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Tunable Metasurface Senses Multidimensional Light-Field Information

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Researchers led by professor Yuanmu Yang from the State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instruments, Tsinghua University have developed a near-infrared spectropolarimeter based on an electrically tunable liquid crystal metasurface. The team demonstrated an approach to achieve simultaneous polarization and spectral measurements through the tunable metasurface, with high-quality-factor guided-mode resonances combined with a computational reconstruction algorithm.

The advancement targets issues with the multiplexing of polarization and spectrum detection mechanisms, which can be used to develop composite systems to simultaneously measure the polarization and spectrum of light. Though the multiplexing approach can yield functional composite systems, the multiplexing inevitably results in a further increased system form factor and complexity, due to the already bulky nature of conventional polarimeters and spectrometers. This hinders their applications in miniaturized platforms.

The team’s approach also offers an alternative to division-of-amplitude polarimeters and spectrometers, which use polarization beamsplitters and dispersive elements. The liquid crystal metasurface could be used in applications that require polarization and spectral measurements, such as in biomedical imaging, remote sensing, and optical communications.

Though metasurface-based spectropolarimetry has been demonstrated by spatially splitting the incident light with different polarization components and wavelengths, such an approach requires a detector array for polarization and spectrum detection at a single spatial location. This prevents its use for spectropolarimetric imaging.

The team turned to an electrically tunable liquid crystal-embedded silicon metasurface, which became the core hardware of its system. The team tailored the metasurface to support multiple, high-quality-factor guided-mode resonances. The metasurface has rich and anisotropic spectral features that could be widely tuned by applying different bias voltages. The system could reconstruct the incident light’s full Stokes parameters and spectrum from the reflected light intensity recorded by a single-pixel photodetector.


In addition, fabrication of the liquid crystal metasurface is fully compatible with the CMOS and liquid crystal on silicon (LCoS) manufacturing processes. This means the system may be mass-produced at a low cost.
Photograph and scanning electron microscopy image of the fabricated liquid crystal metasurface. b,c Experimentally spectropolarimetric reconstruction results for monochromatic (b) and broadband (c) incident light. Courtesy of Yibo Ni et al.
Photograph and scanning electron microscopy image of the fabricated liquid crystal metasurface. Experimentally spectropolarimetric reconstruction results for monochromatic (b) and broadband (c) incident light. Courtesy of Yibo Ni et al.
In its successful demonstration, the metasurface encoded the polarization and spectral information, using its anisotropic high-Q guided-mode resonances. Although the liquid crystal metasurface currently operates in the reflection mode, it is possible to also design a transmissive liquid crystal metasurface to enable more compact integration with the photodetector.

To use the liquid crystal metasurface for spectropolarimetric imaging without sacrificing spatial resolution, the metasurface must be integrated with a proper detector array. Beyond biomedical imaging, remote sensing, and optical communication, the research team’s strategy for development of the metasurface could be extended to construct compact systems that can measure additional light field information, such as the depth of a target scene or the wavefront of the incident light.

The research was published in eLight (www.doi.org/10.1186/s43593-022-00032-0).

Published: November 2022
Glossary
liquid crystal
Liquid crystals are a state of matter that exhibits properties intermediate between those of conventional liquids and solid crystals. In a liquid crystal, the molecules are ordered like those in a crystal, but they can still flow like a liquid. This unique combination of structural order and fluidity gives liquid crystals their distinctive characteristics and makes them valuable in various technological applications, particularly in display technology. Key features and characteristics of...
metasurfaces
Metasurfaces are two-dimensional arrays of subwavelength-scale artificial structures, often referred to as meta-atoms or meta-elements, arranged in a specific pattern to manipulate the propagation of light or other electromagnetic waves at subwavelength scales. These structures can control the phase, amplitude, and polarization of incident light across a planar surface, enabling unprecedented control over the wavefront of light. Key features and characteristics of metasurfaces include: ...
remote sensing
Remote sensing is a method of data collection and observation where information about objects, areas, or phenomena on Earth's surface is gathered from a distance, typically using sensors onboard satellites, aircraft, drones, or other platforms. This technique enables the monitoring and analysis of Earth's surface and atmosphere without direct physical contact. Remote sensing systems capture electromagnetic radiation (such as visible light, infrared, microwave, or radio waves) reflected or...
photodetector
A photodetector, also known as a photosensor or photodiode, is a device that detects and converts light into an electrical signal. Photodetectors are widely used in various applications, ranging from simple light sensing to more complex tasks such as imaging and communication. Key features and principles of photodetectors include: Light sensing: The primary function of a photodetector is to sense or detect light. When photons (particles of light) strike the active area of the photodetector,...
multiplexing
The combination of two or more signals for transmission along a single wire, path or carrier. In most optical communication systems this is referred to as wavelength division multiplexing, in which the combination of different signals for transmission are imbedded in multiple wavelengths over a single optical channel. The optical channel is a fiber optic cable or any other standard optical waveguide.
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