Ultra-Thin Optical Devices Directly Measure Polarization

For the first time, researchers have used ultra-thin layers of 2D structures to create holograms that can measure the polari­zation of light. The new meta­surface holograms could be used to create very fast and compact devices for polari­zation measurements, which are used in spectro­scopy, sensing and communi­cations appli­cations. Now, a multi-insti­tutional group of researchers report using metasurface holograms to effec­tively and quickly determine polari­zation at near-infrared to visible wavelengths. The new work represents a step toward functional metasurface-based devices to support a range of appli­cations from tele­communications to chemical analysis.

Illustration of a metasurface to generate two overlapping holographic images, one that is left-handed circularly polarized and one that is right-handed circularly polarized. By analyzing the interference of the two images, they obtained the amplitude contrast and phase difference between the components of the incident beam. (Source: X. Zhang, J. Han, Tianjin U. / W. Zhang, Oklahoma State U.)

“Holograms made from meta­surfaces are an efficient and effec­tive way to generate high-quality images with sub­wavelength reso­lution,” said research team leader Xueqian Zhang from Tianjin University, China. “Our work uniquely applies metasurface holograms to polari­zation measurements, which could enable camera-size devices that measure polari­zation in one step without moving parts.”

Although sunlight and most household light sources emit unpo­larized light that oscil­lates in all directions, optical components such as filters can be used to produce polarized light that propa­gates in just a single plane – typically vertical or horizontal. Analytical instruments such as spectro­meters can measure how light polari­zation changes after interacting with a material to determine its physical properties. Different light polari­zations can also be used to send multiple signals through optical fibers for tele­communications appli­cations.

Conventional methods for determining polari­zation often require multiple measure­ments, bulky optical setups or precise adjustment of high-quality optical components to indirectly determine the polari­zation state. In the new work, the researchers instead used a meta­surface to determine polari­zation directly by comparing the amplitude and phase of light waves that are polarized at right angles to themselves.

The metasurface generates two over­lapping holo­graphic images, one that is left-handed circularly polarized (LCP) and another that is right-handed circularly po­larized (RCP). Circu­larly polarized light features an electric field oscil­lation plane that rotates to the left or right in a plane perpendicular to the direction of the wave. “The over­lapping images can be simply and quickly captured using a CCD camera,” said Zhang. “By analyzing the inter­ference of the two holo­graphic images, we can obtain the amplitude contrast and phase difference between the LCP and RCP components of the incident beam, thus identi­fying the polari­zation state.”

Key to the new technique was a Gerchberg-Saxton-algorithm, which is widely used in holo­graphic research. The researchers figured out how to modify this algorithm so that it could be used to identify the phase difference between the LCP and RCP components of the incident light in the overlapping holo­graphic images. The researchers demons­trated their new meta­surface holo­graphic approach by using it to measure the polari­zation states of illuminating light beams with known polari­zations. The measured polari­zation states matched well with the known ones, confirming the effec­tiveness of the approach. In the future, the metasurface could be incorporated into a camera’s photo­sensitive area to make a compact device for measuring polari­zation.

The meta­surface the researchers used is based on the Pancha­ratnam-Berry phase method, which features relative phase responses that do not exhibit any dispersion. This allows the metasurface holograms to work over a broad range of wavelengths. “Our method can be extended to many potential appli­cations requiring polari­zation measurement, such as polari­zation spectro­scopy, sensing and communi­cations,” said Zhang. “Polari­zation-encoded holography could also be used for security information trans­mission because only a receiver who knows the desired polari­zation states could decode infor­mation from the final holo­graphic images.” Now that they have proved the concept, the researchers plan to improve the efficiency of the method and will compare its performance with conventional commercial instruments used to measure polari­zation. (Source: OSA)

Reference: X. Zhang et al.: Direct polarization measurement using a multiplexed Pancharatnam-Berry metahologram, Optica 6, 1190 (2019); DOI: 10.1364/OPTICA.6.001190

Link: Micro and Nano Manufacturing, Tianjin University, Tianjin, China

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