Scanning differential microscopy for characterization of reflecting phase-gradient metasurfaces

Abstract

Gap-plasmon based phase-gradient metasurfaces operating in reflection are widely used for the realization of diverse flat optical components, ranging from spectropolarimeters to efficient couplers for surface waves. Successful implementation of carefully designed metasurfaces is however often hampered by technological imperfections that could be related to deviations of geometrical parameters of fabricated nanostructures from the designed ones or material properties, such as the metal and/or dielectric susceptibilities, from the handbook data. While the overall performance of fabricated components might indicate the existence of a potential problem, it is very difficult to identify its origin, which, for example, can simply be related to the deviation in only one cell of the metasurface supercell. We suggest exploiting well-developed experimental techniques of scanning differential heterodyne microscopy (SDHM) to characterize fabricated phase-gradient metasurfaces designed to operate in reflection. We further establish that, by carefully measuring the SDHM response of a gradient metasurface, one should be able of detecting small (~ 5%) amplitude and phase deviations (with respect to the design values) in the optical field reflected by an individual subwavelength-sized cell of the metasurface supercell. Research highlights Сomplex scanning differential heterodyne microscope (SDHM) response of phase-gradient metasurfaces operating in reflection is numerically investigated, revealing a direct relationship between the SDHM phase response and the average phase gradient of the metasurface. Furthermore, we establish that small (~ 5%) amplitude and phase deviations (from the design values) in the complex reflection of an individual subwavelength-sized cell of the metasurface supercell can be detected by analysing the SDHM response.

Characterization of gap-plasmon based metasurfaces using scanning differential heterodyne microscopy

“…The presented study is devoted to experimental verification of efficiency and accuracy of the SDHM characterization for direct inspection of GSP reflective metasurfaces, using as a starting point the analytical considerations and numerical calculations reported previously 10 . Two GSP-based metasurface configurations, representing a binary grating and linear phase gradient, were experimentally characterized with the SDHM operating at the light wavelength of 633 nm.…”

“…Concerning technical limitations in using the SDHM technique for metasurface inspection, the main and most fundamental limitation is related to the resolution limit that any far-field microscopy technique is subjected to. In principle, this limitation can be circumvented, at least to some extent, by increasing the signal-to-noise ratio along with using apriori information about the metasurface under inspection and the SDHM response function 10 . The most relevant technical limitation is thereby related to the fundamental limit of signal-to-noise ratio that is difficult to achieve in practice when using moderately powerful lasers.…”

“…The nonlinear nature of the SDHM response leads, in general case, to a complicated dependence of the amplitudephase response on the actual amplitude-phase properties of the reflecting surface. However, based on the fact that at least one case exists where the amplitude and phase properties of the surface separately affect the amplitude response and phase one 10 , we decided to apply following version of the linearization algorithm:…”

“…2. The magnitude of the phase response is given by 10 : where 2 w is the full width of the focused probe beam, δ is the interval between two focused beams. In our case, if Δ φ = 60° and δ = w = 0.5 μm we obtain φ max = 30° that is in good agreement with the result of the exact calculation shown in Fig.…”

“…In this work, we conduct experimental investigations of an alternative approach to the characterization of reflective metasurfaces that is based on the usage of scanning differential heterodyne microscopy (SDHM). The SDHM application to the characterization of reflective metasurfaces was recently proposed and considered using theoretical considerations and numerical simulations 10 . It is however clear that there are many issues, such as influence of noise, limited resolution and experimental inaccuracies, whose importance for accurate SDHM characterization of metasurfaces can only be evaluated when the SDHM technique is applied in practice.…”