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.
Optical phase-gradient metasurfaces, whose unique capabilities are based on the possibility to arbitrarily control the phase of reflected/transmitted light at the subwavelength scale, are seldom characterized with direct measurements of phase gradients. Using numerical simulations and experimental measurements, we exploit the technique of scanning differential heterodyne microscopy (SDHM) for direct phase and amplitude characterization of gap-plasmon based optical metasurfaces. Two metasurface configurations utilizing the third-order gap surface plasmon (GSP) resonance, representing a binary grating and linear phase gradient, are experimentally characterized with the SDHM operating at the light wavelength of 633 nm. Comparing the experimental performances of these GSP metasurfaces with those expected from the phase and amplitude profiles reconstructed from the SDHM measurements, we verify the efficiency and accuracy of the developed SDHM characterization approach for direct inspection of GSP reflective metasurfaces.
The possibility of achieving superresolution on a microstep phase image in a laser scanning differential heterodyne microscope is studied both heoretically and experimentally. The superresolution is estimated as the width ratio for the amplitude and phase components of the microscope response measured at half the height of the corresponding parts of the response. It is shown theoretically that superresolution greatly exceeding unity can be achieved for an object in the form of a phase microstep introducing a phase shift equal to π. Superresolution of ∼2 is experimentally obtained for certified test micro-objects. A possibility of tuning a test sample into the superresolution regime by shifting a point photodetector in the microscope’s Fourier plane is demonstrated.
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