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. 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).
