Advanced super-resolution microscopy

Abstract

An imaging system that could find a range of uses in biomedical and nanoelectronic research has been developed that can observe both natural and synthetic nanostructures. The new practical tool for superresolution optical microscopy, called saturated transient absorption microscopy (STAM), provides high-resolution images and obviates the need for fluorescent dyes, allowing for study on the nanoscale, as well as of molecules such as proteins and lipids. While standard optical microscopes can only view objects of up to around 300 nm – the ‘‘diffraction limit’’ – the STAM approach captured images of graphite nanoplatelets of only around 100 nm width. A team from Purdue University and the University of California, Irvine, whose researchwas published in the journal Nature Photonics [Wang et al., Nat. Photonics (2013) doi:10.1038/nphoton.2013.97], demonstrated for the first time that it is possible to defeat the diffraction limit in a label-free way, using absorption as a contrast mechanism, to resolve previously unseen details on the nanoscale. The team initially explored carbon nanotubes with transient absorptionmicroscopy, helping themtodistinguish semiconducting nanotubes from metallic nanotubes. However, as their resolution was insufficient to separate adjacent nanotubes, they advanced their work towards the spatial resolution of transient absorption microscopy. STAM is comprised of three laser beams, one shaped like a doughnut that can selectively illuminate specific molecules. Electrons in the atoms of each illuminated molecule become excited and are moved temporarily to a higher energy level, while the others remain in their ground state. A probe laser then creates images that contrast the two molecular states. As team leader Ji-Xin Cheng points out, ‘‘We demonstrate a new scheme for breaking the diffraction limit in optical imaging of non-fluorescent species. Because it is labelfree, the signal is directly from the object so that we can learn more about the nanostructure.’’ The laser excitation technique could go even further, as it is hoped improvements could permit them to see objects only 10 nm in diameter – 30 times smaller than that realized by conventional optical microscopy. Through simplification, the researchers hope to push the spatial resolution and use the system to characterizenanomaterials such as nanotubes and graphene. They also plan to assess whether the system can be usedwith shorterwavelengths of light –with these, the hole in the doughnut would be smaller, perhaps allowing them to focus on even smaller objects. Laurie Donaldson