State-of-the-art infrared and THz near-field microscopy, also known as scattering-type scanning near field microscopy (s-SNOM), complemented by nano-Fourier transform IR spectroscopy (nano-IR), have enabled superior nano-scale imaging with simultaneous monitoring of the local infrared spectral features associated with electronic and lattice excitations in novel materials. With s-SNOM and nano-IR, optical imaging and manipulation of light-matter interactions are possible at nanometer scale, with a >1000-fold increase in the spatial resolution compared to conventional optics. Salient examples include launching and detecting plasmons in graphene, polaritons in hBN, or monitoring phase separations in vanadium dioxide (VO2) thin films[1][2][3].

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Ultrafast spectroscopy is an important tool to interrogate complex materials as it can access the fundamental time scales of electron/lattice motion and at the same time monitor/perturb the relevant energy excitation [1]. In particular, ultrafast THz spectroscopy with meV energy pulses could be utilized to investigate coherent Drude response in transition metal oxides and gaped phenomena in superconductors. Combining optical-pump/optical-probe, optical-pump/THz-probe and THz-pump/THz-probe techniques, one can supply insight into the electron-electron and electron-lattice interactions in strongly correlated materials. Specifically, in the transition metal oxides V2O3 and VO2, dynamic far-infrared conductivity induced by ultrafast optical or THz pulses revealed a giant optical susceptibility against external stimuli, indicating a very strong coupling between the electronic and lattice degrees of freedom in the target materials. The figure above shows an ultrafast pump probe setup coupled to the near-field microscope.

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Complex materials are defined to have no dominant energy scales so that different degrees of freedom (electron, lattice, spin etc.) interplay with each other and contribute collectively to their macroscopic properties. Good examples for complex materials are superconductors, multiferroics, magnetoresistors, heavy-fermion actinide compounds and Mott-insulators. There are enormous interesting features and rich physics in these materials that require thorough studies with all the possible time, space and energy scales one can access. Here are two reviews about the electrodynamics of correlated electron materials  and nanoscale electrodynamics of strongly correlated quantum materials.

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Metamaterials, artificial composites made of sub-wavelength inclusions that have a defined effective permittivity and permeability, are useful alternatives to materials that are naturally available. Metamaterials can also be used as a “mediator” between light and matter to perturb and study the novel optical/THz responses of complex materials. The design and fabrication of metamaterials on different substrates always involve extensive E-beam lithography, photolithography or stencil imprint techniques. The marriage between micro-fabrication (MEMS) and optics has proved to be very successful in terms of generating new functional materials with novel applications at different frequency ranges. A few examples can be found here:[1], [2], [3], [4], [5]

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Plasmonic polaritons, phonon polaritons or different waveguide modes in 2D materials can be imaged by IR s-SNOM.
References: [1], [2], [3], [4], [5]

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Our recent work demonstrated electron-regulated nanoscale polymorphic transitions in silk proteins revealed by near-field infrared imaging and nano-spectroscopy at resolutions approaching the molecular level. The ability to locally probe nanoscale protein structural transitions combined with nanometre-precision electron-beam lithography offers us the capability to finely control the structure of silk proteins in two and three dimensions​
​Nature Communications, 7, 13079 (2016)
​https://www.nature.com/articles/ncomms13079

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