image: Design of the SPMM. view more
Credit: OES
A new publication from Opto-Electronic Science; DOI 10.29026/oes.2023.220026 considers multi-foci metalens for spectra and polarization ellipticity recognition and reconstruction.
As fundamental properties of light, spectra and polarization carry vital information concerning the propagation of light waves. For example, spectral imaging can reflect the material composition of objects, while polarized imaging contains information on the texture of the surface, light polarization, and/or spatial distribution of the optical properties of a scene. Owing to the crucial information provided by light wavelength and polarization, multispectral and polarized imaging technologies are of significant interest in various science and technology fields, including archeology, biology, remote sensing, and astronomy. Conventional multispectral and polarization imaging devices are based on filters and polarization analyzers, which usually require to take multiple shots to collect desired optical information and consist of bulky multi-pass systems or mechanically moving parts and are difficult to integrate into compact and integrated optical systems.
Metasurfaces that achieve full control of light properties, such as phases, amplitudes, and polarization states, have been demonstrated. As two-dimensional optical devices consisting of sub-wavelength nanostructures, metasurfaces are suitable for the design of integrated systems. Today, metasurfaces have been used in many different types of functional optical devices, such as optical displays, orbital angular momentum devices, beam splitters, meta-holography elements, and light-field imaging.
To realize integrated and compact designs, metasurface elements have been used in polarization and multispectral optical systems. However, there remains a lack of metalens devices that can achieve both spectra- and polarization-resolved functionalities simultaneously while keeping a good imaging performance with a large numerical aperture (NA). On the technical side, although at least three projections are required to determine the polarization state, the longitude of the Poincare sphere (also expressed as polarization ellipticity) can also reflect abundant information of the scene.
The research groups of Prof. Wei Xiong, Prof. Jinsong Xia, and Prof. Hui Gao from Huazhong University of Science and Technology proposed a spectra- and polarization ellipticity resolved multi-foci metalens (SPMM) methodology to realize the spectra- and polarization ellipticity resolved imaging without the requirement of any moving parts or bulky spectral and polarization optics.
Unlike previously demonstrated common multispectral or polarization imaging systems, the SPMM can collect the desired optical information by only a single shot due to its twelve spectra- and polarization-dependent images at different locations, which simplifies the process of collecting optical information. In this SPMM design, the positions and intensities of foci/images on the focal/imaging plane can be changed by tuning the polarization ellipticity and/or spectra of incident light beams. Therefore, the as-developed SPMM device possesses both detection and reconstruction abilities of specific polarization ellipticity and discrete wavelengths (or spectral bands) while keeping normal functions of metalens such as focusing and imaging. And the SPMM has a sharing aperture design which possesses superior imaging performance due to the larger NA than that of the as-reported micro-metalens array design with the same fabrication size and focal length. Experimental demonstrations of the SPMM are performed with both coherent and incoherent light to prove its general applicability.
The light from imaged objects contains rich information associated with multiple wavelengths and polarization ellipticity, which is usually lost or ignored in traditional intensity-based imaging methods. To address this issue, the SPMM generates twelve foci or images at different positions, which correspond to six bands of spectra and two orthogonal circular polarization states. Furthermore, the spectra and polarization ellipticity (linear, elliptical, or circular) relating to specific object areas can be resolved and reconstructed by identifying the focusing/imaging positions and corresponding relative intensities.
The design and physical mechanism of the SPMM are based on the principles of geometric phase and holography. To realize a transversely dispersive metalens, the phase distributions of multiple lenses that possess different working wavelengths with corresponding foci at different positions can be encoded to a single metasurface element by the holography principle. The polarization-dependent metalens design can be obtained by adding these two Hadamard product results together. The focal position of this metalens can be switched by changing the polarization of the incident light beam. Therefore, an SPMM with twelve foci can be obtained by combining two transversely dispersive metalenses randomly as a single metasurface element, as shown in Fig. 1.
Compared with the existing special metasurface spectra- or polarization detection elements based on a micro-metalens array, through the demonstration of the SPMM imaging with both ordinary coherent (Fig.2) and incoherent light sources (Fig.3), this work has exhibited its practical potential for the construction of ultra-compact multispectral and polarized imaging devices without the need of a multi-pass design using complicated spectral filters or mechanically moving parts. Moreover, this SPMM concept can be extended to the reconstruction of arbitrary points with both longitude and latitude on the Poincare sphere and achieve much finer partition of spectral bands via improved metalens design and nanofabrication techniques.
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The Micro-Nano Optoelectronics Laboratory of Huazhong University of Science and Technology headed by Professor Wei Xiong (supported by the national youth talent support program), mainly focuses on micro-nano scale laser 3D / 4D printing, laser-induced synthesis and assembly of nano functional materials, ultrafast laser imaging and characterization, metasurface micro nano-optical devices, etc. Relying on Wuhan National Laboratory for Optoelectronics, the team has carried out a series of pioneering work in the interdisciplinary fields of ultra-fast laser micro-nano extreme manufacturing technology and equipment. The team has undertaken several projects, such as the National Key R&D Program of China, and the general projects of the National Natural Science Foundation of China. In recent years, the group has published more than 140 papers in international well-known journals such as Science Advances, Nature Communications, Advanced Materials, Light: Science & Application, Nano Letters, etc., and applied for more than 40 authorized and public invention patents and being cited more than 1000 times. Wei Xiong has presented more than 20 reports at international conferences in this field, such as Photonics West、MRS、ICALEO, etc. He has won the best paper award of ICALEO. He has served as the chairman of ICALEO laser nano processing and manufacturing branch of American Laser Association, co-chairman of laser branch of POEM International Conference, and vice president of Wuhan Laser Society of Hubei Province, Member of extreme manufacturing Committee of Chinese society of mechanical engineering.
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Opto-Electronic Science (OES) is a peer-reviewed, open access, interdisciplinary and international journal published by The Institute of Optics and Electronics, Chinese Academy of Sciences as a sister journal of Opto-Electronic Advances (OEA, IF=9.682). OES is dedicated to providing a professional platform to promote academic exchange and accelerate innovation. OES publishes articles, reviews, and letters of the fundamental breakthroughs in basic science of optics and optoelectronics.
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Gao H, Fan XH, Wang YX, Liu YC, Wang XG et al. Multi-foci metalens for spectra and polarization ellipticity recognition and reconstruction. Opto-Electron Sci 2, 220026 (2023). doi: 10.29026/oes.2023.220026
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