  We experimentally demonstrate long-wave infrared-visible sum-frequency
generation microscopy for imaging polaritonic resonances of infrared (IR)
nanophotonic structures. This nonlinear-optical approach provides direct access
to the resonant field enhancement of the polaritonic near fields, while the
spatial resolution is limited by the wavelength of the visible sum-frequency
signal. As a proof-of-concept, we here study periodic arrays of
subdiffractional nanostructures made of 4H-silicon carbide supporting localized
surface phonon polaritons. By spatially scanning tightly focused incident
beams, we observe excellent sensitivity of the sum-frequency signal to the
resonant polaritonic field enhancement, with a much improved spatial resolution
determined by visible laser focal size. However, we report that the tight
focusing can also induce sample damage, ultimately limiting the achievable
resolution with the scanning probe method. As a perspective approach towards
overcoming this limitation, we discuss the concept of using wide-field
sum-frequency generation microscopy as a universal experimental tool that would
offer long-wave IR super-resolution microscopy with spatial resolution far
below the IR diffraction limit.
