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Schematics of multiplexing metadevices based on coherent wave interferences. Credit: OEA
A recent paper explores the multiplexing of infinite functionalities using a single metasurface, leveraging coherent wave interference.
With advancements in optical sciences and applications, there is a growing demand for multifunctional optical devices capable of integrating as many wave-control functionalities as possible into a single, ultra-compact system.
However, optical devices made from conventional dielectrics rely on the propagation phases of light, which inherently results in bulky device sizes (relative to wavelengths) and/or low efficiencies (due to the absence of magnetic responses). Furthermore, the lack of additional degrees of freedom to manipulate light makes it challenging to use conventional dielectrics to create compact optical devices with multiple functionalities—an issue that significantly hinders optical integration.
Metasurfaces, ultra-thin metamaterials composed of planar subwavelength microstructures with tailored optical responses arranged in specific, pre-designed sequences, offer extraordinary capabilities for controlling light waves and have garnered significant attention in recent years. By designing both the metaatoms and their arrangement, scientists have developed various metasurfaces that locally control scattered waves in terms of phase and polarization, enabling the formation of tailored light beams in the far field based on Huygens’ principle. This approach has led to the realization of numerous wave-manipulation functionalities, including bi- and multifunctional capabilities within a single meta-device.
Multifunctionality in Metasurfaces
Regarding these bi/multi-functional meta-devices, scientists have combined multiple-mechanism generated interface phases to obtain dual/multi-functional light field control, for example, simultaneously utilizing polarization-dependent geometric phases induced by structural rotations and polarization-independent structural resonance phases, realizing different functionalities within one single subwavelength ultra-thin device, greatly advancing the development of integrated optics.
However, existing multi-functional light field control devices mostly require simultaneous variation of multiple different characteristics of the incident light, while employing solely the variation of the incident light polarization can only exhibit no more than two distinct wave-control functionalities, dictated by the number of independent incident polarizations.
To further increase the number of functionalities multiplexed by one single metasurface device, new design strategies need to be developed to overcome the limitation imposed by the number of independent polarization states on the number of independent functionalities.
Novel Design Strategy for Infinite Functionalities
The authors of this article propose an approach to design metadevices exhibiting (in principle) infinite number of wave-control functionalities based on coherent wave interferences tuned by continuously varying the polarization state of incident light, and experimentally verify the concept in the telecom wavelength regime (1550 nm). The research was recently published in the journal Opto-Electronic Advances.
It is proposed that the incident polarization can be projected to the bases of left circular polarization (LCP) and right circular polarization (RCP), i.e. σ0=A+++A–, ± denoting LCP and RCP components respectively. The wave scattered by the metasurface can be denoted by the linear decomposition of the LCP and RCP wave-fronts: A+⋅F+(r)σ+(r)+A-⋅F-(r)σ-(r), F±(r) denoting the LCP and RCP wave-fronts respectively.
Independently design these two wave-fronts of components possessing opposite helicities, the continuous tune of incident polarization, i.e. the ratio of LCP and RCP components, can effectively modulate the wave-front and local polarization of the total field obtained through coherent interference denoted Ff(r)σf(r)=A+⋅F+(r)σ+(r)+A-⋅F-(r)σ-(r), and thus multiplex (in principle) infinite number of wave-control functionalities.
After designing a series of metaatoms with tailored reflection phases and polarization-conversion capabilities, two functional metadevices were constructed and their wave-control functionalities were experimentally calculated under illuminations of light with polarization continuously tuned along a certain path on the Poincare’s sphere.
The experiments demonstrated that:
- The first device generated two distinct, non-overlapping vortex beams with continuously varying strengths.
- The second device produced a single vectorial vortex beam with orbital angular momentum (OAM) and/or local polarization distributions (LPDs) continuously modulated by varying the incident polarization.
The experimental findings were consistent with both numerical simulations and theoretical predictions.
These findings can find numerous applications in practice and can stimulate many future studies. For example, extensions to near-field and far-field complexing and/or transmissive systems are interesting future projects, and using vectorial beams as the incident light can further enrich the wave-manipulation functionalities of the metadevices.
Reference: “Functionality multiplexing in high-efficiency metasurfaces based on coherent wave interferences” by Yuejiao Zhou, Tong Liu, Changhong Dai, Dongyi Wang and Lei Zhou, 3 September 2024, Opto-Electronic Advances.
DOI: 10.29026/oea.2024.240086
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