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Metallic nanoantennas for field-enhanced spectroscopy and microscopy : from the visible to the terahertz

Javier Aizpurua - Center for Materials Physics (CSIC) and Donostia International Physics Center (DIPC) -Paseo Manuel Lardizabal 4, 20018 Donostia-San Sebastián, Spain


Optical antennas are nanoscale metallic structures which act as effective receivers, transmitters and receivers of visible light. These nanoatennas show the ability to focus electromagnetic radiation into tiny spots of nanometer-scale dimensions allowing for more effective field-enhanced visible spectroscopies such as in surface-enhanced Raman spectroscopy (SERS). A review on the basics of the optical response of these optical nanoantennas will be presented, with examples of different canonical nanostructures that show a remarkable field-enhancement effect : metallic nanorings, dimers, nanoshells, nanowires, etctera. In particular, the use of 2 nanorod-like gold nanoantennas is described in detail. By engineering the length of the rod-like nanoantennas, it is possible to extend the field enhancement capability into the infrared range of the spectrum to perform direct surface-enhanced infrared absorption (SEIRA). With use of this concept, we show that it is possible to obtain direct IR spectral information of a few thousand molecules deposited on the antenna. Another option to engineer the optical response of a nanoantenna relies on the manipulation of the antenna gap. We show theoretically and experimentally the modification of the optical response of nanoatennas as a function of the thickness of the antenna gap, bridging together concepts of optics and circuit theory. Furthermore, a metallic tip acting as a scattering antenna can be used to resolve the near-field information of nanoscale metallic particles in the IR, allowing for a resolution of objects as small as /1000 in scattering-type scanning near-field optical microscopy (s-SNOM). The basic details of this field-enhanced microscopy technique will be addressed. Finally, using terahertz radiation with similar antenna tips, we will show that it is possible to squeeze such long wavelength radiation into the nanoscale to obtain quantitative information on the doping concentration of free carriers in state-of-the-art semiconductor devices. These examples show that nanoatennas can assist to perform nanoscale spectroscopy and microscopy from visible to terahertz.

 

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