Ultra-thin optical element for direct measurement of polarization

Nov. 29, 2019

A hologram made with a hypersurface can provide fast, compact polarization measurements for spectroscopy, sensing, and communication. It is reported that researchers from Tianjin University for the first time use ultra-thin two-dimensional structural layers called "supersurfaces" to make A hologram measuring the polarization of light. New hypersurface holograms can be used to create very fast and compact polarization measurement devices for applications such as spectroscopy, sensing, and communications.

A metasurface is a high precision optical components with nanoscale features, and its overall thickness is less than one-fifth of that of human hair. They can be manufactured using standard microelectronics manufacturing techniques for mass production and can be easily integrated into wafer-level optical systems. Despite these promising features, they have not been used in many practical applications. In the Optics Journal of the Optical Society (Optica), researchers from the Cooperative Research Group at Tianjin University in China reported that they used metasurface holograms to efficiently and quickly determine polarizations in the near infrared to visible light bands. This new work represents a step towards hypersurface-based functional devices to support a range of applications from telecommunications to chemical analysis.

Direct measurement of polarization

Although sunlight and most domestic light sources can emit unpolarized light that oscillates in all directions, optical elements such as neutral density filter can produce polarized light, and polarized light can only propagate on one plane, usually vertically or horizontally. Analytical instruments such as spectrum analyzers can measure the polarization change of light after interacting with matter, thereby determining its physical properties. Different light polarizations can also be used to send multiple signals through fiber optics for telecommunication applications.

Traditional polarization measurement methods often require multiple measurements, bulky optical devices or precise adjustment of high-quality optical components to indirectly determine the polarization state. In this new work, researchers have turned to metasurfaces to directly determine polarization by comparing the amplitude and phase of vertically polarized light waves. The metasurface generates two overlapping holographic images, one is left-handed circular polarization (LCP) and the other is right-handed circular polarization (RCP). Circularly polarized light has an electric field oscillation plane that rotates left or right on a plane perpendicular to the direction of the wave.

Zhang Xueqian, research team leader from Tianjin University, said: "The overlapping images can be captured quickly and easily with a CCD camera. By analyzing the interference of two holographic images, we can obtain the amplitude contrast and phase difference between the LCP and RCP components of the incident beam to determine the polarization state." The key to this new technology is an algorithm called Gerchberg-Saxton, which is widely used in holographic research. Researchers have improved the algorithm so that it can be used to identify the phase difference between the LCP and RCP components of incident light in overlapping holograms.



Effective polarization measurement

The researchers demonstrated their new hypersurface holographic method, which is used to measure the polarization state of an illumination beam with a known polarization. The measured polarization state agrees well with the known polarization state, which verifies the effectiveness of the method. In the future, supersurfaces can be integrated into the light-sensitive area of a camera, making it a small device(polarization beamsplitter) for measuring polarization. The metasurfaces used by the researchers are based on the Pancharatnam-Berry phase (also known as geometric phase) method, which is characterized by the relative phase response showing no dispersion. This allows metasurface holograms to operate over a wide range of wavelengths.

"Our approach can be extended to many potential applications that require polarization measurements, such as polarization spectroscopy, sensing, and communications," Zhang said. Polarization-coded holography can also be used for secure information transmission because only receivers that know the required polarization state can decode information from the final holographic image. Now that they have proven the concept, researchers will plan to increase the efficiency of the method and compare its performance with traditional commercial instruments used to measure polarization.

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