Nanotechnology

Multi-focal metals for recognition and reconstruction of spectral ellipticity and polarization

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April 04, 2023

(Nanowerk News) New publication of Opto-Electronic Science (“Multi-foci metalens for recognition and reconstruction of spectral ellipticity and polarization”) considers multi-focal metalens for recognition and reconstruction of spectral ellipticity and polarization.

As the basic properties of light, spectrum and polarization carry important information about the propagation of light waves. For example, a spectral image may reflect the material composition of an object, whereas a polarized image contains information about surface texture, light polarization, and/or the spatial distribution of the optical properties of the scene.

Because of the important information provided by light’s wavelength and polarization, multispectral and polarized imaging technologies are in great demand in various fields of science and technology, including archeology, biology, remote sensing and astronomy.

Conventional multispectral and polarizing imaging devices are based on polarizing filters and analyzers, which usually require taking multiple images to collect the desired optical information and consist of large multi-pass systems or mechanically moving parts and are difficult to integrate into a compact and optical system. integrated. Spectral ellipticity and polarization designs complete multi-focal metals Figure 1. Design spectral ellipticity and polarization of multi-focal metallic finishes. (Image: Compuscript)

A metasurface that achieves complete control over the properties of light, such as phase, amplitude, and polarization state, has been demonstrated. As two-dimensional optical devices consisting of sub-wavelength nanostructures, metasurfaces are suitable for integrated system design. At present, metasurfaces have been used in various types of functional optical devices, such as optical displays, orbital angular momentum devices, beam splitters, meta-holographic elements and light-field imaging.

To realize an integrated and compact design, metasurface elements have been used in polarizing and multispectral optical systems. However, there is still a shortage of metallens devices that can achieve both spectral resolution and polarization functions simultaneously while maintaining good imaging performance with large numerical apertures (NA). On the technical side, although at least three projections are required to determine the polarization state, the Poincare spherical longitude (also expressed as the polarization ellipticity) can also reflect the rich information of the sight.

Prof. research group Wei Xiong, Prof. Jinsong Xia, and Prof. Hui Gao of Huazhong University of Science and Technology proposed the spectral ellipticity and polarization complete multi-foci metalens (SPMM) methodology to realize the spectral ellipticity and polarization complete imaging without the requirement of moving parts or spectral optics and large polarization. Multispectral and polarized imaging using SPMM with a laser source Figure 2. Multispectral and polarized imaging using a SPMM with a laser source. (Image: Compuscript)

Unlike the common multispectral or polarization imaging systems previously demonstrated, the SPMM can collect the optical information of interest with just one shot due to twelve spectra- and polarization-dependent images at different locations, which simplifies the optical information-gathering process.

In this SPMM design, the position and intensity of the focus/image on the focal/imaging plane can be changed by adjusting the ellipticity of the polarization and/or the spectrum of the incident light beam. Therefore, the developed SPMM device has the ability to detect and reconstruct specific polarization ellipticities and discrete wavelengths (or spectral bands) while maintaining the normal functions of metals such as focusing and imaging. And the SPMM has a shared aperture design that has superior imaging performance due to its larger NA than a reported micro-metallens array design with the same fabrication size and focal length.

SPMM experimental demonstrations were performed with coherent and incoherent light to prove its general applicability.

The light from an imaged object contains rich information related to various wavelengths and polarization ellipticities, which are usually lost or ignored in traditional intensity-based imaging methods. To solve this problem, the SPMM generates twelve focuses or images at different positions, which correspond to six spectral bands and two orthogonal circular polarization states.

In addition, spectral ellipticity and polarization (linear, elliptical or circular) associated with a specific object area can be solved and reconstructed by identifying the appropriate focusing/imaging position and relative intensity. Multispectral and polarized imaging uses SPMM with an ordinary white light beam Figure 3. Multispectral and polarized imaging using SPMM with an ordinary white light beam. (Image: Compuscript)

The design and physical mechanism of the SPMM are based on geometric and holographic phase principles. In order to realize transverse dispersive metals, the phase distribution of multiple lenses having different working wavelengths with corresponding focuses at different positions can be encoded onto a single metasurface element by the holographic principle.

The polarization-dependent design of metalens can be obtained by adding these two Hadamard products. The focal position of this metal can be shifted by changing the polarization of the incident light beam. Therefore, an SPMM with twelve foci can be obtained by randomly fusing two transverse dispersive metals as a single metasurface element, as shown in Fig. 1.

Compared with the existing special metasurface spectra or polarization detection elements based on micro-metalens arrays, through the demonstration of SPMM imaging with ordinary coherent (Fig.2) and incoherent (Fig.3) light sources, this work has exhibited its practical potential for the construction of imaging devices. multispectral and ultra-compact polarized without the need for multi-pass designs using complex spectral filters or mechanically moving parts.

In addition, this SPMM concept can be extended to arbitrary point reconstruction with longitude and latitude on the Poincare sphere and achieve much finer spectral band partitioning through improved metalens design and nanofabrication techniques.



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