With the Nobel Prize for Chemistry being awarded last year for innovative research into significantly improved nanometer resolution in microscopy, nanoscale imaging has become a hot topic. The breakthrough was achieved by ensuring the source of illumination was small and closer to the object being imaged. However, there remains the problem that such proximity allows the light source and object to interact with each other, blurring the image.
Conventional microscopy is usually constrained by the diffraction of light around objects, with light waves scattering on striking an object, so that the spatial resolution is only about a half of the wavelength of the light being emitted; for visible light, the diffraction limits the resolution to less than a few hundred nanometers. To ensure visible light can achieve a resolution of only a few nanometers and overcome the diffraction limit, nanometer-sized light sources such as nanoparticles, quantum dots and fluorescent molecules must be sufficiently close to the object being imaged.
In new research from the Joint Quantum Institute at the University of Maryland, a way of sharpening nanoscale microscopy by improving the precise location of the light source has been demonstrated. The study, reported in Nature Communications [Ropp et al. Nat. Commun. (2015) DOI: 10.1038/ncomms7558], used quantum dots – tiny crystals made from a semiconductor material – that emit single photons of light. Nanoscopic mappings of the electromagnetic field profile around silver nano-wires was carried out by placing the quantum dots close by. However, such sub-wavelength imaging has a problem in that an “image dipole” induced in the surface of the nanowire acts to distort awareness of their actual location, and therefore their electromagnetic field measurement.
When a dot approaches the wire, the latter develops an “image” electrical dipole whose emission can interfere with the emission from the dot. As the measured light from the dot is the substance of the imaging process, the presence of light coming from the image dipole can interfere with light coming from the dot, distorting the perceived position of the dot by 10 times more than the expected spatial accuracy of the imaging technique.
The team managed to measure the image-dipole effect and show that it can be corrected under the right circumstances, offering a more accurate map of the electromagnetic fields surrounding the nanowire. This shows there are substantial inaccuracies in super-resolved imaging, and describes a procedure to correct for them, providing a pathway towards probing and correcting image-dipole effects in near-field imaging applications.