Optical properties of many macroscopic systems are defined by the ultrafast dynamics of electronic, vibrational, and spin excitations localized on the nanoscale. Harnessing these excitations is a long-standing goal in science and technology, however challenging due to the lack of spectroscopic techniques that can resolve processes simultaneously on the nanometer spatial and femtosecond temporal scales. I will describe fundamental principles, implementation, and experimental demonstration of a novel type of ultrafast microscopy based on the concept of adiabatic plasmonic nanofocusing. A combined few tens of nanometers and few femtoseconds spatio-temporal resolution is achieved through nonlinear optical four-wave mixing response generated by compressed surface plasmon polaritons on a metallic near-field probe. This work opens up new avenues for probing and control of ultrafast coherent dynamics in materials.

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