Optical Microscopy Employs ‘Nanoantenna’
New optical microscopy techniques such as near-field scanning, Förster resonance energy transfer and structured illumination can detect features smaller than the optical diffraction limit. These methods share one fundamental similarity: They detect photons generated in or transmitted through the sample.
A new optical microscopy technique offers high-resolution images without detecting light from the object under study. Researchers at Eidgenössische Technische Hochschule Zürich in Switzerland, and at Zuse Institut Berlin and Universität Potsdam in Germany are using changes in the electromagnetic properties of a gold “nanoantenna” to detect variations in a sample’s radiative characteristics.
Gold nanoparticles, with diameters of 100 nm or less, act as antennae with a well-defined resonance frequency and linewidth that are influenced by the optical properties of their surroundings. Vahid Sandoghdar of the Swiss institute and his colleagues thus place a gold nanoparticle at the end of an uncoated glass fiber tip and bring the nanoantenna to within 5 to 10 nm of the sample surface. The plasmon modes of the nanoantenna change as a function of the dielectric constant of the surface.
It is this aspect of the work that Sandoghdar finds most intriguing. “We are modifying the intrinsic optical properties of a nanoparticle just by putting it close to something,” he said.
To probe the spectral changes of the nanoantenna, the researchers illuminate the particle with white light from a xenon lamp, coupled through a separate optical fiber that emits the light parallel to the sample plane. Some of the light scatters off the nanoparticle and travels through the sample, where it is spatially filtered and collected. The spectrum of the scattered light varies with changes in the plasmon spectrum of the nanoantenna, which in turn varies with changes in the dielectric constant of the sample.
In a proof-of-principle demonstration of the approach, the scientists deposited a layer of chromium on a microscope coverslip such that there were a number of 2-µmdiameter holes through which the glass was exposed. Over the chromium, the nanoantenna resonance peaked at about 570 nm with a linewidth of 135 nm, but over the glass, the peak was 555 nm and the linewidth, 110 nm. As the sample was scanned, the sharp edge was detected with a position change of 170 µm, well less than the optical diffraction limit.
Although the initial demonstrations used optical scattering to detect changes in plasmon resonance, the nanoantenna modes could be excited electronically. This would eliminate imaging artifacts caused by background scatter of the white-light illumination.
To make the method routine, Sandoghdar suggested, it will be necessary to develop a way to fabricate dielectric tips with gold nanoparticles at their ends, rather than by picking up a nanoparticle with a fiber, as in the experiment.
The researchers have demonstrated the ability to perform plasmon spectroscopy on particles as small as 5 nm.
However, if smaller nanoparticles were used in the microscopy technique, Sandoghdar suggested, a spatial resolution of 30 to 40 nm might be possible.
Physical Review Letters, Nov. 11, 2005, 200801.
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