Characterization of gap-plasmon-based metasurfaces with scanning white-light interferometry

Abstract

Subject of study. The possibility of using the well-established method of scanning white-light interferometry (SWLI), implemented as a commercially available device, is studied for characterizing promising metasurfaces based on third-order plasmon resonance. The characteristics of the samples were determined for two types of experimental metasurfaces, namely, for a binary grating with a period of 12.6 µm and a phase gradient grating with a period of 6.75 µm. Objective. The main goals of this study were to test the fundamental possibility of using a commercially available white-light interferometer to determine the phase characteristics of plasmonic metasurfaces and to estimate experimentally the lateral optical resolution in the phase image of metasurfaces. Main results. Although the method of SWLI was originally intended for measuring a geometric microrelief, it was found that it can be used to determine the phase increment of the optical radiation reflected from the studied metasurfaces in the range of 10°–100° with a relative accuracy of approximately 50% without any modification of the signal processing algorithm. The possibility of obtaining a phase mapping of the surface with such accuracy in this configuration is not obvious because the metasurface is a matrix of nanoelements with the same height of 50 nm, and the phase shift in the reflected optical radiation is created due to third-order gap plasmon resonance. On the obtained optical phase images of metasurfaces, unit (single) cells of the metasurface are reliably resolved owing to a sufficiently high lateral resolution (approximately 450 nm). The results of the characterization of metasurfaces obtained using SWLI are compared with those obtained in a laboratory setup using scanning differential heterodyne microscopy (SDHM). Although the SWLI method demonstrates better lateral resolution compared with the SDHM method, it is less accurate in restoring the phase characteristics of metasurfaces with a phase gradient. Practical significance. The results obtained allow us to conclude that the evaluation of the phase optical characteristics of nanostructured plasmonic metasurfaces by the method of SWLI is possible and requires further experimental and theoretical studies.