Surface Phonon Polariton Resonance Imaging Using Long-Wave Infrared-Visible Sum-Frequency Generation Microscopy

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.