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Metasurfaces are composed of tiny elements called building blocks or meta-atoms, which are used to control light where two different materials meet. These surfaces can be divided into two types: local metasurfaces, where each element works independently, and nonlocal metasurfaces, where elements are connected and work together. Metasurfaces manipulate the way light travels, allowing them to direct light in specific directions, focus it like lenses, or project certain light patterns, such as with holograms. They achieve this through several methods, including guiding light through small structures, using structures that resonate to affect light, and changing the light’s polarization with specially oriented elements. These techniques allow metasurfaces to perform complex optical tasks in compact and efficient ways, making them valuable for advanced technology applications while remaining simple to integrate into devices.

Figure 1: Three methods used to control light in dielectric metasurface, specifically for a deflector metasurface design: a. Propagation phase: light travels through structures with high aspect ratios, where the light's path is influenced by the shape and size of the meta-atoms. b. Resonant phase: This method depends on the natural vibrations of light (Mie resonances) which affect how light is scattered and absorbed. c. Geometric phase: In this mechanism, each meta-atom functions like a filter that changes the orientation of circularly polarized light, and the turning of these meta-atoms adjusts the light's phase, doubling the angle of rotation. (Image source from Kossowski, N., Tahmi, Y., Loucif, A., Lepers, M., Wattellier, B., Vienne, G., Khadir, S., & Genevet, P. (2025). Metrology of metasurfaces: optical properties. npj Nanophotonics)

Despite their significant advantages, metasurfaces pose unique challenges in metrology—the science of measurement. The nanoscale features of metasurfaces demand high-resolution, high-precision measurement techniques to accurately assess their optical properties and ensure performance consistency across production batches.

1.    Amplitude Measurements: Quantifying light transmission/reflection efficiency is essential for evaluating metasurface performance. However, nanoscale structural variations introduce localized losses, leading to inconsistent efficiency across devices. Traditional intensity-based methods struggle to resolve subtle fluctuations critical for applications like metalenses and metadeflectors. 

Figure 2:  Amplitude characterization of metasurfaces a, b Diagrams showing the incident and transmitted light for metasurface without polarization conversion and polarization convertingmetasurface (PBPMS), respectively.  c Example of optical setup used for the characterization of the transmission efficiency of metasurface. d (top) Normalized transmission image of 12 circular metasurface arrays (30 μmin diameter) made of identical Gallium Nitride (GaN) nanopillars of 1 μm height. The diameter of the nanopillars is different for each array, gradually increasing from left to right and from top to bottom. The diameters are chosen to cover the 2π phase range with an interval of π/6 and (bottom) corresponding transmission efficiencies of the 12 arrays. e (top) Normalized transmission image of a metalens designed according to the conventional lens maker formulae, using the nanopillars previously characterized in (d) and (bottom) radially averaged profile of transmitted intensity of themetalens. (Image source from Kossowski, N., Tahmi, Y., Loucif, A., Lepers, M., Wattellier, B., Vienne, G., Khadir, S., & Genevet, P. (2025). Metrology of metasurfaces: optical properties. npj Nanophotonics)

2.    Phase Reconstruction
 Metasurfaces rely on precise phase modulation to shape wavefronts. Direct phase measurement is inherently impossible with photodetectors, which only record intensity. Current solutions, such as interferometry and computational phase retrieval, face trade-offs between accuracy, speed, and complexity, especially for large-area or high-resolution metasurfaces.

Figure 3:   Phase characterization of metasurfaces

a Left side: schematic of a basic reference interferometry system with two beam splitters (BS), an objective lens (OL) and tube lens (TL) for the microscopic system, two mirrors (M), and an attenuator (Att) for the reference arm. Right side: Phase measurement of a metalens measured with reference interferometry and its aberrations. b Left side: shearing interferometry or common path setup. Right side: Example of interferogram measured with QLSI and its associated optical path difference. c Ptychographic system. Left side: schematic of the optical set-up for optical ptychography on a classical measurement .In the center: illustration of the scan beam illumination on the metasurface. Right side: Reconstructed phase of a metalens with ptychographic method. The device used for this demonstration has intentionally been scratched, adding additional complexities and irregularities on the metalens profile to test the performance of the reconstruction algorithm. (Image source from Kossowski, N., Tahmi, Y., Loucif, A., Lepers, M., Wattellier, B., Vienne, G., Khadir, S., & Genevet, P. (2025). Metrology of metasurfaces: optical properties. npj Nanophotonics)

3.    Polarization characterization : 
Dynamic polarization control is central to metasurfaces in imaging, sensing, and communication systems. However, conventional polarization analyzers lack the spatial resolution and compatibility required to map polarization states at the subwavelength scale, limiting their utility in miniaturized metasurface-integrated devices.

Figure 4:  f Schematic of a Mueller matrix characterization setup. g Spectroscopic ellipsometry measurements of the Mueller matrix elements of a gold nanoparticles metasurface. (Image source from Kossowski, N., Tahmi, Y., Loucif, A., Lepers, M., Wattellier, B., Vienne, G., Khadir, S., & Genevet, P. (2025). Metrology of metasurfaces: optical properties. npj Nanophotonics)

Industrializing metasurfaces involves scaling up production without compromising the intricate nanostructures that define their functionality. Achieving uniformity and high yields while maintaining cost-effectiveness and meeting stringent manufacturing standards represents a significant industrial challenge.
Phasics’ QWLSI Technology: An advanced Solution for Metasurface Metrology
To address these challenges, Phasics leverages its patented Quadriwave Lateral Shearing Interferometry (QWLSI) technology.  QWLSI combines high spatial resolution, achromatic operation, and compact design. The technique employs a diffractive grating to split incident light into four coherent waves, generating interferograms that encode both phase gradients and intensity distributions. 
Phasics’ SID4 series wavefront sensor, powered by QWLSI technology, delivers nanometric phase sensitivity and real-time data acquisition. Key applications include:
•    High Resolution and Sensitivity: Essential for inspecting nanostructures and ensuring precise measurements.
•    Optical function validation: Assessing metalens focusing efficiency and aberration control.
•    Imaging quality analysis: Capable of assessing wavefront, imaging quality (PSF & MTF in one shot), and metasurface structure.
•    Robustness and Compatibility: Integrates easily with standard microscopes, enhancing reliability and reproducibility.

As compact, efficient, and highly functional optical devices, metasurfaces push innovation across various industries. The integration of Phasics' metrology solutions into the production and testing phases ensures that these advanced materials meet the rigorous demands of modern optical applications.
For a more comprehensive understanding,see in the publication Kossowski, N., Tahmi, Y., Loucif, A., Lepers, M., Wattellier, B., Vienne, G., Khadir, S., & Genevet, P. (2025). Metrology of metasurfaces: optical properties. npj Nanophotonics.