Looking at optical Fano resonances under a new light

IMAGE: Conventional Fano-resonant metasurfaces can just show light with a particular frequency, a planar wavefront, and direct polarization; the freshly proposed metasurfaces can be customized to be reflective to light with…
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Credit: Overvig and Alù, DOI: 10.1117/1.AP.3.2.026002

In 1961, physicist Ugo Fano offered the very first theoretical description to an anomalous asymmetry observed in the spectral profiles of worthy gases. He presented an impactful analysis of this phenomenon, now called “Fano resonance,” mentioning that if a discrete fired up state of a system falls within the energy variety of a continuum of other possible states, these 2 can hinder each other and trigger irregular peaks and dips in the system’s frequency action.

Though Fano resonance can take place in different physical systems, current development in metasurfaces and nanotechnology has actually accentuated this phenomenon as a possibly effective tool in optics. The traditional understanding of optical Fano resonances is that they are selective in the momentum-frequency domain; simply put, they can just be delighted by planar light waves with particular frequencies and occurrence angles, therefore restricting their applicability. But could this image really be insufficient?

In a recent study published in Advanced Photonics, researchers Adam Overvig and Andrea Alù from the Advanced Research Center, City University of New York, USA, examined Fano-resonant metasurfaces and found new residential or commercial properties that might open their real capacity. Overvig and Alù surpassed the routine metasurfaces traditionally utilized for generating Fano resonances, showing that rigorous periodicity is not really needed to allow this phenomenon, and as a result existing metasurfaces just represent a particular subset of the Fano resonances that can emerge in optical systems.

A basic example works to get the total essence of the research study. A traditional, routine Fano-resonant metasurface provides strong polarization, and both spectral and angular selectivity. This suggests that the system hardly shows light of any offered frequency, occurrence angle, and polarization unless they particularly match those of its Fano resonance (in which case, ideal reflection takes place). As specified in the past, another essential element of such routine metasurfaces is that they can just go through Fano resonances if the event light waves have a planar wavefront. In plain contrast with these restrictions, the scientists showed that it is possible to craft a nonperiodic metasurface that accomplishes ideal reflection, strangely enough accompanied by stage conjugation of the inbound fields, for light waves with an arbitrarily customized wavefront shape and type.

Overvig and Alù mathematically showed that these metasurfaces can be developed by tactically presenting nonperiodic perturbations in otherwise extremely routine photonic crystal pieces. Their work sheds light on yet-unexplored elements of optical Fano resonance, extending the idea beyond traditional understanding.

The proposed technique has several pertinent applications, as summed up by Alù: “Our finding generalizes the concept of a Fano resonance, showing that it is not necessarily associated with a planar wavefront. In practice, this enables a new class of optical devices that are transparent and weakly interacting with the incoming light for most excitations but are somehow triggered by a specific wavefront form, frequency, and polarization, which can be selected by design. Only under this specific excitation condition, the device becomes highly reflective and sends back a time-reversed version of the specific input.”

He elaborates on the performance of such gadgets: “An example can be a transparent surface that can be illuminated from any angle and any frequency and polarization, and it is always transparent. However, if you illuminate it with a localized point source placed at a specific location only, with the precise frequency and polarization, all the input energy is reflected and focused back at the location of the source.”

The presented idea of generalized Fano resonances might lead the way for advanced metamaterials that control light in unique methods, with amazing applications in a diverse variety of situations not restricted to optics, however likewise extendable to acoustics and other wave phenomena.


Read the open gain access to research study short article by Adam Overvig and Andrea Alù, “Wavefront-selective Fano resonant metasurfaces,” Adv. Photonics 3(2) 026002 (2021), doi 10.1117/1.AP.3.2.026002.

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