The plasmonic phenomenon of extra-ordinary optical transmission (EOT)  refers to the interaction of the light and a metallic film perforated with the subwavelength holes. It provides a larger transmission of electromagnetic fields than the expected values by classical optics . Most precedent researches have been based on intensity measurements. Meanwhile, in optical spectroscopy, broadband phase spectra are known to provide richer, linear, and more direct sources of information on target samples compared to traditional intensity spectra . The direct acquisition of phase spectra, however, has proven difficult in plasmonics, owing to the critical requirements for highly stabilised light sources, coherent optical interferometers, and stable optical paths. The frequency comb provides millions of well-defined optical modes over a broad spectrum with a high phase coherence referenced to an atomic clock of a time standard . Herein, this research demonstrates an interferometric approach to measure the plasmonic EOT phase using a frequency comb as a novel light source for plasmonics .
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