Plasmonics for near-field nano-imaging and superlensing

Scientists have long dreamt of an optical microscope that can be used to see a sample in nanometre resolution. Because light propagates through water and air, an optical microscope can be used to see, in vivo, the details of living matter and other materials in their unperturbed natural condition. In addition, an optical microscope provides colour images that contain much richer information than other microscopes in which only monochrome images are produced. In particular, the visible and mid-infrared region of light covers the energies corresponding to the electronic transitions in atoms and to molecular vibrations, allowing optical microscopes to provide information related to the intrinsic properties of the sample. But the resolution attainable with an optical microscope is limited to about 0.5 μm because of the diffraction of light1–3. Therefore, it becomes almost impossible to image a sample at the nanoscale through an optical microscope. Breaking through this limit, an optical microscope was proposed in which a nano-sized metallic probe tip scans the sample surface to form an image with a resolution much better than the diffraction limit4. The mechanism is based on the excitation of localized modes of surface plasmon polaritons (SPP) at the metallic tip, which generates a nano-sized spot of light at the apex of the nano-tip. By using this tiny light-spot as the light source of an optical microscope, extremely high spatial resolution can be obtained. The fundamental idea of exciting SPP to obtain images beyond the diffraction limit of light has been used in other studies as well. For example, the perfect lens or superlens proposed by Pendry also images a sample in nanoscopic resolution by exciting SPP at its resonance frequency5,6. Here we review the mechanism of optical nano-imaging based on the excitation of SPP to provide spatial resolution beyond the diffraction limit. We discuss a variety of metallic nanostructures as probes and look at their applications.