As fundamental properties of light, spectra and polarisation carry vital information concerning the propagation of light waves. For example, spectral imaging can reflect the material composition of objects, while polarised imaging contains information on the texture of the surface, light polarisation and/or spatial distribution of the optical properties of a scene. Owing to the crucial information provided by light wavelength and polarisation, multispectral and polarised imaging technologies are of significant interest in various science and technology fields, including archaeology, biology, remote sensing and astronomy. Conventional multispectral and polarisation imaging devices are based on filters and polarisation analysers, which usually require to take multiple shots to collect the 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 polarisation 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 realise integrated and compact designs, metasurface elements have been used in polarisation and multispectral optical systems. However, there remains a lack of metalens devices that can achieve both spectra- and polarisation-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 polarisation state, the longitude of the Poincare sphere (also expressed as polarisation 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 polarisation ellipticity resolved multi-foci metalens (SPMM) methodology to realise the spectra- and polarisation ellipticity resolved imaging without the requirement of any moving parts or bulky spectral and polarization optics.
Unlike previously demonstrated common multispectral or polarisation imaging systems, the SPMM can collect the desired optical information by only a single shot due to its 12 spectra- and polarisation-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 polarisation ellipticity and/or spectra of incident light beams. Therefore, the as-developed SPMM device possesses both detection and reconstruction abilities of specific polarisation ellipticity and discrete wavelengths (or spectral bands) while keeping the normal functions of the 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 have been 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 polarisation ellipticity, which is usually lost or ignored in traditional intensity-based imaging methods. To address this issue, the SPMM generates 12 foci or images at different positions, which correspond to 6 bands of spectra and 2 orthogonal circular polarisation states. Furthermore, the spectra and polarisation 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 realise 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 polarisation-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 polarisation of the incident light beam. Therefore, an SPMM with 12 foci can be obtained by combining 2 transversely dispersive metalenses randomly as a single metasurface element.
Compared with the existing special metasurface spectra- or polarisation detection elements based on a micro-metalens array, through the demonstration of the SPMM imaging with both ordinary coherent and incoherent (see Figure) light sources, this work has exhibited its practical potential for the construction of ultra-compact multispectral and polarised 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.
