Probing Spectral-Hole-Burning\nin Non-Hermitian Scatterings:\nDifferentiating Far-Field Interference and Near-Field Coupling

Scattering spectra from radiative non-Hermitian systems\noften exhibit\nintricate line shapes, where peaks typically garner the most attention\nfor mode identification. However, in multimode systems, the valleys\nbetween these peaks may contain valuable information. This “coupling”\nfeature arises from the nonorthogonality of modes in both far and\nnear fields, giving rise to diverse and complex spectra-hole-burning\n(SHB) patterns. Traditionally, the interpretation of these SHBs has\nfocused on Rabi splitting or Fano resonances, often concentrating\nsolely on either far-field interference or near-field coupling. However,\nit is essential to recognize that both phenomena coexist in non-Hermitian\nscatterings. In this study, we develop a quantitative quantum model\nto probe scattering SHB by simultaneously extracting near-field coupling\nrates between system quasinormal modes, nonradiative decay rates into\na heat reservoir, and radiative decay rates into a vacuum reservoir\nfor far-field interference. We apply our model to illustrate the concept\nof geometric engineering in tuning the ratio of far-field interference\nand near-field coupling, exemplified by a silver dimer transitioning\nfrom cube-dimer to sphere-dimer or cube-dimer to nanocube-on-mirror\nconfigurations. Through this, we establish a universal design guideline\nfor non-Hermitian scattering by creating a two-mode SHB library based\non arbitrarily tunable far-field interference and near-field coupling.\nThe developed model serves as a generalized diagnostic tool for probing\nthe SHB mechanisms in all types of non-Hermitian scattering problems,\npromising to advance our understanding of intricate phenomena and\nfacilitate the design of tailored optical devices with enhanced performance\nand functionality.