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Metamaterial device makes 16 polarization measurements at once

Metamaterial device makes 16 polarization measurements at once

16 May 2024

Capturing all the ways that an object can affect a light wave’s polarization has always been cumbersome. Now it can be done in an instant.

Five black-and-white images of a beetle arranged in an X shape.
The chiral beetle is unusual for the optical properties of its shell, which acts as a circular polarizer. Shown here are the raw images of the beetle captured by a metamaterial imaging system. Although they’re too small to see, tiny fluctuations in the four outer images encode all the effects the shell might have on any polarization state of light. Credit: Adapted from A. Zaidi et al., Nat. Photon. (2024), doi:10.1038/s41566-024-01426-x

When you’re walking along a beach on a sunny day, polarized sunglasses come in handy: The glare off the water, even though it’s a reflection of unpolarized sunlight, is horizontally polarized and is blocked by the vertically polarized lenses. The ways that objects change light’s polarization state don’t stop there. The shell of the so-called chiral beetle, shown in the images above and below, reflects right-circularly polarized light but not left-circularly polarized light. And cancerous tissue, because of its erratic jumble of collagen fibers, has a different polarizing effect than healthy tissue, whose collagen fibers are straight and parallel.

Fully describing light’s polarization state—whether linear, circular, elliptical, or partially or completely unpolarized—requires four numbers, often represented as the four-component Stokes vector. Reflection off an object can transform each of those components into any of the others, so it’s described by a 4 × 4 matrix called the Mueller matrix. Mueller-matrix measurements are useful in many fields, including cancer screening. But until now, they’ve required bulky optics, careful calibration, and repeated measurements to collect all 16 components.

Now Aun Zaidi (who recently graduated from Harvard University), his PhD adviser Federico Capasso, and their colleagues have developed a device that can image an object’s entire spatially resolved Mueller matrix in a single shot. Instead of conventional optical components, such as polarizers and wave plates, the device uses metamaterials—intricate arrays of tiny dielectric pillars—to control and analyze complex polarization states of light.

The idea is to illuminate the object with structured light—whose polarization varies on a fine spatial scale—to probe the effect of all possible polarizations at once. The researchers use one metamaterial to create the structured light; after bouncing the light off the object of interest, they use a second metamaterial to diffract it into four polarization components, as shown above. (The fifth image in the center is the zero-order, undiffracted light; it carries no additional polarization information.) Analyzing those images pixel by pixel gives the full Mueller matrix, as shown below.

Sixteen pink squares arranged in a 4 × 4 array. The corner squares contain images of the same beetle shown above.
Mueller-matrix images show that the chiral beetle is a homogeneous circular polarizer. The corner components of the matrix, which carry information about circular polarization, show the beetle’s structure clearly. The matrix elements in the second and third rows and columns, which carry information about linear polarization, are nearly zero. Credit: Adapted from A. Zaidi et al., Nat. Photon. (2024), doi:10.1038/s41566-024-01426-x

The chiral beetle was a proof-of-concept test of the technique: Its shell is a homogeneous circular polarizer, and its Mueller matrix is relatively uniform. But the researchers are most interested in applications in which the Mueller matrix varies over an object’s surface, such as a patient with a skin tumor. (A. Zaidi et al., Nat. Photon., 2024, doi:10.1038/s41566-024-01426-x.)

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