Pernille Klarskov, Hyewon Kim, Vicki Colvin, and Daniel Mittleman
http://pubs.acs.org/doi/abs/10.1021/acsphotonics.7b00870?mi=aayia761&af=R&AllField=nano&target=default&targetTab=std
Laser terahertz emission microscopy (LTEM) has become a powerful tool for studying ultrafast dynamics and local fields in many different types of materials. This technique, which relies on acceleration of charge carriers in a material upon femtosecond excitation, can provide insight into the physics of charge transport, built-in fields, grain boundaries or surface states. We describe a new implementation of LTEM with a spatial resolution in the nanoscale regime based on a scattering-type near-field tip-based approach. We observe a spectral reshaping of the signal compared to conventional LTEM, which is analyzed using a resonant antenna model. Our experimental and computational results clarify the importance of the mechanisms for both the plasmonic in-coupling of the near infrared pulses into the near field and the out-coupling of generated terahertz pulses. We demonstrate a tip-size-limited spatial resolution of ~20 nanometers by imaging a gold nanorod using terahertz emission from the underlying substrate. This work enables for the first time the possibility of performing LTEM measurements on individual nanostructures.
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