A tiny polarization controlled metalens creates a flexible 3D sensor that shifts between precision and long-range modes, offering a new path toward adaptable optical measurement tools.
(Nanowerk Spotlight) Optical measurement tools shape how modern industries build and inspect their most precise components. Every smooth lens surface, etched semiconductor pattern, and miniature biomedical device depends on a method that can capture three-dimensional structure without touching the object. Several optical techniques support this work. Each offers a different balance of speed, robustness, and resolution, and each performs better in some settings than in others.
Interferometry measures height by observing how light waves interact with one another. It can reach nanometer sensitivity but reacts strongly to vibration and temperature changes. Structured light projection sweeps large areas quickly by shining coded patterns onto surfaces. It is useful in production environments but does not always reach the depth accuracy needed for fine inspection. Confocal microscopy blocks out-of-focus light to reveal sharp depth information. It often requires mechanical scanning along the optical axis, which slows measurement and adds moving parts.
Chromatic confocal measurement offers another approach. It uses white light and takes advantage of the fact that different wavelengths come to focus at different depths when a lens introduces controlled chromatic aberration. Each wavelength forms its best focus at a specific axial position. A spectrometer detects which wavelength returns from the surface with the highest intensity, and that wavelength maps to the surface height. This avoids mechanical scanning and maintains good precision. Many existing systems, however, depend on stacks of refractive lenses that add weight and size and limit how easily the tool can adapt to new tasks.
Metasurfaces have created new opportunities to rethink these systems. A metasurface is a flat layer patterned with nanostructures that are smaller than the wavelength of light. Each structure, sometimes called a meta atom, changes how light passes through it. By adjusting the geometry of these structures, one can steer, focus, or filter light in ways that traditional glass lenses cannot. Metalenses based on these principles are thin and lightweight. They can provide precise control over phase and polarization and have the potential to make compact optical measurement devices more flexible.
A study published in Advanced Science (“Polarization‐Multiplexed Metalens Enables Switchable and Compact Chromatic Confocal Sensing with Dual‐Mode Precision Control”) presents a chromatic confocal sensor that uses a single metalens to achieve two different measurement modes. The system switches between these modes by rotating the polarization of the incoming light. This allows the device to operate either as a high accuracy sensor or as an extended range sensor without changing hardware. The entire probe has a diameter of 1 mm, making it far smaller than many conventional chromatic confocal probes.
a) Schematic diagram of the conventional chromatic confocal probe. b) Schematic diagram of Metalens-integrated chromatic confocal probe. c) Displacement range of chromatic confocal sensor based on polarization multiplexing metalens (CCS-ML) in x-pol and y-pol modes with working wavelength range from 500–700 m. d) Schematic diagram of 3D topography measurement application by CCS-ML x-pol and y-pol modes. (Image: Reproduced from DOI:10.1002/advs.202517093, CC BY) (click on image to enlarge)
The metalens consists of silicon nitride nanofins arranged on a silicon dioxide substrate. These nanofins act as anisotropic elements. Anisotropic here means that they affect light differently depending on the direction in which its electric field oscillates. When the incoming light is x polarized, the metalens forms a focus with a focal length of 1 mm and a numerical aperture of 0.45.
Numerical aperture measures how tightly a lens can focus light and how steep a surface it can measure without losing signal. When the light is y polarized, the same metalens instead produces a focal length of 4 mm and a numerical aperture of 0.125. These two behaviors create the high accuracy and extended range modes.
The design of each nanofin determines how it delays the phase of x polarized and y polarized light. The researchers used simulations to map how each combination of fin length and width affects phase and transmission. They then selected geometries that reproduced the two lens phase profiles they needed, all within a single patterned surface. They also checked how the response shifts across the full 500–700 nm wavelength band used for measurement.
The full sensor integrates the metalens with a fiber coupled white light source, a fiber circulator, a parabolic mirror, and a spectrometer. Light travels through the fiber, reaches the parabolic mirror, passes through the metalens, and illuminates the sample. Light reflected from the sample returns through the same elements and enters the spectrometer. The circulator separates incoming and outgoing light inside the fiber without extra optics.
In the high accuracy mode, which uses x polarization, the numerical aperture is high and the focus is sharp. Across the 500–700 nm wavelength band, the focal point shifts along a 400 µm range. This mode reaches an axial accuracy of ± 0.25 µm and an axial resolution of 70 nm near 600 nm. Axial accuracy describes how close the measured value is to the true height. Axial resolution describes the smallest height difference the system can distinguish.
In the extended range mode, which uses y polarization, the weaker numerical aperture produces a longer depth of focus and a broader spectral response. Combined with the intensity distribution of the light source, this extends the dispersion range to 1.57 mm. The axial accuracy in this mode is ± 1.45 µm, and the axial resolution reaches 0.3 µm near the same central wavelength.
To calibrate the sensor, the researchers mounted a plane mirror on a translation stage and moved it along the axis in small increments. At each position the system recorded a spectrum. A wavelet-based method reduced noise, and a Gaussian fit extracted the peak wavelength. They then built a calibration curve linking the peak wavelength to the known displacement using a nonlinear least squares fit. The curve for the high accuracy mode matched the expected dispersion. The curve for the extended range mode showed a small shift caused by the intensity distribution of the light source.
To measure axial resolution more precisely, they used a piezoelectric stage capable of motion below 5 nm. They took many repeated measurements at closely spaced positions and treated the distributions of peak wavelengths as Gaussian. Two positions were considered resolvable when the overlap between their wavelength distributions met a defined probability threshold. This produced minimum resolvable steps of 70 nm in the high accuracy mode and 0.3 µm in the extended range mode.
The study then examined three sample types. A standard etched step block with a height near 24 µm was measured with a white light interferometer and with both modes of the metalens sensor. The three results matched closely. A concave spherical mirror with a nominal radius of 100 mm was measured with the high accuracy mode. A least squares fit gave a radius of 100.025 mm, with deviations within about ± 1 µm across a 12 mm measurement region. A glass slide about 1 mm thick was measured with the extended range mode. The sensor detected two peaks corresponding to the upper and lower surfaces. After converting peak wavelengths to depths and correcting for the refractive index of glass, the sensor measured a thickness of 1.034 mm, which agreed with a micrometer reading of 1.033 mm obtained over 100 trials.
These results show that a single metalens with polarization dependent behavior can support a compact and adaptable chromatic confocal sensor. The device offers two operating modes without adding hardware complexity and remains small enough for applications where space is limited. The study highlights the value of metasurfaces in this context because they can control dispersion and polarization with an ultrathin structure. As metasurface technology progresses, similar instruments could provide integrated three-dimensional measurement tools for tasks that benefit from small, lightweight, and flexible optical systems.
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Shanyong Chen (National University of Defense Technology)
, 0000-0001-9639-2448 corresponding author
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