Fourier analysis of near-field patterns generated by propagating polaritons
Minsoo Jang, Sergey G. Menabde, Fatemeh Kiani, Jacob T. Heiden, Vladimir A. Zenin, N. Asger Mortensen, Giulia Tagliabue and Min Seok Jang
Physical Review Applied 22, 014076 (2024)
Scattering-type scanning near-field optical microscope (s-SNOM) has become an essential tool to study polaritons—quasiparticles of light coupled to collective charge oscillations—via direct probing of their near field with a spatial resolution far beyond the diffraction limit. However, extraction of the polariton’s complex propagation constant from the near-field images requires subtle considerations that have not received the necessary attention so far. In this study, we discuss important yet overlooked aspects of the near-field analysis. First, we experimentally demonstrate that the sample orientation inside the s-SNOM may significantly affect the near-field interference pattern of mid-infrared polaritons, leading to an error in momentum measurement up to 7.7%, even for the modes with an effective index of 12.5. Second, we establish a methodology to correctly extract the polariton damping rate from the interference fringes depending on their origin—i.e., the s-SNOM nanotip or the material edge. Overall, our work provides a unified framework for the accurate extraction of the polariton momentum and damping from the near-field interference fringes.